- Email: [email protected]
Impact of Dy on structural, dielectric and magnetic properties of Li-Tb-nanoferrites synthesized by micro-emulsion method
Ceramics International (xxxx) xxxx–xxxx
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Impact of Dy on structural, dielectric and magnetic properties of Li-Tbnanoferrites synthesized by micro-emulsion method ⁎
Muhammad Khurram Abbasa, Muhammad Azhar Khana, , Faiza Mushtaqb, Muhammad Farooq Warsic, Muhammad Sherd, Imran Shakire, Mohamed F. Aly Aboude,f a
Department of Physics, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan Department of Biochemistry, Bahauddin Zakariya University, Multan 60800, Pakistan c Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan d Department of Chemistry, University of Sargodha, Sargodha 40100, Pakistan e Sustainable Energy Technologies (SET) Center, College of Engineering, King Saud University, PO-BOX 800, Riyadh 11421, Saudi Arabia f Mining, Metallurgical and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, Nasr City, Cairo 11371, Egypt b
A R T I C L E I N F O
A BS T RAC T
Keywords: Nano-sized ferrites TGA XRD Dielectric properties Magnetic properties
A series of Dy-doped Li–Ni ferrites of the following composition Li0.5Ni0.48Tb0.02DyxFe2−xO4 (0.2≤x≥0) was synthesized by the microemulsion method. The X-ray diffraction (XRD) analysis indicated that Li0.5Ni0.48Tb0.02DyxFe2−xO4nano-crystalline ferrites exhibited the single-phase spinel structure. The lattice parameter was determined by the Nelson-Riley refinement technique and it increased by increasing the Dy contents. The crystallite size was computed from the Debye Scherrer's formula and it was in range from 27 to 40 nm. The thermal decomposition process was studied by the thermogravimetric analysis and the annealing temperature observed was 980 °C. The real and complex parts of dielectric constant decreases very sharply in the lower frequency region, but in the higher frequency region, the real and complex part of dielectric constant show variable values with Dy contents. The dielectric tangent loss (tan δ) decreases exponentially with Dy contents. The magnitude of the ac conductivity decreases in certain frequency region, as the Dy contents are increased. The possible mechanisms contributing to the above behavior are discussed. The results of these nanocrystalline ferrites are very suitable materials for microwave device applications.
1. Introduction Different metal oxides when combined along with iron oxides as their chief component are named as ferrites and word “ferrite” is deduced from a Latin word “ferum” that stands for “iron in different things”. Ferrites are ceramic materials which are composed by different oxides but the Iron oxide is primary constituent of ferrites [1]. From 1932 to 1935, the magnetic oxides were investigated by the Japanese workers [2]. Nanotechnology is an advanced technology, which relates to the synthesis of nano-particles and their particular applications. Typically, if the particle sizes will be in the range of 1–100 nm then they are generally named as nano-particles or materials [3]. Magnetic ceramics, or ferrites, is a well-established group of magnetic materials. Their resistivity range is 10−2–1011 Ω cm [4]. The way of ferrites, their synthesis, the partnership between crystal structure, consistency and physical properties, the actual modeling of magnetic connections, is essentially interdisciplinary [5]. Recent usage of nano-structured magnets seemed economic because of saving in
⁎
energy consumption and less weight when compared with motors it lead to a step forward to green economy [6]. The magnetization will also depend upon presence of impurities and to what extent internal strain of the material is present. Magnetically soft materials are used in armature stamping and transformer lamination. Hard magnetic materials are normally required in applications where we need a “permanent magnetic field” [7]. A soft magnet could be magnetized and attracted to the other magnet, but if it is prevailing in magnetic field [8]. Their properties are not as permanent magnet so they have various applications as in electronic power transformers. These are also used as rotor and stator materials for generators and motors [9]. AB2O4 is the general molecular formula of spinel ferrites. It has also 8 molecules in its unit cell=A8B16O32. A is a divalent ion (Mg2+, Zn2+, Fe2+ etc.) And B indicates a trivalent ion (Al3+, Fe3+ etc.) [10,11]. The inverse spinel ferrites having the general molecular formula is AB2O4 and the entire of A ions, 1/2 of B ions are found in octahedral sites as well as the other 1/2 of B ions, they are put in tetrahedral sites. Almost all “ferrites” possess that inverse spinel composition. Within the above spinel
Corresponding author. E-mail address: [email protected] (M.A. Khan).
http://dx.doi.org/10.1016/j.ceramint.2017.01.075 Received 8 October 2016; Received in revised form 12 January 2017; Accepted 14 January 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Abbas, M.K., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.01.075
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
preparation of nanocrystalline ferrites is environment friendly and less costly as it requires cheap salts and a small amount of organic solvent [19]. Lithium Chloride (LiCl), Nickel (II) Chloride Hexahydrate (NiCl2· 6H2O), TetraterbiumHeptaoxide (Tb4O7), Dysprosium Nitrate Hydrate (Dy(NO3)3.xH2O), Iron(III) Chloride (FeCl3), CetylTrimethyl Ammonium Bromide (CTAB), Aqueous Ammonia (NH4OH) and distilled water were used as chemical agents for the synthesis of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites. The solutions of chemical reagents named above were mixed in six different beakers and these beakers were placed for stirring. Freshly prepared aqueous ammonia solution was used to increase the pH from acidic to basic media and it is added till pH reached at 10. Thereafter at the room temperature, the mixtures were again continuously stirred, for six hours. All the mixtures were kept after stirring in cupboards for a night. Solutions were continuously washed until the pH decreased to neutral level i.e. 7.0. The precipitates of different compositions in beakers were dried at the 100 °C in oven and grinded by mortar and pestle. Controlled Muffle Furnace Vulcan A-550 was used for annealing of dried and grinded samples of each composition. Annealing temperature was kept at about 980 °C. Annealing was performed continuously for 8 h. The samples after annealing were then packed in air restricted glass vials and stored for the purpose of characterization. The Philips X′Pert Pro 3040/60diffractometer along with CuKα as the source of radiation was applied to examine XRD patterns of the prepared samples [20,21]. The X-ray diffractometer is diffracting the X-rays of known wavelength. The principle of X-ray diffraction is also governed by the Bragg's law [22]. The thermal analyzer SDT Q600 V8.2 Build 100 is used to study the thermal analysis (TGA-DTA-DSC). Both TGA and DTA curves are used for examining the weight loss and DSC is a thermal analytical technique for determination of heat flow relating to a sample's phase transitions as a function of time or temperature. This type of analysis provide information regarding qualitative, physical and chemical changes involving both exothermic and endothermic activities collectively denoted as change in heat capacity [23]. The Wayn WK6500B precision instrument was used to investigate the dielectric properties of the synthesized Li-Ni nano-crystalline ferrites characterized at moderate temperature within a specific range of frequency (1 MHz to 3GHz). The parameters observed were dielectric loss along with its dependence on frequency, dielectric constant and tan δ. The peak values vary at varying frequency range.
ferrites, there is just 1 type of trivalent or divalent metal ions, but reality is that there also exist some spinel ferrites exhibiting interesting permanent magnetic properties in various applications. Generally used empirical formula of spinel ferrites is (A1−xBy)(AxB2−y)O4. They bear a wide range of applications; such as, memory cores, high frequency inductor and write / read heads [5,12]. The spinel crystal lattice have closely packed arrangement made up of a unit cell consisting of 32 oxygen ions forming smallest repeating unit in the crystal system. There are two sites A-site and B-site in crystal structure [10]. The cations of metals are ordered in interstitial sites in such a way that two adjacent octants do not have the same configuration and as a result basic unit comprises 8 such octants [10]. The radii of some metal ions and lattice parameter of some ferrites must involve in spinel ferrites [11]. Garnet ferrites have the general formula R3Fe5O12. The garnet ferrites are the basic materials for high technology devices, magneto optical, memory and microwave applications. These ferrites have been studied as single crystals, ceramics, epitaxial and thin films, etc. Yttrium Iron Garnet (YIG) is used in many fundamental studies [5]. Permanent magnets are made of hard ferrites. So these are used in those applications in which a constant magnetic field without electric current as is necessary. A permanent magnet hold energy inside which it was provided in the course of material preparation, during magnetization of the sample [13]. The lithium ferrite has inverse spinel structure. Octahedral and tetrahedral cations have strong interactions due to their orbital geometry. Curie temperature is used to express the relative strengths of both these interactions. Observed curie temperature of Li ferrite is highest in spinels having a value of 958 K [5]. Lithium ferrites have become attractive these days due to their extensive practical applications in technology with a range of frequency from microwave (1011 Hz) to radio-wave (104 Hz). Devices with lithium ferrite bring forth broad applications in microwave field as phase shifters, microwave absorbers, latching devices, isolators and circulators etc, because of their certain properties as; nonreciprocal microwave property, high saturation magnetization, high resistivity and squared hysteresis loop [14]. Li-Zn ferrites being inexpensive material with considerable properties bear greater technological applicability. It is also observed that their intrinsic factors (Curie temperature, resistivity, initial permeability, dielectric constant and magnetization) mainly depend upon their chemical composition, heat treatment and substituted ions. Lithium ions exhibit strong potential to fill A-site and zinc ions exhibit strong potential to fill B-site [5]. Lattice parameters observed for the sample Li0.3Zn0.4Fe2.3O4 (8.3817 Å) compares considerably with those reported before (8.378 Å) [15]. Each antenna has a better selectivity due to quality value (Q) of the spinel ferrites. In order to decrease the demagnetization effect, the length-diameter ratio of spinel ferrite must be kept larger for a specific proportion of length-diameter and to enhance the sensitivity of the antenna, one should increase existing of the spinel ferrite rods [16]. Ferrites are being extensively used in electronics industry for their properties of high electrical resistivity and high magnetic permeability. Frequency range of ferrites are high (1 MHz to 1GHz) and they are being used in making high frequency electronic components [17]. Soft ferrites with a nano-crystalline structure have various other applications. They are used in radio antennas, radio transmitting circuits, microwave appliances, electronic data processing of digital tapes, sensors and transformer cores for their greater resistivity, high Curie temperature, chemical stability, low eddy current losses and less magnetic coercivity [18]. This article presents the influence of minor amount of substituting Dy ions on structural, dielectric and magnetic properties of Li-Tb-Ni ferrites.
3. Results and discussion 3.1. Thermal analysis Nanocrystalline ferrites having the compositional formula Li0.5Ni0.48Tb0.02DyxFe2−xO4(where x=0, 0.05, 0.1, 0.15, 0.2) were analyzed by the autocatalytic combustion processes of differential scanning calorimeter (DSC), differential thermal analysis (DTA) and thermal gravimetric analysis (TGA). Curves of prepared nanocrystalline ferrites for these techniques are illustrated in Fig. 1. The weight loss percentage along with temperature is noted by TGA analysis curve and the particular annealing temperature of lithium ferrites was determined by the thermal process. A continuous weight loss determined by TGA curve is in the temperature range of 26–980 °C. The decomposition process is involved in several steps presented in Fig. 1 [24]. The total weight loss ~35% is shown by the TGA curve. The smallest weight loss determined is about 3% probably due to loss of water in the form of vapors and in succeeding steps approximately equal to 27.5% loss in weight examined at 298.48 °C but in the last step approximately 4.5% weight is reduced at temperature 298.48–798.50 °C which is because of phase transition of the material. From 930.02 °C, the phase development is started, and after this temperature, a very small weight loss of the material was examined as shown in the Fig. 1. The annealing temperature was determined 980 °C and after this temperature, there is no weight loss of the material [25,26].
2. Experimental work In the present work, we have synthesized nanocrystalline Li0.5Ni0.48Tb0.02DyxFe2–xO4 spinel ferrites (where x=0.00, 0.05, 0.1, 0.15, 0.2) using the normal micro-emulsion method. This method for 2
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
(where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all other compositions. The planes (111), (220), (311), (222), (400), (422), (511) and (440) observed in the X-ray diffraction (XRD) patterns confirmed the formation of FCC cubic spinel ferrite structure. It is detected that the crystallite size (D) of the synthesized samples decrease with increase in Dy concentration [30]. Debye Scherrer's formula was used to measure average crystalline size of these samples as given below;
D = Kλ / β Cos θ
(1)
where D is the crystallite size, K is a constant (K=0.9) and it depends only on the shape of the particle, β is the full width at half maximum (FWHM) of the high intensity peak (311), λ is the X-ray wavelength (λ=1.54) and θ is the diffraction angle (Bragg's Angle) of the peak [31]. The average crystallite size lie in the range 27–40 nm at different x values, as given in Table 1 [32]. A peak of secondary phase (orthorhombic phase) was determined at angle 33° and this peak is known as (hkl) plane of RFeO3. The lattice constant (a) is determined by using many peaks from X-ray diffraction (XRD) patterns and plotted these values as a function of the Dy contents (x) as shown in Fig. 2 [33]. The values of the lattice constant (a) were calculated by using the standard Nelson-Riley refinement method and also cell software. The lattice constant (a) lies in the range from 8.329Å to 8.348 Å at different values of Dy concentration (x) as given in Table 1 [34,35]. It is observed that the lattice constant (a) of the prepared samples increases with increase in Dy contents and the result of Dy contents, on lattice constant (a) and the crystallite-size (D) of all prepared samples is labeled in Fig. 3. It's analyzed that lattice constant (a) increases with increase Dy concentration, but the crystallite size is decreased. The (311) plane was shown the maximum intensity of all the samples [36]. The lattice constant (a) is increased because of larger ionic radii of dysprosium (0.912 Å) and terbium (0.93 Å) than the host cations Fe3+(0.64 Å), Ni2+(0.69 Å) and Li+1 (0.74 Å) [37]. The rare earth material has the natural trend of going towards octahedral site because of their large ionic radii [26]. Following expression is used to determine X-ray density of samples;
Fig. 1. TGA, DTA and DSC curves of the prepared lithium-nickel nanocrystalline ferrite Li0.5Ni0.48Tb0.02DyxFe2−xO4 (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
The differential thermal analysis curve has given several different exothermic and endothermic peaks at varying temperature range. These peaks may be developed due to the decomposition process and different chemical reactions. Decomposition process took place in several steps due to these peaks. 1st exothermic peak was seemed at 94.18 °C, 2nd peak at 151 °C, 3rd peak at 238.56 °C and fourth was observed at 418.036 °C. Similarly, the first endothermic peak was observed at 67.52 °C, the second peak at 125.36 °C, the third peak at 178.45 °C while the fourth peak was appeared at 422.45 °C [27]. Both TGA and DTA curves exhibited the weight loss. Maximum weight loss was determined from temperature 238.56–418.036 °C [28]. The DSC curve shows that when the temperature increases initially, it increases the heat flow, but after 323.45 °C further increase in temperature reduces heat flow [26]. 3.2. XRD analysis
dx = ZM /Na3
The X-ray diffraction patterns of prepared samples of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nano-crystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2) are shown in Fig. 2. All these samples of ferrites were prepared by micro-emulsion technique with an annealing temperature 980 °C. The single phase spinel structure of these samples was confirmed by X-ray diffraction (XRD) [29]. In the first prepared sample, there is no rare earth element but the Tb0.02 and the Dyx
where Z=number of molecules/unit cell (Z=8 for every spinel ferrites), ‘a’ is lattice parameters, M is the molecular mass and N is the Avogadro's number. Table 1 show that the values of the X-ray density (dx) increase gradually by an increase in the value of Dy contents (x). It is also analyzed that the value range of X-ray density is 4.80 g/cm3 to 5.29 g/cm3 as given in the Table 1 [27]. The bulk density (d) of the samples was determined by using the given relation:
D = M /V
(2)
(3)
where M is the mass of the sample (determined by using a digital balance, OHAUS model PA214) and V is the cell volume of the sample. Table 1 show that the value of bulk density (d) increases gradually with an increase in the value of Dy concentration (x). It is also observed that the value range of bulk density (d) is 3.61 g/cm3 to 3.98 g/cm3 as given in the Table 1 [38].
D = m / πr 2h
(4)
where m, h and r are the mass, thickness and radius respectively of the pellets [39].
3.3. Dielectric properties Dielectric properties of prepared nanocrystalline ferrites based on their preparation methods, chemical composition, crystalline size and cationic distribution [21,40].
Fig. 2. XRD patterns of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites.
3
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
Table 1 Lattice constant, crystallite size, X-ray density (dx), bulk density (d) and cell volume (Vcell) of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites. Composition (x)
Lattice constant (a) (Å)
Crystallite size (D) (nm)
X-ray density (dx) (g/cm3)
Vcell (a3)
Bulk density (d) (g/cm3)
x=0.00 x=0.00 x=0.05 x=0.10 x=0.15 x=0.20
8.321 8.329 8.330 8.339 8.339 8.348
39.9 35.7 33.4 31.6 29.9 27.8
4.80 4.84 4.96 5.07 5.18 5.29
576.26 577.98 578.15 580.02 580.06 581.86
3.61 3.64 3.73 3.81 3.90 3.98
Fig. 5. Dielectric loss factor (ε'') vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
Fig. 3. The effect of Dy concentration on crystallite size and lattice constant of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
ε″ = ε′ tan δ
3.3.1. Dielectric constant and dielectric loss in prepared nanocrystalline ferrites The variation of dielectric constant and dielectric loss as a function of frequency of the prepared samples of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2) was taken at room temperature with frequencies range 1 MHz to 3GHz as shown in Figs. 4 and 5 respectively [41]. In the first prepared sample, there is no rare earth element but the Tb0.02 (same in all the samples) and the Dyx (where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all other compositions. The dielectric constant (ε') and complex part of dielectric constant (ε'') were determined by using the following equations:
ε′ = Ct / εoA
(6)
where “c” is capacitance, “t” is total surface area of a sample, “A” is surface area of prepared discs, tan δ is dielectric loss and ε' is permittivity of the free space [42]. Figs. 4 and 5 respectively, exhibited that when the frequency increases, then the real and complex parts of dielectric constant decrease gradually and this decrease is very sharp in the lower frequency region, but at high frequency region, the real and complex parts of the dielectric constant show variable values (larger and smaller). The fluctuation of dielectric loss and dielectric constant (ε') with frequency showed deviation probably because of “Maxwell Wagner” interfacial polarization. The polarization is decreased with the increment in the frequency and also reaches at constant value in the given range of frequency [40]. The complex dielectric constant is decreased more rapidly as shown in the Figs. 4 and 5, while the real part of dielectric constant is not decreasing rapidly [36]. Electron replace between the Fe3+and Fe2+ ions gives rise to the phenomena of local movement of charges in order to produce electric field, which determines the polarization. The electronic exchange between Fe2+ and Fe3+ ions are not obeying the alternating field [43]. According to Koop's theory, the dielectric structures of spinel ferrites composed of considerably conducting grains [44] and these are disjointed by the grain boundaries, so these are usually poor conductors. The hopping of electron between Fe2+ and Fe3+ results in the collective electrons at the grain boundaries. At x=0.05 the value of the dielectric constant (ε') and complex part of dielectric constant (ε'') is maximum as given in Figs. 4 and 5 respectively [39].
(5)
3.3.2. Dielectric loss tangent variation with frequency The variation of dielectric loss tangent (tan δ) with frequency of nano-crystalline ferrites samples of Li0.5Ni0.48Tb0.02DyxFe2−xO4 (where x=0, 0, 0.05, 0.1, 0.15, 0.2) was measured at room temperature in the range (1 MHz to 3GHz) as shown in Fig. 6 [45]. In the first prepared sample, there is no rare earth element but the Tb0.02 (same in all the samples) and the Dyx(where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all
Fig. 4. Dielectric constant (ε') vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2–xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
4
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
Fig. 7. Quality factor vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
Fig. 6. Tangent loss (tan δ) vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
created within the grains, after the certain limits of the temperature constituted due to the very high frequency. So the value of Q decreases because these pores have increased the loss-factor. [43]. In the given frequency range the variations display a considerable quality factor with peaks at certain frequencies. Earlier research describes the variation of loss factor according to varying frequency in substituted lithium ferrites [48]. The substitution of Dy also plays an important role with a low loss factor and high value of Q-factor as compared to the previously carried out researches. Fig. 7 shows the loss in Q-factor at different frequencies [42].
other compositions. Fig. 6 demonstrates that tan δ (dielectric tangent loss) decreases gradually with increase of frequency, which is under consideration but at x=0. The value of the dielectric tangent loss (tan δ) is shown highest. Highest tan δ against frequency appeared when frequency of hopping charge carriers (Fe2+ and Fe3+ ions) coincides with frequency of applied alternating field, at above 1.5 GHz [40]. In the high frequency range, the charged defect dipoles lead to the dielectric tangent loss. Furthermore, the loss peaks in Fig. 6 shift to high frequency side as increase in substitution of Dy content. This may be due to the Dy substitution chooses to go on adjacent octahedral site which strengthens the dipole-dipole interactions and accordingly it limits the rotation of the dipoles [39]. Dielectric tangent loss is specified by following expression:
tan δ = ε″/ ε′
3.3.4. AC conductivity (σac) at room temperature with frequency Fig. 8 shows the variation of AC conductivity (σac) with the frequency of the prepared samples of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2) measured in the frequency range 1 MHz to 3GHz at room temperature [49]. Fig. 9 shows the relation between the log omega and log sigma. It is observed that the value of log sigma rapidly increases with the increase of the value of log omega but at a certain value of log omega, the value of log sigma decreases sharply. In the first prepared sample, there is no rare earth element but the Tb0.02 (same in all the samples) and the Dyx (where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all other compositions. AC conductivity of nano-crystalline ferrites is given by following expression [50]:
(7)
Here tan δ is dielectric loss angle ε'' is complex part of dielectric constant and ε' is dielectric constant. According to previous literature, the dielectric tangent loss value depend upon various factors that are Fe2+contents, stoichiometry and structural homogeneity, also depends on the composition and the synthesis methods [46,47]. Dielectric tangent loss value is low in higher frequency regions and high in low frequency regions [47]. Thinner grain boundaries are effective at low frequency while at high frequencies conducting grains are more efficient [41]. High loss in energy is predicted at low frequency (thin grain boundaries) and hopping charges claim here high energy. That's why; dielectric tangent loss is higher in that region. The loss in energy is usually lower in the higher frequency region (good conducting grains), as in that region a low amount of energy is required for hopping charge carriers. That's why dielectric tangent loss is low in that region [41].
σtot = σ1(T ) + σ2(ω, T )
3.3.3. Variation of quality factor (Q) with frequency Fig. 7 shows the variation of quality factor (Q) as a function of frequency of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0.05, 0.1, 0.15, 0.2) in the range of 1 MHz to 3GHz measured at the room temperature [41]. In first prepared sample, there is no rare earth element but the Tb0.02 (same in all the samples) and the Dyx (where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all other compositions. The quality factor (Q) of all the prepared samples displays decreasing trend up to the frequency 1 GHz and then display a constant increase up to the frequency 2.5 GHz as shown in the Fig. 7. Again a decreasing trend in Q-factor value is shown after this particular frequency region, but some peaks are observed at x=0. This altering trend is assigned to the behavior of the constitution of pores which are
(8)
Fig. 8. AC conductivity (σac) vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
5
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
Fig. 11. Electric modulus (M′) vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
Fig. 9. Log sigma verses log omega for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
3.3.5. Electrical modulus The variation of electrical modulus (M') and imaginary electrical modulus (M'') with the frequency of the prepared samples of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2) measured at constant temperature in the range of frequency 1 MHz to 3GHz as shown in Figs. 11 and 12 respectively [53]. In the first prepared sample, there is no rare earth element but the Tb0.02 and the Dyx (where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all other compositions. The complex electric modulus pattern play pivotal role, it can be used to analyze the electrical response of ferromagnetic specimen that is based on polarization analysis. Five parameters could be represented by drawing Cole-Cole plots viz. electric modulus, dielectric loss, permittivity, admittance and impedance. All these are interdependent as expressed in following relation;
Here σ1 denotes DC conductivity and σ2 depends on frequency, it represents AC conductivity due to hopping electrons at octahedral sites [49,51]. It is observed by the Fig. 8 that in the low frequency region, AC conductivity (σac) of all the samples has a same decreasing trend but the value of the AC conductivity (σac) is highest at x=0.15. Nevertheless the dispersion behavior is evidenced in the higher range of frequencies. Both Koop's phenomenon logical theory and Maxwell-Wagner model reveals that ferrite material's part where conducting grains formed is separately marked by grain boundary's thin layering [43]. In specific regions where Dy content increased, AC conductivity value decreased, that can be justified by the involvement of Dy-ions at octahedral sites of these ferrites [45]. It seems that concentration of Dy-ions has an influence on conductivity. Hopping electrons also responsible for change in conductivity, it also increased as the frequency increases. Electronic exchange at B-site stopped when Fe2+ ions concentration decreased. Hence, the AC conductivity (σac) decreases as the Dy contents increase [52]. It is reported that the decrease in AC conductivity (σac) is due to the substitution of different rare-earths ions in Li-Ni ferrites. Fig. 10 shows the relationship between the effect of Dy concentration and the slope (n) of all prepared samples. It is analyzed that the value of the slope is highest at first and last value of the composition (x) [45].
tan δ = ε″/ ε′ = Y ′/ Y ″ = Z ′/ Z ″ = M ″/ M ′
(9)
Dielectric modulus M*; relaxation process can be easily figured out and its real and imaginary portions can be represented as;
M ′ = ε′/(ε′2 + ε″2 )
(10)
M ″ = ε″/(ε′2 + ε″2 )
(11)
For analysis of the dependence of frequency on “interfacial polarization effect” in Li0.5Ni0.48Tb0.02DyxFe2–xO4 nanocrystalline ferrites (where x=0, 0.05, 0.1, 0.15, 0.2) which established accommodation of
Fig. 12. Imaginary electrical modulus (M′') vs frequency (Hz) at room temperature for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
Fig. 10. Slope (n) verses composition (x) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanoferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
6
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
Fig. 13. Relation between real and imaginary parts of dielectric modulus for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
Fig. 15. The MH-loop for Li0.5Ni0.48Tb0.02Fe2O4 ferrite.
electric charge across the magnetic ceramic particle in altering relaxation peaks and electric modulus is used. Therefore if grain boundaries region have large volume M' versus M'' plot gave best information regarding the semicircles. Furthermore in the complex modulus spectra M'' is inversely proportional to capacitance and M′' is reflected because of the existing dispersion in relaxation times. Non-Debye nature of curves and visual aspects of peaks in the spectra are probably due to the effect of intrinsic distributive nature of these materials. In this article, six semicircles are derived and presented from prepared samples in frequency 1.3–3 GHz as shown by Fig. 13. The fluctuation in semicircle radius of various samples interprets the substituted concentration of Dy-ions [43].
3.4. Magnetic properties The M-H loops were plotted using vibrating sample magnetometer (VSM) of each sample of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2). Hysteresis loop of all samples of ferrites were studied by applying the magnetic field of −15,000 to 15,000 Oe as indicated by Figs. 14–19 [54]. Under this applied magnetic field the prepared samples expressed a clear hysteresis behavior [34]. Magnetic properties of pure and dysprosium (Dy) doped nanocrystalline ferrites examined to be narrow that confirmed the soft magnetic qualities of these nano-ferrites. It is detected from the field dependence of magnetization (M-H curves) that the samples behaved as a delicate soft magnetic material with relevant values of hysteresis loss and saturation magnetization [55]. Some magnetic
Fig. 16. The MH-loop for Li0.5Ni0.48Tb0.02Dy0.05Fe1.95O4 ferrite.
Fig. 17. The MH-loop for Li0.5Ni0.48Tb0.02Dy0.1Fe1.90O4 ferrite.
parameters; coercivity (Hc), saturation magnetization (Mc) and remanant magnetization (Mr) are measured by hysteresis loops for each sample. The variation of above parameters with concentration of dysprosium (Dy) and values of squareness ratio (Mr/Ms) and magnetic moment (ɳB) are shown in Table 2 [24]. The variation of saturation magnetization (Ms), coercivity (Hc) and remnant magnetization (Mr) with the Dy contents are shown in Fig. 21 [18]. It indicates that the remanant magnetization (Mr) of the prepared sample increases by increasing concentration of Dy up to 0.05 and then decreases [38].
Fig. 14. The MH-loop for Li0.5Ni0.5Fe2O4 ferrite.
7
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
Fig. 20. The MH-loops for all prepared samples.
Fig. 18. The MH-loop for Li0.5Ni0.48Tb0.02Dy0.15Fe1.85O4 ferrite.
Fig. 19. The MH-loop for Li0.5Ni0.48Tb0.02Dy0.2Fe1.80O4 ferrite. Fig. 21. The variation of Ms, Hc and Mr with Dy concentration for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
The saturation magnetization value (MS) of un-doped lithium ferrite is higher than that of doped material. It is indicated by Fig. 21 that saturation magnetization value decreases (from 94.22 emu/g to 61.99 emu/g) by the increase of Dy contents. This decrease of magnetization by Dy concentration is because of weakened magnetization of B-sublattice of Dy3+ ions. Fe3+ ions exhibit higher magnetic moment than Dy3+ ions so they replace some amount of Dy3+ ions at B-site. If Fe3+ concentration decreases at B-site, magnetization at B-sublattice decreased and resultantly decrease in saturation magnetization value is also observed in Dy3+ substituted samples [18]. In addition, the decrease of saturation magnetization value probably because of large surface to mass ratio and small size of nanoparticles. The decrease in saturation magnetization is also reported by addition of rare earth ions [55]. The coercivity (Hc) of the ferrites increases (from 92.84Oe to 169.84 Oe) by increasing the Dy contents as depicted in Fig. 21. The coercivity values of the Dy doped samples are less when compared with
pure Li- Ni ferrite. The rise in coercivity value is correlated to presence of 2nd phase near or at grain boundaries that didn't allow the movement to domain walls. Factors like crystallinity, magnetic-anisotropy, magnetic domain size, micro-strain, magnetic particle morphology and size distribution are responsible to determine the coercivity [56]. The increase of coercivity (Hc) may be explained by the increase of magnetic anisotropy and microstructures of samples. It can also be interpreted by critical diameter or domain structure of the particle. Then again coercivity (Hc) increases by decreasing particle diameter. As the particle size of the sample constitute single domain size, so the decrease of coercivity by decreasing substitution of Dy ions is in agreement with above discussion [55]. The Dy3+ ions substituting in the spinel ferrite on account of their electronic configuration probably distorted the crystalline area of the lattice causing internal stress [18]. The magnetic moment per unit is measured as in Bohr magneton;
Table 2 The magnetic parameters of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2). Sr no.
Sample composition
Retentivity (emu/g) Mr
Coercivity (Oe) Hc
Saturation (emu/g) Ms
Squareness ratio Mr/ Ms
Magnetic moment (µB)
1 2 3 4 5 6
Li0.5Ni0.5Fe2O4 Li0.5Ni0.48Tb0.02Fe2O4 Li0.5Ni0.48Tb0.02Dy0.05Fe1.95O4 Li0.5Ni0.48Tb0.02Dy0.1Fe1.90O4 Li0.5Ni0.48Tb0.02Dy0.15Fe1.85O4 Li0.5Ni0.48Tb0.02Dy0.2Fe1.80O4
13.46 13.97 15.86 14.15 13.99 13.66
92.84 119.26 130.57 143.77 155.04 169.84
94.22 76.51 69.47 67.47 64.46 61.99
0.1429 0.1826 0.2283 0.2097 0.2170 0.2204
3.517 2.883 2.684 2.672 2.614 2.573
8
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
nB = M × Ms /5585
[8] L.H.V. Vlack, Elements of Material Science and Engineering, 6th edition, AddisonWesley Co. Publishers Ltd., New York, 1998. [9] D.K.M. Ghauri, Development of Ferrite Magnets, Department of Metallurgical Engg., UET Lahore, 1998. [10] A. Goldman, Modern Ferrite Technology, second edition, New York: Pittsburgh, PA, USA, 1990 [11] G. Montemezzani, A.A. Zozulya, L. Czaia, D.Z. Anderson, M. Zgonik, P. Gunter, Origin of the lobe structure in photorefractive beam fanning, Phy. Rev. A 52 (2) (1995) 1791–1794. [12] B.S. Boyanor, J. Therm. Anal. (1994). [13] C.D. Graham, B.D. Cullity, Introduction to Magnetic Materials, Second edition, IEEE Press, 445 Hoes Lane Piscataway, NJ 08854, 2009. [14] M.K. Rendale, S.N. Mathad, D.C. Kulkarni, V. Puri, Dielectric and magnetic properties of substituted Li-Zn ferrite thick films clouded over a half wavelength microstrip rejection filter, Int. J. Self Propag. High Temp. Synth. 25 (2) (2016) 86–91. [15] A.A. Sattar, H.M. El-Sayed, W.R. Agami, Physical and Magnetic Properties of Calcium-Substituted Li-Zn Ferrite, J. Mater. Eng. Perform. 16 (5) (2007) 573–577. [16] N. Ichinose, Introduction to Fine Ceramics, Ohmsha Publishers Ltd., Japan, 1987. [17] V.R.K. Murthy, B. Viswanathan, Science and Technology, Narosa Publishing House, New Delhi, 1990. [18] J. Jiang, Y.-M. Yang, L.-C. Li, Synthesis and magnetic properties of lanthanumsubstituted lithium–nickel ferrites via a soft chemistry route, Phys. B: Condens. Matter 399 (2) (2007) 105–108. [19] Daliya S. Mathew, Ruey-Shin Juang, An overview of the structure and magnetism of spinel ferrite nanoparticles and their synthesis in microemulsions, Chem. Eng. J. 129 (2007) 51–65. [20] P.R. Arjunwadkar, R.R. Patil, D.K. Kulkarni, Effect of sintering temperature on the structural, electrical and magnetic properties of Li0.5Al1.0Fe2O4 ferrite prepared by combustion method, J. Alloy. Compd. 463 (1–2) (2008) 403–407. [21] E. De Fazio, P.G. Bercoff, S.E. Jacobo, Electromagnetic properties of manganese– zinc ferrite with lithium substitution, J. Magn. Magn. Mater. 323 (22) (2011) 2813–2817. [22] B.D. Cullity, Elements of X-ray Diffraction, 2nd edition, Addison-Wesley Reading, Massachusetts, 1978. [23] E.L. Charsley, Thermal Analysis. Techniques and Applications, Royal Society of Chemistry, Cambridge, 1992. [24] J. Jiang, L. Li, F. Xu, Polyaniline–LiNi ferrite core–shell composite: preparation, characterization and properties, Mater. Sci. Eng.: A 456 (1–2) (2007) 300–304. [25] V. Mohanty, et al., Ce-substituted lithium ferrite: preparation and electrical relaxation studies, J. Mater. Sci. Technol. 30 (4) (2014) 335–341. [26] M. Azhar Khan, et al., High frequency dielectric response and magnetic studies of Zn1−xTbxFe2O4 nanocrystalline ferrites synthesized via micro-emulsion technique, J. Magn. Magn. Mater. 360 (0) (2014) 188–192. [27] T. Namgyal, et al., δ-phase evolution in Cd-doped lithium ferrites: their thermal, electrical and magnetic properties, J. Mol. Struct. 1019 (0) (2012) 103–109. [28] S. Sutradhar, et al., Sol–gel derived nanoparticles of Zn substituted lithium ferrite (Li0.32Zn0.36Fe2.32O4): magnetic and Mössbauer effect measurements and their theoretical analysis, J. Magn. Magn. Mater. 324 (7) (2012) 1317–1325. [29] V. Verma, et al., Influence of silicon substitution on the properties of lithium ferrite, J. Alloy. Compd. 443 (1–2) (2007) 178–181. [30] R.K. Kotnala, et al., Minimizing of power loss in Li–Cd ferrite by nickel substitution for power applications, J. Magn. Magn. Mater. 322 (22) (2010) 3714–3719. [31] I. Soibam, S. Phanjoubam, C. Prakash, Magnetic and Mössbauer studies of Ni substituted Li–Zn ferrite, J. Magn. Magn. Mater. 321 (18) (2009) 2779–2782. [32] N. Singh, et al., Synthesis, microstructure, dielectric and magnetic properties of Cu substituted Ni–Li ferrites, J. Magn. Magn. Mater. 323 (5) (2011) 486–492. [33] A.I. Ali, et al., Effect of the La3+ ions substitution on the magnetic properties of spinal Li-Zn-ferrites at low temperature, J. Mater. Res. Technol. 2 (4) (2013) 356–361. [34] M.F. Al-Hilli, S. Li, K.S. Kassim, Structural analysis, magnetic and electrical properties of samarium substituted lithium–nickel mixed ferrites, J. Magn. Magn. Mater. 324 (5) (2012) 873–879. [35] F. Al-Hilli, M.S. Li, K.S. Kassim, Microstructure, electrical properties and Hall coefficient of europium-doped Li–Ni ferrites, Mater. Sci. Eng.: B 158 (1–3) (2009) 1–6. [36] R. Ali, et al., Impacts of Ni–Co substitution on the structural, magnetic and dielectric properties of magnesium nano-ferrites fabricated by micro-emulsion method, J. Alloy. Compd. 584 (0) (2014) 363–368. [37] M. Maisnam, et al., Structural and DC resistivity behaviour of Li–Mn–Ni ferrites substituted with trace amount of Co2+, Phys. B: Condens. Matter 370 (1–4) (2005) 1–5. [38] S.T. Assar, H.F. Abosheiasha, Structure and magnetic properties of Co–Ni–Li ferrites synthesized by citrate precursor method, J. Magn. Magn. Mater. 324 (22) (2012) 3846–3852. [39] H. Malik, et al., Influence of cobalt substitution on the magnetic properties of zinc nanocrystals synthesized via micro-emulsion route, Ceram. Int. 40 (7, Part A) (2014) 9439–9444. [40] R. Ali, et al., Structural, magnetic and dielectric behavior of Mg1−xCaxNiyFe2−yO4 nano-ferrites synthesized by the micro-emulsion method, Ceram. Int. 40 (3) (2014) 3841–3846. [41] M. Ishaque, et al., Study on the electromagnetic behavior evaluation of Y3+ doped cobalt nanocrystals synthesized via co-precipitation route, J. Magn. Magn. Mater. 372 (0) (2014) 68–73. [42] M.N. Akhtar, et al., Structural, morphological, dielectric and magnetic characterizations of Ni0.6Cu0.2Zn0.2Fe2O4 (NCZF/MWCNTs/PVDF) nanocomposites for
(12)
Here M is molecular weight of a nanoferrite and Ms is saturation magnetization (emu/g) [27]. The squareness ratio (Mr/Ms) of the prepared sample is in the range of 0.142–0.220. The magnetic moment (µB) decreases (from 3.517 to 2.573) with the increase of Dy contents as shown in the Table 2. As the localized 4f electrons are responsible for rare earth ion's magnetic moments so their magnetic dipolar orientation is distorted at room temperature [54]. Final magnetic moment of lattice is M=(Mb-Ma) while Ma and Mb are magnetic moments of A and B sites. A-B intermolecular forces are strong as compared to A-A and A-B, resultantly decrease of saturation magnetization value due to weak A-B interactions. Magnetic moment of each engaged ion is responsible for each prepared composition. The dysprosium having larger ionic radius, always replaced by Fe3+(0.64 Å) ions at octahedral sites in this way minimizing their opportunity to take the tetrahedral sites. Hence, the probability of lower magnetic moment values on octahedral sites increases as compared to tetrahedral sites. The smaller magnetic moment at B-sublattice lowers the magnetization in addition to net decrease in magnetization of samples [54]. 4. Conclusion The micro-emulsion method was adapted to prepare the nanocrystalline dysprosium-doped Li-Ni ferrites having formula Li0.5Ni0.48Tb0.02DyxFe2 – xO4 (where x=0, 0.05, 0.1, 0.15, 0.2). The impacts of Dy concentration on lattice parameters; crystallite size, bulk density, X-ray density and dielectric properties were investigated and analyzed. FCC single phase spinel structure was exhibited by all samples. The increase in Dy substitution caused an increase in lattice parameter, exception is observed in crystalline size that decreased and it lies in the range of 27–40 nm. A peak of secondary phase (orthorhombic phase) was observed for higher Dy concentration. The thermal process was studied by the TGA, DTA and DSC. The coercivity of these ferrites increased from 92.84Oe to 169.84 Oe by increasing the Dy contents. The change in frequency caused variation in dielectric loss and dielectric constant. It exhibited dispersion due to interfacial polarization of Maxwell Wagner type. A maximum in dielectric tangent loss (tan δ) versus frequency shows when the frequency of the moving charge carriers coincides with frequency of the utilized alternating field. The maxima in dielectric tangent loss (tan δ) fall out beyond 1.5 GHz. The decreasing trend in Q-factor value is observed after the particular frequency region, although some peak values are observed at x=0. This study exhibits that the dielectric properties of mixed ferrite mostly depend on the substituted amount of dysprosium. The results of dielectric properties suggested that these nanocrystalline ferrites are suitable materials for microwave devices applications. Acknowledgement One of the authors (Dr. Mohamed F. Al Aboud) is thankful to King Saud University, Riyadh, Saudi Arabia for Research Grant (RG-1437025). References [1] B. Viswanathan, Ferrites Materials, Springer-Verlag Narosa Publishing, New Delhi, 1990. [2] E.C. Snelling, Soft Ferrites. Properties and Applications, Butterworth and Co. (Publishers) Ltd., London, 1988. [3] A.K. Bandyopadhyay, Nano Materials Vol. 1, Newage International(P) Limited, Publishers, Daryaganj, New Delhi–110002, 2008. [4] J. Smit, Ferrites, John Wiley and Sons Publishers, New York, 1959. [5] R. Valenzuela, Magnetic ceramics. Chemistry of Solid State Materials, Cambridge University Press, New York, 1994. [6] R.W. Kelsall, Nanoscale Science and Technology, John Wiley & Sons Ltd, England, 2005. [7] A.H.W. Ngan, R.E. Smallman, Physical Metallurgy and Advanced Materials, Seventh edition, Elsevier Ltd., Burlington, 2007.
9
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
[43]
[44] [45] [46]
[47] [48]
[49]
[50] A.M. Abo El Ata, et al., Studies of AC electrical conductivity and initial magnetic permeability of rare-earth-substituted Li–Co ferrites, J. Magn. Magn. Mater. 297 (1) (2006) 33–43. [51] M.A. Dar, et al., Synthesis and characterization of nano-sized pure and Al-doped lithium ferrite having high value of dielectric constant, J. Alloy. Compd. 493 (1–2) (2010) 553–560. [52] P.P. Hankare, et al., Magnetic and dielectric properties of nanophase manganesesubstituted lithium ferrite, J. Magn. Magn. Mater. 321 (19) (2009) 3270–3273. [53] S.S. Teixeira, M.P.F. Graça, L.C. Costa, Dielectric, morphological and structural properties of lithium ferrite powders prepared by solid state method, J. Non-Cryst. Solids 358 (16) (2012) 1924–1929. [54] M. Asif Iqbal, et al., Effect of Gd-substitiution on physical and magnetic properties of Li1.2Mg0.4 Gdx Fe(2−x)O4 ferrites, J. Alloy. Compd. 579 (0) (2013) 181–186. [55] I. Ahmad, et al., Structural and magnetic properties of erbium doped nanocrystalline Li–Ni ferrites, Ceram. Int. 40 (6) (2014) 7941–7945. [56] J. Jing, L. Liangchao, X. Feng, Structural analysis and magnetic properties of Gddoped Li-Ni ferrites prepared using rheological phase reaction method, J. Rare Earths 25 (1) (2007) 79–83.
multilayer chip inductor (MLCI) applications, Ceram. Int. 40 (10, Part A) (2014) 15821–15829. M. Asif Iqbal, et al., High frequency dielectric properties of Eu+3-substituted Li– Mg ferrites synthesized by sol–gel auto-combustion method, J. Alloy. Compd. 586 (0) (2014) 404–410. M. Ishaque, et al., Investigations on structural, electrical and dielectric properties of yttrium substituted Mg-ferrites, Ceram. Int. 41 (3, Part A) (2015) 4028–4034. M.A. Khan, et al., Magnetic and dielectric behavior of terbium substituted Mg1−xTbxFe2O4 ferrites, J. Alloy. Compd. 519 (0) (2012) 156–160. M.Y. Lodhi, et al., New Mg0.5CoxZn0.5−xFe2O4 nano-ferrites: structural elucidation and electromagnetic behavior evaluation, Curr. Appl. Phys. 14 (5) (2014) 716–720. Z.K. Heiba, et al., Cation distribution and dielectric properties of nanocrystalline gallium substituted nickel ferrite, J. Alloy. Compd. 586 (0) (2014) 773–781. S.C. Watawe, et al., Preparation and dielectric properties of cadmium substituted lithium ferrite using microwave-induced combustion, Mater. Chem. Phys. 103 (2– 3) (2007) 323–328. M.A. Khan, et al., Magnetic, ferromagnetic resonance and electrical transport study of Ni1−xTbxFe2O4 spinel ferrites, Ceram. Int. 40 (2) (2014) 3571–3577.
10
Contents lists available at ScienceDirect
Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Impact of Dy on structural, dielectric and magnetic properties of Li-Tbnanoferrites synthesized by micro-emulsion method ⁎
Muhammad Khurram Abbasa, Muhammad Azhar Khana, , Faiza Mushtaqb, Muhammad Farooq Warsic, Muhammad Sherd, Imran Shakire, Mohamed F. Aly Aboude,f a
Department of Physics, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan Department of Biochemistry, Bahauddin Zakariya University, Multan 60800, Pakistan c Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan d Department of Chemistry, University of Sargodha, Sargodha 40100, Pakistan e Sustainable Energy Technologies (SET) Center, College of Engineering, King Saud University, PO-BOX 800, Riyadh 11421, Saudi Arabia f Mining, Metallurgical and Petroleum Engineering Department, Faculty of Engineering, Al-Azhar University, Nasr City, Cairo 11371, Egypt b
A R T I C L E I N F O
A BS T RAC T
Keywords: Nano-sized ferrites TGA XRD Dielectric properties Magnetic properties
A series of Dy-doped Li–Ni ferrites of the following composition Li0.5Ni0.48Tb0.02DyxFe2−xO4 (0.2≤x≥0) was synthesized by the microemulsion method. The X-ray diffraction (XRD) analysis indicated that Li0.5Ni0.48Tb0.02DyxFe2−xO4nano-crystalline ferrites exhibited the single-phase spinel structure. The lattice parameter was determined by the Nelson-Riley refinement technique and it increased by increasing the Dy contents. The crystallite size was computed from the Debye Scherrer's formula and it was in range from 27 to 40 nm. The thermal decomposition process was studied by the thermogravimetric analysis and the annealing temperature observed was 980 °C. The real and complex parts of dielectric constant decreases very sharply in the lower frequency region, but in the higher frequency region, the real and complex part of dielectric constant show variable values with Dy contents. The dielectric tangent loss (tan δ) decreases exponentially with Dy contents. The magnitude of the ac conductivity decreases in certain frequency region, as the Dy contents are increased. The possible mechanisms contributing to the above behavior are discussed. The results of these nanocrystalline ferrites are very suitable materials for microwave device applications.
1. Introduction Different metal oxides when combined along with iron oxides as their chief component are named as ferrites and word “ferrite” is deduced from a Latin word “ferum” that stands for “iron in different things”. Ferrites are ceramic materials which are composed by different oxides but the Iron oxide is primary constituent of ferrites [1]. From 1932 to 1935, the magnetic oxides were investigated by the Japanese workers [2]. Nanotechnology is an advanced technology, which relates to the synthesis of nano-particles and their particular applications. Typically, if the particle sizes will be in the range of 1–100 nm then they are generally named as nano-particles or materials [3]. Magnetic ceramics, or ferrites, is a well-established group of magnetic materials. Their resistivity range is 10−2–1011 Ω cm [4]. The way of ferrites, their synthesis, the partnership between crystal structure, consistency and physical properties, the actual modeling of magnetic connections, is essentially interdisciplinary [5]. Recent usage of nano-structured magnets seemed economic because of saving in
⁎
energy consumption and less weight when compared with motors it lead to a step forward to green economy [6]. The magnetization will also depend upon presence of impurities and to what extent internal strain of the material is present. Magnetically soft materials are used in armature stamping and transformer lamination. Hard magnetic materials are normally required in applications where we need a “permanent magnetic field” [7]. A soft magnet could be magnetized and attracted to the other magnet, but if it is prevailing in magnetic field [8]. Their properties are not as permanent magnet so they have various applications as in electronic power transformers. These are also used as rotor and stator materials for generators and motors [9]. AB2O4 is the general molecular formula of spinel ferrites. It has also 8 molecules in its unit cell=A8B16O32. A is a divalent ion (Mg2+, Zn2+, Fe2+ etc.) And B indicates a trivalent ion (Al3+, Fe3+ etc.) [10,11]. The inverse spinel ferrites having the general molecular formula is AB2O4 and the entire of A ions, 1/2 of B ions are found in octahedral sites as well as the other 1/2 of B ions, they are put in tetrahedral sites. Almost all “ferrites” possess that inverse spinel composition. Within the above spinel
Corresponding author. E-mail address: [email protected] (M.A. Khan).
http://dx.doi.org/10.1016/j.ceramint.2017.01.075 Received 8 October 2016; Received in revised form 12 January 2017; Accepted 14 January 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
Please cite this article as: Abbas, M.K., Ceramics International (2017), http://dx.doi.org/10.1016/j.ceramint.2017.01.075
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
preparation of nanocrystalline ferrites is environment friendly and less costly as it requires cheap salts and a small amount of organic solvent [19]. Lithium Chloride (LiCl), Nickel (II) Chloride Hexahydrate (NiCl2· 6H2O), TetraterbiumHeptaoxide (Tb4O7), Dysprosium Nitrate Hydrate (Dy(NO3)3.xH2O), Iron(III) Chloride (FeCl3), CetylTrimethyl Ammonium Bromide (CTAB), Aqueous Ammonia (NH4OH) and distilled water were used as chemical agents for the synthesis of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites. The solutions of chemical reagents named above were mixed in six different beakers and these beakers were placed for stirring. Freshly prepared aqueous ammonia solution was used to increase the pH from acidic to basic media and it is added till pH reached at 10. Thereafter at the room temperature, the mixtures were again continuously stirred, for six hours. All the mixtures were kept after stirring in cupboards for a night. Solutions were continuously washed until the pH decreased to neutral level i.e. 7.0. The precipitates of different compositions in beakers were dried at the 100 °C in oven and grinded by mortar and pestle. Controlled Muffle Furnace Vulcan A-550 was used for annealing of dried and grinded samples of each composition. Annealing temperature was kept at about 980 °C. Annealing was performed continuously for 8 h. The samples after annealing were then packed in air restricted glass vials and stored for the purpose of characterization. The Philips X′Pert Pro 3040/60diffractometer along with CuKα as the source of radiation was applied to examine XRD patterns of the prepared samples [20,21]. The X-ray diffractometer is diffracting the X-rays of known wavelength. The principle of X-ray diffraction is also governed by the Bragg's law [22]. The thermal analyzer SDT Q600 V8.2 Build 100 is used to study the thermal analysis (TGA-DTA-DSC). Both TGA and DTA curves are used for examining the weight loss and DSC is a thermal analytical technique for determination of heat flow relating to a sample's phase transitions as a function of time or temperature. This type of analysis provide information regarding qualitative, physical and chemical changes involving both exothermic and endothermic activities collectively denoted as change in heat capacity [23]. The Wayn WK6500B precision instrument was used to investigate the dielectric properties of the synthesized Li-Ni nano-crystalline ferrites characterized at moderate temperature within a specific range of frequency (1 MHz to 3GHz). The parameters observed were dielectric loss along with its dependence on frequency, dielectric constant and tan δ. The peak values vary at varying frequency range.
ferrites, there is just 1 type of trivalent or divalent metal ions, but reality is that there also exist some spinel ferrites exhibiting interesting permanent magnetic properties in various applications. Generally used empirical formula of spinel ferrites is (A1−xBy)(AxB2−y)O4. They bear a wide range of applications; such as, memory cores, high frequency inductor and write / read heads [5,12]. The spinel crystal lattice have closely packed arrangement made up of a unit cell consisting of 32 oxygen ions forming smallest repeating unit in the crystal system. There are two sites A-site and B-site in crystal structure [10]. The cations of metals are ordered in interstitial sites in such a way that two adjacent octants do not have the same configuration and as a result basic unit comprises 8 such octants [10]. The radii of some metal ions and lattice parameter of some ferrites must involve in spinel ferrites [11]. Garnet ferrites have the general formula R3Fe5O12. The garnet ferrites are the basic materials for high technology devices, magneto optical, memory and microwave applications. These ferrites have been studied as single crystals, ceramics, epitaxial and thin films, etc. Yttrium Iron Garnet (YIG) is used in many fundamental studies [5]. Permanent magnets are made of hard ferrites. So these are used in those applications in which a constant magnetic field without electric current as is necessary. A permanent magnet hold energy inside which it was provided in the course of material preparation, during magnetization of the sample [13]. The lithium ferrite has inverse spinel structure. Octahedral and tetrahedral cations have strong interactions due to their orbital geometry. Curie temperature is used to express the relative strengths of both these interactions. Observed curie temperature of Li ferrite is highest in spinels having a value of 958 K [5]. Lithium ferrites have become attractive these days due to their extensive practical applications in technology with a range of frequency from microwave (1011 Hz) to radio-wave (104 Hz). Devices with lithium ferrite bring forth broad applications in microwave field as phase shifters, microwave absorbers, latching devices, isolators and circulators etc, because of their certain properties as; nonreciprocal microwave property, high saturation magnetization, high resistivity and squared hysteresis loop [14]. Li-Zn ferrites being inexpensive material with considerable properties bear greater technological applicability. It is also observed that their intrinsic factors (Curie temperature, resistivity, initial permeability, dielectric constant and magnetization) mainly depend upon their chemical composition, heat treatment and substituted ions. Lithium ions exhibit strong potential to fill A-site and zinc ions exhibit strong potential to fill B-site [5]. Lattice parameters observed for the sample Li0.3Zn0.4Fe2.3O4 (8.3817 Å) compares considerably with those reported before (8.378 Å) [15]. Each antenna has a better selectivity due to quality value (Q) of the spinel ferrites. In order to decrease the demagnetization effect, the length-diameter ratio of spinel ferrite must be kept larger for a specific proportion of length-diameter and to enhance the sensitivity of the antenna, one should increase existing of the spinel ferrite rods [16]. Ferrites are being extensively used in electronics industry for their properties of high electrical resistivity and high magnetic permeability. Frequency range of ferrites are high (1 MHz to 1GHz) and they are being used in making high frequency electronic components [17]. Soft ferrites with a nano-crystalline structure have various other applications. They are used in radio antennas, radio transmitting circuits, microwave appliances, electronic data processing of digital tapes, sensors and transformer cores for their greater resistivity, high Curie temperature, chemical stability, low eddy current losses and less magnetic coercivity [18]. This article presents the influence of minor amount of substituting Dy ions on structural, dielectric and magnetic properties of Li-Tb-Ni ferrites.
3. Results and discussion 3.1. Thermal analysis Nanocrystalline ferrites having the compositional formula Li0.5Ni0.48Tb0.02DyxFe2−xO4(where x=0, 0.05, 0.1, 0.15, 0.2) were analyzed by the autocatalytic combustion processes of differential scanning calorimeter (DSC), differential thermal analysis (DTA) and thermal gravimetric analysis (TGA). Curves of prepared nanocrystalline ferrites for these techniques are illustrated in Fig. 1. The weight loss percentage along with temperature is noted by TGA analysis curve and the particular annealing temperature of lithium ferrites was determined by the thermal process. A continuous weight loss determined by TGA curve is in the temperature range of 26–980 °C. The decomposition process is involved in several steps presented in Fig. 1 [24]. The total weight loss ~35% is shown by the TGA curve. The smallest weight loss determined is about 3% probably due to loss of water in the form of vapors and in succeeding steps approximately equal to 27.5% loss in weight examined at 298.48 °C but in the last step approximately 4.5% weight is reduced at temperature 298.48–798.50 °C which is because of phase transition of the material. From 930.02 °C, the phase development is started, and after this temperature, a very small weight loss of the material was examined as shown in the Fig. 1. The annealing temperature was determined 980 °C and after this temperature, there is no weight loss of the material [25,26].
2. Experimental work In the present work, we have synthesized nanocrystalline Li0.5Ni0.48Tb0.02DyxFe2–xO4 spinel ferrites (where x=0.00, 0.05, 0.1, 0.15, 0.2) using the normal micro-emulsion method. This method for 2
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
(where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all other compositions. The planes (111), (220), (311), (222), (400), (422), (511) and (440) observed in the X-ray diffraction (XRD) patterns confirmed the formation of FCC cubic spinel ferrite structure. It is detected that the crystallite size (D) of the synthesized samples decrease with increase in Dy concentration [30]. Debye Scherrer's formula was used to measure average crystalline size of these samples as given below;
D = Kλ / β Cos θ
(1)
where D is the crystallite size, K is a constant (K=0.9) and it depends only on the shape of the particle, β is the full width at half maximum (FWHM) of the high intensity peak (311), λ is the X-ray wavelength (λ=1.54) and θ is the diffraction angle (Bragg's Angle) of the peak [31]. The average crystallite size lie in the range 27–40 nm at different x values, as given in Table 1 [32]. A peak of secondary phase (orthorhombic phase) was determined at angle 33° and this peak is known as (hkl) plane of RFeO3. The lattice constant (a) is determined by using many peaks from X-ray diffraction (XRD) patterns and plotted these values as a function of the Dy contents (x) as shown in Fig. 2 [33]. The values of the lattice constant (a) were calculated by using the standard Nelson-Riley refinement method and also cell software. The lattice constant (a) lies in the range from 8.329Å to 8.348 Å at different values of Dy concentration (x) as given in Table 1 [34,35]. It is observed that the lattice constant (a) of the prepared samples increases with increase in Dy contents and the result of Dy contents, on lattice constant (a) and the crystallite-size (D) of all prepared samples is labeled in Fig. 3. It's analyzed that lattice constant (a) increases with increase Dy concentration, but the crystallite size is decreased. The (311) plane was shown the maximum intensity of all the samples [36]. The lattice constant (a) is increased because of larger ionic radii of dysprosium (0.912 Å) and terbium (0.93 Å) than the host cations Fe3+(0.64 Å), Ni2+(0.69 Å) and Li+1 (0.74 Å) [37]. The rare earth material has the natural trend of going towards octahedral site because of their large ionic radii [26]. Following expression is used to determine X-ray density of samples;
Fig. 1. TGA, DTA and DSC curves of the prepared lithium-nickel nanocrystalline ferrite Li0.5Ni0.48Tb0.02DyxFe2−xO4 (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
The differential thermal analysis curve has given several different exothermic and endothermic peaks at varying temperature range. These peaks may be developed due to the decomposition process and different chemical reactions. Decomposition process took place in several steps due to these peaks. 1st exothermic peak was seemed at 94.18 °C, 2nd peak at 151 °C, 3rd peak at 238.56 °C and fourth was observed at 418.036 °C. Similarly, the first endothermic peak was observed at 67.52 °C, the second peak at 125.36 °C, the third peak at 178.45 °C while the fourth peak was appeared at 422.45 °C [27]. Both TGA and DTA curves exhibited the weight loss. Maximum weight loss was determined from temperature 238.56–418.036 °C [28]. The DSC curve shows that when the temperature increases initially, it increases the heat flow, but after 323.45 °C further increase in temperature reduces heat flow [26]. 3.2. XRD analysis
dx = ZM /Na3
The X-ray diffraction patterns of prepared samples of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nano-crystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2) are shown in Fig. 2. All these samples of ferrites were prepared by micro-emulsion technique with an annealing temperature 980 °C. The single phase spinel structure of these samples was confirmed by X-ray diffraction (XRD) [29]. In the first prepared sample, there is no rare earth element but the Tb0.02 and the Dyx
where Z=number of molecules/unit cell (Z=8 for every spinel ferrites), ‘a’ is lattice parameters, M is the molecular mass and N is the Avogadro's number. Table 1 show that the values of the X-ray density (dx) increase gradually by an increase in the value of Dy contents (x). It is also analyzed that the value range of X-ray density is 4.80 g/cm3 to 5.29 g/cm3 as given in the Table 1 [27]. The bulk density (d) of the samples was determined by using the given relation:
D = M /V
(2)
(3)
where M is the mass of the sample (determined by using a digital balance, OHAUS model PA214) and V is the cell volume of the sample. Table 1 show that the value of bulk density (d) increases gradually with an increase in the value of Dy concentration (x). It is also observed that the value range of bulk density (d) is 3.61 g/cm3 to 3.98 g/cm3 as given in the Table 1 [38].
D = m / πr 2h
(4)
where m, h and r are the mass, thickness and radius respectively of the pellets [39].
3.3. Dielectric properties Dielectric properties of prepared nanocrystalline ferrites based on their preparation methods, chemical composition, crystalline size and cationic distribution [21,40].
Fig. 2. XRD patterns of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites.
3
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
Table 1 Lattice constant, crystallite size, X-ray density (dx), bulk density (d) and cell volume (Vcell) of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites. Composition (x)
Lattice constant (a) (Å)
Crystallite size (D) (nm)
X-ray density (dx) (g/cm3)
Vcell (a3)
Bulk density (d) (g/cm3)
x=0.00 x=0.00 x=0.05 x=0.10 x=0.15 x=0.20
8.321 8.329 8.330 8.339 8.339 8.348
39.9 35.7 33.4 31.6 29.9 27.8
4.80 4.84 4.96 5.07 5.18 5.29
576.26 577.98 578.15 580.02 580.06 581.86
3.61 3.64 3.73 3.81 3.90 3.98
Fig. 5. Dielectric loss factor (ε'') vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
Fig. 3. The effect of Dy concentration on crystallite size and lattice constant of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
ε″ = ε′ tan δ
3.3.1. Dielectric constant and dielectric loss in prepared nanocrystalline ferrites The variation of dielectric constant and dielectric loss as a function of frequency of the prepared samples of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2) was taken at room temperature with frequencies range 1 MHz to 3GHz as shown in Figs. 4 and 5 respectively [41]. In the first prepared sample, there is no rare earth element but the Tb0.02 (same in all the samples) and the Dyx (where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all other compositions. The dielectric constant (ε') and complex part of dielectric constant (ε'') were determined by using the following equations:
ε′ = Ct / εoA
(6)
where “c” is capacitance, “t” is total surface area of a sample, “A” is surface area of prepared discs, tan δ is dielectric loss and ε' is permittivity of the free space [42]. Figs. 4 and 5 respectively, exhibited that when the frequency increases, then the real and complex parts of dielectric constant decrease gradually and this decrease is very sharp in the lower frequency region, but at high frequency region, the real and complex parts of the dielectric constant show variable values (larger and smaller). The fluctuation of dielectric loss and dielectric constant (ε') with frequency showed deviation probably because of “Maxwell Wagner” interfacial polarization. The polarization is decreased with the increment in the frequency and also reaches at constant value in the given range of frequency [40]. The complex dielectric constant is decreased more rapidly as shown in the Figs. 4 and 5, while the real part of dielectric constant is not decreasing rapidly [36]. Electron replace between the Fe3+and Fe2+ ions gives rise to the phenomena of local movement of charges in order to produce electric field, which determines the polarization. The electronic exchange between Fe2+ and Fe3+ ions are not obeying the alternating field [43]. According to Koop's theory, the dielectric structures of spinel ferrites composed of considerably conducting grains [44] and these are disjointed by the grain boundaries, so these are usually poor conductors. The hopping of electron between Fe2+ and Fe3+ results in the collective electrons at the grain boundaries. At x=0.05 the value of the dielectric constant (ε') and complex part of dielectric constant (ε'') is maximum as given in Figs. 4 and 5 respectively [39].
(5)
3.3.2. Dielectric loss tangent variation with frequency The variation of dielectric loss tangent (tan δ) with frequency of nano-crystalline ferrites samples of Li0.5Ni0.48Tb0.02DyxFe2−xO4 (where x=0, 0, 0.05, 0.1, 0.15, 0.2) was measured at room temperature in the range (1 MHz to 3GHz) as shown in Fig. 6 [45]. In the first prepared sample, there is no rare earth element but the Tb0.02 (same in all the samples) and the Dyx(where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all
Fig. 4. Dielectric constant (ε') vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2–xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
4
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
Fig. 7. Quality factor vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
Fig. 6. Tangent loss (tan δ) vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
created within the grains, after the certain limits of the temperature constituted due to the very high frequency. So the value of Q decreases because these pores have increased the loss-factor. [43]. In the given frequency range the variations display a considerable quality factor with peaks at certain frequencies. Earlier research describes the variation of loss factor according to varying frequency in substituted lithium ferrites [48]. The substitution of Dy also plays an important role with a low loss factor and high value of Q-factor as compared to the previously carried out researches. Fig. 7 shows the loss in Q-factor at different frequencies [42].
other compositions. Fig. 6 demonstrates that tan δ (dielectric tangent loss) decreases gradually with increase of frequency, which is under consideration but at x=0. The value of the dielectric tangent loss (tan δ) is shown highest. Highest tan δ against frequency appeared when frequency of hopping charge carriers (Fe2+ and Fe3+ ions) coincides with frequency of applied alternating field, at above 1.5 GHz [40]. In the high frequency range, the charged defect dipoles lead to the dielectric tangent loss. Furthermore, the loss peaks in Fig. 6 shift to high frequency side as increase in substitution of Dy content. This may be due to the Dy substitution chooses to go on adjacent octahedral site which strengthens the dipole-dipole interactions and accordingly it limits the rotation of the dipoles [39]. Dielectric tangent loss is specified by following expression:
tan δ = ε″/ ε′
3.3.4. AC conductivity (σac) at room temperature with frequency Fig. 8 shows the variation of AC conductivity (σac) with the frequency of the prepared samples of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2) measured in the frequency range 1 MHz to 3GHz at room temperature [49]. Fig. 9 shows the relation between the log omega and log sigma. It is observed that the value of log sigma rapidly increases with the increase of the value of log omega but at a certain value of log omega, the value of log sigma decreases sharply. In the first prepared sample, there is no rare earth element but the Tb0.02 (same in all the samples) and the Dyx (where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all other compositions. AC conductivity of nano-crystalline ferrites is given by following expression [50]:
(7)
Here tan δ is dielectric loss angle ε'' is complex part of dielectric constant and ε' is dielectric constant. According to previous literature, the dielectric tangent loss value depend upon various factors that are Fe2+contents, stoichiometry and structural homogeneity, also depends on the composition and the synthesis methods [46,47]. Dielectric tangent loss value is low in higher frequency regions and high in low frequency regions [47]. Thinner grain boundaries are effective at low frequency while at high frequencies conducting grains are more efficient [41]. High loss in energy is predicted at low frequency (thin grain boundaries) and hopping charges claim here high energy. That's why; dielectric tangent loss is higher in that region. The loss in energy is usually lower in the higher frequency region (good conducting grains), as in that region a low amount of energy is required for hopping charge carriers. That's why dielectric tangent loss is low in that region [41].
σtot = σ1(T ) + σ2(ω, T )
3.3.3. Variation of quality factor (Q) with frequency Fig. 7 shows the variation of quality factor (Q) as a function of frequency of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0.05, 0.1, 0.15, 0.2) in the range of 1 MHz to 3GHz measured at the room temperature [41]. In first prepared sample, there is no rare earth element but the Tb0.02 (same in all the samples) and the Dyx (where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all other compositions. The quality factor (Q) of all the prepared samples displays decreasing trend up to the frequency 1 GHz and then display a constant increase up to the frequency 2.5 GHz as shown in the Fig. 7. Again a decreasing trend in Q-factor value is shown after this particular frequency region, but some peaks are observed at x=0. This altering trend is assigned to the behavior of the constitution of pores which are
(8)
Fig. 8. AC conductivity (σac) vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
5
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
Fig. 11. Electric modulus (M′) vs frequency (Hz) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
Fig. 9. Log sigma verses log omega for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
3.3.5. Electrical modulus The variation of electrical modulus (M') and imaginary electrical modulus (M'') with the frequency of the prepared samples of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2) measured at constant temperature in the range of frequency 1 MHz to 3GHz as shown in Figs. 11 and 12 respectively [53]. In the first prepared sample, there is no rare earth element but the Tb0.02 and the Dyx (where x=0, 0.05, 0.1, 0.15, 0.2) are doped in all other compositions. The complex electric modulus pattern play pivotal role, it can be used to analyze the electrical response of ferromagnetic specimen that is based on polarization analysis. Five parameters could be represented by drawing Cole-Cole plots viz. electric modulus, dielectric loss, permittivity, admittance and impedance. All these are interdependent as expressed in following relation;
Here σ1 denotes DC conductivity and σ2 depends on frequency, it represents AC conductivity due to hopping electrons at octahedral sites [49,51]. It is observed by the Fig. 8 that in the low frequency region, AC conductivity (σac) of all the samples has a same decreasing trend but the value of the AC conductivity (σac) is highest at x=0.15. Nevertheless the dispersion behavior is evidenced in the higher range of frequencies. Both Koop's phenomenon logical theory and Maxwell-Wagner model reveals that ferrite material's part where conducting grains formed is separately marked by grain boundary's thin layering [43]. In specific regions where Dy content increased, AC conductivity value decreased, that can be justified by the involvement of Dy-ions at octahedral sites of these ferrites [45]. It seems that concentration of Dy-ions has an influence on conductivity. Hopping electrons also responsible for change in conductivity, it also increased as the frequency increases. Electronic exchange at B-site stopped when Fe2+ ions concentration decreased. Hence, the AC conductivity (σac) decreases as the Dy contents increase [52]. It is reported that the decrease in AC conductivity (σac) is due to the substitution of different rare-earths ions in Li-Ni ferrites. Fig. 10 shows the relationship between the effect of Dy concentration and the slope (n) of all prepared samples. It is analyzed that the value of the slope is highest at first and last value of the composition (x) [45].
tan δ = ε″/ ε′ = Y ′/ Y ″ = Z ′/ Z ″ = M ″/ M ′
(9)
Dielectric modulus M*; relaxation process can be easily figured out and its real and imaginary portions can be represented as;
M ′ = ε′/(ε′2 + ε″2 )
(10)
M ″ = ε″/(ε′2 + ε″2 )
(11)
For analysis of the dependence of frequency on “interfacial polarization effect” in Li0.5Ni0.48Tb0.02DyxFe2–xO4 nanocrystalline ferrites (where x=0, 0.05, 0.1, 0.15, 0.2) which established accommodation of
Fig. 12. Imaginary electrical modulus (M′') vs frequency (Hz) at room temperature for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
Fig. 10. Slope (n) verses composition (x) for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanoferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
6
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
Fig. 13. Relation between real and imaginary parts of dielectric modulus for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
Fig. 15. The MH-loop for Li0.5Ni0.48Tb0.02Fe2O4 ferrite.
electric charge across the magnetic ceramic particle in altering relaxation peaks and electric modulus is used. Therefore if grain boundaries region have large volume M' versus M'' plot gave best information regarding the semicircles. Furthermore in the complex modulus spectra M'' is inversely proportional to capacitance and M′' is reflected because of the existing dispersion in relaxation times. Non-Debye nature of curves and visual aspects of peaks in the spectra are probably due to the effect of intrinsic distributive nature of these materials. In this article, six semicircles are derived and presented from prepared samples in frequency 1.3–3 GHz as shown by Fig. 13. The fluctuation in semicircle radius of various samples interprets the substituted concentration of Dy-ions [43].
3.4. Magnetic properties The M-H loops were plotted using vibrating sample magnetometer (VSM) of each sample of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2). Hysteresis loop of all samples of ferrites were studied by applying the magnetic field of −15,000 to 15,000 Oe as indicated by Figs. 14–19 [54]. Under this applied magnetic field the prepared samples expressed a clear hysteresis behavior [34]. Magnetic properties of pure and dysprosium (Dy) doped nanocrystalline ferrites examined to be narrow that confirmed the soft magnetic qualities of these nano-ferrites. It is detected from the field dependence of magnetization (M-H curves) that the samples behaved as a delicate soft magnetic material with relevant values of hysteresis loss and saturation magnetization [55]. Some magnetic
Fig. 16. The MH-loop for Li0.5Ni0.48Tb0.02Dy0.05Fe1.95O4 ferrite.
Fig. 17. The MH-loop for Li0.5Ni0.48Tb0.02Dy0.1Fe1.90O4 ferrite.
parameters; coercivity (Hc), saturation magnetization (Mc) and remanant magnetization (Mr) are measured by hysteresis loops for each sample. The variation of above parameters with concentration of dysprosium (Dy) and values of squareness ratio (Mr/Ms) and magnetic moment (ɳB) are shown in Table 2 [24]. The variation of saturation magnetization (Ms), coercivity (Hc) and remnant magnetization (Mr) with the Dy contents are shown in Fig. 21 [18]. It indicates that the remanant magnetization (Mr) of the prepared sample increases by increasing concentration of Dy up to 0.05 and then decreases [38].
Fig. 14. The MH-loop for Li0.5Ni0.5Fe2O4 ferrite.
7
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
Fig. 20. The MH-loops for all prepared samples.
Fig. 18. The MH-loop for Li0.5Ni0.48Tb0.02Dy0.15Fe1.85O4 ferrite.
Fig. 19. The MH-loop for Li0.5Ni0.48Tb0.02Dy0.2Fe1.80O4 ferrite. Fig. 21. The variation of Ms, Hc and Mr with Dy concentration for Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2).
The saturation magnetization value (MS) of un-doped lithium ferrite is higher than that of doped material. It is indicated by Fig. 21 that saturation magnetization value decreases (from 94.22 emu/g to 61.99 emu/g) by the increase of Dy contents. This decrease of magnetization by Dy concentration is because of weakened magnetization of B-sublattice of Dy3+ ions. Fe3+ ions exhibit higher magnetic moment than Dy3+ ions so they replace some amount of Dy3+ ions at B-site. If Fe3+ concentration decreases at B-site, magnetization at B-sublattice decreased and resultantly decrease in saturation magnetization value is also observed in Dy3+ substituted samples [18]. In addition, the decrease of saturation magnetization value probably because of large surface to mass ratio and small size of nanoparticles. The decrease in saturation magnetization is also reported by addition of rare earth ions [55]. The coercivity (Hc) of the ferrites increases (from 92.84Oe to 169.84 Oe) by increasing the Dy contents as depicted in Fig. 21. The coercivity values of the Dy doped samples are less when compared with
pure Li- Ni ferrite. The rise in coercivity value is correlated to presence of 2nd phase near or at grain boundaries that didn't allow the movement to domain walls. Factors like crystallinity, magnetic-anisotropy, magnetic domain size, micro-strain, magnetic particle morphology and size distribution are responsible to determine the coercivity [56]. The increase of coercivity (Hc) may be explained by the increase of magnetic anisotropy and microstructures of samples. It can also be interpreted by critical diameter or domain structure of the particle. Then again coercivity (Hc) increases by decreasing particle diameter. As the particle size of the sample constitute single domain size, so the decrease of coercivity by decreasing substitution of Dy ions is in agreement with above discussion [55]. The Dy3+ ions substituting in the spinel ferrite on account of their electronic configuration probably distorted the crystalline area of the lattice causing internal stress [18]. The magnetic moment per unit is measured as in Bohr magneton;
Table 2 The magnetic parameters of Li0.5Ni0.48Tb0.02DyxFe2−xO4 nanocrystalline ferrites (where x=0, 0, 0.05, 0.1, 0.15, 0.2). Sr no.
Sample composition
Retentivity (emu/g) Mr
Coercivity (Oe) Hc
Saturation (emu/g) Ms
Squareness ratio Mr/ Ms
Magnetic moment (µB)
1 2 3 4 5 6
Li0.5Ni0.5Fe2O4 Li0.5Ni0.48Tb0.02Fe2O4 Li0.5Ni0.48Tb0.02Dy0.05Fe1.95O4 Li0.5Ni0.48Tb0.02Dy0.1Fe1.90O4 Li0.5Ni0.48Tb0.02Dy0.15Fe1.85O4 Li0.5Ni0.48Tb0.02Dy0.2Fe1.80O4
13.46 13.97 15.86 14.15 13.99 13.66
92.84 119.26 130.57 143.77 155.04 169.84
94.22 76.51 69.47 67.47 64.46 61.99
0.1429 0.1826 0.2283 0.2097 0.2170 0.2204
3.517 2.883 2.684 2.672 2.614 2.573
8
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
nB = M × Ms /5585
[8] L.H.V. Vlack, Elements of Material Science and Engineering, 6th edition, AddisonWesley Co. Publishers Ltd., New York, 1998. [9] D.K.M. Ghauri, Development of Ferrite Magnets, Department of Metallurgical Engg., UET Lahore, 1998. [10] A. Goldman, Modern Ferrite Technology, second edition, New York: Pittsburgh, PA, USA, 1990 [11] G. Montemezzani, A.A. Zozulya, L. Czaia, D.Z. Anderson, M. Zgonik, P. Gunter, Origin of the lobe structure in photorefractive beam fanning, Phy. Rev. A 52 (2) (1995) 1791–1794. [12] B.S. Boyanor, J. Therm. Anal. (1994). [13] C.D. Graham, B.D. Cullity, Introduction to Magnetic Materials, Second edition, IEEE Press, 445 Hoes Lane Piscataway, NJ 08854, 2009. [14] M.K. Rendale, S.N. Mathad, D.C. Kulkarni, V. Puri, Dielectric and magnetic properties of substituted Li-Zn ferrite thick films clouded over a half wavelength microstrip rejection filter, Int. J. Self Propag. High Temp. Synth. 25 (2) (2016) 86–91. [15] A.A. Sattar, H.M. El-Sayed, W.R. Agami, Physical and Magnetic Properties of Calcium-Substituted Li-Zn Ferrite, J. Mater. Eng. Perform. 16 (5) (2007) 573–577. [16] N. Ichinose, Introduction to Fine Ceramics, Ohmsha Publishers Ltd., Japan, 1987. [17] V.R.K. Murthy, B. Viswanathan, Science and Technology, Narosa Publishing House, New Delhi, 1990. [18] J. Jiang, Y.-M. Yang, L.-C. Li, Synthesis and magnetic properties of lanthanumsubstituted lithium–nickel ferrites via a soft chemistry route, Phys. B: Condens. Matter 399 (2) (2007) 105–108. [19] Daliya S. Mathew, Ruey-Shin Juang, An overview of the structure and magnetism of spinel ferrite nanoparticles and their synthesis in microemulsions, Chem. Eng. J. 129 (2007) 51–65. [20] P.R. Arjunwadkar, R.R. Patil, D.K. Kulkarni, Effect of sintering temperature on the structural, electrical and magnetic properties of Li0.5Al1.0Fe2O4 ferrite prepared by combustion method, J. Alloy. Compd. 463 (1–2) (2008) 403–407. [21] E. De Fazio, P.G. Bercoff, S.E. Jacobo, Electromagnetic properties of manganese– zinc ferrite with lithium substitution, J. Magn. Magn. Mater. 323 (22) (2011) 2813–2817. [22] B.D. Cullity, Elements of X-ray Diffraction, 2nd edition, Addison-Wesley Reading, Massachusetts, 1978. [23] E.L. Charsley, Thermal Analysis. Techniques and Applications, Royal Society of Chemistry, Cambridge, 1992. [24] J. Jiang, L. Li, F. Xu, Polyaniline–LiNi ferrite core–shell composite: preparation, characterization and properties, Mater. Sci. Eng.: A 456 (1–2) (2007) 300–304. [25] V. Mohanty, et al., Ce-substituted lithium ferrite: preparation and electrical relaxation studies, J. Mater. Sci. Technol. 30 (4) (2014) 335–341. [26] M. Azhar Khan, et al., High frequency dielectric response and magnetic studies of Zn1−xTbxFe2O4 nanocrystalline ferrites synthesized via micro-emulsion technique, J. Magn. Magn. Mater. 360 (0) (2014) 188–192. [27] T. Namgyal, et al., δ-phase evolution in Cd-doped lithium ferrites: their thermal, electrical and magnetic properties, J. Mol. Struct. 1019 (0) (2012) 103–109. [28] S. Sutradhar, et al., Sol–gel derived nanoparticles of Zn substituted lithium ferrite (Li0.32Zn0.36Fe2.32O4): magnetic and Mössbauer effect measurements and their theoretical analysis, J. Magn. Magn. Mater. 324 (7) (2012) 1317–1325. [29] V. Verma, et al., Influence of silicon substitution on the properties of lithium ferrite, J. Alloy. Compd. 443 (1–2) (2007) 178–181. [30] R.K. Kotnala, et al., Minimizing of power loss in Li–Cd ferrite by nickel substitution for power applications, J. Magn. Magn. Mater. 322 (22) (2010) 3714–3719. [31] I. Soibam, S. Phanjoubam, C. Prakash, Magnetic and Mössbauer studies of Ni substituted Li–Zn ferrite, J. Magn. Magn. Mater. 321 (18) (2009) 2779–2782. [32] N. Singh, et al., Synthesis, microstructure, dielectric and magnetic properties of Cu substituted Ni–Li ferrites, J. Magn. Magn. Mater. 323 (5) (2011) 486–492. [33] A.I. Ali, et al., Effect of the La3+ ions substitution on the magnetic properties of spinal Li-Zn-ferrites at low temperature, J. Mater. Res. Technol. 2 (4) (2013) 356–361. [34] M.F. Al-Hilli, S. Li, K.S. Kassim, Structural analysis, magnetic and electrical properties of samarium substituted lithium–nickel mixed ferrites, J. Magn. Magn. Mater. 324 (5) (2012) 873–879. [35] F. Al-Hilli, M.S. Li, K.S. Kassim, Microstructure, electrical properties and Hall coefficient of europium-doped Li–Ni ferrites, Mater. Sci. Eng.: B 158 (1–3) (2009) 1–6. [36] R. Ali, et al., Impacts of Ni–Co substitution on the structural, magnetic and dielectric properties of magnesium nano-ferrites fabricated by micro-emulsion method, J. Alloy. Compd. 584 (0) (2014) 363–368. [37] M. Maisnam, et al., Structural and DC resistivity behaviour of Li–Mn–Ni ferrites substituted with trace amount of Co2+, Phys. B: Condens. Matter 370 (1–4) (2005) 1–5. [38] S.T. Assar, H.F. Abosheiasha, Structure and magnetic properties of Co–Ni–Li ferrites synthesized by citrate precursor method, J. Magn. Magn. Mater. 324 (22) (2012) 3846–3852. [39] H. Malik, et al., Influence of cobalt substitution on the magnetic properties of zinc nanocrystals synthesized via micro-emulsion route, Ceram. Int. 40 (7, Part A) (2014) 9439–9444. [40] R. Ali, et al., Structural, magnetic and dielectric behavior of Mg1−xCaxNiyFe2−yO4 nano-ferrites synthesized by the micro-emulsion method, Ceram. Int. 40 (3) (2014) 3841–3846. [41] M. Ishaque, et al., Study on the electromagnetic behavior evaluation of Y3+ doped cobalt nanocrystals synthesized via co-precipitation route, J. Magn. Magn. Mater. 372 (0) (2014) 68–73. [42] M.N. Akhtar, et al., Structural, morphological, dielectric and magnetic characterizations of Ni0.6Cu0.2Zn0.2Fe2O4 (NCZF/MWCNTs/PVDF) nanocomposites for
(12)
Here M is molecular weight of a nanoferrite and Ms is saturation magnetization (emu/g) [27]. The squareness ratio (Mr/Ms) of the prepared sample is in the range of 0.142–0.220. The magnetic moment (µB) decreases (from 3.517 to 2.573) with the increase of Dy contents as shown in the Table 2. As the localized 4f electrons are responsible for rare earth ion's magnetic moments so their magnetic dipolar orientation is distorted at room temperature [54]. Final magnetic moment of lattice is M=(Mb-Ma) while Ma and Mb are magnetic moments of A and B sites. A-B intermolecular forces are strong as compared to A-A and A-B, resultantly decrease of saturation magnetization value due to weak A-B interactions. Magnetic moment of each engaged ion is responsible for each prepared composition. The dysprosium having larger ionic radius, always replaced by Fe3+(0.64 Å) ions at octahedral sites in this way minimizing their opportunity to take the tetrahedral sites. Hence, the probability of lower magnetic moment values on octahedral sites increases as compared to tetrahedral sites. The smaller magnetic moment at B-sublattice lowers the magnetization in addition to net decrease in magnetization of samples [54]. 4. Conclusion The micro-emulsion method was adapted to prepare the nanocrystalline dysprosium-doped Li-Ni ferrites having formula Li0.5Ni0.48Tb0.02DyxFe2 – xO4 (where x=0, 0.05, 0.1, 0.15, 0.2). The impacts of Dy concentration on lattice parameters; crystallite size, bulk density, X-ray density and dielectric properties were investigated and analyzed. FCC single phase spinel structure was exhibited by all samples. The increase in Dy substitution caused an increase in lattice parameter, exception is observed in crystalline size that decreased and it lies in the range of 27–40 nm. A peak of secondary phase (orthorhombic phase) was observed for higher Dy concentration. The thermal process was studied by the TGA, DTA and DSC. The coercivity of these ferrites increased from 92.84Oe to 169.84 Oe by increasing the Dy contents. The change in frequency caused variation in dielectric loss and dielectric constant. It exhibited dispersion due to interfacial polarization of Maxwell Wagner type. A maximum in dielectric tangent loss (tan δ) versus frequency shows when the frequency of the moving charge carriers coincides with frequency of the utilized alternating field. The maxima in dielectric tangent loss (tan δ) fall out beyond 1.5 GHz. The decreasing trend in Q-factor value is observed after the particular frequency region, although some peak values are observed at x=0. This study exhibits that the dielectric properties of mixed ferrite mostly depend on the substituted amount of dysprosium. The results of dielectric properties suggested that these nanocrystalline ferrites are suitable materials for microwave devices applications. Acknowledgement One of the authors (Dr. Mohamed F. Al Aboud) is thankful to King Saud University, Riyadh, Saudi Arabia for Research Grant (RG-1437025). References [1] B. Viswanathan, Ferrites Materials, Springer-Verlag Narosa Publishing, New Delhi, 1990. [2] E.C. Snelling, Soft Ferrites. Properties and Applications, Butterworth and Co. (Publishers) Ltd., London, 1988. [3] A.K. Bandyopadhyay, Nano Materials Vol. 1, Newage International(P) Limited, Publishers, Daryaganj, New Delhi–110002, 2008. [4] J. Smit, Ferrites, John Wiley and Sons Publishers, New York, 1959. [5] R. Valenzuela, Magnetic ceramics. Chemistry of Solid State Materials, Cambridge University Press, New York, 1994. [6] R.W. Kelsall, Nanoscale Science and Technology, John Wiley & Sons Ltd, England, 2005. [7] A.H.W. Ngan, R.E. Smallman, Physical Metallurgy and Advanced Materials, Seventh edition, Elsevier Ltd., Burlington, 2007.
9
Ceramics International (xxxx) xxxx–xxxx
M.K. Abbas et al.
[43]
[44] [45] [46]
[47] [48]
[49]
[50] A.M. Abo El Ata, et al., Studies of AC electrical conductivity and initial magnetic permeability of rare-earth-substituted Li–Co ferrites, J. Magn. Magn. Mater. 297 (1) (2006) 33–43. [51] M.A. Dar, et al., Synthesis and characterization of nano-sized pure and Al-doped lithium ferrite having high value of dielectric constant, J. Alloy. Compd. 493 (1–2) (2010) 553–560. [52] P.P. Hankare, et al., Magnetic and dielectric properties of nanophase manganesesubstituted lithium ferrite, J. Magn. Magn. Mater. 321 (19) (2009) 3270–3273. [53] S.S. Teixeira, M.P.F. Graça, L.C. Costa, Dielectric, morphological and structural properties of lithium ferrite powders prepared by solid state method, J. Non-Cryst. Solids 358 (16) (2012) 1924–1929. [54] M. Asif Iqbal, et al., Effect of Gd-substitiution on physical and magnetic properties of Li1.2Mg0.4 Gdx Fe(2−x)O4 ferrites, J. Alloy. Compd. 579 (0) (2013) 181–186. [55] I. Ahmad, et al., Structural and magnetic properties of erbium doped nanocrystalline Li–Ni ferrites, Ceram. Int. 40 (6) (2014) 7941–7945. [56] J. Jing, L. Liangchao, X. Feng, Structural analysis and magnetic properties of Gddoped Li-Ni ferrites prepared using rheological phase reaction method, J. Rare Earths 25 (1) (2007) 79–83.
multilayer chip inductor (MLCI) applications, Ceram. Int. 40 (10, Part A) (2014) 15821–15829. M. Asif Iqbal, et al., High frequency dielectric properties of Eu+3-substituted Li– Mg ferrites synthesized by sol–gel auto-combustion method, J. Alloy. Compd. 586 (0) (2014) 404–410. M. Ishaque, et al., Investigations on structural, electrical and dielectric properties of yttrium substituted Mg-ferrites, Ceram. Int. 41 (3, Part A) (2015) 4028–4034. M.A. Khan, et al., Magnetic and dielectric behavior of terbium substituted Mg1−xTbxFe2O4 ferrites, J. Alloy. Compd. 519 (0) (2012) 156–160. M.Y. Lodhi, et al., New Mg0.5CoxZn0.5−xFe2O4 nano-ferrites: structural elucidation and electromagnetic behavior evaluation, Curr. Appl. Phys. 14 (5) (2014) 716–720. Z.K. Heiba, et al., Cation distribution and dielectric properties of nanocrystalline gallium substituted nickel ferrite, J. Alloy. Compd. 586 (0) (2014) 773–781. S.C. Watawe, et al., Preparation and dielectric properties of cadmium substituted lithium ferrite using microwave-induced combustion, Mater. Chem. Phys. 103 (2– 3) (2007) 323–328. M.A. Khan, et al., Magnetic, ferromagnetic resonance and electrical transport study of Ni1−xTbxFe2O4 spinel ferrites, Ceram. Int. 40 (2) (2014) 3571–3577.
10