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Structural, spectral, dielectric and magnetic properties of Tb–Dy doped Li-Ni nano-ferrites synthesized via micro-emulsion route
Journal of Magnetism and Magnetic Materials 419 (2016) 338–344
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Structural, spectral, dielectric and magnetic properties of Tb–Dy doped Li-Ni nano-ferrites synthesized via micro-emulsion route Muhammad Junaid a,n, Muhammad Azhar Khan a,n, F. Iqbal a, Ghulam Murtaza b, Majid Niaz Akhtar c, Mukhtar Ahmad c, Imran Shakir d, Muhammad Farooq Warsi e a
Department of Physics, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan Centre for Advanced Studies in Physics, Government College University, Lahore 54000, Pakistan c Department of Physics, COMSATS Institute of Information Technology, Lahore 54000, Pakistan d Deanship of scientific research, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia e Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 November 2015 Received in revised form 14 June 2016 Accepted 17 June 2016 Available online 18 June 2016
Terbium (Tb) and dysprosium (Dy) doped lithium-nickel nano-sized ferrites (Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 where x ¼0.00 0.08) were prepared by micro-emulsion technique. The X-ray diffraction (XRD) patterns confirmed the single phase cubic spinel structure. The lattice constant was increased due to larger ionic radii of Tb3 þ and Dy3 þ cations. The crystallite size was found in the range 30–42 nm. The FTIR (Fourier transform infrared spectroscopy) spectra revealed two significant absorption bands ( 400–600 cm 1) which indicate the formation of cubic spinel structure. The peaking behavior of dielectric parameters was observed beyond 1.5 GHz. The dielectric constant and dielectric loss were found to decrease by the increase of Tb–Dy contents and frequency. The doping of Tb and Dy in Li–Ni ferrites led to increase the coercive field (120–156 Oe). The smaller magnetic and dielectric parameters suggested the possible utility of these nano-materials in switching and microwave devices applications. & 2016 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline ferrites Thermal properties XRD FTIR Dielectric properties Magnetic properties
1. Introduction Ferrites have attracted the attention of technologist and physicists due to their vast applications such as transformer core, magnetic memories, high frequency circuits and switching devices. Semiconductor and magnetic materials exhibit the interesting properties and these materials can be used in many electronic devices [1,2]. The magnetic properties can be enhanced due to interaction of metallic ions with the oxygen ions in the crystal structure [3]. The structure of spinel ferrites is closed pack and having the chemical formula AB2O4 [4]. Many efforts have been made to improve the basic properties of nano-ferrites by the incorporation of various metals ions and employing various synthesis routes. The rare earth ions are the auspicious substitute for improving the properties of spinel ferrites [5]. Li–Ni soft ferrites are low cost materials and have interesting electrical and magnetic properties. These ferrites have been widely used in the microwave devices such as isolators, phase shifters and circulators [6]. These ferrites have low dielectric losses, high n
Corresponding authors. E-mail addresses: [email protected] (M. Junaid), [email protected] (M.A. Khan). http://dx.doi.org/10.1016/j.jmmm.2016.06.043 0304-8853/& 2016 Elsevier B.V. All rights reserved.
resistivity and high Curie temperature. These intrinsic parameters depend upon the chemical compositions, type of dopants ions and heat treatment [7]. The doping of rare earth ions in the Li–Ni ferrites appreciably optimizes the electrical and magnetic properties. Many researchers have reported the effect of rare earth doping on the properties of Li–Ni, Ni–Zn, Mn–Zn, Mg–Cu, Cu–Zn ferrites. Therefore, the incorporation of rare earth cations indicated interesting properties of the spinel ferrites [8–12]. In the present work, Tb3 þ and Dy3 þ simultaneously doped Li– Ni ferrites were synthesized by the micro-emulsion method. The aim of present work is to investigate the influence of terbium and dysprosium ions doping in lithium-nickel ferrites on the thermal, structural, spectral, dielectric and magnetic properties of Li0.2Ni0.8Fe2O4 ferrites in order to make these ferrites suitable for microwave devices and switching applications.
2. Experimental details Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 nanoferrites were synthesized by micro-emulsion technique. Analytical grade LiCl (99%, sigma Aldrich), NiCl2.6H2O (99.99%, sigma Aldrich), Tb4O7 (99.5%, sigma Aldrich), Dy(NO3)3.H2O (99%, sigma Aldrich), FeCl3 (98%, sigma Aldrich), Cetyltrimethylammonium bromide (CTAB), aqueous
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NH3(35 wt%, BDH) and deionized water (conductivity ̴ 2-4 S) were used for the preparation of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites. The appropriate amounts of above metal salts were dissolved in deionized water. The concentration of Li, Ni, Tb, and Dy was 0.1 M and Fe was 0.2 M taken in beakers. The stirring was done at 60 °C using magnetic stirrer. Aqueous solution of CTAB having the concentration of 0.35 M was added to the mixture of different metal salts. The value of pH was maintained up to 10 by adding aqueous ammonia solution. Then the mixture of all samples was stirred for 6 h continuously. After stirring, all the reaction mixtures of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 were placed in the cupboards for overnight. In this period of time the precipitate were settled down. The washing of all samples was done by the deionized water to reduce the value of pH. The washing was continued until the value of pH reached to the neutral level 7. The precipitates of different compositions were taken in different beakers and these were dried in oven at 100 °C to remove the water. Before grinding the mortar and pestle were washed and cleaned by methanol. After drying the precipitates were grinded in the mortar and pestle. To avoid any contamination the mortar and pestle were washed and dried after each grinding. The grinded samples were then annealed at 970 °C for 5.5 h continuously. The annealing was done by using controlled muffle furnace Vulcan A-550. After annealing the materials were grinded into powders and then packed into capillary tubes for further characterization. Thermo gravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetery (DSC) were done with the help of (SDT Q600V8.2 Build 100). XRD analysis was carried out with the help of Philips X' Pert PRO 3040/60 diffractometer. FTIR spectra were recorded on Nexus 470 spectrometer. The dielectric measurements were done using Wayn Ker WK6500B Precision instrument at 298 K. The vibrating sample magnetometer (VSM) was employed to unfold the magnetic properties.
3. Results and discussion 3.1. Thermal analysis The autocatalytic combustion process of as prepared sample was investigated by TGA, DTA and DSC. Fig. 1 exhibits the TGA, DTA and DSC curves of as prepared dried sample of Li0.2Ni0.8Fe2O4 ferrite. The percentage weight loss versus temperature was
Fig. 1. TGA (black), DTA (blue) and DSC (red) curves of Li0.2Ni0.8Fe2 O4 ferrite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
339
observed from the TGA curve. A continuous weight loss has been observed from TGA curve in the temperature range 0–980 °C. It is observed from the Fig. 1 that the decomposition process was completed through successive steps. TGA curve showed total weight loss ̴ 44%. In the first step, smallest weight loss was observed 3% below 100 °C which was due to the evaporation of water vapors. In the second step around 27% weight loss was observed and it occurred at 279 °C. In the third step 14% weight loss was observed and it occurred in the temperature range of 279–773 °C. The TGA plot revealed that the phase development started at 773 °C and thereafter very small weight loss of the matter was observed. The precursor established a stable phase at 945 °C and beyond this temperature there was no weight loss [13,14]. The DTA curve exhibited some peaks in the entire temperature range. Hence the decomposition occurred in many steps due to number of exothermic peaks. The first exothermic peak appeared at 61 °C. The second peak at 150 °C while third peak was observed at 247 °C and these were attributed to the burning of extra components. Maximum weight loss was observed in the temperature range 247–724 °C [15]. The DSC curve revealed that initially the heat flow increased with the increase in temperature, while after 278 °C the heat flow decreased for further increase in temperature. The decomposition process was found to be strongly exothermic [14]. 3.2. X-ray diffraction analysis XRD patterns of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites are shown in Fig. 2. The samples were annealed at 970 °C. All the peaks in the XRD patterns belong to the fcc cubic spinel structure which shows the formation of single phase spinel structure. A peak of orthorhombic phase (secondary phase) was observed at 33° (indicated by * in Fig. 2) and this peak is identified as TbFeO3.The secondary peak was observed due to excess of Tb3 þ ions because they have the large ionic radii as compared to the host ions. The crystallize size was calculated from the Debye Scherrer's formula [7].
D = 0.9λ/β cos θ
(1)
where D is the crystallize size, λ is the wavelength of X-rays (1.54 Å), β is the full width at half maximum and θ is the angle of diffraction. Initially the crystallize size decreases with the increase
Fig. 2. X-ray diffraction patterns of Li0.2Ni 0.06 and 0.08) ferrites.
0.8Tb0.5xDy0.5xFe2 xO4
(x¼ 0, 0.02, 0.04,
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Table 1 Lattice parameter, crystallize size, cell volume, measured density and X-ray density of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites. Tb–Dy Concentration (x)
lattice constant (a) Å
crystallite size (t) nm
Cell volume (V) Å3
Bulk density g/cm3
X-ray density g/cm3
0 0.02 0.04 0.06 0.08
8.329 7 0.005 8.3357 0.005 8.3367 0.005 8.3377 0.005 8.340 7 0.005
42.50 38.79 30.16 33.25 38.79
577.80 579.05 579.25 579.46 580.09
2.57 2.67 3.23 3.60 3.92
5.15 5.18 5.23 5.28 5.32
Fig. 4. FTIR spectra of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02 and 0.04) ferrites.
3.3. Spectroscopic analysis
Fig. 3. The variation of lattice Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites.
parameter
and
crystallite
size
of
of dopants concentration and then it increased for x¼ 0.06–0.08. The lattice constant was found to increase by the incorporation of Dy3 þ and Tb3 þ cations [6,7]. The average value of lattice constant was calculated using Nelson Riley function. The value of lattice constant, the crystallize size, cell volume, bulk density and X-ray density are given in the Table 1. The most intense peak of all the samples is identified as 311 [16]. The change in lattice constant was described on the basis of ionic radii of the cations involved. The lattice constant increased due to larger ionic radii of Tb3 þ (0.93 Å) and Dy3 þ (0.912 Å) as compared to the host cations Fe3 þ (0.64 Å), Li þ 1 (0.74 Å) and Ni2 þ (0.69 Å). The replacement of smaller ions with the larger ions resulted in the increase of lattice constant. The rare earth cations have the natural tendency to occupy the octahedral site due to their larger ionic radii [14,17]. The volume of unit cell is increased with the increase of dopants concentration (x). The variation of crystallite size and lattice constant are depicted in Fig. 3. The unit cell expand when terbium and dysprosium were added in the lithium nickel ferrites [14]. The X-ray density of the samples was calculated by the relation:
ρx = 8 M /Na3
(2)
where M is the molecular weight of the sample, N is Avogadro's number and a3 volume of the cubic unit cell. The bulk density is calculated by using the relation [18].
ρ b = m/πr 2h
(3)
where h is thickness of the pellet, r is radius of the pellet and m is the mass of the pellet. when the terbium and dysprosium were added in Lithium–Nickel ferrites then the value of X-ray density was increased from 5.15–5.32 (g/cm3) [16].
FTIR provide information about the chemical changes and occupancy of cations on various sites (tetrahedral and octahedral). There are different methods of infrared spectroscopy but FTIR is the most important. Fig. 4 shows the FTIR spectra of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites (x ¼0.00, 0.02 and 0.04). In all these spinel ferrites, two main broad metal-oxygen bands were observed in the range 400–600 cm 1 [19]. The absorption peaks were observed in the range 500–600 cm 1 belong to the tetrahedral site of intrinsic stretching vibrations of the metal while the peaks in the spectral range of 400–500 cm 1 belong to the octahedral-metal stretching vibrations. These bands are the characteristic features of the spinel ferrites. It is clear from the Fig. 4 that when the concentration increased the low frequency band υ1 and high frequency band υ2 bands shifted toward lower wave number range [20]. The high and low frequency bonds (542 cm 1, 550 cm 1, 541 cm 1) and (425 cm 1, 407 cm 1, 411 cm 1) occurred due to stretching vibrations of Fe3 þ –O2– complexes on tetrahedral and octahedral sites. The bond length of tetrahedral site is smaller as compared to the bond length of octahedral site. Due to this reason the vibration of functional group appeared at high frequency in tetrahedral site than the octahedral site. When the doping of terbium and dysprosium increased the lattice parameter increased due to stretching vibrations of Fe3 þ –O2– complexes and play an important role in the deviation of bonds position [21]. From Fig. 4, it is clear that two major bands exist and confirmed the doping of terbium and dysprosium in the lithium nickel ferrites [22]. From the FTIR absorption data, the force constants Ko and Kt were calculated from the standard formulae given below [23,24].
K o = 0.942128 Mυ22 /( M + 32)
Kt =
( 2)1/2 Ko υ1 /υ2
(4)
(5)
where Ko and Kt are force constants for octahedral and tetrahedral sites. M is molecular weight, υ1 is the frequency band of tetrahedral site and υ2 is frequency band of octahedral site. From Table 2, It was observed that Ko first decrease and then increased with the increase of concentration (Tb þ 3 and Dy þ 3) and Kt was
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Table 2 Chemical composition, molecular weight, υ1, υ2, Ro, Rt and force constants of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x¼ 0.00, 0.02 and 0.04) ferrites. Chemical composition
Li0.2Ni0.8Fe2O
Molecular weight (gm/mole) υ1(cm 1) υ2(cm 1) Ko (dyne/cm2)*105 Kt (dyne/cm2)*105 Ro Rt
224 541.5 424.7 1.49 2.68 0.762 0.483
Table 3 Saturation magnetization, retentivity, Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites.
coercivity
and
Li0.2Ni0.8Tb0.02Dy0.02Fe
4
228.2 550.1 407 1.37 2.62 0.763 0.484
squarness
ratio
Retentivity Mr Coercivity Hc Squarness ratio Mr/ Ms
0 0.02 0.04 0.06 0.08
10.5 9.8 9.2 8.5 7.8
120 126 133 142 156
O
4
Li0.2Ni0.8Tb0.04Dy0.04Fe1.92O4 232.4 540.5 411.3 1.4 2.6 0.764 0.485
of
Concentration (x) Saturation magnetization Ms 54 47 41 35 27
1.96
0.19 0.21 0.22 0.24 0.27
gradually decreased with the increase of concentration (Tb þ 3 and Dy þ 3) [23].
R tetra = a ( 3)1/2( u − 0.25) − R o
(6) Fig. 5. The dielectric constant as a function of frequency Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites.
R octa = a ( 5/8 − u) − R o
of
(7)
where a is lattice parameter, u is oxygen positional parameter, Ro is the radius of oxygen ion, Rtetra is the tetrahedral radii and Rocta is the octahedral radii (1.32 Å) [25]. The value of oxygen parameter for an ideal FCC crystal is 0.375 [26]. From Table 2, it is clear that the Rtetra and Rocta both increased with the increase of Tb þ 3 and Dy þ 3 concentration (the bond length increase with the increase of Tb and Dy contents) [25] Table 3. 3.4. Dielectric properties The electrical properties of magnetic ceramics materials are very important. Owing to these properties, the relative speed of electromagnetic signals that travel through the materials is identified by the dielectric constant. The dielectric properties of ferrites depend upon the temperature, frequency of applied electric field, chemical composition, fabrication route and crystallize size. Figs. 5 and 6 show the variation in the dielectric constant and dielectric loss as a function of frequency from 1 MHz to 3 GHz. Fig. 5 shows that the dielectric constant decreases with the increase of frequency. The gradual decrease in the dielectric constant was observed in all ferrite samples and resonance peaks at higher frequencies were exhibited by all ferrites [14,27]. These plots of dielectric constant and dielectric loss revealed dispersion with increasing frequency. This behavior of synthesized samples is due to Maxwell Wagner interfacial polarization and Koop's phenomenological theory [28]. In this model the grains are more effective at high frequency while the grain boundaries are more effective at low frequencies [27]. During sintering the random distribution of oxygen ions at grain boundaries and grain result in interfacial polarization at lower frequency and dipolar, electronic and ionic polarization at high frequency [29]. The polarization was damped with the increase of frequency. The dielectric constant decreases with the increase of frequency. In ferrites the main charge carriers are electrons and motion of
Fig. 6. The dielectric loss as a function of frequency for Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites.
electrons take place between Fe2 þ –Fe3 þ ions which are present at octahedral sites. This phenomenon is known as hopping mechanism of electrons. The decrease in dielectric constant with the increase of frequency is due to the exchange of electrons between Fe2 þ –Fe3 þ ions at octahedral site and this exchange cannot follow the alteration of an ac electric field. Few resonance peaks were observed in the spinel ferrites when the exchange of electrons between Fe2 þ –Fe3 þ ions was equal to the applied ac frequency. This phenomenon is called ferromagnetic resonance [14,30]. If an ion has two equilibrium states C and D and separated by some potential barrier, the jumping probability of both ions was same. The frequency that changes the position of ion is called the natural frequency of that ion. When both the natural and external applied frequencies were same, then the maximum electrical energy was
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Fig. 7. Tan loss as a function of frequency for Li0.2Ni 0.02, 0.04, 0.06 and 0.08) ferrites.
0.8Tb0.5xDy0.5xFe2 xO4
(x ¼0, Fig. 9. The relation between lnω and lnsac of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites.
transferred to the oscillating ions and then the power loss was also increased. The result was the occurrence of resonance phenomenon [28]. Fig. 7 shows the variation in dielectric loss tangent with frequency for Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites. According to Maxwell–Wagner theory both the dielectric constant and tanδ were inversely proportional with the frequency. From Fig. 7 it is evident that when the frequency was increased then tan loss was decreased. At low frequency the hopping frequency of electrons follow the applied field and the loss was comparatively high. When the frequency of applied field was increased then the hopping frequency between Fe2 þ –Fe3 þ ions cannot follow the applied field after certain critical frequency and the loss was remarkably decreased. It is observed that when the concentration of rare earth ions was increased the peaks shifted toward the lower frequency region. When the doping level was increased beyond certain limit then the resonance frequency was decreased [28]. Fig. 8 exhibit the variation in ac conductivity of all synthesized samples. This plot revealed that at low frequency region all the samples show the same trend (increasing trend). The dispersion behavior occurs at high frequency region. Both the Koop's phenomenological theory and Maxwell–Wagner model shows that the ferrites materials consist of conducting grains which are separated by resistive layer of grain boundaries. The conduction and dielectric polarization process are related to each other. On the other
Fig. 8. The ac conductivity as a function of frequency Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites.
for
hand at high frequency the grains are affected and the hopping mechanism between Fe2 þ –Fe3 þ ions was increased, due to this the conductivity was increased [28].
σ ( ω) = Bωn
(8)
According to the above equation the exponent (n) was calculated as function of composition by drawing ln(s) versus ln(ω) (Fig. 9). This shows the straight line with slope that is equal to n and intercept on vertical axis at lnω ¼0 is equal to lnB. When n ¼0 the electrical conduction is independent of frequency. For n r1, electric conduction depends upon frequency. In the present case, the value of n varies between 0.02–0.1 (Fig. 10), which shows that the conduction process follow the hopping conduction mechanism [31]. 3.5. Magnetic properties The incorporation of Tb3 þ and Dy3 þ in lithium-nickel ferrites greatly influenced the magnetic properties. The M–H loops of all the samples are shown in Fig. 11. These hysteresis loops exhibited that Tb–Dy doped lithium-nickle ferrites are soft magnetic materials (as indicated in inset of Fig. 11) because it revealed minimal hysteresis [32]. The saturation magnetization (Ms), remanence (Mr), coercivity (Hc) and squareness ratios are calculated and these
Fig. 10. Slope Vs concentration of Li0.2Ni 0.06 and 0.08) ferrites.
0.8Tb0.5xDy0.5xFe2 xO4
(x¼ 0, 0.02, 0.04,
M. Junaid et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 338–344
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except x¼0.8. It was observed that the lattice parameter increase with the increase of Tb3þ and Dy3 þ concentration. The cell volume and X-ray density were also increased with the increase of dopants concentration. The dielectric parameters were reduced by the incorporation of Tb3 þ and Dy3þ cations. The dielectric constant, dielectric loss and tan loss are decreased with the increase of frequency and concentration of dopants. The saturation magnetization was decreased from 54 emu/g to 27 emu/g while coercivity increased from 120 Oe to 156 Oe by the addition of rare earth ions (Tb and Dy). The smaller values of dielectric parameters and coercivity along with moderate saturation magnetization of these doped ferrites suggested that these nanoferrites are potential candidates for microwave devices and switching applications.
Acknowledgement One of the authors (I. Shakir) is highly thankful to the Deanship of Scientific Research, King Saud University, Riyadh for Prolific Research Group Project no. PRG-1436-25. Fig. 11. MH loops of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x¼ 0, 0.02, 0.04, 0.06 and 0.08) ferrites.
References lie in the range (54–27) emu/g, (10.5–7.8) emu/g, (120–156) Oe [14]. The net magnetic moment in ferrites is the difference of magnetic moment of B-site and A-site which is described by the relation [28].
M = MB − MA
(9)
The net magnetic moment in ferrites is mainly due to the uncompensated electron spin of the individual ions and the spin alignment of two sub-lattices which are arranged in antiparallel mode. According to Neel's molecular field model each ion at the A-site has 12 B-site ions as the nearest neighbors. In magnetization process A–A and B–B interactions have less dominance as compared to A–B interaction. The replacement of Fe3 þ ions with Li þ and Ni2 þ at A-site decreased the magnetization in the A-site. In the replacement of Fe3 þ ions with Tb3 þ and Dy3 þ at B-site leading to decrease the magnetization in the B-site. So the magnetizations of both sides are decreased. The magnetization at B-site is stronger than the A-site. Due to this reason the net magnetization is decreased [21,32]. The coercive force (Hc) is one of the important magnetic parameters. The coercivity mainly depends upon the particle size and anisotropy constant. The coercivity is inversely proportional to the particle size. The coercivity can be altered by heat treatment or deformation. The relationship between the saturation magnetization and Hc is described by the Brown's relation [21,33]:
Hc = 2 K1/μo Ms
(10)
Where K1 is anisotropy constant, μo is permeability, Ms is the saturation magnetization and Hc is coercivity. The above relation indicates that the Hc is inversely proportional to the Ms which is consistent with our experimental results. The remanent magnetization is independent parameter and does not depend upon the Ms and Hc. The value of Mr lies in the range (10.5–7.8) emu/g [32].
4. Conclusions Tb3þ and Dy3 þ doped Lithium-Nickle ferrites powder were prepared by micro-emulsion method. Thermal gravimetric analysis revealed thermal decomposition process and phase development of these ferrites. The XRD analysis of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites exhibited that all the samples have single phase spinel structure
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M. Junaid et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 338–344 ZnxMn1 xFe2O4 (x ¼0.0, 0.25, 0.50, 0.75 and 1.0) mixed ferrites, Mater. Chem. Phys. 131 (2011) 413–419. C. Choodamani, G.P. Nagabhushana, S. Ashoka, B. Daruka Prasad, B. Rudraswamy, G.T. Chandrappa, Structural and magnetic studies of Mg(1 x)ZnxFe2O4 nanoparticles prepared by a solution combustion method, J. Alloys Compd. 578 (2013) 103–109. A. Ghasemi, M. Mousavinia, Structural and magnetic evaluation of substituted NiZnFe2O4 particles synthesized by conventional sol–gel method, Ceram. Int. 40 (2014) 2825–2834. M.A. Khan, M.U. Islam, M. Ishaque, I.Z. Rahman, Effect of Tb substitution on structural, magnetic and electrical properties of magnesium ferrites, Ceram. Int. 37 (2011) 2519–2526. S.C. Watawe, B.D. Sutar, B.D. Sarwade, B.K. Chougule, Infrared studies of some mixed Li–Co ferrites, Int. J. Inorg. Mater. 3 (2001) 819–823. A. Maqsood, K. Khan, M. Anis-ur-Rehman, M.A. Malik, Spectroscopic and magnetic investigation of NiCo nanoferrites, J. Alloys Compd. 509 (2011) 7493–7497. Z. Karimi, Y. Mohammadifar, H. Shokrollahi, S.K. Asl, G. Yousefi, L. Karimi, Magnetic and structural properties of nano sized Dy-doped cobalt ferrite synthesized by co-precipitation, J. Magn. Magn. Mater. 361 (2014) 150–156.
[26] O.M. HEMEDA, Structural and magnetic properties of Co0.6ZnN0.4MnxFe2 xO4, Turk. J. Phys. 28 (2004) 121–132. [27] N. Singh, A. Agarwal, S. Sanghi, P. Singh, Synthesis, microstructure, dielectric and magnetic properties of Cu substituted Ni–Li ferrites, J. Magn. Magn. Mater. 323 (2011) 486–492. [28] M. Asif Iqbal, M.U. Islam, I. Ali, M.A. khan, I. Sadiq, I. Ali, High frequency dielectric properties of Eu þ 3-substituted Li–Mg ferrites synthesized by sol–gel auto-combustion method, J. Alloys Compd. 586 (2014) 404–410. [29] P.J. Harrop, Dielectronics, Butterworth, London, 1972. [30] N. Singh, A. Agarwal, S. Sanghi, Dielectric relaxation, conductivity behavior and magnetic properties of Mg substituted Zn–Li ferrites, Curr. Appl. Phys. 11 (2011) 783–789. [31] M.U.I. Muhammad Irfan, Irshad Ali, M.Asif Iqbal, Nazia Karamat, Hassan M. Khan, Curr. Appl. Phys. 14 (2014) 112–117. [32] D.R. Mane, S. Patil, D.D. Birajdar, A.B. Kadam, S.E. Shirsath, R.H. Kadam, Sol–gel synthesis of Cr3 þ substituted Li0.5Fe2.5O4: cation distribution, structural and magnetic properties, Mater. Chem. Phys. 126 (2011) 755–760. [33] M. Asif Iqbal, M.-u Islam, M.N. Ashiq, I. Ali, A. Iftikhar, H.M. Khan, Effect of Gdsubstitiution on physical and magnetic properties of Li1.2Mg0.4GdxFe(2 x)O4 ferrites, J. Alloys Compd. 579 (2013) 181–186.
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Structural, spectral, dielectric and magnetic properties of Tb–Dy doped Li-Ni nano-ferrites synthesized via micro-emulsion route Muhammad Junaid a,n, Muhammad Azhar Khan a,n, F. Iqbal a, Ghulam Murtaza b, Majid Niaz Akhtar c, Mukhtar Ahmad c, Imran Shakir d, Muhammad Farooq Warsi e a
Department of Physics, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan Centre for Advanced Studies in Physics, Government College University, Lahore 54000, Pakistan c Department of Physics, COMSATS Institute of Information Technology, Lahore 54000, Pakistan d Deanship of scientific research, College of Engineering, King Saud University, PO Box 800, Riyadh 11421, Saudi Arabia e Department of Chemistry, The Islamia University of Bahawalpur, Bahawalpur 63100, Pakistan b
art ic l e i nf o
a b s t r a c t
Article history: Received 23 November 2015 Received in revised form 14 June 2016 Accepted 17 June 2016 Available online 18 June 2016
Terbium (Tb) and dysprosium (Dy) doped lithium-nickel nano-sized ferrites (Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 where x ¼0.00 0.08) were prepared by micro-emulsion technique. The X-ray diffraction (XRD) patterns confirmed the single phase cubic spinel structure. The lattice constant was increased due to larger ionic radii of Tb3 þ and Dy3 þ cations. The crystallite size was found in the range 30–42 nm. The FTIR (Fourier transform infrared spectroscopy) spectra revealed two significant absorption bands ( 400–600 cm 1) which indicate the formation of cubic spinel structure. The peaking behavior of dielectric parameters was observed beyond 1.5 GHz. The dielectric constant and dielectric loss were found to decrease by the increase of Tb–Dy contents and frequency. The doping of Tb and Dy in Li–Ni ferrites led to increase the coercive field (120–156 Oe). The smaller magnetic and dielectric parameters suggested the possible utility of these nano-materials in switching and microwave devices applications. & 2016 Elsevier B.V. All rights reserved.
Keywords: Nanocrystalline ferrites Thermal properties XRD FTIR Dielectric properties Magnetic properties
1. Introduction Ferrites have attracted the attention of technologist and physicists due to their vast applications such as transformer core, magnetic memories, high frequency circuits and switching devices. Semiconductor and magnetic materials exhibit the interesting properties and these materials can be used in many electronic devices [1,2]. The magnetic properties can be enhanced due to interaction of metallic ions with the oxygen ions in the crystal structure [3]. The structure of spinel ferrites is closed pack and having the chemical formula AB2O4 [4]. Many efforts have been made to improve the basic properties of nano-ferrites by the incorporation of various metals ions and employing various synthesis routes. The rare earth ions are the auspicious substitute for improving the properties of spinel ferrites [5]. Li–Ni soft ferrites are low cost materials and have interesting electrical and magnetic properties. These ferrites have been widely used in the microwave devices such as isolators, phase shifters and circulators [6]. These ferrites have low dielectric losses, high n
Corresponding authors. E-mail addresses: [email protected] (M. Junaid), [email protected] (M.A. Khan). http://dx.doi.org/10.1016/j.jmmm.2016.06.043 0304-8853/& 2016 Elsevier B.V. All rights reserved.
resistivity and high Curie temperature. These intrinsic parameters depend upon the chemical compositions, type of dopants ions and heat treatment [7]. The doping of rare earth ions in the Li–Ni ferrites appreciably optimizes the electrical and magnetic properties. Many researchers have reported the effect of rare earth doping on the properties of Li–Ni, Ni–Zn, Mn–Zn, Mg–Cu, Cu–Zn ferrites. Therefore, the incorporation of rare earth cations indicated interesting properties of the spinel ferrites [8–12]. In the present work, Tb3 þ and Dy3 þ simultaneously doped Li– Ni ferrites were synthesized by the micro-emulsion method. The aim of present work is to investigate the influence of terbium and dysprosium ions doping in lithium-nickel ferrites on the thermal, structural, spectral, dielectric and magnetic properties of Li0.2Ni0.8Fe2O4 ferrites in order to make these ferrites suitable for microwave devices and switching applications.
2. Experimental details Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 nanoferrites were synthesized by micro-emulsion technique. Analytical grade LiCl (99%, sigma Aldrich), NiCl2.6H2O (99.99%, sigma Aldrich), Tb4O7 (99.5%, sigma Aldrich), Dy(NO3)3.H2O (99%, sigma Aldrich), FeCl3 (98%, sigma Aldrich), Cetyltrimethylammonium bromide (CTAB), aqueous
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NH3(35 wt%, BDH) and deionized water (conductivity ̴ 2-4 S) were used for the preparation of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites. The appropriate amounts of above metal salts were dissolved in deionized water. The concentration of Li, Ni, Tb, and Dy was 0.1 M and Fe was 0.2 M taken in beakers. The stirring was done at 60 °C using magnetic stirrer. Aqueous solution of CTAB having the concentration of 0.35 M was added to the mixture of different metal salts. The value of pH was maintained up to 10 by adding aqueous ammonia solution. Then the mixture of all samples was stirred for 6 h continuously. After stirring, all the reaction mixtures of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 were placed in the cupboards for overnight. In this period of time the precipitate were settled down. The washing of all samples was done by the deionized water to reduce the value of pH. The washing was continued until the value of pH reached to the neutral level 7. The precipitates of different compositions were taken in different beakers and these were dried in oven at 100 °C to remove the water. Before grinding the mortar and pestle were washed and cleaned by methanol. After drying the precipitates were grinded in the mortar and pestle. To avoid any contamination the mortar and pestle were washed and dried after each grinding. The grinded samples were then annealed at 970 °C for 5.5 h continuously. The annealing was done by using controlled muffle furnace Vulcan A-550. After annealing the materials were grinded into powders and then packed into capillary tubes for further characterization. Thermo gravimetric analysis (TGA), differential thermal analysis (DTA) and differential scanning calorimetery (DSC) were done with the help of (SDT Q600V8.2 Build 100). XRD analysis was carried out with the help of Philips X' Pert PRO 3040/60 diffractometer. FTIR spectra were recorded on Nexus 470 spectrometer. The dielectric measurements were done using Wayn Ker WK6500B Precision instrument at 298 K. The vibrating sample magnetometer (VSM) was employed to unfold the magnetic properties.
3. Results and discussion 3.1. Thermal analysis The autocatalytic combustion process of as prepared sample was investigated by TGA, DTA and DSC. Fig. 1 exhibits the TGA, DTA and DSC curves of as prepared dried sample of Li0.2Ni0.8Fe2O4 ferrite. The percentage weight loss versus temperature was
Fig. 1. TGA (black), DTA (blue) and DSC (red) curves of Li0.2Ni0.8Fe2 O4 ferrite. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)
339
observed from the TGA curve. A continuous weight loss has been observed from TGA curve in the temperature range 0–980 °C. It is observed from the Fig. 1 that the decomposition process was completed through successive steps. TGA curve showed total weight loss ̴ 44%. In the first step, smallest weight loss was observed 3% below 100 °C which was due to the evaporation of water vapors. In the second step around 27% weight loss was observed and it occurred at 279 °C. In the third step 14% weight loss was observed and it occurred in the temperature range of 279–773 °C. The TGA plot revealed that the phase development started at 773 °C and thereafter very small weight loss of the matter was observed. The precursor established a stable phase at 945 °C and beyond this temperature there was no weight loss [13,14]. The DTA curve exhibited some peaks in the entire temperature range. Hence the decomposition occurred in many steps due to number of exothermic peaks. The first exothermic peak appeared at 61 °C. The second peak at 150 °C while third peak was observed at 247 °C and these were attributed to the burning of extra components. Maximum weight loss was observed in the temperature range 247–724 °C [15]. The DSC curve revealed that initially the heat flow increased with the increase in temperature, while after 278 °C the heat flow decreased for further increase in temperature. The decomposition process was found to be strongly exothermic [14]. 3.2. X-ray diffraction analysis XRD patterns of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites are shown in Fig. 2. The samples were annealed at 970 °C. All the peaks in the XRD patterns belong to the fcc cubic spinel structure which shows the formation of single phase spinel structure. A peak of orthorhombic phase (secondary phase) was observed at 33° (indicated by * in Fig. 2) and this peak is identified as TbFeO3.The secondary peak was observed due to excess of Tb3 þ ions because they have the large ionic radii as compared to the host ions. The crystallize size was calculated from the Debye Scherrer's formula [7].
D = 0.9λ/β cos θ
(1)
where D is the crystallize size, λ is the wavelength of X-rays (1.54 Å), β is the full width at half maximum and θ is the angle of diffraction. Initially the crystallize size decreases with the increase
Fig. 2. X-ray diffraction patterns of Li0.2Ni 0.06 and 0.08) ferrites.
0.8Tb0.5xDy0.5xFe2 xO4
(x¼ 0, 0.02, 0.04,
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Table 1 Lattice parameter, crystallize size, cell volume, measured density and X-ray density of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites. Tb–Dy Concentration (x)
lattice constant (a) Å
crystallite size (t) nm
Cell volume (V) Å3
Bulk density g/cm3
X-ray density g/cm3
0 0.02 0.04 0.06 0.08
8.329 7 0.005 8.3357 0.005 8.3367 0.005 8.3377 0.005 8.340 7 0.005
42.50 38.79 30.16 33.25 38.79
577.80 579.05 579.25 579.46 580.09
2.57 2.67 3.23 3.60 3.92
5.15 5.18 5.23 5.28 5.32
Fig. 4. FTIR spectra of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02 and 0.04) ferrites.
3.3. Spectroscopic analysis
Fig. 3. The variation of lattice Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites.
parameter
and
crystallite
size
of
of dopants concentration and then it increased for x¼ 0.06–0.08. The lattice constant was found to increase by the incorporation of Dy3 þ and Tb3 þ cations [6,7]. The average value of lattice constant was calculated using Nelson Riley function. The value of lattice constant, the crystallize size, cell volume, bulk density and X-ray density are given in the Table 1. The most intense peak of all the samples is identified as 311 [16]. The change in lattice constant was described on the basis of ionic radii of the cations involved. The lattice constant increased due to larger ionic radii of Tb3 þ (0.93 Å) and Dy3 þ (0.912 Å) as compared to the host cations Fe3 þ (0.64 Å), Li þ 1 (0.74 Å) and Ni2 þ (0.69 Å). The replacement of smaller ions with the larger ions resulted in the increase of lattice constant. The rare earth cations have the natural tendency to occupy the octahedral site due to their larger ionic radii [14,17]. The volume of unit cell is increased with the increase of dopants concentration (x). The variation of crystallite size and lattice constant are depicted in Fig. 3. The unit cell expand when terbium and dysprosium were added in the lithium nickel ferrites [14]. The X-ray density of the samples was calculated by the relation:
ρx = 8 M /Na3
(2)
where M is the molecular weight of the sample, N is Avogadro's number and a3 volume of the cubic unit cell. The bulk density is calculated by using the relation [18].
ρ b = m/πr 2h
(3)
where h is thickness of the pellet, r is radius of the pellet and m is the mass of the pellet. when the terbium and dysprosium were added in Lithium–Nickel ferrites then the value of X-ray density was increased from 5.15–5.32 (g/cm3) [16].
FTIR provide information about the chemical changes and occupancy of cations on various sites (tetrahedral and octahedral). There are different methods of infrared spectroscopy but FTIR is the most important. Fig. 4 shows the FTIR spectra of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites (x ¼0.00, 0.02 and 0.04). In all these spinel ferrites, two main broad metal-oxygen bands were observed in the range 400–600 cm 1 [19]. The absorption peaks were observed in the range 500–600 cm 1 belong to the tetrahedral site of intrinsic stretching vibrations of the metal while the peaks in the spectral range of 400–500 cm 1 belong to the octahedral-metal stretching vibrations. These bands are the characteristic features of the spinel ferrites. It is clear from the Fig. 4 that when the concentration increased the low frequency band υ1 and high frequency band υ2 bands shifted toward lower wave number range [20]. The high and low frequency bonds (542 cm 1, 550 cm 1, 541 cm 1) and (425 cm 1, 407 cm 1, 411 cm 1) occurred due to stretching vibrations of Fe3 þ –O2– complexes on tetrahedral and octahedral sites. The bond length of tetrahedral site is smaller as compared to the bond length of octahedral site. Due to this reason the vibration of functional group appeared at high frequency in tetrahedral site than the octahedral site. When the doping of terbium and dysprosium increased the lattice parameter increased due to stretching vibrations of Fe3 þ –O2– complexes and play an important role in the deviation of bonds position [21]. From Fig. 4, it is clear that two major bands exist and confirmed the doping of terbium and dysprosium in the lithium nickel ferrites [22]. From the FTIR absorption data, the force constants Ko and Kt were calculated from the standard formulae given below [23,24].
K o = 0.942128 Mυ22 /( M + 32)
Kt =
( 2)1/2 Ko υ1 /υ2
(4)
(5)
where Ko and Kt are force constants for octahedral and tetrahedral sites. M is molecular weight, υ1 is the frequency band of tetrahedral site and υ2 is frequency band of octahedral site. From Table 2, It was observed that Ko first decrease and then increased with the increase of concentration (Tb þ 3 and Dy þ 3) and Kt was
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Table 2 Chemical composition, molecular weight, υ1, υ2, Ro, Rt and force constants of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x¼ 0.00, 0.02 and 0.04) ferrites. Chemical composition
Li0.2Ni0.8Fe2O
Molecular weight (gm/mole) υ1(cm 1) υ2(cm 1) Ko (dyne/cm2)*105 Kt (dyne/cm2)*105 Ro Rt
224 541.5 424.7 1.49 2.68 0.762 0.483
Table 3 Saturation magnetization, retentivity, Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites.
coercivity
and
Li0.2Ni0.8Tb0.02Dy0.02Fe
4
228.2 550.1 407 1.37 2.62 0.763 0.484
squarness
ratio
Retentivity Mr Coercivity Hc Squarness ratio Mr/ Ms
0 0.02 0.04 0.06 0.08
10.5 9.8 9.2 8.5 7.8
120 126 133 142 156
O
4
Li0.2Ni0.8Tb0.04Dy0.04Fe1.92O4 232.4 540.5 411.3 1.4 2.6 0.764 0.485
of
Concentration (x) Saturation magnetization Ms 54 47 41 35 27
1.96
0.19 0.21 0.22 0.24 0.27
gradually decreased with the increase of concentration (Tb þ 3 and Dy þ 3) [23].
R tetra = a ( 3)1/2( u − 0.25) − R o
(6) Fig. 5. The dielectric constant as a function of frequency Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites.
R octa = a ( 5/8 − u) − R o
of
(7)
where a is lattice parameter, u is oxygen positional parameter, Ro is the radius of oxygen ion, Rtetra is the tetrahedral radii and Rocta is the octahedral radii (1.32 Å) [25]. The value of oxygen parameter for an ideal FCC crystal is 0.375 [26]. From Table 2, it is clear that the Rtetra and Rocta both increased with the increase of Tb þ 3 and Dy þ 3 concentration (the bond length increase with the increase of Tb and Dy contents) [25] Table 3. 3.4. Dielectric properties The electrical properties of magnetic ceramics materials are very important. Owing to these properties, the relative speed of electromagnetic signals that travel through the materials is identified by the dielectric constant. The dielectric properties of ferrites depend upon the temperature, frequency of applied electric field, chemical composition, fabrication route and crystallize size. Figs. 5 and 6 show the variation in the dielectric constant and dielectric loss as a function of frequency from 1 MHz to 3 GHz. Fig. 5 shows that the dielectric constant decreases with the increase of frequency. The gradual decrease in the dielectric constant was observed in all ferrite samples and resonance peaks at higher frequencies were exhibited by all ferrites [14,27]. These plots of dielectric constant and dielectric loss revealed dispersion with increasing frequency. This behavior of synthesized samples is due to Maxwell Wagner interfacial polarization and Koop's phenomenological theory [28]. In this model the grains are more effective at high frequency while the grain boundaries are more effective at low frequencies [27]. During sintering the random distribution of oxygen ions at grain boundaries and grain result in interfacial polarization at lower frequency and dipolar, electronic and ionic polarization at high frequency [29]. The polarization was damped with the increase of frequency. The dielectric constant decreases with the increase of frequency. In ferrites the main charge carriers are electrons and motion of
Fig. 6. The dielectric loss as a function of frequency for Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites.
electrons take place between Fe2 þ –Fe3 þ ions which are present at octahedral sites. This phenomenon is known as hopping mechanism of electrons. The decrease in dielectric constant with the increase of frequency is due to the exchange of electrons between Fe2 þ –Fe3 þ ions at octahedral site and this exchange cannot follow the alteration of an ac electric field. Few resonance peaks were observed in the spinel ferrites when the exchange of electrons between Fe2 þ –Fe3 þ ions was equal to the applied ac frequency. This phenomenon is called ferromagnetic resonance [14,30]. If an ion has two equilibrium states C and D and separated by some potential barrier, the jumping probability of both ions was same. The frequency that changes the position of ion is called the natural frequency of that ion. When both the natural and external applied frequencies were same, then the maximum electrical energy was
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Fig. 7. Tan loss as a function of frequency for Li0.2Ni 0.02, 0.04, 0.06 and 0.08) ferrites.
0.8Tb0.5xDy0.5xFe2 xO4
(x ¼0, Fig. 9. The relation between lnω and lnsac of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites.
transferred to the oscillating ions and then the power loss was also increased. The result was the occurrence of resonance phenomenon [28]. Fig. 7 shows the variation in dielectric loss tangent with frequency for Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites. According to Maxwell–Wagner theory both the dielectric constant and tanδ were inversely proportional with the frequency. From Fig. 7 it is evident that when the frequency was increased then tan loss was decreased. At low frequency the hopping frequency of electrons follow the applied field and the loss was comparatively high. When the frequency of applied field was increased then the hopping frequency between Fe2 þ –Fe3 þ ions cannot follow the applied field after certain critical frequency and the loss was remarkably decreased. It is observed that when the concentration of rare earth ions was increased the peaks shifted toward the lower frequency region. When the doping level was increased beyond certain limit then the resonance frequency was decreased [28]. Fig. 8 exhibit the variation in ac conductivity of all synthesized samples. This plot revealed that at low frequency region all the samples show the same trend (increasing trend). The dispersion behavior occurs at high frequency region. Both the Koop's phenomenological theory and Maxwell–Wagner model shows that the ferrites materials consist of conducting grains which are separated by resistive layer of grain boundaries. The conduction and dielectric polarization process are related to each other. On the other
Fig. 8. The ac conductivity as a function of frequency Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x ¼0, 0.02, 0.04, 0.06 and 0.08) ferrites.
for
hand at high frequency the grains are affected and the hopping mechanism between Fe2 þ –Fe3 þ ions was increased, due to this the conductivity was increased [28].
σ ( ω) = Bωn
(8)
According to the above equation the exponent (n) was calculated as function of composition by drawing ln(s) versus ln(ω) (Fig. 9). This shows the straight line with slope that is equal to n and intercept on vertical axis at lnω ¼0 is equal to lnB. When n ¼0 the electrical conduction is independent of frequency. For n r1, electric conduction depends upon frequency. In the present case, the value of n varies between 0.02–0.1 (Fig. 10), which shows that the conduction process follow the hopping conduction mechanism [31]. 3.5. Magnetic properties The incorporation of Tb3 þ and Dy3 þ in lithium-nickel ferrites greatly influenced the magnetic properties. The M–H loops of all the samples are shown in Fig. 11. These hysteresis loops exhibited that Tb–Dy doped lithium-nickle ferrites are soft magnetic materials (as indicated in inset of Fig. 11) because it revealed minimal hysteresis [32]. The saturation magnetization (Ms), remanence (Mr), coercivity (Hc) and squareness ratios are calculated and these
Fig. 10. Slope Vs concentration of Li0.2Ni 0.06 and 0.08) ferrites.
0.8Tb0.5xDy0.5xFe2 xO4
(x¼ 0, 0.02, 0.04,
M. Junaid et al. / Journal of Magnetism and Magnetic Materials 419 (2016) 338–344
343
except x¼0.8. It was observed that the lattice parameter increase with the increase of Tb3þ and Dy3 þ concentration. The cell volume and X-ray density were also increased with the increase of dopants concentration. The dielectric parameters were reduced by the incorporation of Tb3 þ and Dy3þ cations. The dielectric constant, dielectric loss and tan loss are decreased with the increase of frequency and concentration of dopants. The saturation magnetization was decreased from 54 emu/g to 27 emu/g while coercivity increased from 120 Oe to 156 Oe by the addition of rare earth ions (Tb and Dy). The smaller values of dielectric parameters and coercivity along with moderate saturation magnetization of these doped ferrites suggested that these nanoferrites are potential candidates for microwave devices and switching applications.
Acknowledgement One of the authors (I. Shakir) is highly thankful to the Deanship of Scientific Research, King Saud University, Riyadh for Prolific Research Group Project no. PRG-1436-25. Fig. 11. MH loops of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 (x¼ 0, 0.02, 0.04, 0.06 and 0.08) ferrites.
References lie in the range (54–27) emu/g, (10.5–7.8) emu/g, (120–156) Oe [14]. The net magnetic moment in ferrites is the difference of magnetic moment of B-site and A-site which is described by the relation [28].
M = MB − MA
(9)
The net magnetic moment in ferrites is mainly due to the uncompensated electron spin of the individual ions and the spin alignment of two sub-lattices which are arranged in antiparallel mode. According to Neel's molecular field model each ion at the A-site has 12 B-site ions as the nearest neighbors. In magnetization process A–A and B–B interactions have less dominance as compared to A–B interaction. The replacement of Fe3 þ ions with Li þ and Ni2 þ at A-site decreased the magnetization in the A-site. In the replacement of Fe3 þ ions with Tb3 þ and Dy3 þ at B-site leading to decrease the magnetization in the B-site. So the magnetizations of both sides are decreased. The magnetization at B-site is stronger than the A-site. Due to this reason the net magnetization is decreased [21,32]. The coercive force (Hc) is one of the important magnetic parameters. The coercivity mainly depends upon the particle size and anisotropy constant. The coercivity is inversely proportional to the particle size. The coercivity can be altered by heat treatment or deformation. The relationship between the saturation magnetization and Hc is described by the Brown's relation [21,33]:
Hc = 2 K1/μo Ms
(10)
Where K1 is anisotropy constant, μo is permeability, Ms is the saturation magnetization and Hc is coercivity. The above relation indicates that the Hc is inversely proportional to the Ms which is consistent with our experimental results. The remanent magnetization is independent parameter and does not depend upon the Ms and Hc. The value of Mr lies in the range (10.5–7.8) emu/g [32].
4. Conclusions Tb3þ and Dy3 þ doped Lithium-Nickle ferrites powder were prepared by micro-emulsion method. Thermal gravimetric analysis revealed thermal decomposition process and phase development of these ferrites. The XRD analysis of Li0.2Ni0.8Tb0.5xDy0.5xFe2 xO4 ferrites exhibited that all the samples have single phase spinel structure
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