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Effect of chromium ion substitution on the electromagnetic properties of nickel ferrite
Materials Chemistry and Physics 118 (2009) 153–160
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Effect of chromium ion substitution on the electromagnetic properties of nickel ferrite M.A. Gabal ∗ , Y.M. Al Angari Chemistry Department, Faculty of Science, King Abdul Aziz University, Jeddah, KSA, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 5 January 2009 Received in revised form 9 April 2009 Accepted 9 July 2009 Keywords: NiFe2−x Crx O4 XRD Conductivity VSM Magnetic susceptibility
a b s t r a c t Polycrystalline ferrites with general formula NiFe2−x Crx O4 (0 ≤ x ≤ 1) were prepared through oxalate impregnation method. The samples were characterized using DTA–TG, XRD, FT-IR, AC conductivity, VSM and magnetic susceptibility measurements. All compositions show cubic Spinel structure. Lattice constant slightly decreases with increasing chromium content. Average crystallite size lays in the range 147–170 nm. FT-IR studies show two absorption bands (1 and 2 ) near about 600 cm−1 and 400 cm−1 for tetrahedral and octahedral sites, respectively. No shift was observed in band position at frequency 1 by increasing Cr content, meanwhile the frequency 2 is shifted to higher frequencies with increasing Cr content. AC conductivity measurements as a function of temperature showed that the samples behave like semiconductors. The decrease in the conductivity with increasing Cr content is due to limiting the degree of Fe2+ –Fe3+ conduction in the octahedral site by introducing Cr which has preference to substitute octahedral Fe3+ ions. The saturation magnetization decreases linearly with increasing Cr content, whereas coercivity increases. Nèel’s magnetic moments calculated from expected cation distributions in comparison with that from hysteresis loop gives satisfactory results up to x = 0.8. Magnetic susceptibility measurements revealed that all the samples have ferrimagnetic properties which changed to paramagnetic materials by increasing temperature. The Curie temperature and molar magnetic susceptibility were observed to decrease with increasing Cr content. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ferrimagnetic materials are widely used in electro-technical devices due to their electrical and magnetic properties. Concretely, their very high specific resistance, remarkable flexibility in tailoring the magnetic properties, ease of preparation and price considerations make ferrites the first choice materials for microwave applications, magnetic recording, magnetic fluids, biosensors, telecommunications and many other applications [1]. In spinel oxides, the site and valence distribution of the cations decides its structural, magnetic, electrical and chemical properties [2]. The site distribution can be modified by metallic substitutions [3], ball milling [4] and controlled preparative conditions (e.g. heat treatment) [5]. As a result, the need for this type of information has been the subject of much experimental and theoretical interest. Nickel ferrite have been extensively used in electronic devices because of their large permeability at high frequency, remarkably high electrical resistivity, mechanical hardness, chemical stability and cost effectiveness [6]. Substituted nickel ferrites are widely
∗ Corresponding author. Permanent address: Chemistry Department, Faculty of Science, Benha University, Benha, Egypt. Tel.: +20 966557071572. E-mail address: [email protected] (M.A. Gabal). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.07.025
used as magnetic materials due to their high electrical resistivity, low eddy current and dielectric losses [7]. The effect of substitution of Fe3+ by Cr3+ in NiFe2 O4 has been studied by various workers. Ghatage et al. [8] carried out neutron diffraction study to determine lattice parameter (a), oxygen positional parameter (u) and cation distribution in NiCrx Fe2−x O4 system. The magnetic moment (ns) values measured from hysteresis and from neutron diffraction study agree with each other. Lee et al. [9] studied the chromium-substituted nickel ferrites and reported that Ni2+ moves to tetrahedral site within the range 0.2 < x < 0.6. They showed that the magnetic moment and Curie temperature decreases with the chromium substitution. Fayek and Ata Allah [10] reported that Cr3+ content occupy the octahedral sites for a maximum of x = 0.6 and the excess Cr3+ replaces the Fe3+ at the tetrahedral site. The structural and magnetic properties of Cr-substituted NiCrx Fe1−x O4 (0 < x < 1.4) spinel ferrites, prepared through conventional ceramic technique, have been investigated by means of XRD and Mössbauer spectroscopy techniques. Their crystal structures are found to be pure cubic phases. The Mössbauer spectra of the samples showed that the compounds are gradually transferred from perfect inverse spinel to partially normal spinel structure as the Cr3+ substitution increases [11].
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Singh et al. [12] synthesized spinel-type ternary ferrites with composition NiFe2−x Crx O4 (0 ≤ x ≤ 1) by a precipitation method. Their physicochemical and electrocatalytic properties have been investigated using IR, XRD, BET surface area, XPS, impedance and Tafel polarization techniques. The study indicated that substitution of Cr from 0.2 to 1.0 mol in the spinel matrix increased the apparent electrocatalytic activity of the base oxide towards the O2 evolution reaction. Cation distribution has been investigated using XRD, magnetic and Mössbauer spectral studies in chromium-substituted nickel ferrites prepared by aerosol route [13]. Cation distribution indicates that the chromium atom occupy octahedral site up to x = 0.8, and then also enters into tetrahedral site. The cation distribution estimated using XRD, magnetic and Mössbauer measurements matches very well with each other. In this project, NiFe2−x Crx O4 (0 ≤ x ≤ 1) system was synthesized from oxalate precursors using the impregnation method [14]. To our knowledge, there are no detailed studies on Cr-doped nickel ferrites synthesized through oxalate precursors. The prepared ferrites were characterized using XRD, FT-IR, VSM, magnetic susceptibility and electrical conductivity measurements. The aim of the present investigation is to study the influence of Cr3+ ions on the structure, electrical and magnetic properties of nickel ferrites, since the series is expected to move from ferrite of inverse spinel structure with magnetic characteristics which are reasonably understood, to chromites with diluted magnetic properties, to our knowledge are not yet fully understood. 2. Experimental procedure 2.1. Preparation procedure Individual metal oxalates were prepared by precipitation from aqueous solutions containing the calculated amounts of AR metal salt with equimolar quantity of AR oxalic acid. The fine precipitate obtained were filtered, washed with distilled water and dried. Samples of the ferrite system; NiFe2−x Crx O4 (0 ≤ x ≤ 1) were prepared using the impregnation technique [14]. Stoichiometric amounts, as per chemical formula unit, of chromium (III) oxide, NiC2 O4 ·2H2 O and FeC2 O4 ·2H2 O were thoroughly mixed in a porcelain dish. Bidistilled water was added in drops with vigorous stirring to assure complete homogeneity. The wetted mixtures were then dried in a thermostatic oven at 100 ◦ C for about 2 h. The dried mixtures were annealed in a muffle furnace at 1000 ◦ C for 2 h under static air atmosphere. Then, the samples were quenched to room temperature and kept in desiccator. Samples were pelletized to discs of 1 cm in diameter and about 1 mm in thickness by applying a pressure of 2 ton cm−2 on well ground powders. The pellets were then sintered in air for about 2 h at 700 ◦ C and left to cool in a desiccator. 2.2. Techniques Simultaneous differential thermal analysis–thermogravimetry (DTA–TG) behavior of the solid state mixtures was investigated using a Shimadzu DT-60 thermal analyzer (Japan) in the temperature range from room temperature up to 1000 ◦ C at a heating rate of 5 K min−1 . The experiments were preceded in flowing air atmosphere (50 ml min−1 ). X-ray powder diffraction (XRD) patterns of the samples were recorded on a D8 Advanced diffractometer (Bruker AXS, Germany) at a sweep rate 5◦ min−1 and time constant 10 s using Cu K␣1 as the radiation source ( = 1.541841 Å). FT-IR spectra were measured in the frequency range 1000–200 cm−1 with KBr disc technique using a Jasco model FT-IR 310 (Japan). For electrical measurements, the two surfaces of each pellet are polished, coated with silver paste and checked for good conduction. The real part of dielectric constant (ε ) and the AC conductivity () were measured, by the two probe method using LCR bridge model Hioki type 3531 (Japan), as a function of temperature from 300 K to 750 K at different frequencies ranging from 300 Hz to 100 kHz. A non-inductive and calibrated furnace was used for heating the samples with constant rate. The temperature of the samples was measured using a T-type thermocouple connected to a digit-sense thermometer (USA) with junction in contact with the sample. The temperature dependence of DC magnetic susceptibility as a function of magnetic field intensities was performed using Faraday’s method in which the sample was inserted in the point of maximum gradient. The measurements were performed over a temperature range up to a paramagnetic behavior was observed in all the samples. The temperature of the samples was measured using T-type thermocouple with accuracy better than ±1 ◦ C.
The magnetization measurements were carried out using a vibrating sample magnetometer (VSM) at room temperature with an applied magnetic field up to 5 kOe to reach saturation values. The external field was applied in the direction of perpendicular to the sample axis.
3. Results and discussion 3.1. Structural properties 3.1.1. DTA–TG studies Simultaneous DTA–TG curves for the thermal decomposition in air of precursor with chromium content of 0.6 are shown in Fig. 1. From the figure it is clear that the precursor decomposed through three TG steps giving a total weight loss of 52% at 345 ◦ C in accordance with calculated weight loss of 51.7% corresponding to the formation of a mixture of nickel, chromium and iron oxides. Two overlapped endothermic DTA peaks were observed in the temperature range 151–200 ◦ C. According to the calculated weight loss and DTA peaks position [15], the first TG step can be attributed to the dehydration of the nickel oxalate content of the mixture. The second endothermic peak was swamped by a large exothermic peak with peak position at 233 ◦ C. These two peaks are closely corresponding to the second TG step which follows immediately after the completion of the first one. The weight loss accompanying this step amounts to the overlapped dehydration and oxidative decomposition processes of the hydrated ferrous oxalate content of the sample [15]. The mixture is then thermally stable up to a temperature of 295 ◦ C after which the nickel oxalate content starts to decompose in the third TG step. This step is accompanied by an exothermic DTA peak at 333 ◦ C. No further changes were observed in the TG or DTA curves by raising the temperature up to 1000 ◦ C. 3.1.2. X-ray diffraction X-ray diffraction patterns of the investigated ferrite samples are presented in Fig. 2. The presence of the diffraction peaks corresponding to the planes (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (6 2 0) and (5 3 3) in the patterns confirm the formation of ferrites with cubic spinel structure. No additional phases from the starting or other materials were detected. The interplaner distances d (Å) were calculated using Bragg’s law. The calculated and observed values of interplaner distances are in good agreement. The patterns show also a very slight shifting in position towards lower d-spacing values with increasing Cr content. The lattice constant of the samples was determined using the relation [16]: aexp = dh k l
h2 + k2 + l2
(1)
Fig. 1. DTA–TG curves in air of precursor with Cr content of 0.6 at specified heating rate of 5 ◦ C min−1 .
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Table 1 Lattice parameters (a), ionic radiuses (rA , rB ), average crystallite size (L), X-ray densities (X ), bulk density (bulk ), porosity (P) and FT-IR spectral data (1 , 2 ) of NiFe2−x Crx O4 system. (x)
a (Å)
Ionic radius (Å)
aexp
ath
rA
rB
0.0 0.2 0.4 0.6 0.8 1.0
8.3347 8.3133 8.3059 8.2935 8.2912 8.2893
8.3404 8.3350 8.3297 8.3270 8.3244 82.985
0.49 0.49 0.49 0.49 0.49 0.47
1.335 1.332 1.329 1.326 1.323 1.323
L (nm)
X (gm cm−3 )
bulk (gm cm−3 )
P (%)
2 (cm−1 )
1 (cm−1 )
147 170 165 162 151 168
5.38 5.40 5.40 5.40 5.39 5.38
5.33 5.31 5.15 5.08 5.06 4.96
0.9 1.68 4.59 6.05 6.20 7.74
404 396 420 437 447 451
593 594 598 592 599 598
Table 1 lists the values of lattice constant. From the table, it is clear that the lattice constant slightly decreases with increasing chromium content. Similar behavior is already reported by Lee et al. [9] for NiFe2−x Crx O4 systems prepared by ceramic method. This variation in unit cell size may be attributed to the smaller ionic radii of 6-fold-coordinated Cr3+ (0.63 Å) compared to that of 6-foldcoordinated high spin Fe3+ (0.64 Å) [17]. Nickel ferrite [15] is an inverse spinel in which half of the ferric ions preferentially fill the tetrahedral sites (A sites) and the other occupies the octahedral sites (B sites). In chromium-doped ferrites, the Cr ions are known to have strong site preference of B sites [17] leads to the replacement of Fe3+ ions at the octahedral sites. Accordingly, the cation distribution of the system can be represented by the formula [11]: (Fe3+ )[Ni2+ Fe3+ 1−x Crx ]O2− 4 . An exception was obtained with the sample with x = 1, where chromium is observed to be partially distributed between the two sites. This behavior will be discussed through the changing in the corresponding magnetic properties. The theoretical lattice parameter (ath ) can be then calculated using equation [13]: ath =
√ 8 √ (rA + rO ) + 3 (rB + rO ) 3 3
(2)
where rO is the radius of the oxygen ion (0.138 nm), rA and rB are the ionic radii of tetrahedral (A) and octahedral (B) sites, respectively. The values of rA and rB will depend critically on the cation distribution of the given system. The data in Table 1, reveals that the values of the theoretical lattice parameter (ath ), calculated assuming the above suggested cation distribution formula, agree well with that experimentally obtained (aexp ). The X-ray density of all the compositions was calculated using the formula [18]: x =
ZM Na3
(3)
where Z is the number of molecules per unit cell (Z = 8); M is the molecular weight; N the Avogadro’s constant and a is the lattice constant. It was found that, the X-ray density is nearly constant at all compositions (Table 1). This behavior is expected since the decrease in the molecular weight of the sample, attributed to the smaller atomic weight of the chromium compared with that of iron, is accompanied by a decrease in the lattice constant.
The bulk density (bulk ) was measured by the Archimedes principle. The percentage porosity (P) of the samples was then calculated using the relation [18]: P=
1−
bulk x
× 100
The values of the bulk density and porosity were both tabulated in Table 1. From the table it is clear that the bulk density is decreased with increasing the Cr content which indicates less densification. The size of the particles was determined from the diffraction peaks broadening with the use of the Debye-Scherrer equation [18]: D=
0.94 ˇ cos
(4)
where is the wavelength of X-ray; ˇ is the full width at half maximum and is the Bragg’s diffraction angle. The crystallite size of the samples under investigation is presented in Table 1. The present investigation shows that the crystallite size of the samples is slightly changed with increasing Cr content. In most oxidic spinels the oxygen ions are apparently larger than the metallic ions, and in spinel like structure the oxygen parameter has a value of about 0.375 for which the arrangement of O2− ions equals exactly a cubic closed packing, but in actual spinel lattice this ideal pattern is slightly deformed. The oxygen parameter (u) is given by equation [19]:
1 1 u = (rA + rO ) √ + 4 3a
(5)
The oxygen parameter (u) depends on the chemical composition, preparation conditions and heating procedure. The values of the oxygen parameter (u) and the degree of inversion (), as a function of chromium content, were calculated and reported in Table 2. The nearly constant values of oxygen parameter obtained in our work, with increasing x, suggest that the chromium substitutes only the iron present in the octahedral site. Intercationic distances were also calculated using experimental values of lattice constant and oxygen parameter for the investigated system using equations from [20]. The results are reported in Table 2. The distance between magnetic √ ions (hopping length) in octahedral sites is given √ by: (1/4)a 2, whereas for tetrahedral site it is given by: (1/4)a 3 [21]. As a result of decreasing lattice parameter (a) with increasing chromium content (Table 1), the distance
Table 2 Cation distribution, oxygen parameter (u), inversion parameter () and cation–anion distance of NiFe2−x Crx O4 system. Cation distribution
(Fe)[NiFe]O4 (Fe)[NiFe0.8 Cr0.2 ]O4 (Fe)[NiFe0.6 Cr0.4 ]O4 (Fe)[NiFe0.4 Cr0.6 ]O4 (Fe)[NiFe0.2 Cr0.8 ]O4 (Fe0.8 Cr0.2 )[NiFe0.2 Cr0.8 ]O4
u (Å)
0.3795 0.3799 0.3800 0.3802 0.3802 0.3802
1.0 1.0 1.0 1.0 1.0 0.8
Tet. bond (Å)
1.8695 1.8704 1.8702 1.8703 1.8698 1.8542
Oct. bond (Å)
2.0469 2.0384 2.0358 2.0312 2.0306 2.0497
Tet. edge (Å)
3.0528 3.0544 3.0540 3.0542 3.0533 3.0278
Oct. edge (Å) Shared
Unshared
2.8407 2.8240 2.8191 2.8102 2.8094 2.8401
2.9477 2.9403 2.9378 2.9335 2.9320 2.9347
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M.A. Gabal, Y.M.A. Angari / Materials Chemistry and Physics 118 (2009) 153–160 Table 3 Values of conductivity () and activation energy (Ea ) at applied frequency of 100 kHz for NiFe2−x Crx O4 system. Cr content (x)
at 563 K ( −1 cm−1 )
Ea (eV)
0.0 0.2 0.4 0.6 0.8 1.0
6.39 × 10−6 6.02 × 10−6 5.81 × 10−6 5.72 × 10−6 4.93 × 10−6 2.17 × 10−6
0.32 0.43 0.49 0.53 0.58 0.64
between the magnetic ions was found to decrease. This may be explained on the basic of the smaller radius of Cr3+ than that of Fe3+ which makes the magnetic ions become closer to each other and decreasing the hopping length. 3.1.3. FT-IR measurements FT-IR spectra of the investigated samples, measured in the frequency range 1000–200 cm−1 , are shown in Fig. 3. The spectra show
Fig. 2. Characteristic parts of XRD patterns of NiFe2−x Crx O4 system.
Fig. 3. FT-IR spectra of NiFe2−x Crx O4 system.
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two main absorption bands below 700 cm−1 as a common feature of all the ferrites. The bands around 590 cm−1 and 410 cm−1 are assigned as 1 and 2 , respectively. Also the spectrum shows that no shift occurs in band position at frequency 1 by increasing Cr content. Meanwhile, it can be seen that the frequency 2 is shifted to higher frequencies with increasing Cr ion concentration (Table 1) and consequently decreasing iron ions. The absorption band 1 is caused by the stretching vibration of the tetrahedral metal–oxygen bond, and the absorption band 2 is caused by the metal–oxygen vibrations in octahedral sites [22]. The variation in the band position, by increasing Cr content, may be caused due to the variation in the cation–oxygen bond length of the octahedral lattice of the spinel. Possibly, the displacement of Fe3+ ions in the B site by smaller Cr3+ ions will results into a somewhat decrease in the metal–oxygen bond length and consequently increase the wave number of 2 band. The spectra also show that, the intensity of the absorption band (2 ) decreases with increasing Cr concentration. It is known that the intensity ratio is a function of the change of dipole moment with the internuclear distance (d /dr). This value represents the contribution of the ionic bond Fe–O in the lattice. So, the observed decrease in the absorption band (2 ) intensity with increasing Cr content is presumably due to the perturbation occurring in Fe–O bonds by substitution the Cr3+ ions. On the other hand, the electronic distribution of Fe–O bonds is greatly affected when a Cr3+ ion with (3d4 4S2 ) orbitals is introduced in its neighbourhood and this consequently affects (d /dr) of the Fe–O bond [17]. At chromium concentration higher than 0.2, a new band between 1 and 2 bands starts to appear. This band was also slightly shifted to higher frequency by the addition of chromium. A similar band was also detected for ferrites prepared by Lopez et al. [23]. This band is attributed also to the stretching vibrations of metal–oxygen atom. The absence of the low frequency bands in our compounds suggests that the lattice vibrations responsible for these bands are very weak.
3.2. Electrical properties 3.2.1. Electrical conductivity Temperature dependence of electrical conductivity, as a function of frequency, for the investigated samples has been studied from room temperature up to 723 K. Typical plots of ln versus temperature (103 /T) are shown in Fig. 4. As expected for semiconductors, the electrical conductivity of the NiFe2−x Crx O4 system increased with increasing temperature. A closer view to the figure shows the presence of two regions. In the first region, the thermal energy is not enough to liberate charge carriers and consequently the conductivity is hardly changed with temperature i.e. clarifying metallic behavior. In the second region, the value of ln increases linearly with increasing temperature. This is because as temperature increases the lattice vibration increases and the ions become closer to each other. At the same time, when the charge carriers gain addition thermal energy it will have higher probability for hopping between ions present in the octahedral sites which will lead to an increase in the charge carriers’ mobility and consequently the conductivity. The conductivity values, for the investigated samples, taken at 563 K and 100 kHz are presented in Table 3. It has been suggested that conduction in spinels is due to the charge transfer of electrons between cations on B sites of different valencies. In the present study Fe3+ ions at octahedral site are partially replaced by Cr3+ ions which are known to have strong B sites preference. These chromium ions do not participate in the conduction process but limit the degree of Fe2+ –Fe3+ conduction
157
Fig. 4. Relation between ln and reciprocal of absolute temperature at different Cr content (x) as a function of applied frequency for NiFe2−x Crx O4 system.
by blocking up the Fe2+ –Fe3+ transformation. This phenomenon hinders the Verwey-de Boer mechanism between statistically distributed Fe2+ and Fe3+ ions at the equivalent crystallographic lattice sites. A–A hopping does not exist as there are only Fe3+ ions on this sublattice and any Fe2+ ions formed during processing preferentially occupy the B sites. B–B hopping is more dominant than A–B hopping [24]. This explains the observed decrease in conductivity with increasing chromium content in the ferrites (Table 3). The hopping probability depends upon the activation energy, which is associated with the electrical energy barrier experienced by the electrons during hopping. The activation energies are calculated using Arrhenius relation and presented in Table 3. It can be seen from Table 3 that values of activation energy increase with chromium concentration. Also, it is noted that lower activation energy is corresponding to higher conductivity. At lower temperatures the conductivity is observed to be frequency dependent, whereas at higher temperatures the conductivity is appeared to be temperature independent (Fig. 4). By increasing frequency, the conductivity increases as a result of the pumping force of the applied frequency that helps in transferring the charge carriers between the different localized states as well as liberating the trapped charges from the different trapping centers. In the high temperature region, increasing temperature will cause a large lattice vibration which scatters the charge carriers and vanishes the effect of frequency. 3.2.2. Dielectric constant Typical curves indicate the variation of dielectric constant (ε ) as a function of temperature at different frequencies are shown in Fig. 5. From the figure it can be seen that, the relation between the dielectric constant and the absolute temperature can be divided to
158
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Fig. 5. Relation between dielectric constant (ε ) and the absolute temperature as a function of applied frequency for sample with x = 0.0.
Fig. 7. Variation of the saturation magnetization ( s ) and the coercivity (Hc ) with Cr content (x) in NiFe2−x Crx O4 system.
3.3. Magnetic properties three regions as follows: in the first region, the dielectric constant of the samples increases with increasing temperature, until reaching a maximum value at a certain temperature after which ε decreases with increasing temperature (second region). In the third region, ε returns to increase continuously. It is known that, the dielectric constant in general is due to orientational, electronic, ionic and interfacial polarization [25]. In the first region the orientational polarization predominant and by increasing temperature the vibration of ions increases and the number of dipoles which aligned with applied field increases. This leads to an increase in the dielectric constant, and reaches maximum value when the hopping frequency of dipoles nearly equal to the frequency of applied field. This orientational polarization decreases by raising temperature, because the effect of thermal energy on the dipoles becomes higher than that of the electric field. This in turns leads to increasing the randomization, with the result of decreasing ε . In the third region the increase in the temperature liberates more localized dipoles from the atomic bonds so the number of charge carriers increases and under the applied field, the contribution of them in the polarization increases. In general, dielectric constant in ferrites decreases with increasing frequency. The decrease is rapid at lower frequencies and slower at higher frequencies. The decrease in dielectric constant with increase in frequency is a normal dielectric behavior.
3.3.1. Vibrating sample magnetometer (VSM) Fig. 6 shows typical plots for the individually hysteresis loops of investigated samples. The main magnetic properties of NiFe2−x Crx O4 system are listed in Table 4. Fig. 7 plots the saturation magnetization (MS ) and the coercivity (HC ) as a function of chromium content. From the figure it is evident that, the saturation magnetization decreases linearly with increasing chromium content. It is well known that the saturation magnetization can be influenced by extrinsic factors such as microstructure and the bulk density of the ferrites [26]. Decreasing the bulk density, will results in increasing pores number and decreases the spin rotational contribution. Since pores act as pinning centers for the electron spins, thus this will result in decreasing the magnetization [27]. Besides the above extrinsic factor, MS is also influenced by intrinsic factors such as preferential site occupancy of the ions. The decrease in the saturation magnetization with increasing Cr content can be attributed to weaken the inter-site exchange interaction of B-sublattice by the addition of Cr3+ ions which preferably occupy B site. As we know Fe3+ has a larger magnetic moment as compared to that of Cr3+ [13]. Thus, when a small fraction of Cr3+ ions replaced Fe3+ ions at B sites the magnetization of B-sublattice decreases, resulting in the observed decrease in saturation magnetization.
Fig. 6. Magnetization against applied field hysteresis loops for NiFe2−x Crx O4 system.
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Table 4 Magnetic data for NiFe2−x Crx O4 system. Cr content (x)
Hysteresis loop −1
MS (emu g 0.0 0.2 0.4 0.6 0.8 1.0
47.7 42.4 32.5 17.9 9.2 1.7
)
B (x) ( B ) −1
Mr (emu g 13.2 12.4 11.3 7.7 3.9 0.7
)
HC (Oe)
B ( B )
10.4 34.7 43.9 52.3 87.4 237.1
2.00 1.77 1.36 0.743 0.38 0.07
Magnetic susceptibility M (emu (g mol)−1 )
2.00 1.60 1.20 0.80 0.40 0.80
7.1 6.0 3.7 2.4 0.8 –
TC (K) 851 811 771 696 638 –
C
eff ( B )
2.2 0.9 0.1 1.0 0.3 –
4.20 2.62 1.04 2.92 1.56 –
The increase in the coercivity of the samples with increasing Cr content can be attributed again to the decrease in the bulk density which increases the number of pores and results in the increase of demagnetization field [27]. It increases slowly up to the concentration x = 0.8 after which it starts increasing steeply (Fig. 7). This behavior of coercivity may be understood as describe by the Banerjee and O’Reilly [28] on the basis of a new model for cation distribution in which the chromium ions enters into the tetrahedral site when x > 0.8. The presence of Cr3+ ions in tetrahedral site causes a negative trigonal field to be superimposed on the octahedral Cr3+ ions. Due to which a 2-fold degeneracy of the orbital ground state results in an unquenched orbital angular momentum and a large anisotropy. A similar behavior was obtained by Singhal and Chandra for NiFe2−x Crx O4 prepared by aerosol route [13]. In Table 4, Nèel’s magnetic moments per formula units which were calculated from the cation distributions of Table 2 and Eq. (6) were compared with these of the samples by the hysteresis loop method using Eq. (7): B (x) = MB (x) − MA (x)
(6)
B = MW × S /5585
(7)
where MB (x) and MA (x) are the magnetic moments for the B and A sites, S is the saturation magnetization of the hysteresis curve and MW is the molecular weight of each sample. The values of Neel’s magnetic moment were calculated by taking ionic magnetic moment of Fe3+ , Cr3+ and Ni2+ as 5 B , 3 B and 2 B , respectively [13]. The cation distribution calculated using the magnetic moment data clearly indicates that the Cr3+ occupying the octahedral sites up to x = 0.8. However, for the composition x > 0.8, Nèel’s theory do not remain valid in conformity with earlier studies [13]. For NiFe2−x Crx O4 system prepared through usual ceramic method, Ghatage et al. [8] showed that spine1 with x < 0.8 have Nèel type magnetic order while for x ≥ 0.8 the magnetic order is not of Nèel type. 3.3.2. Magnetic susceptibility Fig. 8 correlates the molar magnetic susceptibility ( M ) and absolute temperature for the NiFe2−x Crx O4 system at different magnetic field intensities. From a closer look one can consider the samples as a pure ferrimagnetic material in which the thermal energy was not quite sufficient to disturb the aligned moments of the spins. Near the transition temperature, the thermal energy is high enough to disturb all the aligned spins where M decreases drastically, reaching the paramagnetic region after the transition temperature (TC ). From the point of view of magnetic field intensity and its effect on the magnetic susceptibility, the decrease in M with increasing field intensity can be attributed to the saturation of the ferrimagnetic domains at such high field. The magnetic parameters such as Curie temperature (TC ), Curie constant (C) and magnetic moment ( ) at field intensity of 1660 G, for samples with different chromium content, were calculated by plotting the reciprocal molar magnetic susceptibility versus absolute temperature (Table 4). Two regions characterized by two
Fig. 8. Relation between molar magnetic susceptibility ( M ) and the absolute temperature as a function of different magnetic field intensities for NiFe2−x Crx O4 system.
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straight lines corresponding to ferrimagnetic and paramagnetic behavior, passing through the Curie temperature TC , were obtained. From the table it is clear that, the Curie temperature decreases with increasing chromium content. This means that by increasing chromium, the paramagnetic region (disordered state) is increased on the expense of the ferrimagnetic region, which is in conformity with magnetization results. The lowest interaction was obtained at x = 0.8 which means that the ferrimagnetic grains are widely separated and enclosed by chromium ions. The magnetic moment can be calculated using the relation: √
eff = 2.83 C (8) The obtained effective magnetic moment ( eff ), at different chromium contents, was compared with that of Nèel’s magnetic moment ( B (x)) and hysteresis loop magnetic moment ( B ) (Table 4). From the table it is obvious that, the Nèel’s magnetic moment and hysteresis loop magnetic moment have values agree well with each other up to chromium content of 0.8, which confirms the collinear spin ordering. The susceptibility magnetic moment is observed to be higher than the other values which can be attributed to the increase in the exchange interaction of B site on the expense of that of A site [29]. The molar magnetic susceptibility, for all concentrations, measured at 323 K and 1660 G are presented in Table 4 from which it is noticed that, the M decreases linearly with increasing the chromium content. 4. Conclusion Chromium-substituted nickel ferrites; NiFe2−x Crx O4 was synthesized through impregnation method. XRD patterns revealed pure spinel cubic structure. The lattice constant is slightly decreases with increasing chromium content. Moreover, the crystallite size of the samples is slightly changed with increasing Cr content. The presence of Cr3+ ions causes appreciable changes in the magnetic and electrical properties. All the samples showed a semiconducting behavior with increasing temperature and the conductivity was found to decrease with increasing Cr content. The saturation magnetization was found to decrease with increasing Cr content, whereas coercivity increases. Nèel’s magnetic moments calculated from expected cation distributions in comparison with that from hysteresis loop gives satisfactory results up to x = 0.8. Magnetic susceptibility measurements revealed that all the samples have ferrimagnetic properties. The Curie temperature and molar
magnetic susceptibility were observed to decrease with increasing Cr content. The sample with chromium content 1.0 is advised to be used as permanent magnet, since it exhibits very high coercivity. Acknowledgements The Authors are grateful to King Abdul Aziz University for providing financial support for this work (Proj. No. [E-158-428]). Also, the authors would like to express their thanks to Prof. Dr. M.A. Ahmed, Physics department, Faculty of Science, Materials Science Laboratory (1), Cairo University, Egypt, for extending the facilities of magnetic susceptibility measurements and for suggestions in preparing the revised manuscript. References [1] M. Pardavi-Horvath, J. Magn. Magn. Mater. 215–216 (2000) 171. [2] A. Goldman, Modern Ferrite Technology, Marcel Dekker, New York, 1993. [3] K. Seshan, A.L. Shashimohan, D.K. Chakrabarty, A.B. Biswas, Phys. Status Solidi 68 (1981) 97. [4] V. Sepelak, D. Baabe, K.D. Becker, J. Appl. Phys. 88 (2000) 5884. [5] H.St.C. O’Neill, H. Annersten, D. Virgo, Am. Mineral 77 (1992) 725. [6] K. Ishino, Y. Narumiya, Ceram. Bull. 66 (1987) 1469. [7] T. Abraham, Am. Ceram. Soc. Bull. 73 (1994) 62. [8] A.K. Ghatage, S.A. Patil, S.K. Paranjpe, Solid State Commun. 98 (1996) 885. [9] S.H. Lee, S.J. Yoon, G.J. Lee, H.S. Kim, C.H. Yo, K. Ahn, D.H. Lee, K.H. Kim, Mater. Chem. Phys. 61 (1999) 147. [10] M.K. Fayek, S.S. Ata Allah, Phys. Status Solidi (a) 198 (2003) 457. [11] A.M. Gismelseed, A.A. Yousif, Physica B 370 (2005) 215. [12] R.N. Singh, J.P. Singh, B. Lal, M.J. Thomas, S. Bera, Electrochim. Acta 51 (2006) 5515. [13] S. Singhal, K. Chandra, J. Solid State Chem. 180 (2007) 296. [14] M.A. Gabal, S.S. Ata-Allah, Mater. Chem. Phys. 85 (2004) 104. [15] M.A. Gabal, J. Phys. Chem. Sol. 64 (2003) 1375. [16] B.D. Cullity, Elements of X-ray Diffraction, Addison Wesley Pub. Co. Inc., 1956. [17] A.M. El-Sayed, Ceram. Int. 28 (2002) 651. [18] T.J. Shindea, A.B. Gadkari, P.N. Vasambekar, Mater. Chem. Phys. (2008). [19] M.A. Ahmed, E. Ateia, L.M. Salah, A.A. El-Gamal, Mater. Chem. Phys. 92 (2005) 310. [20] Q.M. Wei, J.B. Li, Y.J. Chen, J. Mater. Sci. 36 (2001) 5115. [21] V.B. Kawade, G.K. Bichile, K.M. Jadhav, Mater. Lett. 42 (2000) 33. [22] R.D. Waldron, Infrared spectra of ferrites, Phys. Rev. 99 (1955) 1727. [23] F.A. Lopez, A. Lopez-Delgado, J.L. Martın de Vidales, E. Vila, J. Alloys Cmpds. 265 (1998) 291. [24] A.M. El-Sayed, Mater. Chem. Phys. 82 (2003) 583. [25] A.K. Singh, T.C. Goel, R.G. Mendiratta, O.P. Thakur, C. Prakash, J. Appl. Phys. 91 (2002) 6626. [26] P. Roy, J. Bera, J. Magn. Magn. Mater. 298 (2006) 38. [27] P. Roy, J. Bera, J. Magn. Magn. Mater. 321 (2009) 247. [28] S.K. Banerjee, W. O’Reilly, IEEE Trans. Magnets 3 (1966) 463. [29] M.A. Gabal, M.A. Ahmed, J. Mater. Sci. 40 (2005) 387.
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Effect of chromium ion substitution on the electromagnetic properties of nickel ferrite M.A. Gabal ∗ , Y.M. Al Angari Chemistry Department, Faculty of Science, King Abdul Aziz University, Jeddah, KSA, Saudi Arabia
a r t i c l e
i n f o
Article history: Received 5 January 2009 Received in revised form 9 April 2009 Accepted 9 July 2009 Keywords: NiFe2−x Crx O4 XRD Conductivity VSM Magnetic susceptibility
a b s t r a c t Polycrystalline ferrites with general formula NiFe2−x Crx O4 (0 ≤ x ≤ 1) were prepared through oxalate impregnation method. The samples were characterized using DTA–TG, XRD, FT-IR, AC conductivity, VSM and magnetic susceptibility measurements. All compositions show cubic Spinel structure. Lattice constant slightly decreases with increasing chromium content. Average crystallite size lays in the range 147–170 nm. FT-IR studies show two absorption bands (1 and 2 ) near about 600 cm−1 and 400 cm−1 for tetrahedral and octahedral sites, respectively. No shift was observed in band position at frequency 1 by increasing Cr content, meanwhile the frequency 2 is shifted to higher frequencies with increasing Cr content. AC conductivity measurements as a function of temperature showed that the samples behave like semiconductors. The decrease in the conductivity with increasing Cr content is due to limiting the degree of Fe2+ –Fe3+ conduction in the octahedral site by introducing Cr which has preference to substitute octahedral Fe3+ ions. The saturation magnetization decreases linearly with increasing Cr content, whereas coercivity increases. Nèel’s magnetic moments calculated from expected cation distributions in comparison with that from hysteresis loop gives satisfactory results up to x = 0.8. Magnetic susceptibility measurements revealed that all the samples have ferrimagnetic properties which changed to paramagnetic materials by increasing temperature. The Curie temperature and molar magnetic susceptibility were observed to decrease with increasing Cr content. © 2009 Elsevier B.V. All rights reserved.
1. Introduction Ferrimagnetic materials are widely used in electro-technical devices due to their electrical and magnetic properties. Concretely, their very high specific resistance, remarkable flexibility in tailoring the magnetic properties, ease of preparation and price considerations make ferrites the first choice materials for microwave applications, magnetic recording, magnetic fluids, biosensors, telecommunications and many other applications [1]. In spinel oxides, the site and valence distribution of the cations decides its structural, magnetic, electrical and chemical properties [2]. The site distribution can be modified by metallic substitutions [3], ball milling [4] and controlled preparative conditions (e.g. heat treatment) [5]. As a result, the need for this type of information has been the subject of much experimental and theoretical interest. Nickel ferrite have been extensively used in electronic devices because of their large permeability at high frequency, remarkably high electrical resistivity, mechanical hardness, chemical stability and cost effectiveness [6]. Substituted nickel ferrites are widely
∗ Corresponding author. Permanent address: Chemistry Department, Faculty of Science, Benha University, Benha, Egypt. Tel.: +20 966557071572. E-mail address: [email protected] (M.A. Gabal). 0254-0584/$ – see front matter © 2009 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2009.07.025
used as magnetic materials due to their high electrical resistivity, low eddy current and dielectric losses [7]. The effect of substitution of Fe3+ by Cr3+ in NiFe2 O4 has been studied by various workers. Ghatage et al. [8] carried out neutron diffraction study to determine lattice parameter (a), oxygen positional parameter (u) and cation distribution in NiCrx Fe2−x O4 system. The magnetic moment (ns) values measured from hysteresis and from neutron diffraction study agree with each other. Lee et al. [9] studied the chromium-substituted nickel ferrites and reported that Ni2+ moves to tetrahedral site within the range 0.2 < x < 0.6. They showed that the magnetic moment and Curie temperature decreases with the chromium substitution. Fayek and Ata Allah [10] reported that Cr3+ content occupy the octahedral sites for a maximum of x = 0.6 and the excess Cr3+ replaces the Fe3+ at the tetrahedral site. The structural and magnetic properties of Cr-substituted NiCrx Fe1−x O4 (0 < x < 1.4) spinel ferrites, prepared through conventional ceramic technique, have been investigated by means of XRD and Mössbauer spectroscopy techniques. Their crystal structures are found to be pure cubic phases. The Mössbauer spectra of the samples showed that the compounds are gradually transferred from perfect inverse spinel to partially normal spinel structure as the Cr3+ substitution increases [11].
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Singh et al. [12] synthesized spinel-type ternary ferrites with composition NiFe2−x Crx O4 (0 ≤ x ≤ 1) by a precipitation method. Their physicochemical and electrocatalytic properties have been investigated using IR, XRD, BET surface area, XPS, impedance and Tafel polarization techniques. The study indicated that substitution of Cr from 0.2 to 1.0 mol in the spinel matrix increased the apparent electrocatalytic activity of the base oxide towards the O2 evolution reaction. Cation distribution has been investigated using XRD, magnetic and Mössbauer spectral studies in chromium-substituted nickel ferrites prepared by aerosol route [13]. Cation distribution indicates that the chromium atom occupy octahedral site up to x = 0.8, and then also enters into tetrahedral site. The cation distribution estimated using XRD, magnetic and Mössbauer measurements matches very well with each other. In this project, NiFe2−x Crx O4 (0 ≤ x ≤ 1) system was synthesized from oxalate precursors using the impregnation method [14]. To our knowledge, there are no detailed studies on Cr-doped nickel ferrites synthesized through oxalate precursors. The prepared ferrites were characterized using XRD, FT-IR, VSM, magnetic susceptibility and electrical conductivity measurements. The aim of the present investigation is to study the influence of Cr3+ ions on the structure, electrical and magnetic properties of nickel ferrites, since the series is expected to move from ferrite of inverse spinel structure with magnetic characteristics which are reasonably understood, to chromites with diluted magnetic properties, to our knowledge are not yet fully understood. 2. Experimental procedure 2.1. Preparation procedure Individual metal oxalates were prepared by precipitation from aqueous solutions containing the calculated amounts of AR metal salt with equimolar quantity of AR oxalic acid. The fine precipitate obtained were filtered, washed with distilled water and dried. Samples of the ferrite system; NiFe2−x Crx O4 (0 ≤ x ≤ 1) were prepared using the impregnation technique [14]. Stoichiometric amounts, as per chemical formula unit, of chromium (III) oxide, NiC2 O4 ·2H2 O and FeC2 O4 ·2H2 O were thoroughly mixed in a porcelain dish. Bidistilled water was added in drops with vigorous stirring to assure complete homogeneity. The wetted mixtures were then dried in a thermostatic oven at 100 ◦ C for about 2 h. The dried mixtures were annealed in a muffle furnace at 1000 ◦ C for 2 h under static air atmosphere. Then, the samples were quenched to room temperature and kept in desiccator. Samples were pelletized to discs of 1 cm in diameter and about 1 mm in thickness by applying a pressure of 2 ton cm−2 on well ground powders. The pellets were then sintered in air for about 2 h at 700 ◦ C and left to cool in a desiccator. 2.2. Techniques Simultaneous differential thermal analysis–thermogravimetry (DTA–TG) behavior of the solid state mixtures was investigated using a Shimadzu DT-60 thermal analyzer (Japan) in the temperature range from room temperature up to 1000 ◦ C at a heating rate of 5 K min−1 . The experiments were preceded in flowing air atmosphere (50 ml min−1 ). X-ray powder diffraction (XRD) patterns of the samples were recorded on a D8 Advanced diffractometer (Bruker AXS, Germany) at a sweep rate 5◦ min−1 and time constant 10 s using Cu K␣1 as the radiation source ( = 1.541841 Å). FT-IR spectra were measured in the frequency range 1000–200 cm−1 with KBr disc technique using a Jasco model FT-IR 310 (Japan). For electrical measurements, the two surfaces of each pellet are polished, coated with silver paste and checked for good conduction. The real part of dielectric constant (ε ) and the AC conductivity () were measured, by the two probe method using LCR bridge model Hioki type 3531 (Japan), as a function of temperature from 300 K to 750 K at different frequencies ranging from 300 Hz to 100 kHz. A non-inductive and calibrated furnace was used for heating the samples with constant rate. The temperature of the samples was measured using a T-type thermocouple connected to a digit-sense thermometer (USA) with junction in contact with the sample. The temperature dependence of DC magnetic susceptibility as a function of magnetic field intensities was performed using Faraday’s method in which the sample was inserted in the point of maximum gradient. The measurements were performed over a temperature range up to a paramagnetic behavior was observed in all the samples. The temperature of the samples was measured using T-type thermocouple with accuracy better than ±1 ◦ C.
The magnetization measurements were carried out using a vibrating sample magnetometer (VSM) at room temperature with an applied magnetic field up to 5 kOe to reach saturation values. The external field was applied in the direction of perpendicular to the sample axis.
3. Results and discussion 3.1. Structural properties 3.1.1. DTA–TG studies Simultaneous DTA–TG curves for the thermal decomposition in air of precursor with chromium content of 0.6 are shown in Fig. 1. From the figure it is clear that the precursor decomposed through three TG steps giving a total weight loss of 52% at 345 ◦ C in accordance with calculated weight loss of 51.7% corresponding to the formation of a mixture of nickel, chromium and iron oxides. Two overlapped endothermic DTA peaks were observed in the temperature range 151–200 ◦ C. According to the calculated weight loss and DTA peaks position [15], the first TG step can be attributed to the dehydration of the nickel oxalate content of the mixture. The second endothermic peak was swamped by a large exothermic peak with peak position at 233 ◦ C. These two peaks are closely corresponding to the second TG step which follows immediately after the completion of the first one. The weight loss accompanying this step amounts to the overlapped dehydration and oxidative decomposition processes of the hydrated ferrous oxalate content of the sample [15]. The mixture is then thermally stable up to a temperature of 295 ◦ C after which the nickel oxalate content starts to decompose in the third TG step. This step is accompanied by an exothermic DTA peak at 333 ◦ C. No further changes were observed in the TG or DTA curves by raising the temperature up to 1000 ◦ C. 3.1.2. X-ray diffraction X-ray diffraction patterns of the investigated ferrite samples are presented in Fig. 2. The presence of the diffraction peaks corresponding to the planes (1 1 1), (2 2 0), (3 1 1), (2 2 2), (4 0 0), (4 2 2), (5 1 1), (4 4 0), (6 2 0) and (5 3 3) in the patterns confirm the formation of ferrites with cubic spinel structure. No additional phases from the starting or other materials were detected. The interplaner distances d (Å) were calculated using Bragg’s law. The calculated and observed values of interplaner distances are in good agreement. The patterns show also a very slight shifting in position towards lower d-spacing values with increasing Cr content. The lattice constant of the samples was determined using the relation [16]: aexp = dh k l
h2 + k2 + l2
(1)
Fig. 1. DTA–TG curves in air of precursor with Cr content of 0.6 at specified heating rate of 5 ◦ C min−1 .
M.A. Gabal, Y.M.A. Angari / Materials Chemistry and Physics 118 (2009) 153–160
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Table 1 Lattice parameters (a), ionic radiuses (rA , rB ), average crystallite size (L), X-ray densities (X ), bulk density (bulk ), porosity (P) and FT-IR spectral data (1 , 2 ) of NiFe2−x Crx O4 system. (x)
a (Å)
Ionic radius (Å)
aexp
ath
rA
rB
0.0 0.2 0.4 0.6 0.8 1.0
8.3347 8.3133 8.3059 8.2935 8.2912 8.2893
8.3404 8.3350 8.3297 8.3270 8.3244 82.985
0.49 0.49 0.49 0.49 0.49 0.47
1.335 1.332 1.329 1.326 1.323 1.323
L (nm)
X (gm cm−3 )
bulk (gm cm−3 )
P (%)
2 (cm−1 )
1 (cm−1 )
147 170 165 162 151 168
5.38 5.40 5.40 5.40 5.39 5.38
5.33 5.31 5.15 5.08 5.06 4.96
0.9 1.68 4.59 6.05 6.20 7.74
404 396 420 437 447 451
593 594 598 592 599 598
Table 1 lists the values of lattice constant. From the table, it is clear that the lattice constant slightly decreases with increasing chromium content. Similar behavior is already reported by Lee et al. [9] for NiFe2−x Crx O4 systems prepared by ceramic method. This variation in unit cell size may be attributed to the smaller ionic radii of 6-fold-coordinated Cr3+ (0.63 Å) compared to that of 6-foldcoordinated high spin Fe3+ (0.64 Å) [17]. Nickel ferrite [15] is an inverse spinel in which half of the ferric ions preferentially fill the tetrahedral sites (A sites) and the other occupies the octahedral sites (B sites). In chromium-doped ferrites, the Cr ions are known to have strong site preference of B sites [17] leads to the replacement of Fe3+ ions at the octahedral sites. Accordingly, the cation distribution of the system can be represented by the formula [11]: (Fe3+ )[Ni2+ Fe3+ 1−x Crx ]O2− 4 . An exception was obtained with the sample with x = 1, where chromium is observed to be partially distributed between the two sites. This behavior will be discussed through the changing in the corresponding magnetic properties. The theoretical lattice parameter (ath ) can be then calculated using equation [13]: ath =
√ 8 √ (rA + rO ) + 3 (rB + rO ) 3 3
(2)
where rO is the radius of the oxygen ion (0.138 nm), rA and rB are the ionic radii of tetrahedral (A) and octahedral (B) sites, respectively. The values of rA and rB will depend critically on the cation distribution of the given system. The data in Table 1, reveals that the values of the theoretical lattice parameter (ath ), calculated assuming the above suggested cation distribution formula, agree well with that experimentally obtained (aexp ). The X-ray density of all the compositions was calculated using the formula [18]: x =
ZM Na3
(3)
where Z is the number of molecules per unit cell (Z = 8); M is the molecular weight; N the Avogadro’s constant and a is the lattice constant. It was found that, the X-ray density is nearly constant at all compositions (Table 1). This behavior is expected since the decrease in the molecular weight of the sample, attributed to the smaller atomic weight of the chromium compared with that of iron, is accompanied by a decrease in the lattice constant.
The bulk density (bulk ) was measured by the Archimedes principle. The percentage porosity (P) of the samples was then calculated using the relation [18]: P=
1−
bulk x
× 100
The values of the bulk density and porosity were both tabulated in Table 1. From the table it is clear that the bulk density is decreased with increasing the Cr content which indicates less densification. The size of the particles was determined from the diffraction peaks broadening with the use of the Debye-Scherrer equation [18]: D=
0.94 ˇ cos
(4)
where is the wavelength of X-ray; ˇ is the full width at half maximum and is the Bragg’s diffraction angle. The crystallite size of the samples under investigation is presented in Table 1. The present investigation shows that the crystallite size of the samples is slightly changed with increasing Cr content. In most oxidic spinels the oxygen ions are apparently larger than the metallic ions, and in spinel like structure the oxygen parameter has a value of about 0.375 for which the arrangement of O2− ions equals exactly a cubic closed packing, but in actual spinel lattice this ideal pattern is slightly deformed. The oxygen parameter (u) is given by equation [19]:
1 1 u = (rA + rO ) √ + 4 3a
(5)
The oxygen parameter (u) depends on the chemical composition, preparation conditions and heating procedure. The values of the oxygen parameter (u) and the degree of inversion (), as a function of chromium content, were calculated and reported in Table 2. The nearly constant values of oxygen parameter obtained in our work, with increasing x, suggest that the chromium substitutes only the iron present in the octahedral site. Intercationic distances were also calculated using experimental values of lattice constant and oxygen parameter for the investigated system using equations from [20]. The results are reported in Table 2. The distance between magnetic √ ions (hopping length) in octahedral sites is given √ by: (1/4)a 2, whereas for tetrahedral site it is given by: (1/4)a 3 [21]. As a result of decreasing lattice parameter (a) with increasing chromium content (Table 1), the distance
Table 2 Cation distribution, oxygen parameter (u), inversion parameter () and cation–anion distance of NiFe2−x Crx O4 system. Cation distribution
(Fe)[NiFe]O4 (Fe)[NiFe0.8 Cr0.2 ]O4 (Fe)[NiFe0.6 Cr0.4 ]O4 (Fe)[NiFe0.4 Cr0.6 ]O4 (Fe)[NiFe0.2 Cr0.8 ]O4 (Fe0.8 Cr0.2 )[NiFe0.2 Cr0.8 ]O4
u (Å)
0.3795 0.3799 0.3800 0.3802 0.3802 0.3802
1.0 1.0 1.0 1.0 1.0 0.8
Tet. bond (Å)
1.8695 1.8704 1.8702 1.8703 1.8698 1.8542
Oct. bond (Å)
2.0469 2.0384 2.0358 2.0312 2.0306 2.0497
Tet. edge (Å)
3.0528 3.0544 3.0540 3.0542 3.0533 3.0278
Oct. edge (Å) Shared
Unshared
2.8407 2.8240 2.8191 2.8102 2.8094 2.8401
2.9477 2.9403 2.9378 2.9335 2.9320 2.9347
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M.A. Gabal, Y.M.A. Angari / Materials Chemistry and Physics 118 (2009) 153–160 Table 3 Values of conductivity () and activation energy (Ea ) at applied frequency of 100 kHz for NiFe2−x Crx O4 system. Cr content (x)
at 563 K ( −1 cm−1 )
Ea (eV)
0.0 0.2 0.4 0.6 0.8 1.0
6.39 × 10−6 6.02 × 10−6 5.81 × 10−6 5.72 × 10−6 4.93 × 10−6 2.17 × 10−6
0.32 0.43 0.49 0.53 0.58 0.64
between the magnetic ions was found to decrease. This may be explained on the basic of the smaller radius of Cr3+ than that of Fe3+ which makes the magnetic ions become closer to each other and decreasing the hopping length. 3.1.3. FT-IR measurements FT-IR spectra of the investigated samples, measured in the frequency range 1000–200 cm−1 , are shown in Fig. 3. The spectra show
Fig. 2. Characteristic parts of XRD patterns of NiFe2−x Crx O4 system.
Fig. 3. FT-IR spectra of NiFe2−x Crx O4 system.
M.A. Gabal, Y.M.A. Angari / Materials Chemistry and Physics 118 (2009) 153–160
two main absorption bands below 700 cm−1 as a common feature of all the ferrites. The bands around 590 cm−1 and 410 cm−1 are assigned as 1 and 2 , respectively. Also the spectrum shows that no shift occurs in band position at frequency 1 by increasing Cr content. Meanwhile, it can be seen that the frequency 2 is shifted to higher frequencies with increasing Cr ion concentration (Table 1) and consequently decreasing iron ions. The absorption band 1 is caused by the stretching vibration of the tetrahedral metal–oxygen bond, and the absorption band 2 is caused by the metal–oxygen vibrations in octahedral sites [22]. The variation in the band position, by increasing Cr content, may be caused due to the variation in the cation–oxygen bond length of the octahedral lattice of the spinel. Possibly, the displacement of Fe3+ ions in the B site by smaller Cr3+ ions will results into a somewhat decrease in the metal–oxygen bond length and consequently increase the wave number of 2 band. The spectra also show that, the intensity of the absorption band (2 ) decreases with increasing Cr concentration. It is known that the intensity ratio is a function of the change of dipole moment with the internuclear distance (d /dr). This value represents the contribution of the ionic bond Fe–O in the lattice. So, the observed decrease in the absorption band (2 ) intensity with increasing Cr content is presumably due to the perturbation occurring in Fe–O bonds by substitution the Cr3+ ions. On the other hand, the electronic distribution of Fe–O bonds is greatly affected when a Cr3+ ion with (3d4 4S2 ) orbitals is introduced in its neighbourhood and this consequently affects (d /dr) of the Fe–O bond [17]. At chromium concentration higher than 0.2, a new band between 1 and 2 bands starts to appear. This band was also slightly shifted to higher frequency by the addition of chromium. A similar band was also detected for ferrites prepared by Lopez et al. [23]. This band is attributed also to the stretching vibrations of metal–oxygen atom. The absence of the low frequency bands in our compounds suggests that the lattice vibrations responsible for these bands are very weak.
3.2. Electrical properties 3.2.1. Electrical conductivity Temperature dependence of electrical conductivity, as a function of frequency, for the investigated samples has been studied from room temperature up to 723 K. Typical plots of ln versus temperature (103 /T) are shown in Fig. 4. As expected for semiconductors, the electrical conductivity of the NiFe2−x Crx O4 system increased with increasing temperature. A closer view to the figure shows the presence of two regions. In the first region, the thermal energy is not enough to liberate charge carriers and consequently the conductivity is hardly changed with temperature i.e. clarifying metallic behavior. In the second region, the value of ln increases linearly with increasing temperature. This is because as temperature increases the lattice vibration increases and the ions become closer to each other. At the same time, when the charge carriers gain addition thermal energy it will have higher probability for hopping between ions present in the octahedral sites which will lead to an increase in the charge carriers’ mobility and consequently the conductivity. The conductivity values, for the investigated samples, taken at 563 K and 100 kHz are presented in Table 3. It has been suggested that conduction in spinels is due to the charge transfer of electrons between cations on B sites of different valencies. In the present study Fe3+ ions at octahedral site are partially replaced by Cr3+ ions which are known to have strong B sites preference. These chromium ions do not participate in the conduction process but limit the degree of Fe2+ –Fe3+ conduction
157
Fig. 4. Relation between ln and reciprocal of absolute temperature at different Cr content (x) as a function of applied frequency for NiFe2−x Crx O4 system.
by blocking up the Fe2+ –Fe3+ transformation. This phenomenon hinders the Verwey-de Boer mechanism between statistically distributed Fe2+ and Fe3+ ions at the equivalent crystallographic lattice sites. A–A hopping does not exist as there are only Fe3+ ions on this sublattice and any Fe2+ ions formed during processing preferentially occupy the B sites. B–B hopping is more dominant than A–B hopping [24]. This explains the observed decrease in conductivity with increasing chromium content in the ferrites (Table 3). The hopping probability depends upon the activation energy, which is associated with the electrical energy barrier experienced by the electrons during hopping. The activation energies are calculated using Arrhenius relation and presented in Table 3. It can be seen from Table 3 that values of activation energy increase with chromium concentration. Also, it is noted that lower activation energy is corresponding to higher conductivity. At lower temperatures the conductivity is observed to be frequency dependent, whereas at higher temperatures the conductivity is appeared to be temperature independent (Fig. 4). By increasing frequency, the conductivity increases as a result of the pumping force of the applied frequency that helps in transferring the charge carriers between the different localized states as well as liberating the trapped charges from the different trapping centers. In the high temperature region, increasing temperature will cause a large lattice vibration which scatters the charge carriers and vanishes the effect of frequency. 3.2.2. Dielectric constant Typical curves indicate the variation of dielectric constant (ε ) as a function of temperature at different frequencies are shown in Fig. 5. From the figure it can be seen that, the relation between the dielectric constant and the absolute temperature can be divided to
158
M.A. Gabal, Y.M.A. Angari / Materials Chemistry and Physics 118 (2009) 153–160
Fig. 5. Relation between dielectric constant (ε ) and the absolute temperature as a function of applied frequency for sample with x = 0.0.
Fig. 7. Variation of the saturation magnetization ( s ) and the coercivity (Hc ) with Cr content (x) in NiFe2−x Crx O4 system.
3.3. Magnetic properties three regions as follows: in the first region, the dielectric constant of the samples increases with increasing temperature, until reaching a maximum value at a certain temperature after which ε decreases with increasing temperature (second region). In the third region, ε returns to increase continuously. It is known that, the dielectric constant in general is due to orientational, electronic, ionic and interfacial polarization [25]. In the first region the orientational polarization predominant and by increasing temperature the vibration of ions increases and the number of dipoles which aligned with applied field increases. This leads to an increase in the dielectric constant, and reaches maximum value when the hopping frequency of dipoles nearly equal to the frequency of applied field. This orientational polarization decreases by raising temperature, because the effect of thermal energy on the dipoles becomes higher than that of the electric field. This in turns leads to increasing the randomization, with the result of decreasing ε . In the third region the increase in the temperature liberates more localized dipoles from the atomic bonds so the number of charge carriers increases and under the applied field, the contribution of them in the polarization increases. In general, dielectric constant in ferrites decreases with increasing frequency. The decrease is rapid at lower frequencies and slower at higher frequencies. The decrease in dielectric constant with increase in frequency is a normal dielectric behavior.
3.3.1. Vibrating sample magnetometer (VSM) Fig. 6 shows typical plots for the individually hysteresis loops of investigated samples. The main magnetic properties of NiFe2−x Crx O4 system are listed in Table 4. Fig. 7 plots the saturation magnetization (MS ) and the coercivity (HC ) as a function of chromium content. From the figure it is evident that, the saturation magnetization decreases linearly with increasing chromium content. It is well known that the saturation magnetization can be influenced by extrinsic factors such as microstructure and the bulk density of the ferrites [26]. Decreasing the bulk density, will results in increasing pores number and decreases the spin rotational contribution. Since pores act as pinning centers for the electron spins, thus this will result in decreasing the magnetization [27]. Besides the above extrinsic factor, MS is also influenced by intrinsic factors such as preferential site occupancy of the ions. The decrease in the saturation magnetization with increasing Cr content can be attributed to weaken the inter-site exchange interaction of B-sublattice by the addition of Cr3+ ions which preferably occupy B site. As we know Fe3+ has a larger magnetic moment as compared to that of Cr3+ [13]. Thus, when a small fraction of Cr3+ ions replaced Fe3+ ions at B sites the magnetization of B-sublattice decreases, resulting in the observed decrease in saturation magnetization.
Fig. 6. Magnetization against applied field hysteresis loops for NiFe2−x Crx O4 system.
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Table 4 Magnetic data for NiFe2−x Crx O4 system. Cr content (x)
Hysteresis loop −1
MS (emu g 0.0 0.2 0.4 0.6 0.8 1.0
47.7 42.4 32.5 17.9 9.2 1.7
)
B (x) ( B ) −1
Mr (emu g 13.2 12.4 11.3 7.7 3.9 0.7
)
HC (Oe)
B ( B )
10.4 34.7 43.9 52.3 87.4 237.1
2.00 1.77 1.36 0.743 0.38 0.07
Magnetic susceptibility M (emu (g mol)−1 )
2.00 1.60 1.20 0.80 0.40 0.80
7.1 6.0 3.7 2.4 0.8 –
TC (K) 851 811 771 696 638 –
C
eff ( B )
2.2 0.9 0.1 1.0 0.3 –
4.20 2.62 1.04 2.92 1.56 –
The increase in the coercivity of the samples with increasing Cr content can be attributed again to the decrease in the bulk density which increases the number of pores and results in the increase of demagnetization field [27]. It increases slowly up to the concentration x = 0.8 after which it starts increasing steeply (Fig. 7). This behavior of coercivity may be understood as describe by the Banerjee and O’Reilly [28] on the basis of a new model for cation distribution in which the chromium ions enters into the tetrahedral site when x > 0.8. The presence of Cr3+ ions in tetrahedral site causes a negative trigonal field to be superimposed on the octahedral Cr3+ ions. Due to which a 2-fold degeneracy of the orbital ground state results in an unquenched orbital angular momentum and a large anisotropy. A similar behavior was obtained by Singhal and Chandra for NiFe2−x Crx O4 prepared by aerosol route [13]. In Table 4, Nèel’s magnetic moments per formula units which were calculated from the cation distributions of Table 2 and Eq. (6) were compared with these of the samples by the hysteresis loop method using Eq. (7): B (x) = MB (x) − MA (x)
(6)
B = MW × S /5585
(7)
where MB (x) and MA (x) are the magnetic moments for the B and A sites, S is the saturation magnetization of the hysteresis curve and MW is the molecular weight of each sample. The values of Neel’s magnetic moment were calculated by taking ionic magnetic moment of Fe3+ , Cr3+ and Ni2+ as 5 B , 3 B and 2 B , respectively [13]. The cation distribution calculated using the magnetic moment data clearly indicates that the Cr3+ occupying the octahedral sites up to x = 0.8. However, for the composition x > 0.8, Nèel’s theory do not remain valid in conformity with earlier studies [13]. For NiFe2−x Crx O4 system prepared through usual ceramic method, Ghatage et al. [8] showed that spine1 with x < 0.8 have Nèel type magnetic order while for x ≥ 0.8 the magnetic order is not of Nèel type. 3.3.2. Magnetic susceptibility Fig. 8 correlates the molar magnetic susceptibility ( M ) and absolute temperature for the NiFe2−x Crx O4 system at different magnetic field intensities. From a closer look one can consider the samples as a pure ferrimagnetic material in which the thermal energy was not quite sufficient to disturb the aligned moments of the spins. Near the transition temperature, the thermal energy is high enough to disturb all the aligned spins where M decreases drastically, reaching the paramagnetic region after the transition temperature (TC ). From the point of view of magnetic field intensity and its effect on the magnetic susceptibility, the decrease in M with increasing field intensity can be attributed to the saturation of the ferrimagnetic domains at such high field. The magnetic parameters such as Curie temperature (TC ), Curie constant (C) and magnetic moment ( ) at field intensity of 1660 G, for samples with different chromium content, were calculated by plotting the reciprocal molar magnetic susceptibility versus absolute temperature (Table 4). Two regions characterized by two
Fig. 8. Relation between molar magnetic susceptibility ( M ) and the absolute temperature as a function of different magnetic field intensities for NiFe2−x Crx O4 system.
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straight lines corresponding to ferrimagnetic and paramagnetic behavior, passing through the Curie temperature TC , were obtained. From the table it is clear that, the Curie temperature decreases with increasing chromium content. This means that by increasing chromium, the paramagnetic region (disordered state) is increased on the expense of the ferrimagnetic region, which is in conformity with magnetization results. The lowest interaction was obtained at x = 0.8 which means that the ferrimagnetic grains are widely separated and enclosed by chromium ions. The magnetic moment can be calculated using the relation: √
eff = 2.83 C (8) The obtained effective magnetic moment ( eff ), at different chromium contents, was compared with that of Nèel’s magnetic moment ( B (x)) and hysteresis loop magnetic moment ( B ) (Table 4). From the table it is obvious that, the Nèel’s magnetic moment and hysteresis loop magnetic moment have values agree well with each other up to chromium content of 0.8, which confirms the collinear spin ordering. The susceptibility magnetic moment is observed to be higher than the other values which can be attributed to the increase in the exchange interaction of B site on the expense of that of A site [29]. The molar magnetic susceptibility, for all concentrations, measured at 323 K and 1660 G are presented in Table 4 from which it is noticed that, the M decreases linearly with increasing the chromium content. 4. Conclusion Chromium-substituted nickel ferrites; NiFe2−x Crx O4 was synthesized through impregnation method. XRD patterns revealed pure spinel cubic structure. The lattice constant is slightly decreases with increasing chromium content. Moreover, the crystallite size of the samples is slightly changed with increasing Cr content. The presence of Cr3+ ions causes appreciable changes in the magnetic and electrical properties. All the samples showed a semiconducting behavior with increasing temperature and the conductivity was found to decrease with increasing Cr content. The saturation magnetization was found to decrease with increasing Cr content, whereas coercivity increases. Nèel’s magnetic moments calculated from expected cation distributions in comparison with that from hysteresis loop gives satisfactory results up to x = 0.8. Magnetic susceptibility measurements revealed that all the samples have ferrimagnetic properties. The Curie temperature and molar
magnetic susceptibility were observed to decrease with increasing Cr content. The sample with chromium content 1.0 is advised to be used as permanent magnet, since it exhibits very high coercivity. Acknowledgements The Authors are grateful to King Abdul Aziz University for providing financial support for this work (Proj. No. [E-158-428]). Also, the authors would like to express their thanks to Prof. Dr. M.A. Ahmed, Physics department, Faculty of Science, Materials Science Laboratory (1), Cairo University, Egypt, for extending the facilities of magnetic susceptibility measurements and for suggestions in preparing the revised manuscript. References [1] M. Pardavi-Horvath, J. Magn. Magn. Mater. 215–216 (2000) 171. [2] A. Goldman, Modern Ferrite Technology, Marcel Dekker, New York, 1993. [3] K. Seshan, A.L. Shashimohan, D.K. Chakrabarty, A.B. Biswas, Phys. Status Solidi 68 (1981) 97. [4] V. Sepelak, D. Baabe, K.D. Becker, J. Appl. Phys. 88 (2000) 5884. [5] H.St.C. O’Neill, H. Annersten, D. Virgo, Am. Mineral 77 (1992) 725. [6] K. Ishino, Y. Narumiya, Ceram. Bull. 66 (1987) 1469. [7] T. Abraham, Am. Ceram. Soc. Bull. 73 (1994) 62. [8] A.K. Ghatage, S.A. Patil, S.K. Paranjpe, Solid State Commun. 98 (1996) 885. [9] S.H. Lee, S.J. Yoon, G.J. Lee, H.S. Kim, C.H. Yo, K. Ahn, D.H. Lee, K.H. Kim, Mater. Chem. Phys. 61 (1999) 147. [10] M.K. Fayek, S.S. Ata Allah, Phys. Status Solidi (a) 198 (2003) 457. [11] A.M. Gismelseed, A.A. Yousif, Physica B 370 (2005) 215. [12] R.N. Singh, J.P. Singh, B. Lal, M.J. Thomas, S. Bera, Electrochim. Acta 51 (2006) 5515. [13] S. Singhal, K. Chandra, J. Solid State Chem. 180 (2007) 296. [14] M.A. Gabal, S.S. Ata-Allah, Mater. Chem. Phys. 85 (2004) 104. [15] M.A. Gabal, J. Phys. Chem. Sol. 64 (2003) 1375. [16] B.D. Cullity, Elements of X-ray Diffraction, Addison Wesley Pub. Co. Inc., 1956. [17] A.M. El-Sayed, Ceram. Int. 28 (2002) 651. [18] T.J. Shindea, A.B. Gadkari, P.N. Vasambekar, Mater. Chem. Phys. (2008). [19] M.A. Ahmed, E. Ateia, L.M. Salah, A.A. El-Gamal, Mater. Chem. Phys. 92 (2005) 310. [20] Q.M. Wei, J.B. Li, Y.J. Chen, J. Mater. Sci. 36 (2001) 5115. [21] V.B. Kawade, G.K. Bichile, K.M. Jadhav, Mater. Lett. 42 (2000) 33. [22] R.D. Waldron, Infrared spectra of ferrites, Phys. Rev. 99 (1955) 1727. [23] F.A. Lopez, A. Lopez-Delgado, J.L. Martın de Vidales, E. Vila, J. Alloys Cmpds. 265 (1998) 291. [24] A.M. El-Sayed, Mater. Chem. Phys. 82 (2003) 583. [25] A.K. Singh, T.C. Goel, R.G. Mendiratta, O.P. Thakur, C. Prakash, J. Appl. Phys. 91 (2002) 6626. [26] P. Roy, J. Bera, J. Magn. Magn. Mater. 298 (2006) 38. [27] P. Roy, J. Bera, J. Magn. Magn. Mater. 321 (2009) 247. [28] S.K. Banerjee, W. O’Reilly, IEEE Trans. Magnets 3 (1966) 463. [29] M.A. Gabal, M.A. Ahmed, J. Mater. Sci. 40 (2005) 387.