ARTICLE pubs.acs.org/JPCC
Autocombustion High-Temperature Synthesis, Structural, and Magnetic Properties of CoCrxFe2xO4 (0 e x e 1.0) B. G. Toksha,† Sagar E. Shirsath,*,† M. L. Mane,† S. M. Patange,‡ S. S. Jadhav,§ and K. M. Jadhav† †
Department of Physics, Dr. Babasaheb Ambedkar Marathwada University, Aurangabad 431004 (MS), India Materials Research Laboratory, Srikrishna Mahavidyalaya Gunjoti, Omerga, Osmanabad 413613 (MS), India § DSM’s Arts, Commerce and Science College, Jintur, Parbhani 431509 (MS), India ‡
ABSTRACT: CoCrxFe2xO4 (0 e x e 1.0) was synthesized by solgel auto combustion method using nitrates of respective elements and by keeping 1:3 ratio of metal nitrate to citrate. Representative sample was investigated by thermogravimeter/ differential thermal analyzer (TG/DTA); then, all samples were annealed at 500 °C for 4 h. The broad peaks in the X-ray diffraction patters (XRD) and evaluation of transmission electron microscopy (TEM) indicate a fine particle nature of the particles. Scanning electron microscopy (SEM) analysis and EDAX indicated that the samples were homogeneous and had the expected FeCoCr ratios. The lattice parameter, bulk density, and particle size are decreased, whereas the X-ray density, specific surface area, and porosity tend to increase with increasing Cr3+ substitution. Cation distribution was estimated using XRD and by employing Bertaut method. Fourier transform infrared spectroscopy (FT-IR) is employed to determine the local symmetry in crystalline solids and to shed light on the ordering phenomenon. Saturation magnetization determined from vibrating sample magnetometer (VSM) decreases linearly with Cr3+ concentration, suggesting that the superexchange interaction Fe(A)OFe(B) link is stronger than that for the Fe(A)OCr(B) link. Coercivity in the Cr-doped cobalt ferrites was larger than that in pure CoFe2O4 compositions.
1. INTRODUCTION Cobalt ferrite has inverse spinel structure and is of the most versatile centrosymmetric magnetic materials. In the inverse spinel structure, the tetrahedral (A) sites are occupied by the Fe3+ ions, and the octahedral sites (B) are occupied by the divalent metal ions (M2+) and Fe3+ in equal proportions. The angle AOB is closer to 180° than the angles BOB and AOA; therefore, the AB pair (FeFe) has a strong superexchange (antiferromagnetic) interaction. The net magnetic moment per molecule in the cobalt ferrite it is 3μB. Cobalt ferrite possesses excellent chemical stability, good mechanical hardness, and a large positive first-order crystalline anisotropy constant, making it a promising candidate for magneto-optical recording media. In addition to precise control of the composition and structure of CoFe2O4, its practical application will require the capability to control particle size on the nanoscale.14 Substitution of other metals for Fe in cobalt ferrite has been proposed as a method to tailor the magnetic and magnetoelastic properties for sensor and actuator applications.5 Chromium (Cr3+) ions with antiferromagnetic nature are known for achieving control over magnetic parameters in developing technologically important materials. Chromium could produce sufficient changes in MFe2O4 solution, and it is known that Cr3+ ions have strong B-site preference.6 The substitution Cr3+ions (3μB) for Fe3+ (5μB) will alter magnetic r 2011 American Chemical Society
properties markedly similar to that of nonmagnetic substitution. Combustion synthesis processes are characterized by high temperatures, fast heating rates, and short reaction times. These features make combustion synthesis an attractive method for the manufacture of technologically useful materials at lower costs compared with conventional ceramic processes. Some other advantages of combustion synthesis are (1) use of relatively simple equipment, (2) formation of high-purity products, (3) stabilization of metastable phases, and (4) formation of virtually any size and shape products, and (5) powders prepared by solgel autocombustion method have good sinterability with a homogeneous composition.7 All facts revealed from literature survey motivated us to prepare cobalt ferrite substituted with chromium. The author realized that citrate nitrate auto combustion route would be suitable for the production. Therefore, an attempt has been made to synthesize CoCrxFe2xO4 spinel ferrite system by solgel auto combustion technique to obtain nanocrystalline particles and to study their structural and magnetic properties.
Received: June 14, 2011 Revised: September 18, 2011 Published: September 19, 2011 20905
dx.doi.org/10.1021/jp205572m | J. Phys. Chem. C 2011, 115, 20905–20912
The Journal of Physical Chemistry C
ARTICLE
Figure 1. Chemical reaction of CoCrxFe2xO4 synthesized by solgel autocombustion method.
Figure 3. EDAX of CoCrxFe2xO4 (0.0 and 1.0). Figure 2. TG-DTA of CoCrxFe2xO4 (x = 0.0).
2. MATERIALS AND PREPARATION The nanocrystalline samples of the series CoCrxFe2xO4 were prepared by citrate nitrate auto combustion route. The A.R. grade citric acid (C6H8O7 3 H2O), cobalt nitrate (Co(NO3)2 3 6H2O), chromium nitrate (Cr(NO3)3 3 9H2O), and ferric nitrate (Fe(NO3)3 3 9H2O) (>99% sd-fine) were used as starting materials. The products of the system were produced by keeping the metal nitrate to citrate ratio 1:3. Reaction procedure was carried out in air atmosphere without protection of inert gases. The metal nitrates were dissolved together in a minimum amount of double-distilled water to get a clear solution. An aqueous solution of citric acid was mixed with metal nitrates solution. The mixed solution was kept on to a hot plate with continuous stirring at 100 °C. During evaporation, the solution became viscous and finally formed a very viscous brown gel. When finally all water molecules were removed from the mixture, the viscous gel began frothing. After a few minutes, the gel automatically ignited and burnt with glowing flints. The decomposition reaction would not stop before the whole citrate complex was consumed. The autoignition was completed within one minute, yielding the brown color ashes termed as a precursor. The as-prepared powders of all samples were heat-treated separately at 500 °C for 4 h to get the final product. The as-prepared and heat-treated powders were used for further characterization. The chemical reaction is shown in Figure 1. The samples were powdered for X-ray investigation. Part of the powder was X-ray examined by a Phillips X-ray diffractometer (model 3710) using Cu Kα radiation (λ = 1.5405 Å). EDAX and scanning electron microscope (SEM) measurements were carried out on JEOL (model JSM-840). Transmission electron
microscope (TEM) measurements were recorded on a Philips apparatus (model CM 200). Fourier transform infrared spectroscopy (FTIR) measurements were carried out in the wavenumber range of 4004000 cm1 on a Perkin-Elmer apparatus (model V755). Magnetization measurements of the samples were carried out using a vibrating sample magnetometer (VSM, Make: Lakeshore, model: 7307).
3. RESULTS AND DISCUSSION A. Structural Properties. Thermal analysis (DTA-TGA) of the precursor citrate combusted powder was carried out in air by using thermogravimetric (TG) and differential thermal analysis (DTA). Simultaneous TG-DTA spectra for the synthesized sample have been presented in Figure 2. The maximum exothermic peak, at appropriate 220 °C, is broad vertical in the DTA curve, accompanied by a weight loss at the same temperature; this indicates that the decomposition of the nitrate-citrate powder has occurred.8 In TG pattern, the system undergoes a weight loss of 6.02% (0.44 mg) between 35 and 220 °C, which can be ascribed to the burning of organic material in the sample. Another fast decrease in weight between 220 and 380 °C of 1.89% (0.14 mg) is observed, corresponding to an obvious exothermic peak on DTA curve, which is caused by the gradual crystallizing process. During 400800 °C, almost no weight loss is detectable, indicating the complete decomposition of the precursor above 400 °C, accompanied by a considerably weak exothermic counterpart on DTA, which is assigned to grain growth and crystal perfection. It may be mentioned here that the formation of the spinel phase is at a temperature 450 °C and the prediction of particle size and further analysis in that direction is irrelevant below this temperature. Therefore, even though the decomposition has completed at 350 °C, we have chosen 500 °C as final calcination temperature for present samples. Thermogram (TG) shows a constant weight after 500 °C with an overall weight-loss of 10% and indicates the complete spinel formation at ∼500 °C. The powder was then heated at the formation temperature, and the XRD patterns were recorded to ensure that the formation was complete. Energy dispersive X-ray (EDAX) analysis was used to investigate the chemical composition for the constituent elements of the synthesized nanoparticles. Representative EDAX spectra for x = 0.0 and 1.0 is shown in Figure 3. The EDAX results showed that products did not contain any impurity elements, and the compositional molar ratio of Co to Fe + Cr was close to 0.5 (Table 1). X-ray diffraction was performed on the as-burnt powders as well as on the 500 °C calcined powder. The X-ray diffraction patterns of all samples are shown in Figure 4a,b. After combustion, the asburnt powders are in a crystalline state, as shown in Figure 4a. The 500 °C heat-treated samples are shown in Figure 4b. Because combusted powder is single phase with a spinel structure. This indicates that the ferrite can be directly formed after the autocombustion of the gel, without heat treatment. The broad peaks in the XRD patterns indicate a fine particle nature of the particles. The peaks can be indexed to the cubic spinel structure of CoFe2O4CoCrFeO4 (JCPDS-ICDD database 03-0864). The positions of the reflection peaks for as-burnt powders are almost identical to the corresponding peaks for the bulk material; this implies that the basic structure of the nanoparticles is essentially the same as that of the bulk material. The reflection peaks of samples become sharper and narrower after the heat treatment at 500 °C, indicating the improvement of crystallinity and increase in the particle size. Variation of the lattice parameter a with Cr concentration (x) in CoCrxFe2xO4 (x = 0 to 1.0) spinel ferrite system is depicted in Figure 5. The results show that the lattice parameter decreases with increasing Cr content. The decrease in a with x can be explained on the basis of a difference in ionic radii of Fe3+ and Cr3+. The smaller ionic radius Cr3+ (0.63 Å) ion replaces the larger Fe3+ (0.67 Å) ions; consequently, the lattice parameter decreases because of shrinkage of unit cell dimension. This indicates difference of Fe3+ and Cr3+ in an oxide solid solution with a spinel-type structure. When doped with smaller sized Cr3+ ions, the spinel cobalt ferrite will shrink. Doping Cr3+ ions in a spineltype structure will induce uniform strain in the lattice as the material is elastically deformed. This effect causes the lattice plane spacing to change and the diffraction peaks shift to a higher 2θ position. Usually, in a solid solution of spinels within the miscibility range, a linear change in the lattice constant with the concentration of the components is observed.11 In the present
Figure 4. (a) XRD patterns of CoCrxFe2xO4 (as prepared) and (b) 500 °C heat-treated.
Figure 5. Variation of lattice constant (a) and X-ray density (dx) with composition x of the ferrite system CoCrxFe2xO4. 20907
dx.doi.org/10.1021/jp205572m |J. Phys. Chem. C 2011, 115, 20905–20912
The Journal of Physical Chemistry C
ARTICLE
Table 2. Crystallite Size Calculated from X-ray Highest Intensity Peak, Particle Size From TEM Image, and Specific Surface Area (S) of As-Prepared and Heat-Treated Samples as-prepared particle size
heat-treated S (m2/g)
comp. x XRD (nm) TEM (nm) 18
particle size
S (m2/g)
XRD (nm)
0.0
19
67.19
35
36.47
0.2
17
76.58
26
50.07
0.4
16
81.93
23
57.00
0.6
15
88.89
21
63.49
0.8 1.0
14 12
96.14 114.71
19 13
70.84 105.88
14
Figure 6. (a) TEM and (b) SAED images of CoCrxFe2xO4 (x = 0.0).
system, as the Cr3+ concentration increases, the lattice constant (angstroms) decreases linearly. This variation of lattice parameter with chromium substitution obeys Vegard’s law12 and is in good agreement with reports available in literature.13,14 X-ray density is calculated using molecular weight, volume of unit cell, and Avogadro’s number. Variation of the X-ray density with Cr concentration (x) in CoCrxFe2xO4 (x = 0 to 1.0) spinel ferrite system is shown in Figure 5. The calculated values of X-ray density show an increase in the value of dx with an increment in chromium concentration in all compositions. The broad shape of the X-ray diffraction lines of the synthesized material reflects the formation of material with the small crystallite size. From the line-broadening analysis and using eq 1 the crystallite size in the produced sample was estimated.15 The full width at half-maximum of the (311) XRD peak was used to estimate the particle size. DXRD ¼
Cλ B1=2 cos θ
ð1Þ
where B1/2 is the full width at half-maximum in (2θ), θ is the corresponding Bragg angle, and C = 0.9. To have the most complete view about crystallite sizes and morphology, the transmission electron microscopy (TEM) images of presently synthesized ferrite nanoparticles were obtained (Figure 6a). The TEM micrographs of synthesized samples revealed that spherical nanoparticles were obtained. The TEM images of samples also show that the particles are aggregated. Particle size of as-prepared typical samples was
Figure 7. Variations of bulk density (dB) and porosity (P) with Cr content x.
determined from TEM micrographs. Particle size determined from TEM images is presented in Table 2. The estimated crystallite size is in good agreement with that observed from the transmission electron microscopy image. Selected area electron diffraction (SAED) patterns of the particle are shown Figure 6b, suggesting the amorphous nature of the sample. The citrate gel process16 is known for synthesizing materials with high surface area (S). Specific surface area for the as-prepared ferrite systems, all compositions, as well as for heat-treated samples is summarized in Table 2. As it can be seen from Table 2, the specific surface area varied in the range 67 114 m2/g in the case of as-prepared samples and increased systematically from 36 to 105 m2/g in the case of heat-treated samples. The values of bulk density (dB) and porosity (P) were calculated using the relation discussed elsewhere.17 As can be observed from Figure 7, bulk density (dB) decreases whereas and porosity (P) increases with Cr content x. Powder morphology was observed via SEM. The typical scanning electron micrographs of the sample (x = 0.0 and 1.0) under investigation are presented in Figure 8. SEM of samples shows that the samples have an agglomerated large grain structure. Heating results in the well-faceted grains to form solid bodies. It is a porous network of sintered bodies exhibiting foamlike structure. 20908
dx.doi.org/10.1021/jp205572m |J. Phys. Chem. C 2011, 115, 20905–20912
The Journal of Physical Chemistry C
ARTICLE
Table 3. Cation Distribution of CoCrxFe2xO4 comp. x
A site
B site
0.0
Co0.25Fe0.75
Co0.75Fe1.25
0.2
Co0.2Fe0.8
Co0.8Cr0.2Fe1.0
0.4
Co0.2Fe0.8
Co0.8Cr0.4Fe0.8
0.6
Co0.2Fe0.8
Co0.8Cr0.6Fe0.6
0.8
Co0.1Fe0.9
Co0.9Cr0.8Fe0.3
1.0
Co0.05Fe0.95
Co0.95Cr1.0Fe0.05
Table 4. Radius of Tetrahedral (rA), Octahedral (rB) Site, Oxygen Position Parameter (u), and Theoretical Lattice Constant (ath) of CoCrxFe2xO4 comp. x
rA (Å)
rB (Å)
u (Å)
ath (Å)
0.0
0.708
1.453
0.3918
8.382
0.2
0.700
1.454
0.3920
8.373
0.4
0.700
1.448
0.3923
8.362
0.6
0.700
1.442
0.3927
8.352
0.8
0.685
1.451
0.3919
8.344
1.0
0.678
1.453
0.3924
8.336
where F is structure factor, P is multiplicity factor, LP is the Lorentz polarization factor, and LP ¼
Figure 8. SEM images of CoCrxFe2xO4 (x = 0.0 and 1.0).
B. Cation Distribution. The investigations of cation distribution provide very useful information for development of materials with desired properties. The cation distribution was obtained from the analysis of intensity of X-ray diffraction patterns. In this method, the observed intensity ratios were compared with the calculated intensity ratios.18 The bulk particles of cobalt ferrite normally exhibit inverse spinel structure with one-half of Fe3+ ions in the A sites and the remaining half of Fe3+ ions and Co2+ ions at the B sites.19 The distribution of Co2+, Cr3+, and Fe3+ cations among the octahedral and tetrahedral sites in the CoCrxFe2xO4 was determined from the X-ray intensity ratio calculations. According to Ohnishi and Teranishi,21 the intensity ratios of planes I(220)/I(400), I(400)/I(440), and I(422)/I(400) are considered to be sensitive to the cation distribution.20 The cation distribution in the present ferrite system was calculated using the R factor method.18 In this method, the best structure is selected so that the value of residual function R is minimized. The value of agreement factor R when converged to a minimum value (for which the theoretical and experimental ratios agree closely) is taken to be the correct. Calculations of intensity for the planes are made for various possible values of the distribution parameter by using the Buerger formula.22
Ihkl ¼ jFj2hkl P 3 LP
ð2Þ
1 þ cos2 2θ sin2 θ 3 cos 2θ
ð3Þ
The atomic scattering factor for various ions was taken from the literature.23 The value of the distribution parameter is reached by comparing theoretical and experimental intensity ratios of the above-mentioned planes. The cation distribution thus estimated is given in Table 3. It can be concluded from Table 3 that it is reasonable to consider that Cr ions reside entirely in the octahedral B sites whereas Co and Fe ions occupy tetrahedral (A) as well as octahedral [B] sites. The ionic configuration based on the site preference energy value for individual cation suggested that the Cr3+ ions with very high octahedral crystal field stabilization energy24 have high octahedral site preference to occupy octahedral B sites in spinel ferrite structure.25 Fe3+ ions do not have any particular preference toward a lattice site, and their accommodation in any of two lattice sites is equally possible;26 Co2+ ions can also occupy both octahedral and tetrahedral sites.27 The integrated intensities calculated by the R factor method are valid at 0 K. Because the observed values are obtained at room temperature, a suitable correction in principle is necessary for more accurate comparison. However, the spinel compounds have high-melting points; the thermal vibration of the atoms at room temperature should not differ greatly from that at absolute zero. Therefore, in our intensity calculations, temperature correction was not taken into consideration. The oxygen parameter is a quantitative measure of the displacement of oxygen ions surrounding a tetrahedral site because the tetrahedral sites in ferrites are small to contain metal ions. The oxygen positional parameter (u) depends on the chemical composition, preparation conditions, and heating procedure.28 The values of oxygen parameter as a function of composition are listed in Table 4. The mean ionic radius of the A-site (rA) and of the B-site (rB) by using accepted cation 20909
dx.doi.org/10.1021/jp205572m |J. Phys. Chem. C 2011, 115, 20905–20912
The Journal of Physical Chemistry C
ARTICLE
Table 5. Saturation Magnetization (Ms), Coercivity (Hc), and Remanant Ratio (R) at Room Temperature of As-Prepared and Heat-Treated Samples heated at 500 °C
as-prepared comp. x Ms (emu/g) Hc (Oe)
Figure 9. FTIR patterns of CoCrxFe2xO4 (x = 0.0, 0.4, and 1.0).
Figure 10. MH plots of CoCrxFe2xO4 (500 °C heat-treated).
distribution is calculated using eqs 4a and 4b. rA ¼ ð0:25 yÞrCo2þ þ ð0:75 þ yÞrFe3þ
ð4aÞ
rB ¼ ð0:75 þ yÞrCo2þ þ ðxÞrCr3þ þ ð1:25 ðx þ yÞÞrFe3þ
ð4bÞ The values of rA and rB are summarized in Table 4. The theoretical lattice constant (ath)29 was calculated using eq 5 and estimated cation distribution and is in agreement with that experimentally obtained lattice constant. pffiffiffi 8 pffiffiffi ½rA þ R0 þ 3½rB þ R0 ath ¼ ð5Þ 3 3 where rA and rB are the site radius of tetrahedral A and octahedral B sites, respectively, and RO is the radius of oxygen. The difference between experimental lattice constant aexp and theoretical lattice constant ath may account for the lattice defects for polycrystalline material and the presence of Fe2+ ions on A or B sites that are not taken into consideration.30 C. Infrared Study. The FTIR spectrum of as-synthesized powder particular compositions of CoCrxFe2xO4 (x = 0.0, 0.4, 1.0) is shown in Figure 9. The two persistent absorption bands correspond to the vibration of tetrahedral and octahedral
R
Ms (emu/g) Hc (Oe)
R
0.0
67.02
605
0.396
70.81
521
0.429
0.2
32.09
910
0.218
46.55
813
0.451
0.4
23.67
1123
0.173
32.03
1090
0.152
0.6
14.16
1329
0.179
18.11
1204
0.160
0.8 1.0
7.70 5.38
1529 2341
0.220 0.234
11.61 7.16
1384 2281
0.229 0.293
complexes at around 600 and 400 cm1 respectively, which is indicative of formation of spinel ferrite structure.31 The FTIR spectrum shows absorption bands in the region 1100 1300 cm1 corresponding to NO3 ions, an absorption band corresponding to carboxyl group (COO) is observed at 1500 cm1, and one more at is observed at 31003450 cm1 corresponding to hydrogen-bonded OH groups. Owing to the high temperature generated during combustion process, all carboxyl, hydroxyl, and nitrate groups appear with less intensity. All samples show two prominent absorption bands, ν1 and ν2, in the range 600 and 400 cm1, respectively. Absorption at ν1 is caused by stretching of tetrahedral metal ion and oxygen bonding, whereas ν2 is caused by vibrations of oxygen in the direction perpendicular to the axis joining the tetrahedral ion and oxygen. Waldron32 studied the vibration spectra of ferrites and attributed the ν1 band to the intrinsic vibrations of the tetrahedral groups and ν2 band to the octahedral groups. IR spectra of ferrites have been studied, and the occurrence of band ν1 around 600 cm1 has been attributed to the intrinsic vibrations of the tetrahedral complexes corresponding to the highest restoring force and band ν2 around 400 cm1 to intrinsic vibrations of octahedral complexes, which are bond-bending vibrations. D. Magnetic Properties. Magnetic hysteresis loops were recorded at room temperature for all compositions of the asprepared as well as annealed samples. Magnetic hysteresis loops of all samples annealed at 500 °C are shown in Figure 10. The saturation magnetization (Ms) for all compositions of the as-prepared and annealed at 500 °C samples is listed in Table 5, indicating that Ms decreases with the increase in chromium concentration. This may be attributed to the weakening of exchange interactions due to Cr3+ ions. Moreover, the saturation magnetization of CoFe2O4 materials is defined by their molecular magnetic moments. When Cr3+ ions were introduced into the inverse spinel CoFe2O4, they replaced some of Fe3+ in B site. Moreover, Cr3+ ions have 3μB, less than 5μB of Fe3+ ions. So, the introduction of Cr3+ ions at octahedral B sites results in the dilution of the magnetization at the B site, which will reduce the saturation magnetization. It can be seen from Table 5 that the saturation magnetization values of as prepared samples are lower than those of the heat-treated samples. The Ms values increased with increase in particle size. With heat treatment, the crystallite sizes increased and the crystallization will be more complete, thus increasing saturation magnetization values. It is known that the magnetic properties of nanosized particles depended on the preparation method and the particle size.33 It was found that the CoFe2O4 synthesized by the present method had lower Hc and higher Ms compared with those reported by other 20910
dx.doi.org/10.1021/jp205572m |J. Phys. Chem. C 2011, 115, 20905–20912
The Journal of Physical Chemistry C
ARTICLE
in μB, nN B , is expressed as nNB ¼ MB ðxÞ MA ðxÞ
ð7Þ
where MA and MB are the A and B sublattice magnetic moments in μB. The values of Neel’s magnetic moment nN B were calculated by taking ionic magnetic moments of Fe3+, Cr3+, and Co2+ as 5μB, 3μB, and 3μB, respectively, shown in Figure 11. The observed and calculated magneton number are decreased and in good agreement with each other. The decrease in calculated magneton number clearly indicates the Cr3+ occupying the octahedral sites.
Figure 11. Variation of magneton number (a) observed, (b) heattreated at 500 °C, and (c) as-prepared samples with Cr content x.
methods.34 The results were related to the fact of the difference in preparation method.35 The coercivity, HC, which is a measure of the magnetic field strength required for overcoming anisotropy to flip the magnetic moments, is clearly affected by the chromium substitution. It can be seen from Table 5 the coercivity values increases with increase in chromium concentration. The coercivity of nanocrystals has a contribution from their finite size, namely, surface anisotropy.36,37 Usually, for a given composition of ferrite, the coercivity increases when the crystal size decreases because the surface becomes much more dominant.38 Therefore, the variation of coercivity in the present case may also be related to the particle size effect. The magnetic coupling at the individual lattice sites can be correlated to magnetic properties such as coercivity. The replacement of a part of Fe3+ cations by Cr3+ ions, which have an unquenched orbital angular momentum and a large anisotropy, obviously affects coupling, which must result in higher coercivity. It is reported that the coercivity varies directly with porosity and anisotropy39 and it may be invoked here that the porosity in present system is increasing with increasing substitution of chromium. The remanence ratio (R), that is, MR/MS, has been calculated for all compositions. The values of remanence ratio are summarized in Table 5. It can be concluded from Table 5 that the values of remanence ratio do not vary systematically with compositions and have the values in the range of 0.49 to 0.15. The magnetocrystalline anisotropy constant bears a direct relation with remanence ratio. A decrease in magnetocrystalline anisotropy constant would mean a decrease in remanence ratio, whereas magnetostrictive constant λ111 contributes negatively to the remanence ratio. In general, the values of remanence ratio of heat-treated samples are found to be greater than that of as prepared sample. The magnitude of remanence depends on the remanant magnetization MR, which in turn depends on the impedance to the domain wall motion, which may be enhanced after heat treatment. The magnetic moment per formula unit in Bohr magneton (μB) was calculated by using the eqs 4a and 4b. The observed magnetic moment (nB obsd) per formula unit in Bohr magneton (μB) was calculated by using the relation. molecular weight Ms ð6Þ 5585 According to the Neel’s two sublattices model of ferrimagnetism,40 the calculated magnetic moment per formula unit (nB calcd) nB ¼
4. CONCLUSIONS A series of Cr3+-substituted CoFe2O4 with a chemical formula CoCrxFe2xO4 (0 e x e 1.0) was successfully synthesized by solgel auto combustion method. The process takes only a few minutes to obtain as-prepared nanocrystalline ferrite powders. The TG-DTA graph revealed that heating of the sample at ∼500 °C is enough for complete crystallization. The XRD pattern revealed that the cubic spinel structure is maintained for all compositions. The decrease in particle size with Cr3+ substitution leads to increase in porosity and specific surface area, which eventually decreases the density of the samples. Cation distribution data suggest that the Cr3+ ions occupy the octahedral B-site and force some of the Co2+ ions to migrate at tetrahedral A-site, whereas Fe3+ ions are distributed over both sites. The theoretical lattice constant and experimental lattice constant are in good agreement with each other. Infrared spectra shows two prominent bands corresponding to spinel ferrite phase. The decrease in saturation magnetization with increasing Cr3+ content occurs because the replacement of Fe3+ by Cr3+ ions weakens the sublattice interaction and lowers the magnetic moments of unit cells. The calculated and observed values of magneton number agree closely to each other, suggesting that the structure is collinear. ’ AUTHOR INFORMATION Corresponding Author
*E-mail:
[email protected]. Phone: +9102402240950. Fax: +9102402361270.
’ REFERENCES (1) Toksha, B. G.; Shirsath, S. E.; Patange, S. M.; Jadhav, K. M. Solid State Commun. 2008, 147, 479–483. (2) Choi, E. J.; Ahn, Y.; Kim, S.; An, D. H.; Kang, K. U.; Lee, B.-G.; Baek, K. S.; Oak, H. N. J. Magn. Magn. Mater. 2003, 262, L198–L202. (3) Hamdeh, H. H.; Hikal, W. M.; Taher, S. M.; Ho, J. C.; Thuy, N. P.; Quy, O. K.; Hanh, N. J. Appl. Phys. 2005, 97, 64310-1–64310-4. (4) Chinnasamy, C. N.; Jeyadevan, B.; Shinoda, K.; Tohji, K.; Djayaprawira, D. J.; Takahashi, M.; Joseyphus, R. J.; Narayanasamy, A. Appl. Phys. Lett. 2003, 83, 2862–2864. (5) Zheng, H.; Wang, J.; Lofland, S. E.; Ma, Z.; Mohaddes-Ardabili, L.; Zhao, T.; Salamanca-Riba, L.; Shinde, S. R.; Ogale, S. B.; Bai, F.; Viehland, D.; Jia, Y.; G. Schlom, D.; Wuttig, M.; Roytburd, A.; Ramesh, R. Science 2004, 303, 661–663. (6) Patange, S. M.; Shirsath, S. E.; Toksha, B. G.; Jadhav, S. S.; Jadhav, K. M. J. Appl. Phys. 2009, 106, 023914–7. (7) Patil, K. C.; Aruna, S. T.; Mimani, T. Curr. Opin. Solid State Mater. Sci. 2002, 6, 507–512. (8) Singhal, S.; Chandra, K. J. Solid State Chem. 2007, 180, 296–300. (9) García-Cerda, L. A.; Rodríguez-Fernandez, O. S.; ResendizHernandez, P. J. J. Alloys Compd. 2004, 369, 182–184. 20911
dx.doi.org/10.1021/jp205572m |J. Phys. Chem. C 2011, 115, 20905–20912
The Journal of Physical Chemistry C
ARTICLE
(10) Mondelaers, D.; Vanhoyland, G.; Van den Rul, H.; D’Haen, J.; Van Bael, M. K.; Mullens, J.; Van Poucke, L. C. Mater. Res. Bull. 2002, 37, 901–914. (11) Raghavender, A. T.; Pajic, D. r.; Zadro, K.; Milekovic, T.; Rao, P. V.; Jadhav, K. M.; Ravinder, D. J. Magn. Magn. Mater. 2007, 316, 1–7. (12) Vegard, L. Z. Phys. 1921, 15, 17–26. (13) Mane, D. R.; Patil, S.; Birajdar, D. D.; Kadam, A. B.; Shirsath, S. E.; Kadam, R. H. Mater. Chem. Phys. 2011, 126, 755–760. (14) Chae, K. P.; Lee, Y. B.; Lee, J. G.; Lee, S. H. J. Magn. Magn. Mater. 2000, 220, 59–64. (15) Patange, S. M.; Shirsath, S. E.; Jangam, G. S.; Lohar, K. S.; Jadhav, S. S.; Jadhav, K. M. J. Appl. Phys. 2011, 109, 053909-1–053909-9. (16) Iqbal, M. J.; Siddiquah, M. R. J. Alloy. Compd. 2008, 453, 513–518. (17) Shirsath, S. E.; Toksha, B. G.; Jadhav, K. M. Mater. Chem. Phys. 2009, 117, 163–168. (18) Weil, L.; Bertaut, E. F.; Bochirol, L. J. Phys. Radium 1950, 11, 208–212. (19) Berchmans, L. J.; Selvan, R. K.; Selva Kumara, P. N.; Augustin, C. O. J. Magn. Magn. Mater. 2004, 279, 103–110. (20) Ohnishi, H.; Teranishi, T. J. Phys. Soc. Jpn. 1961, 16, 35–43. (21) Sivakumar, N.; Narayanasamy, A.; Shinoda, K.; Chinnasamy, C. N.; Jeyadevan, B.; Greneche, J. M. J. Appl. Phys. 2007, 102, 0139161–013916-8. (22) Buerger, M. G. Crystal Structure Analysis; Wiley Interscience: New York, 1960. (23) Cullity, B. D. Elements of X-ray Diffraction; Addison-Wesley: Reading, MA, 1956. (24) Dey, S.; Roy, A.; Das, D.; Ghose, J. J. Magn. Magn. Mater. 2004, 270, 224–229. (25) O’Neill, H. S. C.; Navrotsky, A. Am. Mineral. 1984, 69, 733–753. (26) Jadhav, S. S.; Shirsath, S. E.; Patange, S. M.; Jadhav, K. M. J. Appl. Phys. 2010, 108, 093920-1–093920-6. (27) Allen, G. C.; Jutson, J. A.; Tempest, P. A. J. Nucl. Mater. 1988, 158, 96–107. (28) Ma, L. J.; Chen, L. S.; Chen, S. Y. Solid State Sci. 2009, 11, 176–181. (29) Vijaya Kumar, K.; Ravinder, D. Int. J. Inorg. Mater. 2001, 3, 661–666. (30) Khan, Y.; Kneller, E. J. Magn. Magn. Mater. 1978, 7, 9–11. (31) El-Saadawy, M.; Barakat, M. M. J. Magn. Magn. Mater. 2000, 213, 309–311. (32) Waldron, R. D. Phys. Rev. 1955, 99, 1727–1735. (33) Shirsath, S. E.; Kadam, R. H.; Gaikwad, A. S.; Ghasemi, A.; Morisako, A. J. Magn. Magn. Mater. 2011, 323, 3104–3108. (34) Pradeep, A.; Priyadharsini, P.; Chandrasekaran, G. J. Magn. Magn. Mater. 2008, 320, 2774–2779. (35) Ahn, Y.; Choi, E. J.; Kim, S.; Ok, H. N. Mater. Lett. 2001, 50, 47–52. (36) Zhao, D.; Wu, X.; Guan, H.; Han, E. J. Supercrit. Fluids 2007, 42, 226–233. (37) Shirsath, S. E.; Jadhav, S. S.; Toksha, B. G.; Patange, S. M.; Jadhav, K. M. J. Appl. Phys. 2011, 110, 013914-1–013914-8. (38) Batoo, K. M.; Kumar, S.; Lee, C. G.; Alimuddin Curr. Appl. Phys. 2009, 9, 1397–1406. (39) Shafi, K. V. P. M.; Gedanken, A.; Prozorov, R.; Balogh. J. Chem. Mater. 1998, 10, 3445–3450. (40) Neel, L. Ann. Phys. 1948, 3, 137–198.
20912
dx.doi.org/10.1021/jp205572m |J. Phys. Chem. C 2011, 115, 20905–20912
