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The nanocrystalline magnetic materials have been investigated extensively due to their unique properties such as superparamagnetism and spin glass behaviour which are generally attributed to surface (rather than) disorder.

The nanocrystalline magnetic materials have been investigated extensively due to their unique properties such as superparamagnetism and spin glass behaviour which are generally attributed to surface (rather than) disorder. Nanoparticles of spinel ferrites displayenhanced properties by virtue of their uniqueelectronic and physical structure which may be harnessedfor technological applicationssuch as ferrofluids, magneticdrug delivery, high-density information storage [1-6] etc. The use of ferrites for certain applications depends on their electrical and magnetic properties, which in turn are sensitive to the preparation conditions as well as the type and amount of substitutions[5]. The magnetic properties of ferrite in their nano regime are found to undergo changes due to superparamagnetism, surface spin effects and also with their cation distribution which depends on the method of synthesis [7,8]. In their spinel structure the oxygen ions form an fcc lattice and the cations occupy the interstitial positions leading to an anti-parallel alignment of magnetic moments called super exchange interaction. Nanoferrites are found to exhibit superparamagnetic behavior with their coercivity approaching zero.

The spinel Zn-Cr ferrite is quite important in the field of microwave industry due to its magnetic and dielectric properties. We have reported dielectric properties of this series of samples elsewhere [9]. Magnetic behaviour of nanoparticles of ZnCr_xFe_2-xO₄with x=0.2, 0.4, 0.6 and 0.8 synthesized using sol-gel methodhave been studied in this report.

The nanocrystalline ZnCr_xFe_2-xO₄ ferrite, where x varies in steps of 0.2 were prepared by the sol-gel method, in which the analytical grade Zn(NO₃)₂. 6H₂O, Cr(NO₃)₃.9H₂O, Fe(NO₃)₂.9H₂O and citric acid  were used as raw materials. The molar ratio of nitrates to citric acid is kept 1:1. All the nitrates and citric acid solutions were mixed together with continuous stirring. A small amount of ammonia is added to the solution to adjust the pH to about 7. The mixed solution was slowly heated to 90 ^°C with stirring constantly to transform it into a Xero-gel. Further heating resulted in burning of the gel in a self propagating combustion manner until all the gel gets burnt out completely to form a loose powder. The synthesis conditions were kept steady for the preparation of each samples (having different x values) to get a narrow size distribution more or less identical for all the samples.

The XRD measurements have been performed using Cu K_α radiation (λ=1.54 Å) The dc magnetization measurements have been made on the PARC make vibrating sample magnetometer (VSM) model 155. A closed cycle refrigerator cryostat has been used, for the temperatures 300 K to 20 K.

X-Ray Diffraction

Figure 1 shows the typical XRD patterns obtained for the ZnCr_xFe_2-xO₄ with x = 0.2, 0.4, 0.6 and 0.8 which confirm that the samples are formed in single phase cubic spinel structure.All the diffraction peaks of the synthesized particles, match well with the reported values (JCPDS file) [20]. No diffraction peaks of other impurities were observed, which confirm the formation of impurity free ZnCr_xFe_2-xO₄samples. The lattice constants of the ZnCr_xFe_2-xO₄nanoparticles were determined using the following relationship.

a = d_hkl √(h²+k²+l²)      (1)

Considerably broadened lines in the XRD pattern are indicative of the presence of nano-size particles. The maximum intensity peak (311) of all the samples were fitted with a Gaussian shape to determine the exact peak position as well as the full width half the maximum (FWHM). The average particle sizes were estimated with the help of the Debye-Scherrer [4] equation.

where t is the thickness (diameter) of the particle, l is the X-ray wavelength (1.54 Å), B_M and B_S are the measured peak broadening and instrumental broadening in radian respectively and q_B in the Bragg angle of the reflection. The estimated average particle size for all the samples is ~10 nm.

 

Fig. 1. X-ray diffraction of the nanocrystalline samples ZnCr_xFe_2-xO₄recorded with CuK_α radiation.

 

Fig. 2. (a) M-H curves recorded at 300 K for samples ZnCr_xFe_2-xO₄; (b) Hysteresis curves recorded at 20 K for samples ZnCr_xFe_2-xO₄.

Fig. 2 (a) shows M-H curves recorded at 300 K for all the four samples which clearly indicates zero retentivity and coercivity, suggesting a superparamagnetic behaviour at 300 K. The hysteresis curves recorded at 20 K shown in Fig. 2 (b) provide non-zero values of retaintivity and coercivity, for the four samples.The values of saturation magnetization (M_s) were obtained by plotting the M versus 1/H curves and extrapolating the data to 1/H→0. The obtained values of M_sare 33.9, 33.2, 33.0 and 32.5emu/g,for x=0.2, 0.4, 0.6 and 0.8 respectively. It is clear that saturation magnetization decreases with increase in Cr content. However, the values of M_s are much less than ~68emu/g, the value for bulk sample of Fe₃O₄. Taking the analogy observed in NiFe₂O₄ [10] and in our earlier work on nanoparticles of Cu_0.2Ni_0.8Fe₂O₄ [11] by magnetization measurements, we may attribute this behaviour to the much reduced saturation magnetization in the nanoparticle samples to the frozen disordered spins at the surface.

Coey [12] explained the reduction of saturation magnetization of nanosized ferrites by considering a spin configuration that differs from the Neel type found in the bulk particles. The spins are canted at the surface of the nanoparticles i.e. the ions in the surface layer are inclined at various angles with respect to the direction of the net moment. In this way, the particle magnetization cannot be seen as uniform through the nanoparticle and it is the result of a magnetic ordered core and a surrounding surface shell of disordered spins. Thus the canted spins are in a surface layer and they freeze into a spin-glass-like phase at low temperatures. As a consequence, the surface spins have multiple configurations for any orientation of the core magnetization. The superexchange interaction between the magnetic cations is antiferromagnetic and it is mediated by an intervening oxygen ion. In bulk materials, the ferrimagnetic order is due to the prevalence of the inter-sublattice exchange respect to the intra-sublattice exchange. A change of the effective coordination of surface cations results in a distribution of net exchange fields, both positive and negative with respect to a cation’s sublattice. The exchange bonds are broken if an oxygen ion is missing from the surface or if organic molecules are bonded to the surface [13]. In addition, the superexchange is very sensitive to bond angles and lengths, which would likely be modified near the surface. Fig. 3 shows a schematic representation of arrangements of spins in a magnetic ferrite nanoparticles.

 

 

Fig. 3. Schematic representation (not up to scale) of arrangement of spins in a magnetic ferrite nanoparticle. Each unit cell of a ferrite molecule gives one spin moment and one nanoparticle of 60 Å size contains ~200 ferrite molecules.

Fig. 4. M-T curve in the range of 20 K-300 K in an external field of 100 Oe recorded in zero filed cooling (ZFC) and field cooling (FC) modes for samples ZnCr_xFe_2-xO₄.

Fig. 4 shows M-T curve in the range of 20 K-300 K in an external field of 100 Oe recorded in zero filed cooling (ZFC) and field cooling (FC) modes for nano sample. Appearance of peaks in the ZFC curves due to blocking mechanism owing to the competition between the thermal energy and the magnetic anisotropy energy of nanoparticles. When the nanoparticles are cooled in zero magnetic field, the magnetization direction of each nanoparticle align with its easy axis among the nanoparticles, overall susceptibility becomes almost zero since the easy axes of nanoparticles are random and the applied field is too small to overcome the anisotropy alone. Above T_B, magnetic anistropy is overcome by thermal activation and consequently the nanoparticles become superparamagnetic and show paramagnetic like behavior, which is also evident from Fig. 3. The broader peaks in the ZFC curves are due to large distribution of magnetic anisotropy constant (K) and distribution of particle sizes. The blocking temperatures (T_B) obtained are ~135 K, 140 K, 150 K and 160 K for samples with x=0.2, 0.4, 0.6 and 0.8 respectively. Fig 4 also shows a departure of FC curves from the ZFC, which is a characteristic feature of superparamagnetic behaviour and such a departure is suggestive of temporal relaxation.

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