Temperature-dependent magnetization, anisotropy and conductivity of CoFe_2−x Sn_x O₄ ( x   =  0.025, 0.05, 0.075): appearance of grain boundary conductivity at high temperatures

The temperature dependence of magnetization, anisotropy, ac and dc conductivity of CoFe_2−x Sn_x O₄ ( x   =  0.025, 0.05, 0.075) were investigated and the results are reported. All the compounds were prepared by a solid-state reaction, and the formation of the compounds in the cubic inverse spinel phase was confirmed from their Rietveld refined x-ray diffraction (XRD) patterns and Raman spectra. Increments in the lattice constant were observed upon the partial substitution of Fe³⁺ by Sn⁴⁺. The presence of all elements and their ionic states were confirmed from x-ray photoelectron spectroscopic studies. Magnetic hysteresis loops were measured for each compound at temperature 20 K and 50–300 K (in steps of 50 K) using a superconducting quantum interference device vibrating sample magnetometer. Both magnetization and magnetic anisotropy showed a decrease in values with increasing Sn substitution. Room temperature (RT) magnetization is seen to decrease from 80–65.91 emu g⁻¹ with increasing Sn concentration from x   =  0 (CoFe₂O₄) to 0.075 (CoFe_1.925Sn_0.075O₄). The high field regimes of the hysteresis loops were modeled using the law of approach to the saturation magnetization equation. The temperature variation of magnetization and magnetic anisotropy are explained on the basis of a one-ion model. Complex impedance spectroscopy studies at RT show that the conductivity in these materials is predominantly due to the intrinsic bulk grains. With increasing the temperature, evolution of the grain boundary conduction is clearly seen through the appearance of a second semi-circle in the complex impedance plots. The RT total dc conductivity value of CoFe_2−x Sn_x O₄ ( x   =  0, 0.025, 0.05, 0.075) is found to be 5.78  ×  10⁻⁸, 8.56  ×  10⁻⁸, 1.44  ×  10⁻⁷ and 1.11  ×  10⁻⁷ S cm⁻¹ respectively. The observation of well-distinguishable grain and grain boundary conductions and the low conductivity values in the CoFe_2−x Sn_x O₄ ( x   =  0, 0.025) materials indicates that these materials are promising candidates for high-frequency applications. (paper)