Structural, Magnetic, Electrical, and Magnetoelectric Properties of Sm- and Ho-Substituted Nickel Ferrites

Sm- and Ho-substituted and pure nickel ferrite materials, namely, NiFe 1.925 Sm 0.075 O 4, NiFe 1.925 Ho 0.075 O 4, and NiFe 2 O 4, have been synthesized by the solid-state chemical reaction, and their structural, magnetic, dc electrical conductivity, ferromagnetic, ferroelectric, and dielectric properties have been evaluated. Sm- and Ho-substituted nickel ferrites crystallize in the cubic inverse spinel phase with a very small amount of SmFeO 3 and HoFeO 3 as the additional phase, respectively. X-ray diffraction studies indicate rhombohedral distortion in NiFe 1.925 Sm 0.075 O 4 and NiFe 1.925 Ho 0.075 O 4 compounds. The existence of ferroelectricity in NiFe 1.925 Sm 0.075 O 4 and NiFe 1.925 Ho 0.075 O 4 has been confirmed from the ferroelectric loops, and the respective transition temperatures are 543 and 677 K. The respective magnetocapacitance values are −1.8% and −1.2%. Magnetoelectric coefficients observed at 500 Oe in the NiFe 1.925 Sm 0.075 O 4 and NiFe 1.925 Ho 0.075 O 4 compounds are 1.82 and 1.84 mV cm −1 Oe −1, respectively. A considerable increase in the saturation magnetostriction value has been observed upon the substitution of Sm. Substitution of Sm and Ho for Fe at the B site increases the dielectric constant compared to that of pure nickel ferrite. Frequency variation of the dielectric constant shows a dispersion that can be modeled with a modified Debye function, which considers the possibility of more than one ion contributing to the relaxation. Electrical conductivity curves confirm the improved resistivity of the NiFe 1.925 Sm 0.075 O 4 and NiFe 1.925 Ho 0.075 O 4 compounds compared to that of pure nickel ferrite. Analysis of the temperature-dependent conductivity indicates that the small polaron and variable-range-hopping mechanisms are operative in the 220−300 and 160−220 K temperature regions, respectively.