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.

This study describes the relative effect on the permanent magnet characteristics viz. remanence (Br), intrinsic coercivity (Hci), Curie temperature (TC), and rectangularity of the intrinsic demagnetization curve, when Al, Cu, Ga, and Nb are added individually to NdFeB. Each elemental addition causes significant improvement in Hci but the level of improvement differs from one additive element to the other. The addition of Nb is favored over other elements for realizing maximum enhancement in Hci and rectangularity of the demagnetization curve. The microstructural features of the sintered samples of NdFeB with elemental addition show the formation of a new phase, in addition to the phases (φ,η, and Nd-rich) generally found in the ternary sample. The factors influencing the permanent magnet characteristics of sintered samples are the distribution of the Nd-rich phase in the intergranular region, the size and distribution of the minor phases at the grain junctions, the formation and distribution of new phases due to alloying addition, and the solubility of the dopant element in various phases coexisting in the sample.

We report on the impedance spectroscopic characterization of Sm and Ho doped Ni ferrite materials, namely NiO.Fe 1.925 Sm 0.075 O 3 and NiO.Fe 1.925 Ho 0.075 O 3 , to demonstrate their improved electrical properties compared to pure NiO.Fe 2 O 3 . Sm and Ho doped Ni ferrites crystallize in the cubic inverse spinel phase with a very small amount of SmFeO 3 and HoFeO 3 as the additional phase, respectively. Atomic force microscopy measurements indicate that the bulk grains are approximately 2-5 m in size while the grain boundaries are thin compared to bulk grains. Frequency variation of the dielectric constant shows the dispersion that can be modeled with a modified Debye's function, which considers the possibility of more than one ion, contributing to the relaxation. The resistivity values ͑at 3.5 KHz͒ of NiO . Fe 2 O 3 , NiO . Fe 1.925 Sm 0.075 O 3 , and NiO . Fe 1.925 Ho 0.075 O 3 compounds are found to be 0.1 ϫ 10 4 ⍀ m, 0.5 ϫ 10 4 ,⍀ m and 0.8 ϫ 10 4 ⍀ m, respectively. Impedance spectroscopic analysis indicates the different relaxation mechanisms and their variation with temperature, bulk grain and grain-boundary contributions to the electrical conductivity ͑R g ͒, and capacitance ͑C g ͒ of these materials. While the conductivity in pure NiFeO 4 is predominantly due to intrinsic bulk contribution ͑R g = 213 k⍀ and C g = 4.5 ϫ 10 −8 F͒, NiO.Fe 1.925 R 0.075 O 3 ͑R = Sm,Ho͒ exhibits distinct grain and grain-boundary contributions to the conductivity.

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