Effect of High-Anisotropic Co²⁺ Substitution for Ni²⁺ on the Structural, Magnetic, and Magnetostrictive Properties of NiFe₂O₄: Implications for Sensor Applications

This work reports on the effect of substituting a low-anisotropic and low-magnetic cation (Ni 2+, 2μ B ) by a high-anisotropic and high-magnetic cation (Co 2+, 3μ B ) on the crystal structure, phase, microstructure, magnetic properties, and magnetostrictive properties of NiFe 2 O 4 (NFO). Co-substituted NFO (Ni 1– x Co x Fe 2 O 4, NCFO, 0 ≤ x ≤ 1) nanomaterials were synthesized using glycine-nitrate autocombustion followed by postsynthesis annealing at 1200 °C. The X-ray diffraction measurements coupled with Rietveld refinement analyses indicate the significant effect of Co-substitution for Ni, where the lattice constant ( a ) exhibits a functional dependence on composition ( x ). The a -value increases from 8.3268 to 8.3751 Å (±0.0002 Å) with increasing the “ x ” value from 0 to 1 in NCFO. The a – x functional dependence is derived from the ionic-size difference between Co 2+ and Ni 2+, which also induces grain agglomeration, as evidenced in electron microscopy imaging. The chemical bonding of NCFO, as probed by Raman spectroscopy, reveals that Co( x )-substitution induced a red shift of the T 2g (2) and A 1g (1) modes, and it is attributed to the changes in the metal–oxygen bond length in the octahedral and tetrahedral sites in NCFO. X-ray photoelectron spectroscopy confirms the presence of Co 2+, Ni 2+, and Fe 3+ chemical states in addition to the cation distribution upon Co-substitution in NFO. Chemical homogeneity and uniform distribution of Co, Ni, Fe, and O are confirmed by EDS. The magnetic parameters, saturation magnetization ( M S ), remnant magnetization ( M r ), coercivity ( H C ), and anisotropy constant ( K 1 ) increased with increasing Co-content “ x ” in NCFO. The magnetostriction (λ) also follows a similar behavior and almost linearly varies from −33 ppm ( x = 0) to −227 ppm ( x = 1), which is primarily due to the high magnetocrystalline anisotropy contribution from Co 2+ ions at the octahedral sites. The magnetic and magnetostriction measurements and analyses indicate the potential of NCFO for torque sensor applications. Efforts to optimize materials for sensor applications indicate that, among all of the NCFO materials, Co-substitution with x = 0.5 demonstrates high strain sensitivity (−2.3 × 10 –9 m/A), which is nearly 2.5 times higher than that obtained for their intrinsic counterparts, namely, NiFe 2 O 4 ( x = 0) and CoFe 2 O 4 ( x = 1).