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Effect of High-Anisotropic Co<sup>2+</sup> Substitution for Ni<sup>2+</sup> on the Structural, Magnetic, and Magnetostrictive Properties of NiFe<sub>2</sub>O<sub>4</sub>: Implications for Sensor Applications
This work reports on the effect of substituting a low-anisotropic
and low-magnetic cation (Ni2+, 2μB) by
a high-anisotropic and high-magnetic cation (Co2+, 3μB) on the crystal structure, phase, microstructure, magnetic
properties, and magnetostrictive properties of NiFe2O4 (NFO). Co-substituted NFO (Ni1–xCoxFe2O4,
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 Co2+ and Ni2+, 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 T2g(2) and A1g(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 Co2+, Ni2+, and Fe3+ 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
(MS), remnant magnetization (Mr), coercivity (HC), and anisotropy
constant (K1) 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 Co2+ 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, NiFe2O4 (x = 0) and CoFe2O4 (x = 1)