Crystal Structure, Microstructure, Magnetic Properties, and Dielectric Properties of Dy3+ Substituted Co–Ni Mixed Ferrite

A comparative and exploration study of intrinsic, rare-earth (RE) and rare-earth and transition-metal (TM) cosubstituted cobalt ferrite (CoFe2O4; CFO) materials has been performed by studying their crystal growth, phase-stability, morphology, magnetic, and electrical properties comprehensively. Pristine cobalt ferrite (CoFe2O4), RE-ion (Dy3+) substituted CFO (CoFe1.95Dy0.05O4; RE-CFO), and TM-RE (nickel/dysprosium; Ni2+/Dy3+) cosubstituted CFO (Co1–x Ni x Fe1.95Dy0.05O4; x = 0.1, 0.5; RE-TM-CFO) samples were synthesized by a standard solid-state reaction method. X-ray diffraction (XRD) together with Rietveld refinement confirms the formation of the anticipated phase, which corresponds to the space group Fd3̅m (227), along with the presence of a small impurity phase (DyFeO3) in the samples where Dy3+ was substituted. Surface morphology studies by means of scanning electron microscopy specifies the consequence of Ni2+/Dy3+ on the microstructure as well as the presence of the orthoferrite phase of DyFeO3 at the grain boundaries. Brunauer–Emmett–Teller (BET) analyses indicate that the specific surface area and pore contribution increases, whereas the pore width decreases for Dy-substitution in Co–Ni mixed composition, i.e., Co1–x Ni x Fe1.95Dy0.05O4; x = 0.5. Magnetization (M) versus magnetic field (H) measurements at variable temperature (5–400 K) confirm the ferrimagnetic nature of all the CFO, RE-CFO, and TM-RE-CFO samples. Saturation magnetization (M s), coercive field (H c), and remnant magnetization (M r) are observed to be decreased with Ni2+/Dy3+ substitution. However, the cubic anisotropy does not follow a systematic trend. A maximum magnetostriction coefficient λ11 = −103 ppm is attained for Co0.9Ni0.1Fe1.95Dy0.05O4 along with the strain derivative dλ/dH = 0.1108 ppm/Oe at a lesser magnetic field of 500 Oe, making it one of the best candidates for a stress or magnetic field sensing application. Moreover, this sample may be suitable as one of the constituent phases in magneto-electric materials. The dielectric constant dispersion measurements as a function of frequency revealed the typical dielectric behavior of the ferrites. Concurrently, the presence of a single semicircle arc in the complex impedance study confirms that only grains are contributing to the conduction mechanism. Furthermore, as expected, temperature-dependent complex impedance measurements show a decrease of grain resistance (R g) and relaxation time (τ), whereas the grain capacitanc