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 (CoFe 2 O 4; CFO) materials has been performed by studying their crystal growth, phase-stability, morphology, magnetic, and electrical properties comprehensively. Pristine cobalt ferrite (CoFe 2 O 4 ), RE-ion (Dy 3+ ) substituted CFO (CoFe 1.95 Dy 0.05 O 4; RE-CFO), and TM-RE (nickel/dysprosium; Ni 2+ /Dy 3+ ) cosubstituted CFO (Co 1– x Ni x Fe 1.95 Dy 0.05 O 4; 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 Fd 3̅ m (227), along with the presence of a small impurity phase (DyFeO 3 ) in the samples where Dy 3+ was substituted. Surface morphology studies by means of scanning electron microscopy specifies the consequence of Ni 2+ /Dy 3+ on the microstructure as well as the presence of the orthoferrite phase of DyFeO 3 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., Co 1– x Ni x Fe 1.95 Dy 0.05 O 4; 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 Ni 2+ /Dy 3+ substitution. However, the cubic anisotropy does not follow a systematic trend. A maximum magnetostriction coefficient λ 11 = −103 ppm is attained for Co 0.9 Ni 0.1 Fe 1.95 Dy 0.05 O 4 along with the strain derivative dλ/d H = 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 capacitance ( C g ) is observed to be enhanced. Overall, the results point to the outcome and highlight the TM-RE mixed CFO (Co 0.9 Ni 0.1 Fe 1.95 Dy 0.05 O 4 ) is suitable for designing the magneto-mechanical devices.