Evidence of anomalous conventional and spontaneous exchange bias, high coercivity in Fe doped NiCr2O4 spinel

Abstract

NiCr_2−xFe_xO₄ (x = 0 and 0.2) polycrystalline ceramics have been synthesized successfully through a simple co-precipitation technique to study the evolution of structural and magnetic properties by doping Fe. X-ray diffraction (XRD) reveals that the high-temperature cubic phase (space group Fdm) observed at 320 K in bulk NiCr₂O₄ is stabilized at room temperature by decreasing the particle size to nanometer in x = 0 as well as after incorporating 20 at% Fe in the NiCr₂O₄ lattice. The cation distribution obtained from X-ray absorption fine structure (XAFS) analysis illustrates that while in x = 0, Ni²⁺ and Cr³⁺ ions occupy the tetrahedral (A) and octahedral (B) sites, respectively, x = 0.2, Fe³⁺ and Cr³⁺ ions occupy the A and B sites, respectively, and Ni²⁺ ions are distributed among the A and B sites. This transformation from the normal to mixed spinel structure strongly affects the magnetic properties. While the paramagnetic to long-range ferrimagnetic ordering temperature T_C is enhanced from 71 to 192 K, significantly large coercive field (H_C) of ∼29 kOe is observed for x = 0.2 as compared to the H_C ∼13 kOe for x = 0. Moreover, unusually large conventional and spontaneous exchange bias fields of ∼26 and ∼2.6 kOe are observed for x = 0.2, which is absent for x = 0. The presence of anomalous exchange bias field is ascribed to the unidirectional exchange anisotropy between the two magnetic sublattices at A and B sites. The training effect of the exchange bias field is discussed using a phenomenological model, which considers the contribution from irreversible uncompensated spins that modify the exchange anisotropy at the interface between A and B magnetic sublattices. In addition, diffuse neutron scattering (DNS) with XYZ analysis is employed for both compositions to clearly illustrate the low-temperature peculiar magnetic phase transitions such as spin spiral transition, T_S and spin lock-in transition, T_l. The DNS demonstrates that while T_l decreases from 10 K to 7 K with the incorporation of Fe in the NiCr₂O₄ lattice, T_S significantly increases from 28 K to 50 K.