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

Abstract

NiCr2−xFexO4 (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.

Structural Transformations and Magnetic Properties of Mixed Spinel‐Type NiCr1.7Fe0.3O4 Nanoparticles

“…It can be noted that the estimated T C for NiCr 1.7 Fe 0.3 O 4 nanoparticles is almost 29 K higher compared with T C ≈230 K reported for its bulk counterpart by Park et al [ 18 ] This enhancement in T C corroborates well with the enhanced T C observed for x = 0, 0.1 and 0.2 nanoparticles compared to their bulk counterparts. [ 11,12 ] Moreover, a significant increase in T C compared with the T C in NiCr 2 O 4 nanoparticles is attributed to the increase in exchange interaction between the magnetic cations occupying the A and B sites. Below T C , the irreversibility between the M FC and M ZFC is observed at irreversible temperature, T irr ≈ 244 K. The large bifurcation between the M FC and M ZFC below T irr suggests the presence of uncompensated spins accompanied by strong magnetic anisotropy.…”

“…The development of unidirectional exchange anisotropy between the FM ordered moments at the A site and the AFM ordered moments at the B site, after partially substituting the Cr‐site with magnetic/nonmagnetic cations (i.e., Al, Fe, and Mn), has been reported to cause exchange bias phenomena in NiCr 2– x Fe x /Al/Mn x O 4 systems. [ 9,11,15 ] The exchange bias field, H EB , is estimated using the relation, and the vertical shift in magnetization Δ M is estimated using the relation . The estimated values for H EB and Δ M are plotted as a function of temperature, as shown in Figure 9 .…”

“…With each cycle, rearrangement of uncompensated AFM spins takes place and it modifies the AFM magnetic anisotropy at the FM/AFM interface. [ 11,25,26 ] This modified AFM magnetic anisotropy basically results in two different groups of interfacial uncompensated AFM spins. In one group, spins are strongly exchanged coupled to FM spins with small relaxation time, whereas in the other group, spins are mostly frozen with relatively large relaxation time.…”

“…[ 7,8 ] Interestingly, these phase transitions in NiCr 2 O 4 could be easily tuned by the partial substitution of transition elements for the Ni/Cr‐site, which could further offer new enhanced properties such as exchange bias, magnetocaloric effect, magnetic switching effect, magnetization reversal, large coercivity, and so on. [ 9–12 ] The exchange bias phenomenon which is manifested by the asymmetrical magnetic hysteresis loop about the field and/or magnetization axis has important application in the spin valves, read heads for recording devices, and magnetic tunnel junction‐based devices. [ 13,14 ] Exchange bias is generally induced by the unidirectional exchange anisotropy at the interface between irreversible and reversible magnetic phases.…”

“…[ 15 ] In our recent report, we have shown unusually high exchange bias field of ≈875 and ≈2069 kA m −1 after substituting the Cr‐site with 10 and 20 at% of Fe in NiCr 2 O 4 lattice, respectively. [ 11,12 ]…”