Structural Transformations and Magnetic Properties of Mixed Spinel‐Type NiCr_1.7Fe_0.3O₄ Nanoparticles

Abstract

Structural and magnetic properties of NiCr1.7Fe0.3O4 nanoparticles, synthesized by a facile coprecipitation method, are examined using temperature‐dependent X‐ray diffraction, neutron scattering with XYZ polarizations, and magnetic measurements. The structural analysis demonstrates the stabilization of the cubic phase down to 200 K, followed by a low‐temperature tetragonal phase at 100 K and an orthorhombic phase below 50 K. The refined occupancies of cations demonstrate that the NiCr1.7Fe0.3O4 is a mixed spinel compound, wherein B sites are populated by ≈85% Cr3+ and ≈15% Ni2+ cations and A sites are populated by ≈70% Ni2+ and ≈30% Fe3+ cations. As a consequence, the reduction in the occupancy of Jahn–Teller active Ni2+ (triply degenerate: e4 t4) at A site is attributed to the stabilization of the cubic phase below room temperature. Temperature‐dependent magnetization and neutron scattering with XYZ polarizations measurements reveal the ferrimagnetic nature of the magnetic phase and the absence of spin‐spiral ordering in NiCr1.7Fe0.3O4 nanoparticles. Furthermore, field‐dependent magnetization measurement demonstrates an exchange bias field of ≈134.5 kA m−1 at 5 K. The training effect measurement is performed to discuss the origin of exchange bias based on different phenomenological models such as power‐law and multiexponent models.

Coexistence of tetragonal and cubic phase induced complex magnetic behaviour in CoMn2O4 nanoparticles

“…H CEB with loop cycle continuously decreases, shown in figure 11. Similar kind of behaviour is reported for NiCr 1.7 Fe 0.3 O 4 normal spinel, where training effect is due to the change in distorted spin configuration with loop cycle [54]. The percentage of training effect (%TE) parameter, which denotes the relative decrease in H CEB starting from the first to last hysteresis loop, is estimated using the relation:…”

“…where H CEB 1 and H CEB n are the exchange bias fields of first and nth hysteresis loops, respectively [54]. It is observed that %TE abruptly decreases by 82% from the first to second cycle and thereafter, it decreases gradually and tends to saturate with the number of cycles reaching six.…”

“…This is because of the Mn 3+ occupying the B site accompanied with Yafet-Kittel spin structure. While VMS in NiCr 1.7 Fe 0.3 O 4 is attributed to the impediment of rotation of ferromagnetic moment (FM) moments of A site by the uncompensated antiferromagnetic moment (AFM) moments at B site [54], in NiFe 2 O 4 it is due to the exchange coupling between the spin-glass like phase and ferrimagnetic phase [55]. The very high exchange bias of 28.587 kOe obtained at 5 K seems to be overestimated due to the nonsaturation of magnetization and high irreversibility in magnetization during the hysteresis cycle.…”