Structural phase transition and spontaneous exchange bias in NiCr 1.9 Fe 0.1 O 4 nanoparticles: XRD, EXAFS and magnetic measurements

Abstract

Here, we report structural and magnetic phase transitions in NiCr 1.9 Fe 0.1 O 4 nanoparticles of size ∼60 nm synthesized through cost-effective co-precipitation route. X-ray diffraction pattern shows a stable cubic phase at RT in contrast to tetragonal phase in bulk NiCr 2 O 4 . Decreasing the temperature, a complete tetragonal symmetry is observed at 50 K, while a coexistence of cubic and tetragonal phases in the intermediate temperature range. Interestingly, no tetragonal to orthorhombic phase transformation has been observed down to 12 K as observed in NiCr 2 O 4 at 70 K. The local structure examined from temperature dependent EXAFS demonstrates that although no change in distribution of cations among tetrahedral (A)and octahedral (B)sites down to 10 K has been observed, Fe 3+ ions occupy A site replacing equivalent amount of Ni 2+ towards B site which is contradicting to the bulk NiCr 2 O 4 where Ni 2+ occupies only A site. Magnetic studies demonstrate a two-fold increase in para to long-range ferrimagnetic transition, T C , due to increase in A-B exchange interaction and no change in spiral ordering temperature, T S indicates strong B-B interaction. In addition, these nanoparticles exhibit unusually high spontaneous exchange bias of ∼1.265 kOe at 60 K. We discuss the temperature dependent exchange bias on the basis of competition between uncompensated B site moments and ferromagnetically ordered spins at A site. The tunable exchange bias without field highlight the potential applications of these materials in spintronics devices. © 2019 Elsevier Ltd and Techna Group S.r.l.