Magnetic anisotropy, magnetization reversal and switching in Ni 4 Nb 2 O 9 single crystals.

Ni 4 Nb 2 O 9 is an insulating compensated ferrimagnet with T N = 77 K and T comp = 33 K. We report here the study of the magnetic anisotropy using millimeter-size crystals grown in an image furnace. The magnetization measurements, vs temperature, performed with H aligned along the three main crystallographic axes, show similar Curie-Weiss temperatures ( Θ p ≈ 190 K) and rather similar effective paramagnetic moments (from 3.5 μ B to 3.6 μ B ). This suggests that the strongest magnetic interaction is the antiferromagnetic one, coupling the ferromagnetic distorted honeycomb layers and zigzag ribbons via face sharing NiO 6 octahedra. This strong antiferromagnetic coupling is supported by DFT calculations that do not evidence any inter site ferromagnetic interaction, leading to total compensation between magnetic moments of both Ni 2+ sites. Measurements vs magnetic field below T N reveal an anisotropic behaviour, with square magnetization loops for H in the ab plane, whereas linear M ( H ) curves without hysteresis are observed for H‖c . This anisotropy between ab plane and c axis occurs also in the magnetization reversal (MR), which is observed in the ab plane only. Starting from M ( H ) virgin curves collected just below T comp = 33 K with H‖a or H‖b , the memory-like effect was tested through magnetization switching induced by H or T alternating changes. Below T comp , smaller H is needed to switch M symmetrically for H along b than along a , and, for T switching (2 K interval, constant H ), a larger M change is obtained along a than along b . The comparison with ferrimagnetic oxides which exhibit MR, like spinels or rare earth orthoferrites, shows that Ni 4 Nb 2 O 9 is unique since only one magnetic cation over two sites in octahedral coordination is at play, thus providing a unique platform to study M switching but also a challenge for theoretical interpretation.