Magnetic anisotropy, magnetization reversal and switching in Ni4Nb2O9 single crystals

Ni4Nb2O9 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 (Theta(p) approximate to 190 K) and rather similar effective paramagnetic moments (from 3.5 mu B to 3.6 mu B). This suggests that the strongest magnetic interaction is the antiferromagnetic one, coupling the ferromagnetic distorted honeycomb layers and zigzag ribbons via face sharing NiO6 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 Ni2+ 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.