Hidden magnetic instability in the substituted multiferroics (Nd,Tb)Fe3(BO3)4

https://doi.org/10.1103/PhysRevB.109.014421

In the substituted ${\mathrm{Nd}}_{1-x}{\mathrm{Tb}}_x{\mathrm{Fe}}_3{\left({\mathrm{BO}}_3\right)}_4$ $(x=0.1\text{and}x=0.2)$, possessing almost easy-axis magnetic structure at low temperatures, an unusual two-step transition in fields along the trigonal $c$ axis was observed by magnetization and single-crystal neutron diffraction studies. At the first step, only part of the Tb Ising-type moments flip to the $c$ axis, which is accompanied by a significant deviation of the antiferromagnetic Fe spins from the $c$ axis. At the second step, the remaining Tb moments flip and the Fe moments flop into the basal plane. The ob- served evolution is qualitatively explained by a model assuming small deviations of Tb moments from the trigonal axis due to local environment distortions, which leads to nonequivalence of the Tb ions with respect to effective Tb-Fe exchange and external field. Thus, an intrinsic “hidden” instability of the magnetic system in the magnetic field occurs.