Magnetooptical Study of Spin-Orientation Phase Transition in   $NdFe_3(BO_3)_4$ Single Crystal
  The magnetic field dependences of birefringence in a $NdFe_3(BO_3)_4$ single crystal have been measured in the case where the direction of light propagation coincides with the trigonal crystal axis $C_3$ ($k\|C_3$), and the external magnetic field is oriented along the second-order axis $C_2$ ($H\|C_2$). In the temperature range, in which an incommensurate phase exists with the formation of a long-period antiferromagnetic helix, the strongly pronounced jumps in the field dependence of birefringence are revealed and identified as a firstorder spin-orientation phase transition. The phase transition was accompanied by a hysteresis in the field dependences of birefringence. The $H-T$ phase diagram for a $NdFe_3(BO_3)_4$ single crystal has been plotted in the case where the magnetic field is oriented along the crystal axis $C_2$ ($H\|C_2$). 
V. A. Bedarev, M. I. Pashchenko, D. N. Merenkov, L. N. Bezmaternykh,   V. L. Temerov
cond-mat.mtrl-sci