Magnetoelectric and magneto-optical measurements on Nickel-Bromine boracite Ni 3 B 7 O 13 Br

Various anomalies of Faraday rotation, magnetoelec. coeff. α23 (T), α32 (T), coercive field and birefringence of 3 principal cuts as well as new direct observations of magnetic domains of NiBr boracite single crystals show that the weak ferromagnetic phase m'm2' of NiBr boracite, which appears at 29 K transforms at 21 K into another ferromagnetic phase of triclinic Shubnikov point group 1. The strong anomalies of spontaneous birefringence in the 2 ferromagnetic phases could come from the appearance of the order parameter and the magnetostriction. A magnetic precursor effect on the birefringence appears already in the paramagnetic phase.