Optical and neutron diffraction study of the magnetic phase diagram of NiBr2

The presence of a magnetic phase transition at 22.5K in NiBr 2 (T N =52K) has been confirmed by neutron diffraction and by the appearance of new sharp electric-dipole absorption lines in the optical spectrum near 6080 AA. The latter may be 'switched off' by an external magnetic field parallel to the basal plane equal to 2.75 T at 1.3K. By measuring the variation with temperature of this critical field, the phase diagram of NiBr 2 in the HT plane has been established. Powder neutron diffraction measurements confirm that, from the Neel temperature down to 22.5K, the magnetic structure of NiBr 2 is that of a simple metamagnet, with sheets of ferromagnetically coupled spins in turn coupled together antiferromagnetically, the spins lying in the basal plane. The temperature dependence of the intensity of the (009) magnetic peak in the single-crystal neutron diffraction show that, below 22.5K, the spins become canted out of the basal plane by about 54 degrees .