Polarized light studies of ferromagnetic/ferroelectric/ferroelastic domain patterns in NiCl and NiBr boracite

Ferromagnetic stripe domains were observed in polarized light along the optic axes of the orthorhombic NiCl and NiBr boracites below ∼9K and ∼30K, respectively. The spontaneous magnetization vector is found to lie along the principal refractive index nγ. Measurements of spontaneous Faraday rotation and spontaneous birefringence down to 4.2K are presented