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.
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