Basal-plane birefringence and magnetic ordering in hexagonal antiferromagnets CsNiX3 (X=C1, Br).

Abstract

The basal-plane birefringence (for light propagating along the c axis) was measured in the hexagonal antiferromagnets ${\mathrm{CsNiCl}}_{3}$ and ${\mathrm{CsNiBr}}_{3}$. The results confirmed the sequence of magnetic ordering suggested in NMR and neutron diffraction studies, that is, the basal-plane components of the magnetic moments were ordered in the low-temperature phase (T${T}_{N2}$), but not in the intermediate phase (${T}_{N2}$${T}_{N1}$). It was newly found that a multidomain structure with respect to the direction of the spin plane is established in the low-temperature phase in the absence of external field, and that a single-domain structure is stabilized for the field perpendicular to the c axis above about 0.3 T. The magnetic ordering process of the z(\ensuremath{\parallel}c) component is discussed with respect to the temperature dependence of the birefringence in the ac plane.