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

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<$T_{N1}$). 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(∥c) component is discussed with respect to the temperature dependence of the birefringence in the ac plane.