Magnetic Ordering in RbNiCl3

Abstract

The magnetic structure of RbNi${\mathrm{Cl}}_3$ has been carefully reexamined by single-crystal neutron-diffraction techniques in light of a discrepancy between two previously reported powder neutron-diffraction results. Data were taken on a small single crystal at 4.5°K and were corrected for absorption and extinction. The results confirm the basic picture of a multidomain triangular arrangement with the moments lying in a plane perpendicular to the basal plane of the hexagonal crystal. However, a slight modification of this structure consistent with the symmetry and the presence of a small $c$-axis anisotropy yields a significantly better fit to the data. The magnetization was measured from 4.5 to 11.1°K [$T_N=(11.15\mathrm{}\pm 0.05)°$K], and an attempt was made to determine the exponent $\beta$. The best fit to a simple power law gives $2\beta =0.50\mathrm{}\pm 0.03$. However, a reasonable correction to the power law yields $2\beta =0.60\mathrm{}\pm 0.03$. The ${\mathrm{Ni}}^{2+}$ moment extrapolated to 0°K is $(1.3\mathrm{}\pm 0.1){\mu }_B$, indicative of very substantial zero-point effects.