Magnetization studies in rare-earth orthochromites. VII. LuCrO3

The magnetization and principal magnetic susceptibilities of LuCr${\mathrm{O}}_3$ are measured from ambient temperature down to 4.2°K. It is found that LuCr${\mathrm{O}}_3$ exhibits a weak ferromagnetic moment along the $a$ crystallographic axis below $T_N=111\mathrm{}$°K. By applying a magnetic field along the antiferromagnetic $c$ axis, the antiferromagnetic vector can be rotated smoothly in the $a-c$ plane until it coincides with $a$ at and above a critical field $H_{\mathrm{cr}}$. This field increases with decreasing temperature, its value at $T=4.2\mathrm{}$°K being $H_{\mathrm{cr}}=3.6\mathrm{}\pm 0.4$ kOe. By comparing the temperature dependence of the spontaneous $a$-axis magnetization with that of the $c$-axis magnetization extrapolated to zero field, a quantitative separation between the Dzyaloshinsky-Moriya antisymmetric exchange $D$ and single-ion anisotropy ($A_{\mathrm{xz}}$) contributions to the canting is made. We find that $\frac{A_{\mathrm{xz}}}{D}<0.05\mathrm{}$ for the case of LuCr${\mathrm{O}}_3$. As a consequence, a temperature-independent constant-canting-angle model is adopted. The temperature dependence of the reduced magnetization can then be analyzed using statistical-mechanical models developed for antiferromagnets. For $0.60<\mathrm{}\frac{T}{T_N}<0.99\mathrm{}$ the magnetization exhibits power-law behavior with $\beta \simeq \frac{1}{3}$. For $\frac{T}{T_N}<0.5\mathrm{}$, its behavior is well described by spin-wave models. A comparison of the experimental results with theoretical curves calculated over the entire temperature range using molecular-field and several Green's-function models is also presented.