Low-Temperature Sublattice Magnetization Of Antiferromagnetic CrCl3

The temperature dependence and the magnetic-field dependence of the Cr${\mathrm{Cl}}_3$ sublattice magnetization have been studied in the temperature range 0.39-4.02°K by means of the ${\mathrm{Cr}}^{53}$ nuclear magnetic resonance. The experimental results are in quantitative agreement with predictions of the isotropic Heisenberg exchange model in the low-temperature spin-wave approximation. The antiferromagnetic interlayer-exchange interaction in this hexagonal layer-type crystal is shown to be sufficiently weak, compared to the ferromagnetic intralayer-exchange interaction, to justify its treatment as an effective anisotropy field. In addition to the exchange interactions, both dipolar and single-ion, $D{\left(S^y\right)}^2$, anisotropies are required to fit the data. The resulting two-dimensional ferromagnetic model accurately accounts for the strong temperature dependence of the sublattice magnetization and the approach of this dependence to linearity with increasing temperature. A fit of the theory to the experimental data gives an intralayer exchange constant $\frac{J}{k}=4.52\mathrm{}\pm 0.05°$K. The intensity enhancement of the ${\mathrm{Cr}}^{53}$ resonance in weak external magnetic fields, as well as in zero field, is discussed. The strong field-induced enhancement is explained by a domain rotation model which is based on the torque exerted by the driving field $H_1$ on the net field-induced magnetization, in the presence of a restoring force provided by a weak magnetocrystalline anisotropy ($K{sin}^{2}3\theta$) in the (001) plane. Comparison of calculated and observed intensities as a function of field strength yields $K=150\mathrm{}\pm 50$ ergs/${\mathrm{cm}}^3$ and a spin flopping transition in the (001) plane at a field strength of ∼ 160 Oe. The enhancement mechanism in external fields leads to selective excitation resulting in observable splittings of the nuclear resonance in polycrystalline samples when $H_1\parallel H$. The parallel magnetic susceptibility of the Cr${\mathrm{Cl}}_3$ spin system has been calculated from these splittings at several temperatures between 1 and 4°K. The results (e.g., 0.28±0.01 emu/mole at 4.00°K) agree within experimental error with susceptibilities calculated from the spin-wave model using the interaction constants obtained from the temperature dependence of the sublattice magnetization.