Magnetic Properties of the Hexagonal Antiferromagnet CsMnF3

The magnetic properties of the hexagonal antiferromagnetic CsMn${\mathrm{F}}_3$ have been investigated by magnetic susceptibility, torsion, electron resonance, and nuclear-antiferromagnetic double resonance. Torsion measurements establish a transition to an antiferromagnetically ordered state at 53.5°K. A weak sixfold anisotropy in the transverse plane and a large axial anisotropy along the $c$ axis corresponding, respectively, to the fields $\frac{36K_3}{M}=1.1\mathrm{}$ Oe and $\frac{K_1}{M}=-7500\mathrm{}$ Oe are detected. Susceptibility measurements at 4.2°K establish an exchange field $H_E=3.5\mathrm{}\times {10}^5$ Oe. The temperature dependence of $K_3$ was observed from 4.2°K to the transition temperature and compared with spin-wave and molecular field theory. From paramagnetic resonance measurements an isotropic $g$ value of 1.9989±0.003 is determined. Magnetic resonance measurements below the transition temperature with the applied field in the transverse plane show a weak sixfold anisotropy consistent with the torsion measurements. Measurements out of the transverse plane confirm the large axial anisotropy. In the temperature range from 0.3 to 4.2°K there is an additional temperature dependent anisotropy field $H_{A,T}=\frac{9.15}{T}$ Oe directed along the sublattices. This field arises from the hyperfine interaction with the ${\mathrm{Mn}}^{55}$ nuclear magnetization. Assuming parallel ordering within the transverse planes with adjacent planes alternately magnetized, a calculation of the classical dipolar interactions and of the ligand field anisotropy arising from the displacement of the nearest neighbor fluorines gives a combined axial anisotropy field $\frac{K_1}{M}=-7965\mathrm{}$ Oe. The in-plane anisotropy due to second-order dipolar interactions is estimated to be ≈2 Oe in reasonable agreement with observation. The strong coupling between the nuclei and electrons affords an opportunity to observe the ${\mathrm{Mn}}^{55}$ nuclear resonance indirectly by monitoring the position of the electron resonance field. A saturation of the nuclear magnetization is observed at 668 Mc/sec which is (3±1)% smaller than the calculated average hyperfine field of 689±7 Mc/sec. This indicates the presence of a zero-point reduction in the electron spin.