Magnetic Properties of the Hexagonal Antiferromagnet CsMn<mml:math xmlns:mml="http://www.w3.org/1998/Math/MathML" display="inline"><mml:mrow><mml:msub><mml:mrow><mml:mi mathvariant="normal">F</mml:mi></mml:mrow><mml:mrow><mml:mn>3</mml:mn></mml:mrow></mml:msub></mml:mrow></mml:math>

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\ifmmode^\circ\else\textdegree\fi{}K. A weak sixfold anisotropy in the transverse plane and a large axial anisotropy along the $c$ axis corresponding, respectively, to the fields $\frac{36{K}_{3}}{M}=1.1$ Oe and $\frac{{K}_{1}}{M}=\ensuremath{-}7500$ Oe are detected. Susceptibility measurements at 4.2\ifmmode^\circ\else\textdegree\fi{}K establish an exchange field ${H}_{E}=3.5\ifmmode\times\else\texttimes\fi{}{10}^{5}$ Oe. The temperature dependence of ${K}_{3}$ was observed from 4.2\ifmmode^\circ\else\textdegree\fi{}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\ifmmode\pm\else\textpm\fi{}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\ifmmode^\circ\else\textdegree\fi{}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 $\frac{{K}_{1}}{M}=\ensuremath{-}7965$ Oe. The in-plane anisotropy due to second-order dipolar interactions is estimated to be \ensuremath{\approx}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\ifmmode\pm\else\textpm\fi{}1)% smaller than the calculated average hyperfine field of 689\ifmmode\pm\else\textpm\fi{}7 Mc/sec. This indicates the presence of a zero-point reduction in the electron spin.

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