Antiferromagnetism of ${\mathrm{Cs}}_2$Mn${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O

Single-crystal magnetic susceptibilities of triclinic ${\mathrm{Cs}}_2$Mn${\mathrm{Cl}}_4$·2${\mathrm{H}}_2$O have been measured between 0.35 and 20°K. Over this interval the susceptibility exhibits a uniaxial anisotropy about a direction parallel to the O-Mn-O axis of each octahedral complex ${\left[\mathrm{M}\mathrm{n}{\mathrm{Cl}}_4\mathrm{·}2{\mathrm{H}}_2\mathrm{O}\right]}^{2-}$. The temperature variations of ${\chi }_{\mathrm{II}}$ and ${\chi }_{\perp }$ are those of a uniaxial antiferromagnet with $T_N=1.8\mathrm{}°$K and the preferred direction of spin alignment along or near the O-Mn-O axis. Quantitative estimates of the magnetic dipolar anisotropy indicate it to be an order of magnitude too small to account for the observations above $T_N$. This anisotropy can be described by a term $D{S_z}^2$ in the single-ion Hamiltonian with $D=-0.1\mathrm{}$ ${\mathrm{cm}}^{-1\mathrm{}}$. In the ordered state, this relatively large anisotropy produces a gap $k{T}_{\mathrm{AE}}$, in the spin-wave spectrum with an estimated $T_{\mathrm{AE}}\approx 1.6°$K. A gap of roughly this magnitude is suggested by the temperature variation of ${\chi }_{\mathrm{II}}$ below 1°K. The nearest-neighbor exchange coupling is estimated to be $zJ=-0.54\mathrm{}$k.