Magnetic phase transitions in anisotropic Heisenberg antiferromagnets. I. MnCl2.4H2O

Differential magnetic susceptibility measurements have been used to study the magnetic phase transitions in antiferromagnetic Mn${\mathrm{Cl}}_2$.4${\mathrm{H}}_2$O between 0.3 and 1.6 K. The antiferromagnetic to spin-flop (SF) transition is shown to be a first-order phase transition below 0.4 K. The SF to paramagnetic transition is observed to have a predominantly $T^{\frac{5}{2}}$ temperature dependence, while spin-wave theory predicts a $T^{\frac{3}{2}}$ dependence. From the values of the critical fields, extrapolated to $T=0\mathrm{}$, the exchange and anisotropy constants are determined. The results are consistent with a Heisenberg model of isotropic exchange with a biaxial single-ion-type anisotropy.