ac Magnetic Susceptibility and $H-T$ Phase Diagram of Mn${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O between 0.26 and 1.50°K in Fields up to 27 kOe

The field-induced magnetic phase transitions in Mn${\mathrm{Cl}}_2$·4${\mathrm{H}}_2$O were investigated by measuring the differential magnetic susceptibility between 0.26 and 1.5°K in external magnetic fields up to 27 kOe. With the external field along the most preferred axis of the crystal ($c^{\prime }$ direction), both the antiferromagnetic-to-spin-flop (AF-to-SP) and the spin-flop-to-paramagnetic (SF-to-P) transitions were observed below 1.3°K. The AF-to-SF transition appears to be a first-order transition, as predicted by molecular-field theory. The temperature dependence of the phase boundary is given by $H_c=H_{c0\mathrm{}}+bT$, where $H_{c0\mathrm{}}=7550\mathrm{}\pm 10$ Oe and $b=434\mathrm{}\pm 10$ Oe/deg. Within the spin-flop state ${\chi }_{c^{\prime }}$ is roughly constant and approximately equal to the value of ${\chi }_{\perp }$ in zero field. At the SF-to-P phase boundary, ${\chi }_{c^{\prime }}$ drops rapidly to zero. The temperature dependence of the SF-to-P transition is given by $H_c=H_{c0\mathrm{}}(1-C{T}^n)$ up to about 0.7°K, where ${\mathrm{H}}_{c0\mathrm{}}=20.6\mathrm{}\pm 0.03$ kOe, $n=1.82\mathrm{}\pm 0.10$, and $c=0.228\mathrm{}\pm 0.002$.