Field-induced magnetic ordering inNiCl2⋅4SC(NH2)2

Abstract

We have investigated the low-temperature T field-induced ordering in ${\mathrm{NiCl}}_{2}\ensuremath{\cdot}4\mathrm{SC}({\mathrm{NH}}_{2}{)}_{2},$ from magnetization measurements. This is an $S=1$ system with a large single-ion uniaxial anisotropy leaving a singlet ground state. An external magnetic field $(\mathbf{B})$ parallel to the symmetry axis induces an antiferromagnetically ordered phase for ${B}_{c1}<B<{B}_{c2}.$ The establishment of this long-range order can be regarded as a Bose-Einstein condensation of magnons. The phase boundaries are predicted to obey power laws $[{B}_{c}(T)\ensuremath{-}{B}_{c}(0)]\ensuremath{\propto}{T}^{\ensuremath{\alpha}}.$ The determined phase boundaries could be fitted to power laws yielding ${\ensuremath{\alpha}}_{1}=2.63\ifmmode\pm\else\textpm\fi{}0.10$ for ${B}_{c1}(T)$ and ${\ensuremath{\alpha}}_{2}=2.55\ifmmode\pm\else\textpm\fi{}0.10$ for ${B}_{c2}(T).$