Bose-Einstein Condensation of Ni spin degrees of freedom observed from susceptibility measurements at high magnetic field in NiCl$_{2}$-4SC(NH$_2$)$_2$

The organic compound NiCl$_{2}$-4SC(NH$_2$)$_2$ is a new candidate for Bose-Einstein condensation (BEC) of spins system. Involving a technique based on a self-resonant LC-oscillator, we measured the change in magnetic susceptibility of NiCl$_{2}$- 4SC(NH$_2$)$_2$ in magnetic field up to 18 Tesla, and temperatures down to 60 mK. We have found the existence of two phase boundaries. One appears at H$_{c1}\sim$2 T, where the gap between the $S_{z}$= 0 ground state and the $S_{z}$=-1 excited state is overcome by the Zeeman effect and the spin condensation starts. The other boundary is at H$_{c2}\sim$12 T, where the system saturates. Our results on temperature and angular dependence of critical fields will be discussed in connection with previous experimental reports and theoretical predictions for BEC.