High-field magnetic torque measurement in the spin gap system (CH3)2CHNH3CuCl3

High-field magnetic torque measurement was carried out in the spin gap system (CH3)(2)CHNH3CUCl3. It was observed that the magnitude of the magnetic torque tau is almost zero until the critical field and then increases rapidly, which indicates the existence of magnetic quantum phase transition from the spin-gap phase to the field-induced magnetic ordered phase. From the temperature dependence of T, a cusplike minimum indicative of the Bose-Einstein condensation (BEC) of magnons was observed for H perpendicular to C-plane, while a kink anomaly was observed for H perpendicular to A-plane at the field-induced transition temperature. It was found that this difference in behavior between H perpendicular to A-plane and H perpendicular to C-plane can be interpreted as the breaking of the magnon BEC picture due to a rotational symmetry breaking. An additional anomaly was also observed in the field-induced magnetic ordered phase at the intermediate angle of the applied magnetic field in the A-plane and discussed in terms of spin-flop transition, spin-reorientation transition, and spin-lattice correlations. In order to investigate spin-lattice correlations in (CH3)(2)CHNH3CUCl3, we performed magnetostriction measurement and observed that magnetostriction appears with the disappearance of the spin gap.