High-field magnetic torque measurement in the spin gap system (CH ₃)₂CHNH₃CuCl₃

High-field magnetic torque measurement was carried out in the spin gap system (CH₃)₂CHNH₃CuCl₃. It was observed that the magnitude of the magnetic torque τ 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 τ, a cusplike minimum indicative of the Bose-Einstein condensation (BEC) of magnons was observed for H ⊥ C-plane, while a kink anomaly was observed for H ⊥ A-plane at the field-induced transition temperature. It was found that this difference in behavior between H ⊥ A-plane and H ⊥ 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)₂CHNH ₃CuCl₃, we performed magnetostriction measurement and observed that magnetostriction appears with the disappearance of the spin gap.