High-field Magnetic Torque Measurements in (CH$_3$)$_2$CHNH$_3$CuCl$_3$

TL;DR

This study uses high-field magnetic torque and magnetostriction measurements to explore the magnetic phase transitions, Bose-Einstein condensation of magnons, and spin-lattice correlations in (CH$_3$)$_2$CHNH$_3$CuCl$_3$, revealing symmetry breaking effects.

Contribution

It provides detailed experimental evidence of magnetic quantum phase transitions and magnon BEC behavior, highlighting the effects of rotational symmetry breaking in this spin gap system.

Findings

Observation of a sharp increase in magnetic torque at the critical field.

Detection of cusp-like minimum indicating magnon BEC for H perpendicular to C-plane.

Identification of additional anomalies related to spin-flop and spin-reorientation transitions.

Abstract

High-field magnetic torque measurements were carried out in the spin gap system (CH$_3$)$_2$CHNH$_3$CuCl$_3$. 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 the magnetic quantum phase transition from the spin-gap phase to the field-induced magnetic ordered phase. From the temperature dependence of the magnetic torque $\tau$, the cusp-like minimum indicative of the Bose-Einstein condensation (BEC) of magnons was observed for $H \perp$ C-plane, while the kink-anomaly was observed for $H \perp$ A-plane at the field-induced transition temperature. It was found that this different behavior between $H \perp$ A-plane and $H \perp$ C-plane can be interpreted as the breaking of the magnon BEC picture due to the rotational symmetry breaking. The 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 the spin-flop transition, the spin-reorientation transition and the spin-lattice correlations. In order to investigate the spin-lattice correlations in (CH$_3$)$_2$CHNH$_3$CuCl$_3$, we performed the magnetostriction measurements and observed that the magnetostriction appears by vanishing of the spin gap.