We report on a comprehensive thermodynamic study of a quasi-two-dimensional (quasi-2D) quantum magnet ${\mathrm{Cu}}_2{\left(\mathrm{OH}\right)}_3\mathrm{Br}$ which in the 2D layer can be viewed as strongly coupled alternating antiferromagnetic and ferromagnetic chains. In an applied magnetic field transverse to the ordered spins below $T_N=9.3$ K, a field-induced phase transition from the 3D ordered to a disordered phase occurs at $B_c=16.3$ T for the lowest temperature, which is featured by an onset of a one-half plateaulike magnetization. By performing quantum Monte Carlo simulations of the relevant 2D model, we find that the plateaulike magnetization corresponds to a partial symmetry restoration and the full polarization in the ferromagnetic chains. Our numerical simulations also show that the magnetization saturation occurs with full symmetry restoration at a much higher field of $B_s\simeq 95$ T, corresponding to a 1D quantum phase transition in the antiferromagnetic chains. We argue that the experimentally observed field-induced phase transition at $B_c$ follows from the partial symmetry restoration and the concomitant dimensional reduction.

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• Magnetism
• Magnetocaloric effect
• Magnetostriction
• Quantum phase transitions
• Specific heat
• Thermal expansion
• Triangular lattice
• Magnetization measurements
• Quantum Monte Carlo
• Ultrasound techniques