Pressure-Induced Phase Transition In The J(1)-J(2) Square Lattice Antiferromagnet Rbmoopo4cl

We report results of magnetization and P-31 NMR measurements under high pressure up to 6.4 GPa on RbMoOPO4Cl, which is a frustrated square-lattice antiferromagnet with competing nearest-neighbor and next-nearest-neighbor interactions. Anomalies in the pressure dependencies of the NMR shift and the transferred hyperfine coupling constants indicate a structural phase transition at 2.6 GPa, which is likely to break mirror symmetry and triggers significant change of the exchange interactions. In fact, the NMR spectra in magnetically ordered states reveal a change from the columnar antiferromagnetic (CAF) order below 3.3 GPa to the Neel antiferromagnetic (NAF) order above 3.9 GPa. The spin lattice relaxation rate 1/T-1 also indicates a change of dominant magnetic fluctuations from CAF-type to NAF-type with pressure. Although the NMR spectra in the intermediate pressure region between 3.3 and 3.9 GPa show coexistence of the CAF and NAF phases, a certain component of 1/T-1 shows paramagnetic behavior with persistent spin fluctuations, leaving a possibility for a quantum disordered phase. The easy-plane anisotropy of spin fluctuations with unusual nonmonotonic temperature dependence at ambient pressure gets reversed to the Ising anisotropy at high pressures. This unexpected anisotropic behavior for a spin 1/2 system may be ascribed to the strong spin-orbit coupling of Mo-4d electrons.