We report results of magnetization and ${}^{31}\mathrm{P}$ NMR measurements under high pressure up to 6.4 GPa on ${\mathrm{RbMoOPO}}_4\mathrm{Cl}$, 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 Néel 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.

中文翻译：

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J1-J2方格反铁磁体RbMoOPO4Cl中的压力诱导相变

我们报告磁化强度和 ${}^{31}\mathrm{P}$ 高达6.4 GPa的高压下的NMR测量 ${\mathrm{RbMoOPO}}_4氯$，这是一种受挫的方格反铁磁体，具有最近邻和次近邻竞争的竞争。NMR位移和转移的超精细偶合常数的压力相关性异常表明在2.6 GPa处存在结构相变，这很可能破坏镜像对称性并触发交换相互作用的显着变化。实际上，处于磁有序状态的NMR光谱揭示了从低于3.3 GPa的柱状反铁磁（CAF）阶到高于3.9 GPa的Néel反铁磁（NAF）阶的变化。自旋晶格弛豫率$\mathrm{1个}/{Ť}_{\mathrm{1个}}$还表明随着压力从CAF型到NAF型的主要磁涨落变化。尽管在3.3和3.9 GPa之间的中压区域的NMR谱显示出CAF和NAF相共存，但其中的某些成分$\mathrm{1个}/{Ť}_{\mathrm{1个}}$显示出具有持续自旋波动的顺磁行为，从而可能存在量子无序相。在环境压力下具有非寻常的非单调温度依赖性的自旋涨落的易平面各向异性被逆转为高压下的伊辛各向异性。自旋1/2系统的这种意外的各向异性行为可能归因于Mo-$4d$ 电子。