The coupling between spin and charge degrees of freedom in a crystal gives rise to magneto-optical effects with applications in the sensitive detection of local magnetic order, optical modulation and data storage. In two-dimensional magnets these effects manifest themselves in the large magneto-optical Kerr effect^1,2, spontaneous helical light emission^3,4 from ferromagnetic (FM) monolayers and electric-field induced Kerr rotation^5,6,7 and giant second-order non-reciprocal optical effects⁸ in antiferromagnetic (AFM) bilayers. Here we demonstrate the tuning of inelastically scattered light through symmetry control in atomically thin chromium triiodide (CrI₃). In monolayers, we found an extraordinarily large magneto-optical Raman effect from an A_1g phonon mode due to the emergence of FM order. The linearly polarized, inelastically scattered light rotates by ~40°, more than two orders of magnitude larger than the rotation from the magneto-optical Kerr effect under the same experimental conditions. In CrI₃ bilayers, the same phonon mode becomes Davydov-split into two modes of opposite parity, which exhibit divergent selection rules that depend on inversion symmetry and the underlying magnetic order. We demonstrate the magneto-electrical control over these selection rules by activating or suppressing Raman activity for the odd-parity phonon mode and the magneto-optical rotation of scattered light from the even-parity phonon mode. Our work underlines the unique opportunities provided by two-dimensional magnets to control the combined time-reversal and inversion symmetries to manipulate Raman optical selection rules and for exploring emergent magneto-optical effects and spin–phonon coupled physics.

晶体中自旋和电荷自由度之间的耦合会产生磁光效应，并应用于局部磁序的灵敏检测、光调制和数据存储。在二维磁体中，这些效应表现在大磁光克尔效应^1,2、来自铁磁 (FM) 单层的自发螺旋光发射^3,4和电场诱导的克尔旋转^5,6,7和巨大的第二-反铁磁 (AFM) 双层中的非互易光学效应⁸。在这里，我们展示了通过原子级薄三碘化铬（CrI ₃）。在单层中，由于 FM 阶的出现，我们发现 A _1g声子模式产生了非常大的磁光拉曼效应。线偏振的非弹性散射光旋转约 40°，比相同实验条件下磁光克尔效应的旋转大两个数量级以上。在铬₃在双层结构中，相同的声子模式变为 Davydov 分裂成两种相反宇称的模式，它们表现出不同的选择规则，这些规则取决于反转对称性和潜在的磁序。我们通过激活或抑制奇宇称声子模式的拉曼活动和偶宇称声子模式的散射光的磁光旋转来证明对这些选择规则的磁电控制。我们的工作强调了二维磁体提供的独特机会，可以控制组合的时间反转和反转对称性以操纵拉曼光学选择规则，并探索涌现的磁光效应和自旋声子耦合物理。