The leading order nonlinear (NL) susceptibility, χ₃, in a paramagnet is negative and diverges as T → 0. This divergence is destroyed when spins correlate and the NL response provides unique insights into magnetic order. Dimensionality, exchange interaction, and preponderance of quantum effects all imprint their signatures in the NL magnetic response. Here, we study the NL susceptibilities in the proximate Kitaev magnet α-RuCl₃, which differs from the expected antiferromagnetic behavior. For T < T_c = 7.5 K and field B in the ab-plane, we obtain contrasting NL responses in low (<2 T) and high field regions. For low fields, the NL behavior is dominated by a quadratic response (positive χ₂), which shows a rapid rise below T_c. This large χ₂ > 0 implies a broken sublattice symmetry of magnetic order at low temperatures. Classical Monte Carlo (CMC) simulations in the standard K − H − Γ model secure such a quadratic B dependence of M, only for T ≈ T_c with χ₂ being zero as T → 0. It is also zero for all temperatures in exact diagonalization calculations. On the other hand, we find an exclusive cubic term (χ₃) that describes the high field NL behavior well. χ₃ is large and positive both below and above T_c crossing zero only for T > 50 K. In contrast, for B ∥ c-axis, no separate low/high field behaviors are measured and only a much smaller χ₃ is apparent.

顺磁体中的主阶非线性 (NL) 磁化率χ ₃为负，并且随着T  → 0 发散。当自旋相关时，这种发散被破坏并且 NL 响应提供了对磁序的独特见解。维度、交换相互作用和量子效应的优势都在 NL 磁响应中留下了它们的特征。在这里，我们研究了邻近的 Kitaev 磁体α -RuCl ₃的 NL 磁化率，这与预期的反铁磁行为不同。对于Ť  <  Ť _Ç  = 7.5 K和字段乙在AB-平面，我们在低 (<2 T) 和高场区域获得对比 NL 响应。对于低场，NL 行为由二次响应（正χ ₂）支配，显示出低于T _c的快速上升。这个大的χ ₂  > 0 意味着在低温下磁序的亚晶格对称性被破坏。标准K  −  H  − Γ 模型中的经典蒙特卡罗 (CMC) 模拟确保M 的这种二次B依赖关系，仅对于T  ≈  T _c且χ ₂为零作为T → 0. 在精确对角化计算中，所有温度也为零。在另一方面，我们发现一个独特的三次项（χ ₃）描述了高场NL行为良好。χ ₃大且正，低于和高于T _c仅对于T  > 50 K过零。相比之下，对于B  ∥  c轴，没有测量到单独的低/高场行为，只有小得多的χ ₃是明显的。