AI 中文总结
本研究针对异磁铁、非共线反铁磁体等无净磁化磁体，建立基于序参量η的朗道理论，提出用阶数为(n-1)的磁化率作为实验可观测量，统一了不同磁体的磁有序选择描述并验证于两类代表性材料。
AI 中文摘要
传统的磁有序磁场选择依赖于塞曼耦合，然而，这种耦合在没有净磁化的磁体中会消失，这类磁体正迅速增多，包括异磁铁（AMs）、非共线反铁磁体（nc-AFMs）以及PT对称反铁磁体（PT-AFMs）。本文表明，磁体与均匀磁场根本耦合的物理量并非磁化强度，而是标记朗道自由能两个时间反转相关极小值的二元序参量η。我们基于η在磁点群（MPG）对称性约束下建立了序选择的朗道理论，其中η与磁场的奇次多项式耦合。在此框架内，线性项为铁磁塞曼耦合，而阶数n=3、5、7、9的高阶耦合自然出现在AMs和nc-AFMs中；相比之下，联合PT对称性会禁止任何此类耦合。因此，用于识别AMs和nc-AFMs磁有序的主要实验可观测量是阶数为(n-1)的磁化率，而非净磁化强度。我们对全部122种MPG分类了主导耦合阶数及对应的多项式形式，并在两种代表性材料——异磁铁MnF₂和非共线反铁磁体MnTe₂中验证了该框架。我们进一步为异磁铁体系构建了对称性允许的自旋模型，以揭示高阶耦合的微观起源，并基于自旋模型参数明确给出耦合系数。本工作统一了有净磁化和无净磁化磁体的磁有序选择描述，为这种反直觉物理提供了微观起源，并为区分本征磁场选择与外禀切换提供了指纹。
英文摘要
Conventional field selection of magnetic order relies on the Zeeman coupling, which, however, vanishes in magnets without net magnetization, a rapidly growing class including altermagnets (AMs), noncollinear antiferromagnets (nc-AFMs), and PT-symmetric antiferromagnets (PT-AFMs). Here we show that the quantity that fundamentally couples a magnet to a uniform magnetic field is not the magnetization, but the binary order parameter eta that labels the two time-reversal-related minima of the Landau free energy. We develop a Landau theory of order selection based on eta under the constraints of magnetic point-group (MPG) symmetry, in which eta couples to odd-degree polynomials in the magnetic field. Within this framework, the linear term is the ferromagnetic Zeeman coupling, while higher-order couplings with leading degree n = 3, 5, 7, and 9 naturally appear in AMs and nc-AFMs. In contrast, combined PT symmetry forbids any such coupling. Consequently, it is the order-(n-1) magnetic susceptibility, rather than the net magnetization, that serves as the primary experimental observable for identifying the magnetic order of AMs and nc-AFMs. For all 122 MPGs, we classify the leading coupling degree and the corresponding polynomial forms. We demonstrate our framework in two representative materials: the AM MnF2 and the nc-AFM MnTe2. We further construct a symmetry-allowed spin model for an AM system to reveal the microscopic origin of the higher-order coupling and establish the coupling coefficient explicitly in terms of the spin-model parameters. Our work unifies the description of magnetic-order selection across magnets with and without net magnetization, offers a microscopic origin for this counterintuitive physics, and provides fingerprints for distinguishing intrinsic field selection from extrinsic switching.
