Ferromagnetism can be characterized by various distinct phenomena such as non-zero magnetization (inducing magnetic attraction/repulsion), diagonal piezomagnetism, nonreciprocal circular dichroism (such as Faraday effect), odd-order (including linear) anomalous Hall effect, and magneto-optical Kerr effect. We identify all broken symmetries requiring each of the above phenomena, and also the relevant magnetic point groups (MPGs) with those broken symmetries. All ferromagnetic point groups, relevant for ferromagnets, ferrimagnets, and weak ferromagnets, can certainly exhibit all these phenomena, including non-zero magnetization. Some of the true antiferromagnets, which are defined as magnets with MPGs that do not belong to ferromagnetic point groups, can display these phenomena through magnetization induced by external perturbations such as applied current, light illumination, and uniaxial stress, which preserve the combined symmetry of spatial inversion together with time reversal. Such MPGs are identified for each external perturbation. Since high-density and ultrafast spintronic technologies can be enabled by antiferromagnets, our findings will be essential guidance for future magnetism-related science as well as technology.

铁磁性可以通过各种不同的现象来表征，例如非零磁化（引起磁吸引/排斥）、对角压磁、非互易圆二色性（例如法拉第效应）、奇数阶（包括线性）反常霍尔效应和磁光效应克尔效应。我们识别出需要上述每种现象的所有对称破缺，以及具有这些破缺对称性的相关磁点组（MPG）。与铁磁体、亚铁磁体和弱铁磁体相关的所有铁磁点群当然可以表现出所有这些现象，包括非零磁化强度。一些真正的反铁磁体，被定义为具有不属于铁磁点组的 MPG 的磁体，可以通过外部扰动（例如施加电流、光照和单轴应力）引起的磁化来显示这些现象，从而保留了组合对称性空间反转和时间反转。此类 MPG 是针对每个外部扰动进行识别的。由于反铁磁体可以实现高密度和超快自旋电子技术，因此我们的研究结果将为未来与磁相关的科学和技术提供重要指导。