Trompe L'oeil Ferromagnetism

Ferromagnetism can be characterized by 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 of ferromagnetic point groups, relevant for ferromagnets, ferri-magnets and weak ferromagnets, can certainly exhibit all of those phenomena, including non-zero magnetization. Some of true antiferromagnets, which are defined as magnets whose MPGs do not belong to ferromagnetic point groups, can show those phenomena through magnetization induced by external perturbations such as applied current, electric fields, light illumination, and strain. Such MPGs are identified for each external perturbation. A number of exemplary materials for specific ferromagnetic-like phenomena will be discussed. Since high-density and ultrafast spintronic technologies can be enabled by antiferromagnets, our findings will be an essential guidance for the future magnetism-related science as well as technology.