Abstract
Macroscopic responses of magnets are often governed by magnetization and, thus, have been restricted to ferromagnets. However, such responses are strikingly large in the newly developed topological magnets, breaking the conventional scaling with magnetization. Taking the recently discovered antiferromagnetic (AF) Weyl semimetals as a prime example, we highlight the two central ingredients driving the significant macroscopic responses: the Berry curvature enhanced because of nontrivial band topology in momentum space, and the cluster magnetic multipoles in real space. The combination of large Berry curvature and multipoles enables large macroscopic responses such as the anomalous Hall and Nernst effects, the magneto-optical effect, and the novel magnetic spin Hall effect in antiferromagnets with negligible net magnetization, but also allows us to manipulate these effects by electrical means. Furthermore, nodal-point and nodal-line semimetallic states in ferromagnets may provide the strongly enhanced Berry curvature near the Fermi energy, leading to large responses beyond the conventional magnetization scaling. These significant properties and functions of the topological magnets lay the foundation for future technological development such as spintronics and thermoelectric technology.
Highlights
The concept of topology in momentum-space electronic structure has reshaped our understanding of materials properties, driving the discovery of various classes of systems such as topological insulators and nodal-point and nodal-line semimetals [1,2,3, 5, 6]
It is one of the Kagome metals that has attracted significant attention [9, 12, 25,26,27, 99] and harbors one type of the chiral magnetic structures, which are known to provide a variety of interesting phenomena [17]. It exhibits anomalous Hall effect (AHE) at room temperature, and it can be switched by a weak magnetic field [9]. This discovery indicates that the AHE arises not from the internal magnetic field as in ferromagnets but from the large fictitious field or Berry curvature, which is driven by nontrivial topology of the electronic band structure of the material (Figure 5b), which is discussed in terms of the Weyl fermions in Section 3.3 [10]
Since the discovery of electromagnetism, external magnetic fields produced by ferromagnets as well as electric currents have been essential for realizing motors and power generation
Summary
The concept of topology in momentum-space electronic structure has reshaped our understanding of materials properties, driving the discovery of various classes of systems such as topological insulators and nodal-point and nodal-line semimetals [1,2,3, 5, 6]. We review the recent highlights in the study of topological antiferromagnets and the associated developments in the AF spintronics, which provides the basis for developing nonvolatile memory that operates at a much faster speed and at much lower power consumption Another surprising property of topological magnets is the giant anomalous Nernst effect (ANE) [11, 35,36,37]. We note these topological magnets can be made of only naturally abundant elements, for example, iron and manganese These render a magnet very useful for developing energy-harvesting technology to power Internet-of-things (IoT) sensors and wearable devices [37, 39]. We will review the wide range of exciting phenomena found in the bulk and interface of topological magnets that have outstanding potential for real-life applications, and will focus on the magnets with transition temperature higher than room temperature
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