Antiferromagnets, magnetic materials with the internal magnetic moment offset to zero, exhibit fascinating physical properties and have high application potential. First, the spin precession frequency resonant at the terahertz band of antiferromagnets has a higher spin storage density than that of ferromagnets. Second, the stray field of antiferromagnets is almost zero, which provides a strong ability to resist external interference. Third, antiferromagnets are often accompanied by many complex electronic states that give rise to some novel effects. However, owing to the characteristics of weak or zero net magnetic moments, the detection and regulation of antiferromagnetic materials have been challenging for a long time.
In recent years, ultrathin and monolayer-exfoliated two-dimensional materials have provided new opportunities for research on antiferromagnets. The low-dimensional scale has improved the interaction among the lattice, electronic spin, and charge, which not only gives rise to complex and rich magnetic states but also paves the way for exploring low-dimensional magnetism and its applications by using interdisciplinary research fields such as optoelectronics. Research based on optical means has many advantages such as enabling microscopic-level, high-speed, noncontact, high-sensitivity analyses with a high space-time and high-energy resolution. In addition, it is conducive to the observation of various magnetic responses of antiferromagnets under extreme physical conditions. The developments of magneto-optical principles and optical detection technology have resulted in the use of various laser spectrum and polarization detection schemes to solve the challenges presented by antiferromagnetic material research. Therefore, it is necessary to summarize research advances on two-dimensional antiferromagnets in the existing magneto-optical field.
After a brief introduction in Section 1, the basic structure, properties, and classification of typical van der Waals antiferromagnets, including chromium trihalide and transition metal phosphorus sulfide (Fig. 2), are introduced in Section 2, along with some other common materials (Table 1). Starting from different magnetic coupling characteristics (interlayer or intralayer antiferromagnetic coupling), the magnetic ordering of these antiferromagnetic materials in combination with the molecular configuration and chemical composition are reviewed.
In Section 3, multiple magneto-optical effects are discussed for magnetic thin films (Fig. 3). In addition to the well-known Faraday effect, magneto-optical Kerr effect, Zeeman effect, magnetic dichroism, and Viogt and Cotton-Mouton effects, various specific magnetic changes in matters resulting from the interactions between light and matters are also included. For example, two-dimensional FePS3 exhibits giant linear dichroism because the destruction of the rotational symmetry by the antiferromagnetic order renders the electron energy band anisotropic (Fig. 4). The spin photovoltaic effect is demonstrated in multilayer CrI3, the interlayer magnetic order directly affects the magnitudes of the photocurrent and tunneling current in the direction vertical to the heterojunction. The circular polarization of the photocurrent also reveals a correlation between the magnetic state and the photon energy (Fig. 5). In terms of spectral detection, the second-harmonic emission and scattering spectra that are closely related to the magnetic order are also reviewed (Fig. 6). Subsequently, the ability of the magneto-optical effect to clearly reflect the regulation of two-dimensional antiferromagnets by means of magnetic field, electric field, temperature field, stress regulation (Fig. 7), passive regulation, and ultrafast lasers is discussed (Fig. 8).
In the spectral research presented in Section 4, as a link between macro- and micro-quantum phenomena, elementary excitation quasiparticles are shown to greatly facilitate related research on low-dimensional condensed matter physics. Under antiferromagnetic conditions, the disturbance of the magnetic order may affect the properties and mutual coupling of various quasiparticles in the material. The magnons produced by the collective excitation of the laser to the spintronic system facilitate the loading of electrical information, and their frequency determines the switching speed of the spintronic devices. For applications, the energy of the magnons can be adjusted using an all-optical method and an electrostatic doping method controlled by the gate voltage (Fig. 9). On the other hand, research on the transport of magnons in two-dimensional antiferromagnetic materials has also been conducted and these transport phenomena could potentially be studied by all-optical imaging. Research on the same Bosonic excitons in two-dimensional antiferromagnets is another important research topic. The Frenkel-like excitons in CrI3 have circular-polarized and polaronic characteristics (Fig. 10), while the Wannier-like excitons in NiPS3 prefer linear polarization and high coherence in the emission spectra (Fig. 11). For another low-energy excited Bosonic phonon, strong coupling magnon-phonon states are formed through the tuning effect of a large magnetic field (Fig. 12). The dispersive anti-cross feature of this polaron state is clearly reflected in the spectra, which can provide a new research platform for the magneto-optical control of antiferromagnetic materials.
Section 5 summarizes the prospects for further research and the application of two-dimensional antiferromagnets in magneto-optical and related fields.
Semiconductor microelectronics, which are based on the charge properties of electrons, have brought about revolutionary advancements to modern information technology. However, necessary improvements in the computing and information-processing capabilities of devices cannot only be achieved by manipulating and optimizing the charge properties of electrons. The manipulation of electronic spins, one of the most basic characteristics of magnetic materials, has enabled magnetic devices to become considerably successful, resulting in a series of cutting-edge applications with the advantages of nonvolatility and low-calorie requirements.
This paper reviews the progress in the field of magneto-optical research pertaining to antiferromagnets from the perspectives of optical characterization and regulation. After introduction of the basic principles, the main research objects of the interactions between laser and two-dimensional antiferromagnets are extended to the forms of the macroscopic magneto-optical effect and microscopic elementary excitation quasiparticle, respectively.
Compared with ferromagnets, the unique advantages of antiferromagnets in magneto-optical research are undoubtedly expected to accelerate research in spintronics-related fields. Research on antiferromagnets based on the magneto-optical effect is foreseen to focus on more complex and extreme systems in non-collinear, helical, topological, multiferroic, spin-liquid magnetic states, etc. The control of these magnetic states would enable interlayer electronic coupling to be effectively adjusted to facilitate the detection, manipulation, emission, and tracking of spin information through optical means. This approach could introduce huge optical nonlinearity, efficient spin filtering, high conversion efficiency between charge current and spin current, high electron mobility, long spin diffusion length, and other characteristics, and expand the research scope of magnetism, photonics, and other interdisciplinary disciplines. Although the ability to conduct antiferromagnetic writing and reading under two-dimensional conditions would also need to be fundamentally improved, future breakthroughs in two-dimensional antiferromagnetic opto-spintronics can certainly be anticipated.
