Noncollinear spin textures in low-dimensional magnetic systems have been studied for decades because of their extraordinary properties and promising applications derived from the chirality and topological nature. However, material realizations of topological spin states are still limited. Employing first-principles and Monte Carlo simulations, we propose that monolayer chromium trichloride (CrCl3) can be a promising candidate for observing the vortex/antivortex type of topological defects, so-called merons. The numbers of vortices and antivortices are found to be the same, maintaining an overall integer topological unit. By perturbing with external magnetic fields, we show the robustness of these meron pairs and reveal a rich phase space to tune the hybridization between the ferromagnetic order and meron-like defects. The signatures of topological excitations under external magnetic field also provide crucial information for experimental justifications. Our study predicts that two-dimensional magnets with weak spin-orbit coupling can be a promising family for realizing meron-like spin textures.

Multiferroic materials with coupled ferroelectric and ferromagnetic properties are important for multifunctional devices due to their potential ability of controlling magnetism via electric field, and vice versa. The recent discoveries of twodimensional ferromagnetic and ferroelectric materials have ignited tremendous research interest and aroused hope to search for two-dimensional multiferroics.However, intrinsic two-dimensional multiferroic materials and, particularly, those with strong magnetoelectric couplings are still rare to date. In this paper, using firstprinciples simulations, we propose artificial two-dimensional multiferroics via a van der Waals heterostructure formed by ferromagnetic bilayer chromium triiodide (CrI3) and ferroelectric monolayer Sc2CO2. In addition to the coexistence of ferromagnetism and ferroelectricity, our calculations show that, by switching the electric polarization of Sc2CO2, we can tune the interlayer magnetic couplings of bilayer CrI3 between ferromagnetic and antiferromagnetic states. We further reveal that such a strong magnetoelectric effect is from a dramatic change of the band alignment induced by the strong build-in electric polarization in Sc2CO2 and the subsequent change of the interlayer magnetic coupling of bilayer CrI3. These artificial multiferroics and enhanced magnetoelectric effect give rise to realizing multifunctional nanoelectronics by van der Waals heterostructures.

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Magnetic vortices are characterized by the senses of in-plane magnetization chirality and by the polarity of the vortex core. The electrical control of vortex polarity and chirality is highly demanded not only for fundamental understanding on spin dynamics in nano-disks under different circumstances, but also for technological applications, such as magnetic non-volatile memories and spin torque oscillators for neuromorphic computing. Here we report a novel approach that enables one to electrically control both the vortex chirality and polarity with low energy consumption. Thorough micromagnetic simulations, we show that in thin nano-disks of diameter larger than 160 nm, with the presence of current-induced Oersted field, the dynamic transformation of the edge solitons is able to efficiently switch both vortex chirality and polarity with low current under certain circumstances. We then developed an approach to directly write any of the four vortex states by electrical current pulses from a random state. We further investigated the switching phase diagram as a function of disk diameters. The results show that the switching process is highly size-dependent. As disk diameter is smaller than 160 nm, the switch of VC chirality and polarity always takes place at the same time, resulting in an unchanged handedness before and after switch. Furthermore, the critical switch current can be as low as 6 2 3 10 / cm A × , indicating a possible way for low current switch of vortex chirality in small disks.defines the handedness of the vortex with 1 CP = and 1 CP = − being catalyzed as left and right handed vortices, respectively. Being one of the most interesting magnetic soliton, vortex can be used as an information carrier and is currently attracting much more attention for a number of applications. Magnetic vortex have been proposed as memory bit in non-volatile storage for many years [3,4], for its multi-bit information storage and high stability [5,6]. Very recently, vortex was introduced as a building block for a robust sensor application in the automotive industry [7], where a large linear range is discovered. In addition, vortex-based spin torque nano-oscillators have been demonstrated being building blocks for neuromorphic computing, which is one of research topics towards low-power artificial intelligence application [ 8 , 9 ]. A full understanding on the electrical control of vortex dynamics and the switching process is a key towards those applications. In the past decades, great efforts have been made for searching effective methods to control the vortex polarity and chirality.The VC is very stable, and a static field of above 0.5 T field is required to switch its polarity, while to switch vortex chirality requires even more energy [5,6]. Fortunately, further studied showed that it can be efficiently switched through a dynamic process [10].When excited, the VC will be driven into gyrotropic precession, and polarity reversal happens when the VC reaches a certain critical velocity [ 11,12,13], through the formation and...

It is of fundamental importance but challenging to simultaneously identify atomic and magnetic configurations of two-dimensional van der Waals materials. In this work, we show that the nonreciprocal second-harmonic generation (SHG) can be a powerful tool to answer this challenge. Despite the preserved lattice inversion symmetry, the interlayer antiferromagnetic order and spin-orbit coupling generate enhanced SHG in PT-symmetric bilayer chromium triiodide (CrI3). Importantly, the in-plane polarization-resolved SHG is sensitive to subtly different interlayer structures that cannot be told by linear optical spectra. Beyond bilayer, we further predict that the intensity and angleresolved SHG can be employed to identify both interlayer atomic and magnetic configurations of trilayer CrI3. Our first-principles results agree with available measurements and show the potential of SHG as a non-contacting approach to explore correlations between interlayer structures and magnetic orders of emerging ultra-thin magnetic materials.