Spirals and Skyrmions in Antiferromagnetic Triangular Lattices

We study realizations of spirals and skyrmions in two-dimensional antiferromagnets with a triangular lattice on an inversion-symmetry-breaking substrate. As a possible material realization, we investigate the adsorption of transition-metal atoms (Cr, Mn, Fe, or Co) on a monolayer of MoS2, WS2, or WSe2 and obtain the exchange, anisotropy, and Dzyaloshinskii-Moriya interaction parameters using first-principles calculations. Using energy minimization and parallel-tempering Monte Carlo simulations, we determine the magnetic phase diagrams for a wide range of interaction parameters. We find that skyrmion lattices can appear even with weak Dzyaloshinskii-Moriya interactions, but their stability is hindered by magnetic anisotropy. However, a weak easy plane magnetic anisotropy can be beneficial for stabilizing the skyrmion phase. Our results suggest that Cr/MoS2, Fe/MoS2, and Fe/WSe2 interfaces can host spin spirals formed from the 120∘ antiferromagnetic states. Our results further suggest that for interfaces, such as Fe/MoS2, the Dzyaloshinskii-Moriya interaction is strong enough to drive the system into a three-sublattice skyrmion lattice in the presence of experimentally feasible external magnetic field

Spirals and skyrmions in antiferromagnetic triangular lattices

We study realizations of spirals and skyrmions in two-dimensional antiferromagnets with a triangular lattice on an inversion-symmetry-breaking substrate. As a possible material realization, we investigate the adsorption of transition-metal atoms (Cr, Mn, Fe, or Co) on a monolayer of MoS$_2$, WS$_2$, or WSe$_2$ and obtain the exchange, anisotropy, and Dzyaloshinskii-Moriya interaction parameters using first-principles calculations. Using energy minimization and parallel-tempering Monte-Carlo simulations, we determine the magnetic phase diagrams for a wide range of interaction parameters. We find that skyrmion lattices can appear even with weak Dzyaloshinskii-Moriya interactions, but their stability is hindered by magnetic anisotropy. However, a weak easy plane magnetic anisotropy can be beneficial for stabilizing the skyrmion phase. Our results suggest that Cr$/$MoS$_2$, Fe$/$MoS$_2$, and Fe$/$WSe$_2$ interfaces can host spin spirals formed from the 120$^{\circ}$ antiferromagnetic states. Our results further suggests that for other interfaces, such as Fe$/$MoS$_2$, the Dzyaloshinskii-Moriya interaction is strong enough to drive the system into a three-sublattice skyrmion lattice in the presence of experimentally feasible external magnetic field.Comment: 11 pages, 10 figure

Breakdown of the Drift-Diffusion Model for Transverse Spin Transport in a Disordered Pt Film

Spin-accumulation and spin-current profiles are calculated for a disordered Pt film subjected to an in-plane electric current within the nonequilibrium Green\u27s function approach. In the bulklike region of the sample, this approach captures the intrinsic spin Hall effect found in other calculations. Near the surfaces, the results reveal qualitative differences with the results of the widely used spin-diffusion model, even when the boundary conditions are modified to try to account for them. One difference is that the effective spin-diffusion length for transverse spin transport is significantly different from its longitudinal counterpart and is instead similar to the mean-free path. This feature may be generic for spin currents generated via the intrinsic spin Hall mechanism because of the differences in transport mechanisms compared to longitudinal spin transport. Orbital accumulation in the Pt film is only significant in the immediate vicinity of the surfaces and has a small component penetrating into the bulk only in the presence of spin-orbit coupling, as a secondary effect induced by the spin accumulation

First-Principles Calculations of Voltage-Controlled Magnetic Anisotropy and Spin-Orbit Torque in Magnetic Materials

Voltage-controlled magnetic anisotropy and spin-orbit torques are promising and efficient methods of switching magnetization in spintronic devices. In this dissertation, first-principles methods are used to study them in real material systems. First, the magnetic anisotropy in MgO-capped antiferromagnetic MnPt films and its voltage control are studied using first-principles calculations. The results show that the magnetic anisotropy can be widely tuned by adjusting the film thickness, particularly in Pt-terminated films. The linear voltage control coefficients are found to be large, indicating the potential of MgO-capped MnPt films as a versatile platform for magnetic memory and antiferromagnonic applications. Next, spin-orbit torques in a Mn2Au/heavy-metal bilayer are studied using the non-equilibrium Green\u27s function (NEGF) technique. Spin-orbit coupling in the bulk of Mn2Au generates a strong fieldlike torquance, which is parallel on the two sublattices and scales linearly with the conductivity, and a weaker dampinglike torquance that is antiparallel on the two sublattices. Interfaces with heavy metal generate parallel dampinglike torques of opposite signs that are similar in magnitude to those in ferromagnetic bilayers and similarly insensitive to disorder. The dampinglike torque efficiency depends strongly on the termination of the interface and on the presence of spin-orbit coupling in Mn2Au, suggesting that the dampinglike torque is not due solely to the spin-Hall effect in the heavy metal layer. Interfaces also induce antiparallel fieldlike and dampinglike torques that can penetrate deep into Mn2Au. These results can help in the design and optimization of antiferromagnetic spintronic devices. Last, spin-orbit torques and their disorder dependence in an L11-ordered CuPt/CoPt bilayer are studied using the NEGF technique. The C3v symmetry at the interface gives rise to 3m torques which can enable field-free switching of perpendicular magnetization. It is found that 3m fieldlike torque accounts for 20% of damplinglike torque. Our results can be further used as input to study the magnetization dynamics using micromagnetics and provide insights into the mechanisms for spin-orbit torques in this system

Voltage-controlled magnetic anisotropy in antiferromagnetic MgO-capped MnPt films

The magnetic anisotropy in MgO-capped MnPt films and its voltage control are studied using first-principles calculations. Sharp variation of the magnetic anisotropy with film thickness, especially in the Pt-terminated film, suggests that it may be widely tuned by adjusting the film thickness. In thick films the linear voltage control coefficient is as large as 1.5 and -0.6 pJ/Vm for Pt-terminated and Mn-terminated interfaces, respectively. The combination of a widely tunable magnetic anisotropy energy and a large voltage-control coefficient suggest that MgO-capped MnPt films can serve as a versatile platform for magnetic memory and antiferromagnonic applications

Spirals and skyrmions in antiferromagnetic triangular lattices

We study realizations of spirals and skyrmions in two-dimensional antiferromagnets with a triangular lattice on an inversion-symmetry-breaking substrate. As a possible material realization, we investigate the adsorption of transition-metal atoms (Cr, Mn, Fe, or Co) on a monolayer of MoS2, WS2, or WSe2 and obtain the exchange, anisotropy, and Dzyaloshinskii-Moriya interaction parameters using first-principles calculations. Using energy minimization and parallel-tempering Monte Carlo simulations, we determine the magnetic phase diagrams for a wide range of interaction parameters. We find that skyrmion lattices can appear even with weak Dzyaloshinskii-Moriya interactions, but their stability is hindered by magnetic anisotropy. However, a weak easy plane magnetic anisotropy can be beneficial for stabilizing the skyrmion phase. Our results suggest that Cr/MoS2, Fe/MoS2, and Fe/WSe2 interfaces can host spin spirals formed from the 120∘ antiferromagnetic states. Our results further suggest that for interfaces, such as Fe/MoS2, the Dzyaloshinskii-Moriya interaction is strong enough to drive the system into a three-sublattice skyrmion lattice in the presence of experimentally feasible external magnetic field

Crystal Structure and Dzyaloshinski–Moriya Micromagnetics

The relationship between atomic-scale and micromagnetic Dzyaloshinski–Moriya (DM) interactions has been investigated. By analyzing the Lifshitz invariants for different point groups, we have found that there is no unique link between the absence of inversion symmetry and DM interactions. The absence of inversion symmetry is a necessary condition for a net DM interaction in crystals, but several noncentrosymmetric point groups have zero DM interactions. In many cases, the key consideration is whether the crystals are polar and/or chiral. For example, MnSi-type spin spirals, which violate helical spin symmetry, are caused by the insertion of chiral atomic-scale building blocks into an achiral cubic lattice, and the scalar interaction parameter D used to describe the spirals is only loosely related to the DM vector D. It contains, in fact, magnetostatic and magnetocrystalline contributions of unknown magnitude. Finally, we discuss some aspects of the micromagnetism of the skyrmionics of nanoparticles and granular nanostructures

Crystal Structure and Dzyaloshinski–Moriya Micromagnetics

The relationship between atomic-scale and micromagnetic Dzyaloshinski–Moriya (DM) interactions has been investigated. By analyzing the Lifshitz invariants for different point groups, we have found that there is no unique link between the absence of inversion symmetry and DM interactions. The absence of inversion symmetry is a necessary condition for a net DM interaction in crystals, but several noncentrosymmetric point groups have zero DM interactions. In many cases, the key consideration is whether the crystals are polar and/or chiral. For example, MnSi-type spin spirals, which violate helical spin symmetry, are caused by the insertion of chiral atomic-scale building blocks into an achiral cubic lattice, and the scalar interaction parameter D used to describe the spirals is only loosely related to the DM vector D. It contains, in fact, magnetostatic and magnetocrystalline contributions of unknown magnitude. Finally, we discuss some aspects of the micromagnetism of the skyrmionics of nanoparticles and granular nanostructures