Bulk Dzyaloshinskii-Moriya Interaction Research Articles

Nucleation and Stability of Toron Chains in Non-Centrosymmetric Magnetic Nanowires.

This work analyzes the magnetic configurations of cylindrical nanowires with a bulk Dzyaloshinskii-Moriya interaction and easy-plane anisotropy. We show that this system allows the nucleation of a metastable toron chain even when no out-of-plane anisotropy exists in the nanowire's top and bottom surfaces, as usually required. The number of nucleated torons depends on the nanowire length and the strength of an external magnetic field applied to the system. The size of each toron depends on the fundamental magnetic interactions and can be controlled by external stimuli, allowing the use of these magnetic textures as information carriers or nano-oscillator elements. Our results evidence that the topology and structure of the torons yield a wide variety of behaviors, revealing the complex nature of these topological textures, which should present an exciting interaction dynamic, depending on the initial conditions.

Compositional gradient induced enhancement of Dzyaloshinskii–Moriya interaction in Pt/Co/Ta heterostructures modulated by Pt–Co alloy intralayers

The Dzyaloshinskii–Moriya interaction (DMI) is an antisymmetric exchange in magnetic structures with broken inversion symmetry. In magnetic heterostructures, the bulk DMI may coexist with the conventional interfacial DMI contributing to the chiral magnetic behavior of the system. Here, we report the enhancement of DMI in Pt/Co heterostructures wherein Pt-Co alloy intralayers with compositional gradients are sandwiched between Pt and Co. Due to the composition gradient-induced bulk magnetic asymmetry and the face-centered cubic (fcc) (111) multilayer crystal structure, the DMI shows a complex behavior attributed to both the interfacial (i-DMI) and bulk-like (b-DMI) contributions. While the b-DMI is defined by the presence of composition differences and structure symmetry, the i-DMI hosts the interfacial contributions in the multilayer structure. Our approach allows the creation of graded magnetic thin films, where the structural inversion symmetry is broken solely due to the predefined compositional gradient of an intralayer, thereby giving rise to an ‘all-interface-bulk’ DMI with the controllable spatial distribution.

High-temperature non-centrosymmetric magnets for skyrmionics

Such topological spin textures as magnetic skyrmions and antiskyrmions have attracted significant interest in recent years owing to their rich variety of underlying physics and potential applications in next-generation magnetic devices. In the domain of applications, it is essential to stabilize the topological spin textures over a wide range of temperatures, including room temperature, and manipulate them with various external stimuli. Significant developments have been made in room-temperature skyrmions and antiskyrmions arising from the Dzyaloshinskii–Moriya interaction (DMI) in several magnetic materials with broken inversion symmetry. In this Perspective, we review recent progress in non-centrosymmetric magnets with bulk DMI, which host skyrmions and antiskyrmions above room temperature. We first provide an overview of room-temperature Bloch-type skyrmions and the robustness of their metastability, the variety of their forms, and their dynamics in Co–Zn–Mn alloys with a β-Mn-type chiral structure. We then focus on room-temperature antiskyrmions as well as their topological transformations in Heusler alloys with D2d symmetry and Pd-doped (Fe,Ni)3P with S4 symmetry. The robust skyrmions and antiskyrmions, with versatile tunability in these non-centrosymmetric materials at room temperature, represent a step toward the long-sought milestone of “skyrmionics.”

Field-driven side-by-side magnetic domain wall dynamics in ferromagnetic nanostrips

There has been a plethora of studies on domain wall dynamics in magnetic nanostrips, mainly because of its versatile non-linear physics and potential applications in data storage devices. However, most of the studies focus on out-of-plane domain walls or in-plane head-to-head (tail-to-tail) domain walls. Here, we numerically study the field-driven dynamics of in-plane side-by-side domain walls in ferromagnetic strips, which can be stable in the presence of an in-plane easy-axis anisotropy transverse to the strip. The domain walls move in a rigid-body manner at low field, and show complex Walker breakdown behavior at high field. We observe a multi-step Walker breakdown through vortex nucleation in wide strips. In the presence of Dzyaloshinskii-Moriya interaction (DMI), the first Walker breakdown field first decreases then increases with interfacial DMI, while keeps increasing with bulk DMI. These findings complement the current understanding on domain wall dynamics.

Current-driven spiral domain wall in a ferrimagnet near the magnetization compensation point

A spiral domain wall emerges when the Dzyaloshinskii-Moriya vector is along the easy axis. While the ferromagnetic spiral wall has been well studied, its ferrimagnetic counterpart has nevertheless not been explored. Here, using the collective variable approach, we derive the spiral domain-wall solutions in a ferrimagnetic nanostrip with a longitudinal easy axis and a bulk Dzyaloshinskii-Moriya interaction. At the magnetization compensation point, the wall rotates and propagates steadily in a wide current range. The (angular) velocity of this wall increases almost linearly with the experimentally feasible currents. Also, the wall exhibits a relativistic-like contraction with increasing velocity. Near this point, the wall moves smoothly with its velocity linearly depending on the current below the Walker breakdown. Above this critical current, all the internal parameters of the wall, such as the azimuthal angle, width, and spiral pitch, as well as their rates of change, oscillate periodically. The wall moves forward with its velocity varying periodically. These different behaviors at or near the compensation point are ascribed to the tunable demagnetization effect. The adjustability of FiM parameters, combining with the twist induced by the Dzyaloshinskii-Moriya interaction, can effectively change the DW's propagation and rotation.

Micromagnetics of thin films in the presence of Dzyaloshinskii–Moriya interaction

In this paper, we study the thin-film limit of the micromagnetic energy functional in the presence of bulk Dzyaloshinskii–Moriya interaction (DMI). Our analysis includes both a stationary [Formula: see text]-convergence result for the micromagnetic energy, as well as the identification of the asymptotic behavior of the associated Landau–Lifshitz–Gilbert equation. In particular, we prove that, in the limiting model, part of the DMI term behaves like the projection of the magnetic moment onto the normal to the film, contributing this way to an increase in the shape anisotropy arising from the magnetostatic self-energy. Finally, we discuss a convergent finite element approach for the approximation of the time-dependent case and use it to numerically compare the original three-dimensional (3D) model with the 2D thin-film limit.

Enhancement of the Walker limit by bulk Dzyaloshinskii-Moriya interaction

In this work, dynamics of chiral domain walls in long and narrow magnetic heterostructures based on non-centrosymmetric chiral magnets with bulk Dzyaloshinskii-Moriya interactions (DMI) and perpendicular magnetic anisotropy is systematically investigated. The driving forces can be out-of-plane magnetic fields and in-plane currents, correspondingly both steady and precessional flows are considered. Their dividing points (the Walker critical field and current density) are obtained as functions of bulk DMI strength ($D_{\mathrm{b}}$) and the ratio ($\kappa$) of total (crystalline plus shape) anisotropy in the hard axis over that in the easy one. When far beyond Walker breakdown, the dependence curve of wall velocity on external in-plane bias field takes parabolic shape around the compensation point where the total in-plane field disappears. The center shift is determined by $D_{\mathrm{b}}$, $\kappa$, and the wall's topological charge, thus can be used to measure the bulk DMI strength in chiral magnets.

Longitudinal spin Seebeck effect in pyrochlore iridates with bulk and interfacial Dzyaloshinskii-Moriya interaction

The longitudinal spin-Seebeck effect (SSE) in magnetic insulator$|$non-magnetic metal heterostructures has been theoretically studied primarily with the assumption of an isotropic interfacial exchange coupling. Here, we present a general theory of the SSE in the case of an antisymmetric Dzyaloshinskii-Moriya interaction (DMI) at the interface, in addition to the usual Heisenberg form. We numerically evaluate the dependence of the spin current on the temperature and bulk DMI using a pyrochlore iridate as a model insulator with all-in all-out (AIAO) ground state configuration. We also compare the results of different crystalline surfaces arising from different crystalline orientations and conclude that the relative angles between the interfacial moments and Dzyaloshinskii-Moriya vectors play a significant role in the spin transfer. Our work extends the theory of the SSE by including the anisotropic nature of the interfacial Dzyaloshinskii-Moriya exchange interaction in magnetic insulator$|$non-magnetic metal heterostructures and can suggest possible materials to optimize the interfacial spin transfer in spintronic devices.

Rotating magnetic field driven antiferromagnetic domain wall motion: Role of Dzyaloshinskii-Moriya interaction

In this work, we study the rotating magnetic field driven domain wall (DW) motion in antiferromagnetic nanowires, using the micromagnetic simulations of the classical Heisenberg spin model. We show that in low frequency region, the rotating field alone could efficiently drive the DW motion even in the absence of Dzyaloshinskii-Moriya interaction (DMI). In this case, the DW rotates synchronously with the magnetic field, and a stable precession torque is available and drives the DW motion with a steady velocity. In large frequency region, the DW only oscillates around its equilibrium position and cannot propagate. The dependences of the velocity and critical frequency differentiating the two motion modes on several parameters are investigated in details, and the direction of the DW motion can be controlled by modulating the initial phase of the field. Interestingly, a unidirectional DW motion is predicted attributing to the bulk DMI, and the nonzero velocity for high frequency is well explained. Thus, this work does provide useful information for future antiferromagnetic spintronics applications.

Bulk Dzyaloshinskii-Moriya interaction in amorphous ferrimagnetic alloys.

Symmetry breaking is a fundamental concept that prevails in many branches of physics1-5. In magnetic materials, broken inversion symmetry induces the Dzyaloshinskii-Moriya interaction (DMI), which results in fascinating physical behaviours6-14 with the potential for application in future spintronic devices15-17. Here, we report the observation of a bulk DMI in GdFeCo amorphous ferrimagnets. The DMI is found to increase linearly with an increasing thickness of the ferrimagnetic layer, which is a clear signature of the bulk nature of DMI. We also found that the DMI is independent of the interface between the heavy metal and ferrimagnetic layer. This bulk DMI is attributed to an asymmetric distribution of the elemental content in the GdFeCo layer, with spatial inversion symmetry broken throughout the layer. We expect that our experimental identification of a bulk DMI will open up additional possibilities to exploit this interaction in a wide range of materials.

Topological Hall effect at above room temperature in heterostructures composed of a magnetic insulator and a heavy metal

Non-volatile memory and computing technology rely on efficient read and write of ultra-tiny information carriers that do not wear out. Magnetic skyrmions are emerging as a potential carrier since they are topologically robust nanoscale spin textures that can be manipulated with ultralow current density. To date, most of skyrmions are reported in metallic films, which suffer from additional Ohmic loss and thus high energy dissipation. Therefore, skyrmions in magnetic insulators are of technological importance for low-power information processing applications due to their low damping and the absence of Ohmic loss. Moreover, they attract fundamental interest in studying various magnon-skyrmion interactions11. Skyrmions have been observed in one insulating material Cu2OSeO3 at cryogenic temperatures, where they are stabilized by bulk Dzyaloshinskii-Moriya interaction. Here, we report the observation of magnetic skyrmions that survive above room temperature in magnetic insulator/heavy metal heterostructures, i.e., thulium iron garnet/platinum. The presence of these skyrmions results from the Dzyaloshinskii-Moriya interaction at the interface and is identified by the emergent topological Hall effect. Through tuning the magnetic anisotropy via varying temperature, we observe skyrmions in a large window of external magnetic field and enhanced stability of skyrmions in the easy-plane anisotropy regime. Our results will help create a new platform for insulating skyrmion-based room temperature low dissipation spintronic applications.

An achiral ferromagnetic/chiral antiferromagnetic bilayer system leading to controllable size and density of skyrmions

Magnetic skyrmions are topologically protected domain structures related to the Dzyaloshinskii-Moriya interaction (DMI). To understand how magnetic skyrmions occur under different circumstances, we propose a model for skyrmion formation in a bilayer system of ferromagnetic/antiferromagnetic (FM/AFM) films, in which the bulk DMI is only present in the AFM film. Micromagnetic simulations reveal that skyrmions are formed in this system due to the competition between the DMI and demagnetization energies. A critical interfacial exchange energy (Ai = 6.5 mJ/m2) is determined, above which the competition occurs at its full extent. More skyrmions are formed with increasing external magnetic field till a critical value above which the external field is too large and thus leading to the annihilation of skyrmions. The spacing between two skyrmions can be as small as 45 nm. Our results may give technological implications for future skyrmion applications.

Asymmetric skyrmion Hall effect in systems with a hybrid Dzyaloshinskii-Moriya interaction

We examine the current-induced dynamics of a skyrmion that is subject to both structural and bulk inversion asymmetry. There arises a hybrid type of Dzyaloshinskii-Moriya interaction (DMI) which is in the form of a mixture of interfacial and bulk DMIs. Examples include crystals with symmetry classes C$_n$ as well as magnetic multilayers composed of a ferromagnet with a noncentrosymmetric crystal and a nonmagnet with strong spin-orbit coupling. As a striking result, we find that, in systems with a hybrid DMI, the spin-orbit-torque-induced skyrmion Hall angle is asymmetric for the two different skyrmion polarities ($\pm 1$ given by out-of-plane core magnetization), even allowing one of them to be tuned to zero. We propose several experimental ways to achieve the necessary straight skyrmion motion (with zero Hall angle) for racetrack memories, even without antiferromagnetic interactions or any interaction with another magnet. Our results can be understood within a simple picture by using a global spin rotation which maps the hybrid DMI model to an effective model containing purely interfacial DMI. The formalism directly reveals the effective spin torque and effective current that result in qualitatively different dynamics. Our work provides a way to utilize symmetry breaking to eliminate detrimental phenomena as hybrid DMI eliminates the skyrmion Hall angle.

Spin-orbit-torque-induced skyrmion dynamics for different types of spin-orbit coupling

We investigate current-induced skyrmion dynamics in the presence of Dzyaloshinskii-Moriya interaction and spin-orbit spin-transfer torque corresponding to various types of spin-orbit coupling. We determine the symmetries of Dzyaloshinskii-Moriya interaction and spin-orbit spin-transfer torque based on linear spin-orbit coupling model. We find that like interfacial Dzyaloshinskii-Moriya interaction (Rashba spin-orbit coupling) and bulk Dzyaloshinskii-Moriya interaction (Weyl spin-orbit coupling), Dresselhaus spin-orbit coupling also has a possibility for stabilizing skyrmion and current-induced skyrmion dynamics.

Field-driven Domain Wall Motion in Ferromagnetic Nanowires with Bulk Dzyaloshinskii-Moriya Interaction.

Field-driven domain wall (DW) motion in ferromagnetic nanowires with easy- and hard-axis anisotropies was studied theoretically and numerically in the presence of the bulk Dzyaloshinskii-Moriya interaction (DMI) based on the Landau-Lifshitz-Gilbert equation. We propose a new trial function and offer an exact solution for DW motion along a uniaxial nanowire driven by an external magnetic field. A new strategy was suggested to speed up DW motion in a uniaxial magnetic nanowire with large DMI parameters. In the presence of hard-axis anisotropy, we find that the breakdown field and velocity of DW motion was strongly affected by the strength and sign of the DMI parameter under external fields. This work may be useful for future magnetic information storage devices based on DW motion.