Interfacial Dzyaloshinskii-Moriya Interaction Research Articles

Reversal of domain wall chirality with ferromagnet thickness in W/(Co)FeB/MgO systems

We investigate the effect of the Dzyaloshinskii-Moriya interaction (DMI) on domain wall (DW) configurations in W/(Co)FeB/MgO structures with varying ferromagnet (FM) thicknesses. The DW chirality and effective DMI field are evaluated from field-induced DW motion. The results indicate a reversal of DW chirality with the FM thickness irrespective of the FM material (CoFeB or FeB) and the crystallographic phase of W (α or β phase). The observed change in the magnitude of the DMI field is supported by an additional measurement of spin-orbit torque assisted magnetization switching under in-plane magnetic fields. The present findings offer previously unknown insights into the origin of interfacial DMI and indicate the co-existence of multiple factors governing DW chirality in systems with broken inversion symmetry.

Dzyaloshinskii-Moriya interaction in symmetric epitaxial [Co/Pd(111)]N superlattices with different numbers of Co/Pd bilayers

Heavy metal/ferromagnetic superlattices are of great interest since N\'eel skyrmions may be stabilized in them due to an interfacial Dzyaloshinskii-Moriya interaction even at room temperature. Here we report on evidence of the strong Dzyaloshinskii-Moriya interaction in symmetric epitaxial ${[\mathrm{Co}/\mathrm{Pd}(111)]}_{N}$ superlattices with high perpendicular magnetic anisotropy. The structural and magnetic properties of epitaxial ${[\mathrm{Co}/\mathrm{Pd}(111)]}_{N}$ superlattices were investigated dependent on the number of Co/Pd bilayers. Based on comparison of the periodicity of experimentally obtained demagnetized patterns and simulated magnetic structures, a strong increase of the effective Dzyaloshinskii-Moriya interaction constant with increase of the period $N$ of ${[\mathrm{Co}/\mathrm{Pd}(111)]}_{N}$ superlattices is established. The origin of Dzyaloshinskii-Moriya interaction and its behavior in the symmetric ${[\mathrm{Co}/\mathrm{Pd}(111)]}_{N}$ system is discussed.

Field-free spin-orbit torque switching of a perpendicular ferromagnet with Dzyaloshinskii-Moriya interaction

Leveraging on interfacial Dzyaloshinskii-Moriya interaction (DMI) induced intrinsic magnetization tilting in nanostructures, a parametric window enabling field-free spin-orbit torque (SOT) magnetization switching in a perpendicular ferromagnet is established. The critical current density (Jc) bounds for SOT switching are highly dependent on the DMI, producing a distorted diamond-shaped region bounded by the Jc-DMI curves. The widest Jc interval is found for DMI values between 0.5 mJ/m2 and 0.8 mJ/m2. Geometrical modulation, of the ferromagnetic layer, reveals that the circular structure is optimum for minimizing the switching energy while maximizing the parametric window. For all the structures investigated, the SOT induced reversal process is via domain wall nucleation and propagation, and the switching is practical at room temperature.

Emergence of Topological Hall Effect in a SrRuO3 Single Layer.

Topological Hall effect (THE), appearing as bumps and/or dips in the Hall resistance curves, is considered as a hallmark of the skyrmion spin texture originated from the inversion symmetry breaking and spin-orbit interaction. Recently, Néel-type skyrmion is proposed based on the observed THE in 5d transition metal oxides heterostructures such as SrRuO3 /SrIrO3 bilayers, where the interfacial Dzyaloshinskii-Moriya interaction (DMI), due to the strong spin-orbit coupling (SOC) in SrIrO3 and the broken inversion symmetry at the interface, is believed to play a significant role. Here the emergence of THE in SrRuO3 single layers with thickness ranging from 3 to 6 nm is experimentally demonstrated. It is found that the oxygen octahedron rotation in SrRuO3 also has a significant effect on the observed THE. Furthermore, the THE may be continuously tuned by an applied electrical field. It is proposed that the large SOC of Ru ions together with the broken inversion symmetry, mainly from the interface, produce the DMI that is responsible for the observed THE. The emergence of the gate-tunable DMI in SrRuO3 single layer may stimulate further investigations of new spin-orbit physics in strong SOC oxides.

Direct observation of unusual interfacial Dzyaloshinskii-Moriya interaction in graphene/NiFe/Ta heterostructures

Graphene/ferromagnet interface promises a plethora of new science and technology. The interfacial Dzyaloshinskii Moriya interaction (iDMI) is essential for stabilizing chiral spin textures, which are important for future spintronic devices. Here, we report direct observation of iDMI in graphene/Ni80Fe20/Ta heterostructure from non-reciprocity in spin-wave dispersion using Brillouin light scattering (BLS) technique. Linear scaling of iDMI with the inverse of Ni80Fe20 thicknesses suggests primarily interfacial origin of iDMI. Both iDMI and spin-mixing conductance increase with the increase in defect density of graphene obtained by varying argon pressure during sputter deposition of Ni80Fe20. This suggests that the observed iDMI originates from defect-induced extrinsic spin-orbit coupling at the interface. The direct observation of iDMI at graphene/ferromagnet interface without perpendicular magnetic anisotropy opens new route in designing thin film heterostructures based on 2-D materials for controlling chiral spin structure such as skyrmions and bubbles, and magnetic domain-wall-based storage and memory devices.

Simultaneous Control of Interlayer Exchange Coupling and the Interfacial Dzyaloshinskii–Moriya Interaction in Ru-Based Synthetic Antiferromagnets

We present a Ru-based synthetic antiferromagnet (SAF) made with asymmetric ferromagnetic building blocks (Pt/Co/Ir or Ir/Co/Pt) to couple their interfacial Dzyaloshinskii-Moriya interaction (DMI) with strong interlayer exchange coupling (IEC). This combination may lead to new types of antiferromagnetic domain walls (DWs), which are conceptually presented in this letter. These include an energetically frustrated configuration and an “isolated” spin chain where a net magnetization exists only at the DW. While most SAF designs incorporate a Co/Ru/Co IEC layer, chiral SAFs require Pt and/or Ir at the Ru/Co interface. In all cases except Pt/Ru/Pt, a significant IEC field is preserved over a range of Ru thicknesses, with the largest occurring for Ir/Ru/Ir. These results demonstrate that ferromagnetic film stacks with purportedly strong DMI can be incorporated into SAFs without compromising IEC.

Magnetic Radial Vortex Core and Coupled Edge-Soliton Pair Gyration Dynamics

We studied the gyration dynamics of a magnetic radial vortex core by injecting spin-polarized current into a nanodisk through a nanocontact. The simulation results indicate that the radial vortex core can rotate in an approximately circular orbit with a stable frequency. As the current density increases, the radius and frequency of rotation increase, while the rotation stabilization time decreases. Simultaneously, the intensity of the interfacial Dzyaloshinskii–Moriya interaction has a great influence on the frequency and the maximum drive current density of the gyration. The Oersted field generated by the current can reduce the stabilization time. The radial vortex core transforms into an edge soliton pair and rotates stably along the boundary of the disk for a certain range of current densities. This work provides deeper insight into the gyration dynamics of magnetic radial vortices and the potential for the development of nano-oscillators based on radial vortices.

Electric Field-Induced Creation and Directional Motion of Domain Walls and Skyrmion Bubbles.

Magnetization dynamics driven by an electric field could provide long-term benefits to information technologies because of its ultralow power consumption. Meanwhile, the Dzyaloshinskii-Moriya interaction in interfacially asymmetric multilayers consisting of ferromagnetic and heavy-metal layers can stabilize topological spin textures, such as chiral domain walls, skyrmions, and skyrmion bubbles. These topological spin textures can be controlled by an electric field and hold promise for building advanced spintronic devices. Here, we present an experimental and numerical study on the electric field-induced creation and directional motion of topological spin textures in magnetic multilayer films and racetracks with thickness gradient and interfacial Dzyaloshinskii-Moriya interaction at room temperature. We find that the electric field-induced directional motion of chiral domain wall is accompanied by the creation of skyrmion bubbles at certain conditions. We also demonstrate that the electric field variation can induce motion of skyrmion bubbles. Our findings may provide opportunities for developing skyrmion-based devices with ultralow power consumption.

Electric field control of magnetic domain wall motion via modulation of the Dzyaloshinskii-Moriya interaction.

We show that the electric field (EF) can control the domain wall (DW) velocity in a Pt/Co/Pd asymmetric structure. With the application of a gate voltage, a substantial change in DW velocity up to 50 m/s is observed, which is much greater than that observed in previous studies. Moreover, modulation of a DW velocity exceeding 100 m/s is demonstrated in this study. An EF-induced change in the interfacial Dzyaloshinskii-Moriya interaction (DMI) up to several percent is found to be the origin of the velocity modulation. The DMI-mediated velocity change shown here is a fundamentally different mechanism from that caused by EF-induced anisotropy modulation. Our results will pave the way for the electrical manipulation of spin structures and dynamics via DMI control, which can enhance the performance of spintronic devices.

Bidirectional propagation of tilting domain walls in perpendicularly magnetized T shaped structure with the interfacial Dzyaloshinskii-Moriya interaction

Understanding of domain wall (DW) propagation in a complex structure is an essential first step toward the development of any magnetic-domain based devices including spin-based logic or magnetic memristors. Interfacial Dzyaloshinskii-Moriya interaction (iDMI) in the structure with broken inversion symmetry induces an asymmetrical DW configuration with respect to the direction of in-plane field. Dynamic behaviors of field-driven DW within the film with perpendicular magnetic anisotropy is influenced by DW tilt from the iDMI effect and the corners in the T-shaped structure of the DW path. Images from Kerr microscopy reveal that the iDMI effective field contributes to a tilted structure of DW configuration and evolution along its propagation. With the combination of iDMI and T-shaped structure, we observed two distinguished bidirectional DW propagations in two output branches and distinct arriving times at the destination pads with a uniform external field. Micromagnetic simulation results is compared with the observed dynamics of a DW configuration in the structure providing an additional confirmation of the interpreted results.

Pt concentration dependence of the interfacial Dzyaloshinskii–Moriya interaction, the Gilbert damping parameter and the magnetic anisotropy in Py/Cu1−xPtx systems

Physical effects generated by spin–orbit coupling (SOC) are extremely appealing to numerous applications related to magnetic information processing and to the rapidly developing field of spin-orbitronics. In this paper we have investigated major tendencies in the behavior of three of them, namely the Gilbert damping constant, the magnetic anisotropy and the interfacial Dzyaloshinskii–Moriya interaction (iDMI), as a function of Pt concentration in Py (5 nm)/Cu1−xPtx bilayers, using a combined experimental approach based on ferromagnetic resonance and Brillouin light scattering techniques. The Gilbert damping constant and the magnetic anisotropy demonstrate similar simultaneous growth with Pt concentration. The iDMI presents a very weak value until the Pt concentration is about x = 45% and then increases rapidly until full Pt concentration. Magnetic anisotropy and the damping factor are found to be linearly correlated. On the other hand, above x = 45%, the iDMI increases with respect to the damping factor, thus with magnetic anisotropy. These behaviors show that these physical phenomena are correlated and suggest that they are closely associated with SOC. Moreover, this work opens up the possibility of independent tuning of different SOC-related parameters to tailor-made values optimal for a specific application.

Observation of room-temperature magnetic skyrmions in Pt/Co/W structures with a large spin-orbit coupling

Magnetic skyrmions have significant potential for applications in storage and logic devices, but the ability to control skyrmion motion is key to their success. To realize controlled skyrmion motion, vertical spin current-driven methods employing, e.g., the spin Hall or inverse spin galvanic effect, are efficient; thus, magnetic heterostructures featuring large spin-orbit torques are appealing. In this paper, we report on the observation of room-temperature magnetic skyrmions in Pt/Co/W multilayers. The interfacial Dzyaloshinskii-Moriya interaction was estimated to be $0.19\ifmmode\pm\else\textpm\fi{}0.05\phantom{\rule{0.16em}{0ex}}\mathrm{mJ}/{\mathrm{m}}^{2}$ based on the asymmetric domain-wall motion occurring upon the application of in-plane magnetic fields. The evolution of the magnetic structures from labyrinth domains to skyrmions with diameters of around 145 nm under magnetic fields was observed by performing Lorentz transmission electron microscopy. The skyrmion nucleation fields could be tuned by varying the repetition number. Large spin Hall angle systems such as Pt/Co/W multilayers are appealing for achieving current-driven skyrmion motion in future racetrack and logic applications.

Propagation and interaction of magnetic solitons in a ferromagnetic thin film with the interfacial Dzyaloshinskii-Moriya interaction

Abstract We study the propagation and interaction of magnetic solitons in a ferromagnetic thin film with the interfacial Dzyaloshinskii-Moriya interaction. The governing Landau-Lifshitz-Gilbert equation can be transformed into an integrable nonlinear Shrodinger-type equation in the long wave limit. With the use of the Hirota bilinear method, the analytical expressions of one-, two-, and three-soliton solutions are constructed. We find that the Dzyaloshinskii-Moriya interaction can change the velocities of magnetic solitons and affect the colliding position of the two solitons. In particular, a reverse effect of the propagation direction of the magnetic soliton is observed. Our results have potential applications in storage and communication technologies.

Disentangling factors governing Dzyaloshinskii domain-wall creep in Co/Ni thin films using PtxIr1−x seed layers

We characterize asymmetric growth of magnetic bubble domains in perpendicularly magnetized Co/Ni multi-layers grown on Pt$_x$Ir$_{1-x}$ seedlayers by application of perpendicular and in-plane magnetic fields. Using a refined model of domain wall creep that incorporates contributions from the anisotropic elastic energy, $\varepsilon$, and a chirality-dependent prefactor, $v_0$, we elucidate factors that govern the mobility of Dzyaloshinskii domain walls as a function of seedlayer composition. The interfacial Dzyaloshinskii-Moriya Interaction magnitude is found to decrease monotonically with $x_{Ir}$, which is independently confirmed by Brillouin light scattering (BLS). Moreover, the persistence of significant asymmetry in velocity curves across the full composition range supports previous assertions that a chirality-dependent attempt frequency akin to chiral damping could play a critical role in the observed trends. This work helps resolve fundamental questions about the factors governing Dzyaloshinskii DW creep and demonstrates varying Pt-Ir seedlayer composition as a method to tune DMI.

Magnetic coupling of ferromagnetic SrRuO3 epitaxial layers separated by ultrathin non-magnetic SrZrO3/SrIrO3

Ferromagnetic multilayers with asymmetric interfaces and perpendicular magnetic anisotropy can stabilize non-trivial magnetic structures due to interfacial Dzyaloshinskii-Moriya interactions. Magnetic interlayer coupling between ferromagnetic layers separated by non-magnetic insulating spacers is an additional important ingredient for the properties of multilayers. We addressed the magnetic coupling between ferromagnetic SrRuO3 epitaxial layers separated by ultrathin spacers of SrIrO3/SrZrO3, so that inversion symmetry is broken at the top and bottom interfaces. Major and minor magnetization loops allowed us to assess the type and strength of the magnetic coupling. We inferred that the magnetic coupling of SrRuO3 layers through non-magnetic insulating interlayers (≤1.6 nm thick) is weakly ferromagnetic.

Breathers and rogue waves in a ferromagnetic thin film with the Dzyaloshinskii-Moriya interaction

We report a theoretical research on nonlinear localized excitations in a ferromagnetic thin film with an interfacial Dzyaloshinskii-Moriya interaction. Employing the Generalized Darboux transformation, analytical forms for magnetic breathers and the first- to third-order rogue wave solutions are exactly constructed. It is found that the introduction of the Dzyaloshinskii-Moriya interaction may lead to a reverse effect of the propagation direction of the magnetic breather, and varying the value of the Dzyaloshinskii-Moriya interaction parameter can change the velocity of the magnetic breather. Furthermore, our results display that first- to third-order magnetic rogue waves will rotate clockwise as the intensity of the Dzyaloshinskii-Moriya interaction increases.

Voltage-controlled magnetic anisotropy and voltage-induced Dzyaloshinskii-Moriya interaction change at the epitaxial Fe(001)/MgO(001) interface engineered by Co and Pd atomic-layer insertion

We fabricated epitaxial multilayers with Co and Pd atomic-layer insertion at Fe(001)/MgO(001) interface and systematically investigated voltage-controlled magnetic anisotropy (VCMA) and voltage-induced interfacial Dzyaloshinskii-Moriya interaction $(i\mathrm{DMI})$ change. The VCMA and $i\mathrm{DMI}$ change was characterized by spin-wave spectroscopy. We found that the VCMA was increased by Co and Pd insertions to \ensuremath{-}180 fJ/Vm in the Fe/Co/Pd/MgO system. We also found a large voltage-induced $i\mathrm{DMI}$ change of 65 fJ/Vm. The postannealing dependence of the VCMA and $i\mathrm{DMI}$ change was also characterized to investigate the influence of the change in chemical ordering around the MgO interface.

Readable racetrack memory via ferromagnetically coupled chiral domain walls

Current-induced motion of domain walls (CIMDWs) with the interfacial Dzyaloshinskii-Moriya interaction (DMI) in heavy metal (HM)/ferromagnetic (FM) metal multilayers has attracted attention owing to their potential applications in novel magnetic memories. In recent years, the CIMDW at ultrahigh speed has been observed in a synthetic antiferromagnetic (SAF) multilayer with a high storage density because of the weak stray field. However, due to the zero net magnetization, the reading of information from the SAF multilayer is still challenging. In this work, we propose a readable racetrack memory consisting of a synthetic ferromagnetic multilayer composed of two FM layers with an interlayer FM coupling. One FM layer had an isotropic DMI, while the other had an anisotropic DMI. This difference of DMIs resulted in the opposite tilting directions of the DW planes in the two layers. This tilting was inhibited by a strong interlayer FM coupling, resulting in an increase in the DW velocity and the reduction of the minimum allowed spacing between two adjacent DWs. In addition, the FM coupling enhanced the stray field, and the stored information could be read conveniently using a conventional reading head. Therefore, our proposal paves the way for the fabrication of a racetrack memory with high reading speed, large storage density, and good readability.

Determination of the Dzyaloshinskii-Moriya interaction in exchange biased Au/Co/NiO systems

The determination of the interfacial Dzyaloshinskii-Moriya interaction for perpendicularly magnetized thin layered films is not trivial and therefore needs some experimental efforts to determine its sign and magnitude, especially in the exchange biased system. Here, we developed a method proposed by D.-S. Han et al. [Nano Lett. 16, (2016) 4438], which opens a way to investigate this interaction using a conventional PMOKE magnetometer, also in exchange biased systems. Using our approach we demonstrated that the Au/Co/NiO layered system has a strong negative Dzyaloshinskii-Moriya interaction, which is independent of the direction of perpendicular interlayer exchange bias coupling.

Stabilization and Reversal of Skyrmion Lattice in Ta/CoFeB/MgO Multilayers.

Recently, magnetic skyrmion has attracted much attention due to its potential application in racetrack memory and other nanodevices. In bulk chiral magnets with non-centrosymmetric crystal structures, skyrmion lattice phase has been extensively observed. However, in film or multilayers with interfacial Dzyaloshinskii-Moriya interaction, individual skyrmion is often observed. Here, we report a short-ordered skyrmion lattice observed in [Ta(5.0 nm)/CoFeB(1.5 nm)/MgO(1.0 nm)]15 multilayer in a remnant state. The structure, stabilization, and reversal of these skyrmions are discussed. Applying a slightly tilted in-plane magnetic field caused reversal of the skyrmion lattice. This reversal came from disappearance of skyrmions and nucleation of new skyrmions in the interstitial regions of the lattice. Also, we investigated how the skyrmion lattice depended on the CoFeB thickness. Our findings provide a pathway to stabilize and reverse the skyrmions in multilayers films.