Antiferromagnetic Skyrmion Research Articles

Electron transport through antiferromagnetic spin textures and skyrmions in a magnetic tunnel junction

An ideal layered $\hat{x}$-polarized antiferromagnet (AFM) between two antialigned $\pm \hat{z}$ polarized ferromagnetic (FM) contacts transmits no current due to a $\pi$ phase difference of the matrix elements coupling the spin degenerate states to the two FM contacts. The ratio of the two matrix elements coupling the two spin degenerate $\hat{x}$ AFM states to a $\hat{z}$ FM contact is determined by the exchange energy and the eigenstate energy. Inserting a normal metal layer or tunnel barrier layer between one FM contact and the AFM alters this phase difference, and, due to the unequal weighting of the two spins at the interface, it also breaks the spin degeneracy of the two AFM states. The broken symmetry of the matrix elements combined with the broken degeneracy of the AFM states, result in a Fano resonance in the transmission and a turn-on of the $T_{\uparrow,\downarrow}$ transmission channel. Such a magnetic tunnel junction geometry with two antialigned $\pm \hat{z}$ FM contacts can electrically detect an AFM skyrmion. The AFM skyrmion serves as an analogue of the oblique polarizer in the triple polarizer experiment. Resistances and resistance ratios are calculated and compared for FM and AFM skyrmions in a magnetic tunnel junction.

Accurate manipulation of single skyrmion by probe ring

Magnetic skyrmions, a new type of information carriers, are extensively investigated for their potential applications in next-generation data storage and computing. In this work, we propose a mechanical probe ring to implement the accurate manipulation of a single skyrmion by a voltage-controlled magnetic anisotropy effect, which is analogous to the manipulation of atoms using a scanning tunneling microscope. Combined with the observation process, we can use the probe to move skyrmions accurately in real time. We investigate the effects of various factors on the performance by micromagnetic simulations in order to give a guide for the design and application of this probe. We prove that this method can manipulate not only ferromagnetic skyrmions but also antiferromagnetic skyrmions, which is significant for the study of physical properties and electronic applications of those particle-like spin textures.

Spin textures in chiral magnetic monolayers with suppressed nearest-neighbor exchange

High tunability of two dimensional magnetic materials (by strain, gating, heterostructuring or otherwise) provides unique conditions for studying versatile magnetic properties and controlling emergent magnetic phases. Expanding the scope of achievable magnetic phenomena in such materials is important for both fundamental and technological advances. Here we perform atomistic spin-dynamics simulations to explore the (chiral) magnetic phases of atomic monolayers in the limit of suppressed first-neighbors exchange interaction. We report the rich phase diagram of exotic magnetic configurations, obtained for both square and honeycomb lattice symmetries, comprising coexistence of ferromagnetic and antiferromagnetic spin-cycloids, as well as multiple types of magnetic skyrmions. We perform a minimum-energy path analysis for the skyrmion collapse to evaluate the stability of such topological objects, and reveal that magnetic monolayers could be good candidates to host the antiferromagnetic skyrmions that are experimentally evasive to date.

Traveling skyrmions in chiral antiferromagnets

Skyrmions in antiferromagnetic (AFM) materials with the Dzyaloshinskii-Moriya (DM) interaction are expected to exist for essentially the same reasons as in DM ferromagnets (FM). It is shown that skyrmions in antiferromagnets with the DM interaction can be traveling as solitary waves with velocities up to a maximum value that depends on the DM parameter. Their configuration is found numerically. The energy and the linear momentum of an AFM skyrmion lead to a proper definition of its mass. We give the details of the energy-momentum dispersion of traveling skyrmions and explore their particle-like character based on exact relations. The skyrmion number, known to be linked to the dynamics of topological solitons in FM, is, here, unrelated to the dynamical behavior. As a result, the solitonic behavior of skyrmions in AFM is in stark contrast to the dynamical behavior of their FM counterparts.

Enhanced Stability of Antiferromagnetic Skyrmion during Its Motion by Anisotropic Dzyaloshinskii–Moriya Interaction

Searching for new methods to enhance the stability of antiferromagnetic (AFM) skyrmion during its motion is an important issue for AFM spintronic devices. Herein, the spin‐polarized current‐induced dynamics of a distorted AFM skyrmion is numerically studied, based on the Landau–Lifshitz–Gilbert simulations of the model with an anisotropic Dzyaloshinskii–Moriya (DM) interaction. It is demonstrated that the DM interaction anisotropy induces the skyrmion deformation, which suppresses the distortion during the motion and enhances the stability of the skyrmion. Moreover, the effect of the DM interaction anisotropy on the skyrmion velocity is investigated in detail, and the simulated results are further explained by Thiele's theory. This work unveils a promising strategy to enhance the stability and the maximum velocity of AFM skyrmion, benefiting future spintronic applications.

Skyrmions in antiferromagnets: Thermal stability and the effect of external field and impurities

Calculations of skyrmions in antiferromagnets (AFMs) are presented, and their properties compared with skyrmions in corresponding ferromagnets (FMs). The rates of skyrmion collapse and escape through the boundary of a track, as well as the binding to and collapse at a non-magnetic impurity, are calculated as a function of an applied magnetic field. The activation energy for skyrmion annihilation is the same in AFMs and corresponding FMs in the absence of an applied magnetic field. The pre-exponential factor in the Arrhenius rate law is, however, different because skyrmion dynamics is different in the two systems. An applied magnetic field has opposite effects on skyrmions in the two types of materials. In AFMs, the rate of collapse of skyrmions as well as the rate of escape through the edge of a magnetic strip decreases slightly with increasing field, while these rates increase strongly for a skyrmion in the corresponding FMs when the field is directed antiparallel to the magnetization in the center of the skyrmion. A non-magnetic impurity is less likely to trap a skyrmion in AFMs, especially in the presence of a magnetic field. This, together with the established fact that a spin polarized current moves skyrmions in AFMs in the direction of the current, while in FMs skyrmions move at an angle to the current, demonstrates that skyrmions in AFMs have several advantageous properties over skyrmions in FMs for memory and spintronic devices.

Traps for pinning and scattering of antiferromagnetic skyrmions via magnetic properties engineering

Micromagnetic simulations have been performed to investigate the controllability of the skyrmion position in antiferromagnetic nanotracks with their magnetic properties modified spatially. In this study, we have modeled magnetic defects as local variations on the material parameters, such as the exchange stiffness, saturation magnetization, perpendicular magnetocrystalline anisotropy, and Dzyaloshinskii–Moriya constant. Thus, we have observed not only pinning (potential well) but also scattering (potential barrier) of antiferromagnetic skyrmions, when adjusting either a local increase or a local reduction for each material parameter. In order to control the skyrmion motion, it is very important to impose certain positions along the nanotrack where the skyrmion can stop. Magnetic defects incorporated intentionally in antiferromagnetic racetracks can be useful for such a purpose. In order to provide guidelines for experimental studies, we vary both material parameters and the size of the modified region. The results obtained show that the efficiency of skyrmion traps depends on a suitable combination of magnetic defect parameters. Furthermore, we discuss the reason why skyrmions are either attracted or repelled by a region magnetically modified.

Realization of Isolated and High-Density Skyrmions at Room Temperature in Uncompensated Synthetic Antiferromagnets.

Magnetic skyrmions are vortex-like spin textures with nontrivial spin topology and novel physical properties that show promise as an essential building block for novel spintronic applications. Skyrmions in synthetic antiferromagnets (SAF) have been proposed long-term to have many advantages than those in ferromagnetic materials, which suffer from fundamental limits for size and efficient manipulation. Thus, experimental realization of skyrmions in SAF is intensely pursued. Here we show the observation of zero-field stable magnetic skyrmions at room temperature in SAF [Co/Pd]/Ru/[Co/Pd] multilayers with Lorentz transmission electron microscope, where uncompensated moments of the SAF provide a medium for the skyrmion characterization. Isolated skyrmions and high-density skyrmions via magnetic field and electromagnetic coordinated methods have been observed, respectively. These created high-density skyrmions maintain at zero-field even when both the current and magnetic field are removed. The use of skyrmions in SAF would advance the process toward practical nonvolatile memories based on spin topology.

3D analytical model of skyrmion-like structures in an antiferromagnet with DMI

The purpose of the research is the construction of the analytical model for description of a number of topological objects in a two-sublattice antiferromagnet with uniaxial magnetic anisotropy and Dzyaloshinskii-Moriya interaction (DMI) including vortices, antivortices, skyrmions, antiskyrmions, skyrmioniums and their bound states, which are the exact dynamic solutions of the nonlinear Landau-Lifshitz equations. “Relativistic contraction” of skyrmion-like topological object size in the direction of motion is demonstrated for “subcritical” case when its velocity is less than spin wave velocity in antiferromagnet. Lorentz-like “supercritical” transformation are found for skyrmion-like magnetic structures moving with velocity greater than spin wave velocity in antiferromagnet. In particular, the results of the analytical model are applied for an antiferromagnet in the form of cylindrical nanoshell; in this case, there are more than one solution of the nonlinear Landau-Lifshitz equations for the same boundary and initial conditions. It means that vortices, skyrmions, skyrmioniums and their bound states represent the ground state and the excited states in an antiferromagnetic cylindrical nanoshell. The particular cases of the exact static solutions of the nonlinear Landau-Lifshitz equations include the well-known one-dimensional solutions such as Bloch domain, Neel domain wall, Shirobokov domain structure, antiferromagnetic vortices, two-dimensional Belavin-Polyakov soliton, three-dimensional Hodenkov soliton and target type soliton. The results of this paper can be used for further development of theory of the antiferromagnetic soliton and skyrmion physics. Besides, the exact dynamic solutions of the nonlinear Landau-Lifshitz equations can serve as the reference solutions for testing the results of micromagnetic simulations.

Controlling the deformation of antiferromagnetic skyrmions in the high-velocity regime

The authors acknowledge fruitful discussions with A. Abbout, C. A. Akosa, J.-V. Kim, and A. Thiaville. A.S., F.Z., and A.M. were supported by the King Abdullah University of Science and Technology (KAUST). R.T. and G.F. acknowledge the project ThunderSKY, funded by the Hellenic Foundation for Research and Innovation (HFRI) and the General Secretariat for Research and Technology (GSRT), under Grant Agreement No. 871.

Current-Induced Dynamics of the Antiferromagnetic Skyrmion and Skyrmionium

Antiferromagnetic (AFM) skyrmionium composed of two topological AFM skyrmions shares the merits of an AFM skyrmion, for example, high mobility and no skyrmion Hall effect. Here, we analytically and numerically study the dynamics of the AFM skyrmion and skyrmionium induced by spin currents. Our calculations demonstrate that the current-induced spin-transfer torques can drive AFM skyrmion and skyrmionium with the same speed, while their steady motion speeds induced by spin-orbit torques are different. Furthermore, it is found that due to the existence of the effective AFM texture mass, the AFM skyrmion and skyrmionium obey the momentum theorem, and the time evolution of the position induced by alternating currents presents a phase. Besides, a spin torque nano-oscillator based on the AFM skyrmionium can produce high frequencies, similar to that based on the AFM skyrmion. Numerical simulations are in good agreement with the analytical solutions. Our results demonstrate the inertial dynamics of the AFM skyrmion and skyrmionium and may provide guidelines for building skyrmion-based spintronic devices.

Field-induced double spin spiral in a frustrated chiral magnet

Magnetic ground states with peculiar spin textures, such as magnetic skyrmions and multifunctional domains are of enormous interest for the fundamental physics governing their origin as well as potential applications in emerging technologies. Of particular interest are multiferroics, where sophisticated interactions between electric and magnetic phenomena can be used to tailor several functionalities. We report the direct observation of a magnetic field induced long-wavelength spin spiral modulation in the chiral compound Ba{}_{3}TaFe{}_{3}Si{}_{2}O{}_{14}, which emerges out of a helical ground state, and is hallmarked by the onset of a unique chirality-dependent contribution to the bulk electric polarization. The periodicity of the field-induced modulation, several hundreds of nm depending on the field value, is comparable to the length scales of mesoscopic topological defects such as skyrmions, merons, and solitons. The phase transition and observed threshold behavior are consistent with a phenomenology based on the allowed Lifshitz invariants for the chiral symmetry of langasite, which intriguingly contain all the essential ingredients for the realization of topologically stable antiferromagnetic skyrmions. Our findings open up new directions to explore topological correlations of antiferromagnetic spintronic systems based on non-collinear magnetic systems with additional ferroic functionalities.

Skyrmion based random bit generator

Magnetic skyrmions are topologically quasi-particles offering a wide range of applications ranging from memory to oscillator and neuromorphic elements such as neurons and synapses. However, their trajectory driven by currents exhibits the so-called skyrmion Hall angle that is a limiting factor for the above-mentioned applications. Here, we study the stochastic dynamics of synthetic antiferromagnetic skyrmions driven by the spin-Hall effect in a racetrack memory with bifurcation. We show the limit of their application in programmable logic and a possible direction to use this design as a skyrmion based random bit generator.

Spin-torque nano-oscillators based on radial vortex in the presence of interface Dzyaloshinskii-Moriya interaction

Spin-torque and spin-Hall nano-oscillators based on topological magnetic solitons, including domain wall, circular vortex, Skyrmions and antiferromagnetic Skyrmions have sparked extensively scientific interest, which hold the advantage of miniature size, wideband tunability and high levels of integration with CMOS. Here, we present that the radial vortex state can also be brought into persistent self-oscillatory periodic motion by applying spin polarized current in a nanocontact cylinder magnetic multilayers structure. Using micromagnetic simulation, we show that the threshold current density of radial vortex oscillator is one order of magnitude lower than traditional spin-torque nano-oscillators. More importantly, the radial vortex oscillator exhibits roughly tripled time-varying averaged magnetization oscillation in comparison with the circular vortex oscillators. Therefore, the microwave output power can be greatly enhanced and the energy dissipation can be markedly reduced. The above-mentioned findings provide an in-depth knowledge of radial vortex oscillators, it may facilitate the practical application of radial vortex oscillators.

Formation and current-induced motion of synthetic antiferromagnetic skyrmion bubbles

Skyrmion, a topologically-protected soliton, is known to emerge via electron spin in various magnetic materials. The magnetic skyrmion can be driven by low current density and has a potential to be stabilized in nanoscale, offering new directions of spintronics. However, there remain some fundamental issues in widely-studied ferromagnetic systems, which include a difficulty to realize stable ultrasmall skyrmions at room temperature, presence of the skyrmion Hall effect, and limitation of velocity owing to the topological charge. Here we show skyrmion bubbles in a synthetic antiferromagnetic coupled multilayer that are free from the above issues. Additive Dzyaloshinskii-Moriya interaction and spin-orbit torque (SOT) of the tailored stack allow stable skyrmion bubbles at room temperature, significantly smaller threshold current density or higher speed for motion, and negligible skyrmion Hall effect, with a potential to be scaled down to nanometer dimensions. The results offer a promising pathway toward nanoscale and energy-efficient skyrmion-based devices.

Electric and antiferromagnetic chiral textures at multiferroic domain walls.

Chirality, a foundational concept throughout science, may arise at ferromagnetic domain walls1 and in related objects such as skyrmions2. However, chiral textures should also exist in other types of ferroic materials, such as antiferromagnets, for which theory predicts that they should move faster for lower power3, and ferroelectrics, where they should be extremely small and possess unusual topologies4,5. Here, we report the concomitant observation of antiferromagnetic and electric chiral textures at domain walls in the room-temperature ferroelectric antiferromagnet BiFeO3. Combining reciprocal and real-space characterization techniques, we reveal the presence of periodic chiral antiferromagnetic objects along the domain walls as well as a priori energetically unfavourable chiral ferroelectric domain walls. We discuss the mechanisms underlying their formation and their relevance for electrically controlled topological oxide electronics and spintronics.

Dynamics of an antiferromagnetic skyrmion in a racetrack with a defect

The antiferromagnetic skyrmion is a promising building block for future antiferromagnetic spintronic devices, which has several advantages, including ultrafast dynamics and zero skyrmion Hall angle. The understanding of the pinning and depinning of an antiferromagnetic skyrmion to defects is a prerequisite for skyrmion-based in-line motion applications, such as racetrack memory. Here, we numerically study the effect of two types of defects caused by the local decrease/increase of perpendicular magnetic anisotropy on the current-induced dynamics of an antiferromagnetic skyrmion, where the critical depinning current density of the skyrmion motion for different defects is determined under the framework of micromagnetics. We also provide an explanation for the complex behaviors of the antiferromagnetic skyrmion via energy landscape, which shows that the skyrmion always prefers to stay at a specific place with minimal energy. Furthermore, the current-induced motion of a skyrmion in an antiferromagnetic racetrack with defects is compared to that in a ferromagnetic racetrack. Our results are useful for the understanding of antiferromagnetic skyrmion physics at low temperatures and could provide guidelines for designing applications based on antiferromagnetic skyrmions.

Room-temperature stabilization of antiferromagnetic skyrmions in synthetic antiferromagnets.

Room-temperature skyrmions in ferromagnetic films and multilayers show promise for encoding information bits in new computing technologies. Despite recent progress, ferromagnetic order generates dipolar fields that prevent ultrasmall skyrmion sizes, and allows a transverse deflection of moving skyrmions that hinders their efficient manipulation. Antiferromagnetic skyrmions shall lift these limitations. Here we demonstrate that room-temperature antiferromagnetic skyrmions can be stabilized in synthetic antiferromagnets (SAFs), in which perpendicular magnetic anisotropy, antiferromagnetic coupling and chiral order can be adjusted concurrently. Utilizing interlayer electronic coupling to an adjacent bias layer, we demonstrate that spin-spiral states obtained in a SAF with vanishing perpendicular magnetic anisotropy can be turned into isolated antiferromagnetic skyrmions. We also provide model-based estimates of skyrmion size and stability, showing that room-temperature antiferromagnetic skyrmions below 10 nm in radius can be anticipated in further optimized SAFs. Antiferromagnetic skyrmions in SAFs may thus solve major issues associated with ferromagnetic skyrmions for low-power spintronic devices.

Topological spin Hall effects and tunable skyrmion Hall effects in uniaxial antiferromagnetic insulators

Recent advances in the physics of current-driven antiferromagnetic skyrmions have observed the absence of a Magnus force. We outline the symmetry reasons for this phenomenon, and show that this cancellation will fail in the case of spin polarized currents. Pairing micromagnetic simulations with semiclassical spin wave transport theory, we demonstrate that skyrmions produce a spin-polarized transverse magnon current, and that spin-polarized magnon currents can in turn produce transverse motion of antiferromagnetic skyrmions. We examine qualitative differences in the frequency dependence of the skyrmion Hall angle between ferromagnetic and antiferromagnetic cases, and close by proposing a simple skyrmion-based magnonic device for demultiplexing of spin channels.

Topological Magnons and Edge States in Antiferromagnetic Skyrmion Crystals.

Antiferromagnetic skyrmion crystals are magnetic phases predicted to exist in antiferromagnets with Dzyaloshinskii-Moriya interactions. Their spatially periodic noncollinear magnetic texture gives rise to topological bulk magnon bands characterized by nonzero Chern numbers. We find topologically protected chiral magnonic edge states over a wide range of magnetic fields and Dzyaloshinskii-Moriya interaction values. Moreover, and of particular importance for experimental realizations, edge states appear at the lowest possible energies, namely, within the first bulk magnon gap. Thus, antiferromagnetic skyrmion crystals show great promise as novel platforms for topological magnonics.