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Skyrmion-electronics: Writing, deleting, reading and processing magnetic skyrmions toward spintronic applications
Xichao Zhang, Yan Zhou, Kyung Mee Song, Tae-Eon Park, Jing Xia, Motohiko Ezawa, Xiaoxi Liu, Weisheng Zhao, Guoping Zhao, Seonghoon Woo
TL;DR
Spintronic applications require controllable ways to write, delete, read, and process magnetic skyrmions. This topical review synthesizes theoretical, computational, and experimental advances since 2009, covering field-, current-, and thermally driven operations and applications including storage, logic, and neuromorphic devices. It also identifies future directions involving skyrmions and related topological textures.
Problem
Spintronic applications require methods to write, delete, read, and process magnetic skyrmions across relevant driving mechanisms.
Method
The paper reviews theoretical, computational, and experimental findings on skyrmion operations and spintronic applications.
Results
The review covers information storage, logic computing gates, neuromorphic devices, and other processing applications based on magnetic skyrmions.
Takeaways & Limitations
The review connects skyrmion writing, deleting, reading, and processing with potential spintronic applications and future research on related topological textures.
Takeaways & Limitations
No review article can cover the topic exhaustively.
Abstract
from arXiv · showhide
The field of magnetic skyrmions has been actively investigated across a wide range of topics during the last decades. In this topical review, we mainly review and discuss key results and findings in skyrmion research since the first experimental observation of magnetic skyrmions in 2009. We particularly focus on the theoretical, computational and experimental findings and advances that are directly relevant to the spintronic applications based on magnetic skyrmions, i.e. their writing, deleting, reading and processing driven by magnetic field, electric current and thermal energy. We then review several potential applications including information storage, logic computing gates and non-conventional devices such as neuromorphic computing devices. Finally, we discuss possible future research directions on magnetic skyrmions, which also cover rich topics on other topological textures such as antiskyrmions and bimerons in antiferromagnets and frustrated magnets.
1. Background
Topological spin textures are characterized by dimensionality and topological charge, with skyrmions emerging as a central 2D texture for spintronic applications. Since 2009, experiments established skyrmion lattices and individual skyrmions across low-temperature and room-temperature material platforms, alongside demonstrations of motion, writing, deleting, and application-relevant processing.
- 1.1. Topological Spin Textures: Topological spin textures can be one-, two-, or three-dimensional and usually carry integer or half-integer topological charges.Their behaviors are determined or affected by their topological characteristics.
- 1.1. Topological Spin Textures: Magnetic thin films and multilayers are preferred platforms for nanoscale spintronic applications because they are better understood.The review limits its discussion mainly to 2D topological spin textures in magnetic materials.
- 1.1. Topological Spin Textures: DM interactions stabilize skyrmions, while interface-induced DM interactions can produce Néel-type skyrmions and other interactions can stabilize antiskyrmions.Examples include skyrmions with Q = +1 and Q = -1, and antiskyrmions stabilized instead of skyrmions.
2. Writing and Deleting Skyrmions
Magnetic skyrmions can be written, deleted, and manipulated using magnetic fields, spin-polarized currents, electric fields, lasers, and imprinting. These approaches support localized creation and switching, while strong fields can annihilate skyrmions and current-driven methods incur Joule-heating costs.
- Skyrmions encode binary information through their presence or absence, making reliable creation and annihilation prerequisites for storage applications.The review identifies finding effective writing and deleting methods as a vital task.
- Magnetic Field: Magnetic fields control skyrmion formation, size, switching, dynamics, and deletion across chiral magnetic films and confined structures.Small out-of-plane fields usually support formation, whereas large fields can collapse skyrmions into a ferromagnetic state.
- Local Electric Field: Electric fields enable reversible writing, deleting, switching, displacement, chirality control, and thermal-motion control in several magnetic structures.One demonstration directionally displaced skyrmion bubbles about 10 micrometres, after which removing the field annihilated them.
- Laser: Lasers provide a route to write and delete skyrmions, with writing and deleting speed controlled by temperature and skyrmion-bubble density controlled experimentally.Laser-created textures include skyrmionium textures, and laser writing and deleting were realized in FeGe.
- Imprinting: Imprinting offers theoretical and experimental routes to stabilize skyrmion lattices or create textures, including configurations stable at room temperature without DM interaction.The reviewed proposals include nanoimprinting and imprinting with magnetic structures such as Co nanodots.
3. Reading Skyrmions
Reading skyrmions combines real-space microscopy, which reveals their textures and dynamics, with electrical methods needed for device read-out. Multiple techniques image skyrmion lattices, isolated skyrmions, ultrafast behavior, and electrical signatures, each with specific capabilities and limitations.
- Real-space imaging: Real-space imaging identifies skyrmion topology from in-plane and out-of-plane spin textures and can support topological-charge calculations using vorticity and helicity.Some techniques also provide sub-ns temporal resolution for studying ultrafast dynamics such as excitation modes.
- Real-space imaging: LTEM first imaged a two-dimensional Bloch-type skyrmion lattice in Fe0.5Co0.5Si in 2010 and later observed lattices in several additional materials.These included FeGe, Cu2OSeO3, MnSi, Mn1-xFexGe, and Fe-Gd alloys.
- Real-space imaging: LTEM has limited direct sensitivity to Néel-type configurations in zone-axis mode, but tilting, off-focused imaging, and aberration-corrected DPC-STEM enabled their observation.DPC-STEM was introduced for direct imaging of magnetic textures including skyrmions.
- Other microscopy methods: SP-STM, X-ray methods, SPLEEM, X-ray holography, and MOKE microscopy extended observation to nanoscale, ultrafast, room-temperature, and dynamic skyrmion phenomena.Examples include isolated PdFe/Ir skyrmions, sub-100 nm skyrmions, ferrimagnetic writing and deleting, and current-driven dynamics.
- Other microscopy methods: MOKE microscopy directly observed room-temperature skyrmions and their creation from stripe domain walls, but its spatial resolution is usually limited to about 1 μm.The limitation arises from the wavelength of the light used for observation.
4. Processing Skyrmions
Skyrmion processing uses spin-polarized currents and spin-orbit torques to drive, create, route, and manipulate skyrmions. The section also describes device concepts spanning racetrack memory, logic, transistor-like structures, and neuromorphic computing.
- Current-driven dynamics: The Thiele equation is derived by projecting the Landau-Lifshitz-Gilbert dynamics onto translational motion, with gyrocoupling and dissipation determining skyrmion motion.Spin-transfer torques can induce both translational and rotational modes.
- Current-driven dynamics: The skyrmion Hall effect produces a transverse velocity under spin-current driving, while finite-width channels can confine skyrmions and yield steady-state velocity characteristics.The transverse motion is associated with the skyrmion’s topological dynamics.
- Current-driven dynamics: Vertical spin current drives skyrmions more efficiently than in-plane current, and room-temperature pulses of 5 × 10^11 A m^-2 drove skyrmions above 100 m s^-1.The high-speed motion was demonstrated in Pt/CoFeB/MgO multilayers.
- Current-driven dynamics: Ferrimagnetic skyrmions can move at ~50 m s^-1 with |θSkHE| ~20°, and the skyrmion Hall effect can vanish at angular-momentum compensation.These properties support reduced transverse motion in ferrimagnetic systems.
- Information processing: Skyrmion processing concepts include racetrack memories, hybrid skyrmion–domain-wall devices, logic gates based on duplication and merging, and transistor-like devices.The reviewed logic operations include AND, OR, NOT, NAND, NOR, XOR, and XNOR.
- Neuromorphic and probabilistic computing: Simulations demonstrated skyrmion-based artificial synapses with potentiation, depression, short-term plasticity, long-term potentiation, and pattern-recognition computing.Thermally activated skyrmion diffusion was also proposed for probabilistic computing.
5. Summary and Outlook
The outlook extends skyrmionics beyond conventional ferromagnets to antiferromagnetic, ferrimagnetic, synthetic-antiferromagnetic, frustrated, and two-dimensional van der Waals materials. These platforms offer altered dynamics and additional skyrmion degrees of freedom, but electrical detection remains a key challenge for zero-net-magnetization systems.
- Antiferromagnetic and ferrimagnetic skyrmions: Antiferromagnetic skyrmions consist of coupled opposite-charge sublattice skyrmions whose Magnus forces cancel, enabling straight motion and potentially higher speeds.Synthetic antiferromagnetic bilayer skyrmions likewise have zero topological charge and no skyrmion Hall effect under external driving.
- Open challenges: Efficient electrical manipulation, particularly detection, of antiferromagnetic spin textures with zero net magnetization remains an important practical challenge.The challenge directly affects the application of antiferromagnetic materials in skyrmion-electronics.
- Antiferromagnetic and ferrimagnetic skyrmions: Ferrimagnetic skyrmions retain a nonzero but reduced skyrmion Hall effect, while compensated ferrimagnets can exhibit zero skyrmion Hall effect.Their intermediate magnetic properties can support high-speed motion with reduced edge destruction.
- Antiferromagnetic and ferrimagnetic skyrmions: Antiferromagnets, synthetic antiferromagnets, and ferrimagnets are proposed as skyrmion-hosting materials with improved dynamic performance compared with conventional ferromagnets.Antiferromagnetic skyrmions move along nanotracks without transverse edge-directed motion, whereas ferrimagnetic skyrmions retain some shift.
- Frustrated magnets: Frustrated magnets can host skyrmions with arbitrary vorticity and helicity, helicity-independent energy in some cases, and coupled helicity–center-of-mass dynamics.These dynamics can produce rotational skyrmion motion and current-induced helicity locking–unlocking transitions.
- Frustrated magnets: Experiments observed skyrmion states in frustrated Fe3Sn2 and Gd2PdSi3, including a giant topological Hall response in the latter material.The Gd2PdSi3 result indicated a field-induced skyrmion state.
- Two-dimensional magnets: Van der Waals magnets such as Fe3GeTe2 and Cr3Ge2Te6 experimentally demonstrated stabilized Néel-type skyrmions, motivating further two-dimensional spintronic research.Broken inversion symmetry and strong spin–orbit coupling were suggested as mechanisms supporting Dzyaloshinskii–Moriya interaction in some two-dimensional magnets.
Meron, Antimeron, and Bimeron
The review surveys skyrmion-like textures beyond conventional skyrmions, emphasizing their topology, dynamics, experimental realization, and possible spintronic uses. It also highlights detection, conversion, and unresolved identification issues relevant to device design.
- Antiskyrmion: Antiskyrmions have Qv = -1, opposite to skyrmions with Qv = +1, and exhibit different in-plane spin textures and current-induced motion.
- Antiskyrmion: Skyrmions and antiskyrmions can encode binary information in one device, with skyrmions representing “1” and antiskyrmions representing “0”.
- Antiskyrmion: The topological Hall effect can distinguish skyrmions from antiskyrmions because its sign is proportional to the skyrmion number.
- Skyrmionium: Skyrmioniums have topological charge Q = 0 and have been experimentally observed, while spin currents and spin waves can drive them through narrow nanotracks.
- Biskyrmion: Biskyrmions have |Q| = +2, can form by exciting or merging Qv = +1 skyrmions, and have exhibited current-driven motion experimentally.
- Bimeron: Bimerons are meron–antimeron pairs with |Q| = 1 in easy-plane magnets, and theory suggests spin-transfer torques can drive isolated bimerons.