To exploit nanometric magnetic skyrmions as information carriers in high-density storage devices, a method is needed that creates intended number of skyrmions at specified places in the device preferably at a low energy cost. We theoretically propose that using a system with a fabricated hole or notch, the controlled creation of individual skyrmions can be achieved even when using an external magnetic field applied to the entire specimen. The fabricated defect turns out to work like a catalyst to reduce the energy barrier for the skyrmion creation.
From a theoretical perspective, we demonstrate that nanometric magnetic skyrmions are created by application of a circularly polarized microwave magnetic field to a thin-plate Dzyaloshinskii-Moriya ferromagnet with fabricated rectangular holes. This phenomenon is caused by an effective steady magnetic field perpendicular to the microwave-polarization plane induced by the rotating magnetic field and the intense interference of spin waves excited by this magnetic field due to the hole structure, which causes reversals of local magnetizations and results in the formation of skyrmions. Our proposa…
Inevitable for the basic principles of skyrmion racetrack-like applications is not only their confined motion along one-dimensional channels but also their controlled creation and annihilation. Helical magnets have been suggested to naturally confine the motion of skyrmions along the tracks formed by the helices, which also allow for high-speed skyrmion motion. We propose a protocol to create topological magnetic structures in a helical background. We furthermore analyse the stability and current-driven motion of the skyrmions in a helical background with in-plane uniaxial anisotropy fixing th…
Néel-type magnetic skyrmions in multilayer films have attracted significant amount of attention recently for their stability at room temperature and capability of motion driven by a low-density electrical current, which can be potentially applied to spintronic devices. However, the thermal effect on the formation of the skyrmions and their behavior has rarely been studied. Here, we report a study on the creation of skyrmions in [Pt/Co/Ta]10 multilayer samples at different temperatures using an in-situ Lorentz transmission electron microscopy. By imaging the magnetization reversal process from …
We study the creation of quantum skyrmions in quadratic nanoscopic lattices of quantum spins coupled by Dzyaloshinkii-Moriya and exchange interactions. We numerically show that different kinds of quantum skyrmions, characterized by the magnitude of their spin expectation values and strong differences in their stability, can appear as ground state and as metastable excitations. In dependence on the coupling strengths and the lattice size, an adiabatic rotation scheme of the magnetization at the lattice boundaries allows for a controlled creation of quantum skyrmions.
We determine sizes and activation energies of kπ-skyrmions on a discrete lattice using the Landau- Lifshitz-Gilbert equation and the geodesic nudged elastic band method. The employed atomic material parameters are based on the skyrmionic material system Pd/Fe/Ir(111). We find that the critical magnetic fields for collapse of the 2π-skyrmion and 3π-skyrmion are very close to each other and considerably lower than the critical field of the 1π-skyrmion. The activation energy protecting the structures does not strictly decrease with increasing k as it can be larger for the 3π-skyrmion than for the…
In this work, we utilize voltage controlled magnetic anisotropy (VCMA) to manipulate magnetic skyrmions that are fixed in space. Memory devices based on this strategy can potentially be of smaller footprint and better energy efficiency than current-controlled motion-based skyrmionic devices. To demonstrate VCMA induced manipulation of skyrmions, we fabricate antiferromagnet/ferromagnet/oxide heterostructure films where skyrmions can be stabilized without any external magnetic field due to the presence of exchange bias. These isolated skyrmions were annihilated by applying a voltage pulse that …
Magnetic skyrmions are topological magnetic spin structures exhibiting particle-like behaviour. They are of strong interest from a fundamental viewpoint and for application, where they have potential to act as information carriers in future low-power computing technologies. Importantly, skyrmions have high physical stability because of topological protection. However, they have potential to deform according to their local energy environment. Here we demonstrate that, in regions of high exchange energy density, skyrmions may exhibit such extreme deformation that spontaneous merging with nearest…
Magnetic skyrmions are intriguing topological spin textures that promise future high-density spintronic devices. The creation of magnetic skyrmions has been understood based on the energetics of skyrmions, but the detailed dynamic process of the skyrmion creation remains unclear. Here topological evolution in conversion from uneven domains to Néel-skyrmions is investigated using micromagnetic simulations. We find that, rather than the overall topological charge, annihilation of novel topological defects, i.e., recently suggested domain wall skyrmions, dominantly govern the skyrmion creation pr…
Magnetic skyrmions are prime candidates as information carriers for spintronic devices due to their topological nature and nanometric size. However, unavoidable inhomogeneities inherent to any material leads to pinning or repulsion of skyrmions that, in analogy to biology concepts, define the phenotype of the skyrmion-defect interaction, generating complexity in their motion and challenging their application as future bits of information. Here, we demonstrate that atom-by-atom manufacturing of multi-atomic defects, being antiferromagnetic or ferromagnetic, permits the breeding of their energy …
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 fi…
Magnetic skyrmions are topologically nontrivial spin textures which hold great promise as stable information carriers in spintronic devices at the nanoscale. One of the major challenges for developing novel skyrmion-based memory and logic devices is fast and controlled creation of magnetic skyrmions at ambient conditions. Here we demonstrate the single ultrafast (35-fs) laser pulse-induced generation of skyrmion bubbles and skyrmion bubble lattices from a ferromagnetic state in sputtered ultrathin magnetic films at room temperature. The skyrmion bubble density increases with the laser fluence …
Voltage manipulation of skyrmions is a promising path towards low-energy spintronic devices. Here, voltage effects on skyrmions in a GdOx/Gd/Co/Pt heterostructure are observed experimentally. The results show that the skyrmion density can be both enhanced and depleted by the application of an electric field, along with the ability, at certain magnetic fields to completely switch the skyrmion state on and off. Further, a zero magnetic field skyrmion state can be stablized under a negative bias voltage using a defined voltage and magnetic field sequence. The voltage effects measured here occur o…
We fabricate devices in which a magnetic nanopillar spin valve makes contact to a Co/Pt bilayer thin film with perpendicular magnetic anisotropy, in order to achieve local control of domains in the Co/Pt bilayer underneath the nanopillar. The goal is to develop the ability to nucleate, detect, and annihilate magnetic skyrmions in the Co/Pt using spin-polarized currents from the nanopillar. We demonstrate the ability to distinguish the local behavior of the Co/Pt film beneath the nanopillar from the extended film and show that the two can switch independently of each other. This allows us to is…
A magnetic skyrmionium (also called 2$π$-skyrmion) can be understood as a skyrmion - a topologically non-trivial magnetic whirl - which is situated in the center of a second skyrmion with reversed magnetization. Here, we propose a new optoelectrical writing and deleting mechanism for skyrmioniums in thin films, as well as a reading mechanism based on the topological Hall voltage. Furthermore, we point out advantages for utilizing skyrmioniums as carriers of information in comparison to skyrmions with respect to the current-driven motion. We simulate all four constituents of an operating skyrmi…
Optimized control protocols for stable skyrmion creation using deep reinforcement learning
Korea Advanced Institute of Science and Technology（韩国科学技术院）；Asia Pacific Center for Theoretical Physics（亚太理论物理中心）；Pohang University of Science and Technology（浦项科技大学）
Magnetic skyrmions are the self-organized topological spin textures behaving like particles. Because of their fast creation and typically long lifetime, experimental verification of skyrmion's creation/annihilation processes has been challenging. Here we successfully track skyrmions dynamics in defect-introduced Co9Zn9Mn2, by using pump-probe Lorentz transmission electron microscope. Following the nanosecond-photothermal excitation, we resolve 160-nm-skyrmion's proliferation at <1 ns, contraction at 5 ns, drift from 10 ns to 4 microsecond and coalescence at 5 microsecond. These motions relay t…
Magnetic skyrmions are nanometric spin textures of outstanding potential for spintronic applications due to unique features governed by their non-trivial topology. It is well known that skyrmions of definite chirality are stabilized by the Dzyaloshinskii-Moriya exchange interaction (DMI) in bulk non-centrosimmetric materials or ultrathin films with strong spin-orbit coupling in the interface. In this work, we report on the detection of magnetic hedgehog-skyrmions at room temperature in confined systems with neither DMI nor perpendicular magnetic anisotropy. We show that soft magnetic (permallo…
We formulate and study the general boundary conditions dictating the magnetization profile in the vicinity of an interface between magnets with dissimilar properties. Boundary twists in the vicinity of an edge due to Dzyaloshinskii-Moriya interactions have been first discussed in [Wilson et al., Phys. Rev. B 88, 214420 (2013)] and in [Rohart and Thiaville, Phys. Rev. B 88, 184422 (2013)]. We show that in general case the boundary conditions lead to the magnetization profile corresponding to the Néel, Bloch, or intermediate twist. We explore how such twists can be utilized for creation of skyrm…
Magnetic skyrmions hold promise for spintronic devices. To explore the dynamical properties of skyrmions in devices, a nanoscale method to image spin textures in response to a stimulus is essential. Here, we apply a technique for operando electrical current pulsing of chiral magnetic devices in a Lorentz transmission electron microscope. In ferromagnetic multilayers with interfacial Dzyaloshinskii-Moriya interaction (DMI), we study the creation and annihilation of skyrmions localized by point-like pinning sites due to defects. Using a combination of experimental and micromagnetic techniques, w…
To enable functional skyrmion based spintronic devices, the controllable generation and manipulation of skyrmions is essential. While the generation of skyrmions by using a magnetic geometrical constriction has already been demonstrated, this approach is difficult to combine with a subsequent controlled manipulation of skyrmions. The high efficiency of skyrmion generation from magnetic constrictions limits the useful current density, resulting in stochastic skyrmion motion, which may obscure topological phenomena such as the skyrmion Hall effect. In order to address this issue, we designed a n…
Topological solitons have been studied for decades in classical field theories, and have started recently to impact condensed matter physics. Among those solitons, magnetic skyrmions are two-dimensional particle-like objects with a continuous winding of the magnetization, and magnetic Hopfions are three-dimensional topological solitons that can be formed from a closed loop of a twisted skyrmion string. Whereas intense research is underway with magnetic skyrmions towards a fundamental understanding and potential applications in advanced storage and logic devices, the experimental creation and c…
Magnetic skyrmions have been the focus of intense research with promising applications in memory, logic and interconnect technology. Several schemes have been recently proposed and demonstrated to nucleate skyrmions. However, they either result in an uncontrolled skyrmion bubble production or are mostly targeted towards integration with racetrack memory device. In this work, we propose a novel scheme for a controlled single skyrmion nucleation in a confined nanowire geometry with sub-100 nm width using a generalized approach of "localized spin current injection" technique in material systems e…
Various transitions that a magnetic skyrmion can undergo are found in calculations using a method for climbing up the energy surface and converging onto first order saddle points. In addition to collapse and escape through a boundary, the method identifies a transition where the skyrmion divides and forms two skyrmions. The activation energy for this duplication process can be similar to that of collapse and escape. A tilting of the external magnetic field for a certain time interval is found to induce the duplication process in a dynamical simulation. Such a process could turn out to be an im…
We study how impurities influence the current-induced dynamics of magnetic Skyrmions moving in a racetrack geometry. For this, we solve numerically the generalized Landau-Lifshitz-Gilbert equation extended by the current-induced spin transfer torque. In particular, we investigate two classes of impurities, non-conducting and magnetic impurities. The former are magnetically rigid objects and yield to an inhomogeneous current density over the racetrack which we determine separately by solving the fundamental electrostatic equations. In contrast, magnetic impurities leave the applied current dens…
Magnetic textures can be manipulated by electric currents via the mechanisms of spin-transfer and spin-orbit-torques. We review how these torques can be exploited to create chiral magnetic textures in magnets with broken inversion symmetries, including domain walls and skyrmions. These chiral textures can also be moved by (electric) currents and obey very rich dynamics. For example, magnetic domain walls feature the famous Walker breakdown, and magnetic whirls are subject to the skyrmion Hall effect, which is rooted in their real-space topology. These properties led to a variety of potential n…
We demonstrate numerically on large spin lattices that one can write skyrmions in a thin magnetic film with a magnetic dipole of a few tens of nanometer in size. Nucleation of non-chiral skyrmions as well as chiral skyrmions formed by the Dzyaloshinskii-Moriya interaction has been investigated. Analytical model is developed that agrees with numerical results. It is shown that skyrmions can be written though a number of scenarios that depend on the experimental technique and parameters of the system. In one scenario, that branches into subscenarios of different topology, the magnetic dipole on …
Magnetic skyrmions and their anti-particles, the antiskyrmions, are stable magnetic solitons existing down to the nanometer scale. Their stability and size as well as the possibility to propel them by, e.g., electric currents make them promising candidates for the use in memory devices, such as racetrack memories. Skyrmions and antiskyrmions share those same advantages individually, but may annihilate each other when they coexist in the same device. Yet, combining them to represent logical bits of 0 (skyrmion) and 1 (antiskyrmion) in one device opens new possibilities to create new types of de…
In the present work, using micromagnetic simulation, we show that the magnetic coupling effect plays a very important role in the process of creation of skyrmions in a coupled system of spin-torque nano-oscillators (STNO). First, we have determined the magnetic ground state in an isolated STNO for different values of perpendicular uniaxial anisotropy (PUA) and Dzyaloshinskii-Moriya interaction (DMI). Next, we have applied a perpendicular pulse polarized current density (J) and found that it is possible to create a metastable Néel skyrmion from a disk whose ground state is a single magnetic dom…
