Reversible conversions between Domain wall and skyrmions in nanowires [Nature Communications, 5, 4652, (2014)]
In this article, we presented the first theoretical demonstration of integration of
domain-wall and skyrmion physics in ferromagnetic nanowire and show how they can be mutually converted to each
other, which will lead to the combined advantages of these two very different spin textures. This work were
subsequently confirmed by experiments [Science, Vol. 349 no. 6245 pp. 283-286, 2015] and triggered an
extensive
wave of study of interplay between topologically different textures and excitations as evidenced by its
citation rate [cited 172 times in 4 years]. It is one of the top 1% highly cited papers in the academic field
of Physics in 2016-2018 by Web of Science.
It has been extensively cited by many famous groups in this field including Nobel Laureate Albert Fert’s
review
paper in 2017 [A. Fert, N. Reyren and V. Cros, Nature Reviews | Materials, 2, 17031 (2017)] — “A similar
approach based on the conversion of a domain wall in a conduit into a skyrmion has been studied numerically
(#citing our work) and is at the basis of several conceptual skyrmion-based devices for logic applications, as
discussed later.”
Dynamically stabilized skyrmions without DDI or DMI [Nature Communications, 6, 8193, (2015)]
In order to unleash their extraordinary potential as information carriers, control of individual skyrmions has to be achieved. So far, this has only been shown for a very rare and exotic class of materials where an unusual chiral exchange interaction called the Dzyaloshinskii-Moriya interaction (DMI) plays an important role. Based on theory and micromagnetics simulations, we discovered that skyrmions can be dynamically stabilized in ultra-thin films by their own precessional motion without the need for DMI, not unlike a spinning top. From a more fundamental perspective, precession is hence a third independent possibility to stabilize a skyrmion, without the need for the conventional stabilization from either dipolar energy or DMI. Once created, they can be transported over distances of several hundreds of nanometers, which means that skyrmions can be created and manipulated in materials that have never before been considered for skyrmionics. In addition, the dynamical skyrmions might also serve as nanoscale microwave oscillators with larger precessing amplitude and thus enhanced microwave power output. This theoretical work has been soon confirmed and verified by the other groups in this field such as [Phys. Rev. B 93, 094407 (2016); Sci. Rep. 5, 16184 (2015); Phys. Rev. Lett. 114, 137201 (2015)]. It has been highlighted by Phys.org, Science daily, IEEE Spectrum, Storage Newsletter, alphagalileo etc.
(a)Figure | A bubble skyrmion stabilized by dipolar interactions which may exist as a left- or right-handed version. (b,c) DMI stabilized skyrmions: (b) A chiral skyrmion as favoured in B20-type materials such as MnSi. (c) A hedgehog skyrmion as favoured by interfacial DMI. (d) Dynamically stabilized magnetic skyrmion which requires neither dipolar interactions nor DMI and which exhibits precession around the (vertical) easy-axis anisotropy. (e) For vanishing dipolar interactions (DDI), DMI and Oe fields, and in absence of damping, the skyrmion precesses uniformly and breathing disappears.
Magnon spintronics based on chiral domain wall [NPG Asia Materials, 8, e246 (2016)]
To realize practical application, the attenuation of spin waves due to boundary scattering and multimode dispersion in magnonic waveguides must be greatly reduced. Up to now, there have been not good solutions to this problem. Here, we propose and demonstrate by micromagnetic simulations to use the longitudinal chiral domain wall imprinted into a waveguide with the interfacial Dzyaloshinskii-Moriya interaction (DMI) to introduce a deep potential well and guide spin waves forming an ultra narrow internal spin-wave channel (~10 nm). Spin waves along this channel can prevent scattering arising from boundary roughness and multimode coexistence and thus should exhibit reduced attenuation and enhanced coherence, which is highly desired in building real spin-wave devices. Moreover, we show that the spin-wave transmission in this waveguide with the DMI can be switched on and off at the frequencies lower than a threshold frequency by changing the static domain state. This property is explored to construct logical NAND gate by connecting two logical NOT gates in parallel.
Figure | Channeled SW propagation, wave characteristics and confining potentials for the SDW and single-domain states. SW propagation patterns at specified frequencies in the 60-nm-wide waveguide for the (a) SDW and (b) single-domain states. (c) The beamwidth as a function of frequency. The beamwidth is defined as the FWHM of the SW amplitude distribution over the waveguide width. (d) SW dispersion relation. (e) Internal-field landscape for the SDW and single-domain states.
Skyrmion-based magnonic crystals [Nano Letters, 15, 4029, (2015)]
A linear array of periodically spaced and individually controllable skyrmions is introduced as a magnonic crystal. It is numerically demonstrated that skyrmion nucleation and annihilation can be accurately controlled by a nanosecond spin polarized current pulse through a nanocontact. Arranged in a periodic array, such nanocontacts allow the creation of a skyrmion lattice that causes a periodic modulation of the waveguide’s magnetization, which can be dynamically controlled by changing either the strength of an applied external magnetic field or the density of the injected spin current through the nanocontacts. The skyrmion diameter is highly dependent on both the applied field and the injected current. This implies tunability of the lowest band gap as the skyrmion diameter directly affects the strength of the pinning potential. The calculated magnonic spectra thus exhibit tunable allowed frequency bands and forbidden frequency bandgaps analogous to that of conventional magnonic crystals where, in contrast, the periodicity is structurally induced and static. In the dynamic magnetic crystal studied here, it is possible to dynamically turn on and off the artificial periodic structure, which allows switching between full rejection and full transmission of spin waves in the waveguide. These findings should stimulate further research activities on multiple functionalities offered by magnonic crystals based on periodic skyrmion lattices.
Figure | Skyrmion-Based Dynamic Magnonic Crystal
Skyrmion logic [Scientific Reports, 5, 9400, (2015)]
For practical applications, magnetic skyrmions have been widely explored as information carriers for potential applications in high-density magnetoresistance random access memory (MRAM). For the first time, we have proposed that magnetic skyrmions can also be utilized for logic applications in this work. We show that the conversion, duplication and merging of isolated skyrmions with different chirality and topology are possible. We also demonstrate a mutual conversion of a skyrmion with another form of skyrmion (i.e., a bimeron). These results provide important design guidelines for utilizing the topology of nanoscale spin textures as information carriers in spin logic devices. This work has laid down the foundation for an entirely new field — skyrmion logic applications. Our paper has been cited 275 times in 2 years and ranked as one of the top 100 cited articles amongst ~ 11,000 papers published in Scientific Reports in 2015, which leads to the award of Top 100 citation Badge by Scientific Reports. It received enough number of citation to place it in the top 0.1% highly cited papers in the academic field of Physics in 2016-2017 by Web of Science. It has been cited by many famous groups in this field including Nobel Laureate Albert Fert’s review paper in 2017 [A. Fert, N. Reyren and V. Cros, Nature Reviews | Materials, 2, 17031 (2017)] — “Based on these additional functionalities, skyrmion logic gates AND and OR have been recently proposed (#citing our paper), realizing the first step toward a complete logical architecture with the goal of overtaking existing spin logic devices, obtaining the same functionality with a higher level of integration for some functionalities.”
Figure | Skyrmion logical OR operation.
Dynamically stabilized skyrmions without DDI or DMI [Nature Communications, 6, 8193, (2015)]
Magnetic skyrmions are topologically protected nanoscale objects, which are promising building blocks for novel magnetic and spintronic devices. Here, we investigate the dynamics of a skyrmion driven by a spin wave in a magnetic nanowire. It is found that (i) the skyrmion is first accelerated and then decelerated exponentially; (ii) it can turn L-corners with both right and left turns; and (iii) it always turns left (right) when the skyrmion number is positive (negative) in the T- and Y-junctions. Our results will be the basis of skyrmionic devices driven by a spin wave.
Figure | Snapshots of a skyrmion driven by magnons parallel to the nanostrip. The inset of (a1) shows the structure of the Néel-type skyrmion in our simulation, which is indicated by blue circles in all snapshots. The color scale shows the in-plane component of the magnetizationmx,which is rescaled to [-0.1, +0.1] in order to show the magnon profile more clearly. A length scale is also provided. This work was published in the special issue of Focus on Magnetic Skyrmions, invited by Nobel Laureate Albert Fert.
Antiferromagnetic skyrmions and ferrimagnetic skyrmions [Nature Communications, 7, 10293 (2016); Nature Electronics, 1, 288-296, (2018); Nature Communications, 9, 959, (2018)]
One major roadblock to the transmission of a skyrmion in information processing devices is the skyrmion Hall effect, i.e., a skyrmion exhibits the Hall effect driven by electric currents, owing to the presence of the Magnus force which originates from its nontrivial topology. The skyrmion Hall effect may result in the destruction of a moving skyrmion at the edge of a device, as it does not move parallel with the direction of the current. Therefore, it is essential to find an effective solution to avoid the skyrmion Hall effect during skyrmion motion. One solution is to utilize antiferromagnetically exchange coupled ferromagnetic (FM) bilayer or antiferromagnetic/ferrimagnetic skyrmions as the skyrmion transmission channel instead of utilizing a FM monolayer nanotrack. It has been extensively cited by many high-profile review papers of magnetic skyrmions such as i) news & views in Nature Physics — Gong Chen, Skyrmion Hall effect, Nature Physics, 13, 113 (2017); ii) W. Jiang et al., Skyrmions in Magnetic Multilayers, Physics Report, in press (2017). iii) D Sander et al., The 2017 Magnetism Roadmap, J. Phys. D: Appl. Phys., 50, 363001 (2017). In addition to synthetic AFM skyrmions, the work of antiferromagnetic skyrmions has been extensively cited by many famous groups in this field such as [T. Jungwirth, X. Marti, P. Wadley and J. Wunderlich, Antiferromagnetic spintronics, Nature Nanotechnology, 11, 231 (2016)] and in Nobel Laureate Albert Fert’s famous review paper in 2017 [A. Fert, N. Reyren and V. Cros, Nature Reviews |Materials, 2, 17031 (2017)] — “The current-induced motion of skyrmions has also been investigated in more complex systems, such as in two perpendicularly magnetized ferromagnetic layers strongly coupled by antiferromagnetic interactions (Citing our paper).”
Figure | Illustration of an AFM skyrmion spin texture in a 2D AFM monolayer with the two square antiparallel magnetic sublattices. The color scale represents the magnetization direction, which has been used throughout this paper: orange is into the plane, green is out of the plane, white is in-plane. It was selected as highlights of 2018 by Science News.
Figure | Spin configurations of a skyrmion, a meron and a DWpair. (a–f) A skyrmion is created from a DW pair driven by spin transfer torque from the narrow part to the wide part of the nanowire. The spin direction is shown by yellow arrow. A yellow dot in the blue region shows that the spin is almost up, whereas a yellow dot in the red region shows that it is almost down. No dots are observed when the spin is almost perfectly up or down. (g–l) A meron is created.