Magnetic topologies and textures

Magnetic topologies and textures

Details

Skyrmion dynamics and topological torque

Swirling spin configurations, known as magnetic skyrmions (Rößler et al., Nature 442, 797–801 (2006)), were recently observed in chiral magnets with the bulk/interfacial Dzyaloshinskii-Moriya interaction (DMI) (see e.g. Mühlbauer et al., Science 323, 915–919 (2009); C. Moreau-Luchaire et.al, Nat. Nanotechnol. 11, 444 (2016)). These entities are now considered as promising candidates for next generation information storage and processing applications (A. Fert et al., Nature Nanotech. 8, 152 (2013)). Their rich topological properties, combined with ultralow energy required to manipulate, give skyrmions edge over their predecessors such as domain wall and magnetic vortex (Schulz et al. Nat. Phys. 8, 301–304 (2012)). 
We looked at these beautiful discoveries with both enthusiasm and (hopefully sane) skepticism. As a matter of fact, preliminary proposals to use the topological protection of skyrmions for high-density storage were based on perfect systems with well-controlled defects. However, two limitations appeared quite quickly: first magnetic skyrmions possess an emergent orbital angular momentum related to their magnetic texture. As such, they experience the so-called skyrmion Hall effect (e.g., Jiang et al. Nature Physics (2016)) that distorts their trajectory and can be detrimental to their robust manipulation. We indeed discovered that such an effect is related to a “topological torque”, i.e., a spin torque that is only present in non-trivial topological objects (skyrmions and vortices) and that increases significantly upon reducing the size of the skyrmion [1,2,3]. Moreover, size reduction should also enhance the non-adiabaticity of the system, which also increases the skyrmion Hall effect [4,5]. So, bottom line, going small might be an issue.
Second, magnetic skyrmions are achieved in magnetic multilayers grown by sputtering, i.e., the realistic systems consist of polycrystalline samples. Therefore, in addition to the enhanced skyrmion Hall effect upon size reduction, the pinning is actually very strong, as shown by recent experiments (see e.g., Legrand et al., Nano Letters 17, 2703 (2017)). To better understand how magnetic skyrmion interaction with respect to disorder, we develop micromagnetic and analytical models and study the impact of the topological torques on skyrmion depinning (unpublished).
All studies indicate that the celebrated “topological protection” of skyrmions may not be as extraordinary as initially expected in the sense that it does not make ferromagnetic skyrmions much more robust against disorder, nor faster. However, recently experiments suggest that they can travel at least as fast as magnetic domain walls, which is already quite an achievement…and this is only the beginning. We are now moving towards their antiferromagnetic counterpart, the antiferromagnetic skyrmions (Barker & Tretiakov, PRL 116, 147203 (2016)), which do not show skyrmion Hall effect but do experience topological torques [6]…now it starts to become exciting again! 

                                       


Schematics of the topological torque: it acts like a reaction to topological spin Hall effect. The left panel is a artist view while the right panel shows the non-equilibrium spin accumulation profile in a magnetic vortex. See Ref. [1].

Group members

Akshaykumar Salimath


Collaborators

Mathias Klaui, J. Gutenberg University, Mainz, Germany
Kyung-Jin Lee, Korea University, Korea
Giovanni Finocchio and R. Tomasello , Università degli Studi di Messina
Oleg Tretiakov, Tohoku University, Japan
Gen Tatara, RIKEN, Tokyo, Japan



Skyrmion dynamics and topological torque


Moving magnetic textures have the ability to “pump” spin and charge currents, an effect tagged spin motive force (see Barnes and Maekawa PRL 98, 246601 (2007)). Magnetic skyrmion should of course show the same physics, but because of their non-trivial topological one expects this effect to be much larger. As a matter of fact, while a one-dimensional domain wall is enable to put a charge current in steady state (it needs to be in a turbulent regime – see the beautiful work by Yang et al. PRL 102, 067201 (2009)), a magnetic skyrmion can!

Moving magnetic textures have the ability to “pump” spin and charge currents, an effect tagged spin motive force (see Barnes and Maekawa PRL 98, 246601 (2007)). Magnetic skyrmion should of course show the same physics, but because of their non-trivial topological one expects this effect to be much larger. As a matter of fact, while a one-dimensional domain wall is enable to put a charge current in steady state (it needs to be in a turbulent regime – see the beautiful work by Yang et al. PRL 102, 067201 (2009)), a magnetic skyrmion can!

To demonstrate this idea, we use a time-dependent tight-binding model, and study the motion of a skyrmion in a nanowire. We find that steadily moving magnetic skyrmions can indeed pump a charge current along the wire, and analyze this phenomenon using the scattering theory of adiabatic time-dependent textures. Of course, pumping spins consumes energy, which renormalizes the magnetic damping. We also propose and demonstrate that this effect has a direct impact on the collective motion of skyrmions. Since each skyrmion participate in the pumped charge current, they mutually apply torques on each other, which globally enhances the mobility (unpublished). Think about slipstreaming during bicycle races: by sticking together, bicycle riders save energy by improving their air penetration, and therefore overall speed. 


Group members

Adel Abbout


Collaborators

Xavier Waintal, Joseph Weston, CEA Grenoble, France



Spin transmission through frustrated systems

The transmission of spin signal through magnetic insulators has recently experiences a major burst with the development of spin caloritronics (Bauer et al., Nature Materials 5, 391 (2012)) and the demonstration of efficiency spin current transmission through YIG (Kawajira et al., Nature 464, 262 (2010)). Nowadays, a great deal of attention is given to the investigation of spin superfuidity in magnetic materials. In the superfluid phase, matter behaves as a fluid with zero viscosity and flows without dissipation (E. Sonin, Adv. Phys. 59, 181 (2010)). It has been shown that the easy-plane ferromagnetic or antiferromagnetic and either metallic and insulating thin films exhibit spin superfluidity in which the spin current is dissipation less over long distances (S. Takei and Y. Tserkovnyak, Phys. Rev. Lett. 112, 227201 (2014)).
Our intention is to explore the ability of frustrated systems to transport spin information. Indeed, frustrated magnets and spin liquids are characterized by finite range order. Investigating the manner spin current are transmitted through such channel could therefore give information about the nature of magnetic order (unpublished).
In magnetic insulators, the electrons are not free to move but bound to the atomic cores. The interaction between magnetic moments can lead to magnetic ordering below a certain temperature and to spin-wave excitations, whose quanta are quasiparticles. These magnons can also carry information and be utilized in logic devices, which are studied in the research area of spin-electronics in insulators: magnonics (Chumak, Nature Physics 11, 453 (2015)). Such devices are free of ohmic heating due to the absence of charge flow, making them very attractive with regard to low waste energy production and power consumption. In this perspective, we particularly interested in magnetic insulators displaying a certain degree of chirality, either through Dzyaloshinskii-Moriya or 4-spin interactions. We recently demonstrated that ferromagnetic magnons exert a torque on the magnetic order parameter that resembles strikingly the spin-orbit torque observed in metallic systems [7]. More to come on this subject…

Group members

Collaborators
Kyung-Jin Lee, Korea University, Korea

Publications

[1] Enhanced non-adiabaticity in vortex cores due to the emergent Hall effect, A. Bisig, C. A. Akosa, J.-H. Moon, J. Rhensius, C. Moutafis, A. von Bieren, J. Heidler, G. Kiliani, M. Kammerer, M. Curcic, M. Weigand, T. Tyliszczak, B. Van Waeyenberge,  H. Stoll, G. Schutz, K.-J. Lee, A. Manchon, and M. Klaui, Phys. Rev. Lett. 117, 277203 (2016).
[2] Intrinsic nonadiabatic topological torque in magnetic skyrmions and vortices, C.A. Akosa, P.B. Ndiaye, A. Manchon, Physical Review B 95 (5), 054434.
[3] Topological Hall and spin Hall effects in disordered skyrmionic textures, P. B. Ndiaye, C. A. Akosa, and A. Manchon, Phys. Rev. B 95, 064426 (2017).
[4] Role of spin diffusion in current-induced domain wall motion for disordered ferromagnets, Collins Ashu Akosa, Won-Seok Kim, Andre Bisig, Mathias Klaui, Kyung-Jin Lee, and Aurelien Manchon, Phys. Rev. B 91, 094411 (2015).
[5] Steady motion of skyrmions and domains walls under diffusive spin torques, R. Gabriel Elias, N. Vidal-Silva, and A. Manchon, Phys. Rev. B 95, 104406 (2017).
[6] Theory of Topological Spin Hall Effect in Antiferromagnetic Skyrmion: Impact on Current-induced Motion, C. A. Akosa, O. A. Tretiakov, G. Tatara, A. Manchon, arXiv:1709.02931 (2017).
[7] Magnon-Mediated Dzyaloshinskii-Moriya Torque in Homogeneous Ferromagnets, A. Manchon, P. Birame Ndiaye, J.-H. Moon, H.-W. Lee, and K.-J. Lee, Phys. Rev. B 90, 224403  (2014).