Periodic Reporting for period 2 - MAGicSky (Magnetic Skyrmions for Future Nanospintronic Devices)
Berichtszeitraum: 2016-09-01 bis 2018-08-31
The groundbreaking idea at the core of the MAGicSky proposal was to use the magnetic nanoscale quasi-particle that are the magnetic skyrmions and their exceptional properties, most of them associated to their topological nature, as a support of information in a new generation of spintronic devices. Predicted about 30 years ago, a skyrmion lattice phase was observed only 20 years later at low temperature and under large magnetic fields in non-centro-symmetric magnetic crystals such as MnSi of CoFeGe and using experimental techniques such as small angle neutron scattering or Lorentz TEM. Neither the compounds that were investigated not the experiments used to characterize them were not adapted with any kind of applications. The strategy adopted in MAGicSky to rely on skyrmions stabilized at room temperature in thin films and multilayers compatible which are compatible with industrial process on a long term took its infancy with the pioneer works realized by UHAM partner a few years before MAGicSky demonstrating that skyrmions stabilized by a large interfacial DM interaction can be very small with diameters in the nm range. Based on this discovery, we proposed to develop a complementary approach mixing model epitaxial systems with sputtered grown samples together with a strong theoretical support first to reach room temperature (RT) stabilization. Then, our second big objective was, because skyrmions behave as quasi-particles that can be moved by spin transfer torques, be created or annihilated, to rely on all these remarkable properties, that by the way were almost not studied yet in thin films and multilayers, to make them suitable for “abacus”-type applications in information storage devices, logic technologies and more recently for magnonic and neuromorphic hardware nanodevices.
One of our strategic choices in MAGicSky is to tailor the interface-DM interaction to observe by different imaging techniques some isolated skyrmions at RT in magnetic multilayers (MML) by stacking ultra-thin layers of transition metals (Co, Fe) and spin-orbit metals (Pt, Ir, W, Rh…) alternately. The concerted effort made during the first period enabled us to achieve this first challenging objective with the observation by several imaging techniques (STXM, MFM) and on different MML systems such as Pt/Co/Ir or Pt/Co/AlOx multilayers, some magnetic skyrmions at RT. This key advance has generated a large interest and was crucial for the success of MAGicSky as it was a prerequisite for the addressing the other fundamental objectives. Beyond this achievement, a lot of efforts combining experimental results in epitaxial MML systems and theoretical calculations by e.g. first principle calculations, spin dynamics and Monte Carlo simulations have been performed in the second reporting period to evaluate how the important magnetic parameters can be tuned by changing materials and composition to improve and optimize the skyrmion characteristics. One important challenge was notably to better understand and improve their thermal stability while decreasing their size. To this aim, we rely on elaborating samples with multiple repetitions instead of single thin films. The prize to pay for that is that the interlayer dipolar fields then become more important and can strongly modified the skyrmion shape and size. A combination of experiments, numerical simulations and modelling have been done during this period to tackle this issue, for example with the identification of hybrid chiral skyrmions in some cases, and proposed some guidance for the choice of magnetic parameters to eventually obtain ultra-small and mobile skyrmions at RT. Another important issue has been to improve understanding the interaction with defects or pinning centers.
One of our strategic choices in MAGicSky is to tailor the interface-DM interaction to observe by different imaging techniques some isolated skyrmions at RT in magnetic multilayers (MML) by stacking ultra-thin layers of transition metals (Co, Fe) and spin-orbit metals (Pt, Ir, W, Rh…) alternately. The concerted effort made during the first period enabled us to achieve this first challenging objective with the observation by several imaging techniques (STXM, MFM) and on different MML systems such as Pt/Co/Ir or Pt/Co/AlOx multilayers, some magnetic skyrmions at RT. This key advance has generated a large interest and was crucial for the success of MAGicSky as it was a prerequisite for the addressing the other fundamental objectives. Beyond this achievement, a lot of efforts combining experimental results in epitaxial MML systems and theoretical calculations by e.g. first principle calculations, spin dynamics and Monte Carlo simulations have been performed in the second reporting period to evaluate how the important magnetic parameters can be tuned by changing materials and composition to improve and optimize the skyrmion characteristics. One important challenge was notably to better understand and improve their thermal stability while decreasing their size. To this aim, we rely on elaborating samples with multiple repetitions instead of single thin films. The prize to pay for that is that the interlayer dipolar fields then become more important and can strongly modified the skyrmion shape and size. A combination of experiments, numerical simulations and modelling have been done during this period to tackle this issue, for example with the identification of hybrid chiral skyrmions in some cases, and proposed some guidance for the choice of magnetic parameters to eventually obtain ultra-small and mobile skyrmions at RT. Another important issue has been to improve understanding the interaction with defects or pinning centers.
In this second period, it was planned that a significant part of the effort will be put on the demonstration of the three basic functions required for the devices. First, for the nucleation of skyrmions, several different physical stimuli i.e. sweeping of magnetic fields, electric fields, and current pulses have been investigated both experimental and theoretically and trying for each of them to evaluate how they could be implemented in future devices. It is to be emphasized that, to our knowledge, the first electric field assisted nucleation of skyrmion and the first deterministic nucleation by short current pulses have been demonstrated within MAGicSky project. Second, for the electrical detection, once again, results from MAGicSky are the first showing that electrical detection of single skyrmion through non collinear magnetoresistance (NCMR) effects, a new effect introduced during this period, or change in Hall resistivity, both of them being transferable to future devices. Beyond the technical progress for the detection, we have tried to better understand the origin of the Hall signal associated to skyrmions in multilayers and notably concluded that the Topological Hall contribution is not the dominant mechanisms, however leaving some opened questions for the theoreticians as part of the Hall signal seems to be different from other classical contributions to Hall effect. Third, another important objectives during this second period has been to displace small skyrmion at room temperature using spin transfer effects. We succeeded to demonstrate some flow motion of skyrmions with velocities up to a few 10 m/s for current densities in the range of a few 10^11 A/cm², which is today at the start of the art. However, we also concluded that the role of material inhomogeneities and defects remains important and often perturb or even impede the displacement of skyrmions. Note that all groups working in this area have to face the same issues, meaning that a concerted effort on material science is still needed to improve the material properties as well as the efficiency of the spin transfer torques in order to find solutions and overcome these problems.
The advances on the fundamental objectives that have been achieved during the second period leading to the demonstration of generation, manipulation and detection of skyrmions in MML systems, have allowed us to go for the design and the fabrication of several types of skyrmions devices using some of MML systems identified as being the most promising. Different geometries of skyrmion-tracks including local point contact for current induced nucleation and electrical electrodes for longitudinal and transverse measurements have been prepared by either lithography process, notably from MML systems made of repetition of Pt/Co/Ir, Pt/Co/AlOx or more recently Pt/Co/Ru. In the perspective of preparing devices for characterization of the modes associated to skyrmions by local FMR, submicronic disks equipped with antenna has been also prepared. Finally, we report on test measurements of race track skyrmion devices demonstrating that the three functionalities have successfully proven at room temperature.
Finally, the consortium has organized an international workshop dedicated to the physics of skyrmions and the potential applications for a new generation of spintronic devices (Skymag 2). It has been held in Paris in the first week of May 2017, that all the feedbacks from the more than 150 participants were excellent. It has been clearly a great opportunity to disseminate the results of MAGicSky to the ever-growing community of colleagues working on skyrmions and more generally spin-orbitronic effects.
Finally, the consortium has organized an international workshop dedicated to the physics of skyrmions and the potential applications for a new generation of spintronic devices (Skymag 2). It has been held in Paris in the first week of May 2017, that all the feedbacks from the more than 150 participants were excellent. It has been clearly a great opportunity to disseminate the results of MAGicSky to the ever-growing community of colleagues working on skyrmions and more generally spin-orbitronic effects.