In this very active field in spintronics, most recent studies rely on ferromagnetic thin films or multilayers containing chiral walls, spin spirals or skyrmions. The aim of SPiCY will be to further extend the investigation of chiral and topological properties to new families of ferrimagnetic and even antiferromagnetic materials. This is of high interest as these novel topological textures shall present very interesting perspectives to stabilize topological objects of ultimate size at room temperature and to achieve ultra-fast dynamics [CARE2018, LEGR2019].
Skyrmions are whirling spin structures that cover smoothly every spin direction, that are compact (spatially limited), and topologically non-trivial. They are energetically favored by the Dzyaloshinskii-Moriya (DM) interaction present in some non-centrosymmetric systems. It was their observation in thin films that lead to the proposition of skyrmions as the basic elements of magnetic data storage. In 2013, pioneer simulations of skyrmions in Co/Pt bi-layers realized by UMPhy and LPS [SAMP2013] indicated that interfacial DMI could stabilize isolated skyrmions in nanostructures, and that the spin Hall effect (SHE) in the Pt layer could move them efficiently with little hindrance from border shape defects. These features — a stable, small, and compact spin structure that is efficiently driven by current — show that skyrmions can overcome their domain walls counterparts and be the most promising information carrier in dense and fast spintronic data storage and processing devices [FERT2017].
These predictions have triggered a surge of studies of skyrmions. In less than 5 years, skyrmions have been observed in many material systems [MORE2016, BOUL2016], and their nucleation and propagation with current has been demonstrated [WOO2016, HRAB2018]. However, several issues remain for stabilization of ultra-small and fast skyrmions. Indeed, the dipolar coupling stemming from a large magnetization promotes larger entities and skyrmions move deflected sideways by a gyrotropic force proportional to their velocity. This force can annihilate a skyrmion propagating in a narrow track, a problem for the envisioned devices.
Antiferromagnetic materials, due to their zero magnetization and angular momentum have been proposed as a solution, to overcome the drawback of ferromagnets. However, they are hard to elaborate and probe and therefore have been rarely studied or used in applications. Promising alternatives are the synthetic antiferromagnets and ferrimagnets that keep the advantage of ferromagnets in term of elaboration, a low magnetization and should mimic the dynamics of antiferromagnets, as already demonstrated on domain walls [YAN2015, CARE2018].
Synthetic antiferromagnets (SAFs) are substitutes for antiferromagnets that are composed of nanometer-thick ferromagnetic layers coupled antiferromagnetically through a non-magnetic spacer layer. Like antiferromagnets, SAFs provide vanishing long-range dipolar interactions. However, because the two subsystems are separated by a spacer of about 1nm, small, but measurable, local dipolar fields are present, which has allowed the study of synthetic antiferromagnetic skyrmions (see Fig. 1) [LEGR2019]. Moreover, the DMI in SAFs may be conveniently controlled by employing the knowledge gathered from ferromagnetic layer studies.
In the class of ferrimagnetic materials, rare-earth/transition metal (RE-TM) alloys e.g. GdFe, TbFe, GdCo etc. exhibit continuously tunable MS and LS with alloy composition or temperature, interesting spintronic properties (large spin polarization), large perpendicular anisotropy, amorphous crystalline structure (potentially much better homogeneity), and interfacial DM interaction and Spin Hall Effect (SHE) with a Pt adjacent layer. Several exciting results have very recently appeared, highlighting the interest of RE-TM: room-temperature- stable, 25-nm-sized skyrmions were obtained with no bias field, in Pt/CoGd films [CARE2018] and skyrmions moved under current (50 m/s at 300 GA/m2) with a reduced gyrotropic deflection in multilayers of [Pt/GdFeCo/MgO]20 [WOO2018]
Skyrmions in SAFs or in ferrimagnets would have many advantages over their conventional, ferromagnetic skyrmions analogs, and in addition may prove easier to be studied than in antiferromagnetic material. The teams involved in SPiCY have a long-developed expertise on the main pillars of this topic: the physics of spin transfer torques, the understanding of chiral domain walls or skyrmions dynamics but also in the elaboration of materials with DM interaction as well as in micromagnetic modelling.
Synthetic antiferromagnetic skyrmions have already been obtained in which perpendicular magnetic anisotropy, antiferromagnetic coupling and chiral order were concomitantly adjusted. Utilizing interlayer coupling to an adjacent bias layer, it was demonstrated that spin-spiral states obtained in a SAF with vanishing perpendicular magnetic anisotropy can be turned into isolated skyrmions [LEGR2019]. The deposition and nano-fabrication of homogeneous RETM films with precise and stable composition has been achieved [HALT2018]. In particular, recent measurements in Pt/GdFeCo show a SHE efficiency similar to the ones of Pt/Ferromagnet and demonstrate that the DW precession fully vanishes with a record mobility at the temperature for which the net angular momentum is compensated (TAC) [HALT2019].
Phd hired in SpiCY
- PhD on static and dynamic properties of skyrmions in ferrimagnetic systems
Partners involved: LPS (lead), UMPhy, SOLEIL
This PhD is planned to specifically study the properties of magnetic skyrmions in ultrathin ferrimagnetic films. - PhD on topological insulators/magnetic systems for spin-charge conversion
Partners involved: UMPhy (lead), C2N, SOLEIL
This PhD is planned to study spin to charge current conversion from a BiSb topological insulator. - PhD on ultrafast spintronic
Partners involved: SPEC (lead), UMPhy, SOLEI
This PhD work here aims at assessing the potential of new spintronic components at the picosecond timescale. - PhD on non-linear spin-wave dynamics in non-reciprocal spin-wave bus
Partners involved: C2N (lead), SPEC, UMPhy
This PhD is planed to study non-linear spin-wave dynamics in Bi substituted Yttrium Iron Garnet (YIG) thin films grown by PLD at UMPhy [SOUM2018] and in Heusler Co2MnX alloys (X = Ge, Si) grown by MBE at Institut Jean Lamour, Nancy, both families of materials having record damping values in the low 10-4.
References
[CARE2018] L. Caretta, et al.
Fast current-driven domain walls and small skyrmions in a compensated ferrimagnet
Nat. Nanotechnol.13, 1154 (2018)
[LEGR2019] W. Legrand et al.
Room-temperature stabilization of antiferromagnetic skyrmions in synthetic antiferromagnets
Nature Materials, Sept (2019)
[SAMP2013] J. Sampaio et al.
Nucleation, stability and current-induced motion of isolated magnetic skyrmions in nanostructures
Nature Nano. 8, 839 (2013)
[FERT2017] A. Fert, N. Reyren, V. Cros
Magnetic skyrmions: advances in physics and potential applications
Nature Review Materials 2, 17031 (2017)
[MORE2016] C. Moreau-Luchaire et al
Additive interfacial chiral interaction in multilayers for stabilization of small individual skyrmions at room temperature
Nat. Nanotech. 11, 444 (2016)
[BOUL2016] O. Boulle, et al.
Room-temperature chiral magnetic skyrmions in ultrathin magnetic nanostructures
Nat. Nanotech. 11, 449 (2016)
[WOO2016] S. Woo et al.,
Observation of room-temperature magnetic skyrmions and their current-driven dynamics in ultrathin metallic ferromagnets
Nat. Mat. 15, 501 (2016)
[HRAB2018] A. Hrabec et al.
Current-induced skyrmion generation and dynamics in symmetric bilayers
Nature. Com. (2017)
[YAN2015] S.H., Yang et al.
Domain-wall velocities of up to 750 m/s driven by exchange-coupling torque in synthetic antiferromagnets
Nat. Nanotechnol. 10, 221 (2015)
[CARE2018] L. Caretta, et al.
Fast current-driven domain walls and small skyrmions in a compensated ferrimagnet
Nat. Nanotechnol.13, 1154 (2018)
[WOO2018] S. Woo et al.
Current-driven dynamics and inhibition of the skyrmion Hall effect of ferrimagnetic skyrmions in GdFeCo films
Nat. Commun.9, 959 (2018)
[HALT2018] E. Haltz et al.
Deviations from bulk behaviour in TbFe(Co) thin films:Interfaces contribution in the biased composition
Physical Review Materials 2, 104410 (2018)
[HALT2019] E. Haltz et al
Precession-free domain wall dynamics in compensated ferrimagnets
http://arxiv.org/abs/1908.08867