1 - Physics of domain walls in ultrathin magnetic wires with perpendicular magnetic anisotropy
The search for a medium that allows high information storage density combined with low power consumption, has motivated the study of low dimensional magnetic systems. In such materials, lowered symmetry gives rise to a new category of dominating interactions, whose interplay leads to exotic magnetization patterns. One example of such systems are magnetic thin film multilayers lacking inversion symmetry, which give rise to the Dzyaloshinskii-Moriya interaction (DMI), an anti-symmetric exchange interaction occurring at the interface between a ferromagnetic layer and a heavy metal substrate with large spin-orbit coupling. In ultrathin magnetic wires, interfacial DMI plays a fundamental role in the stabilization of chiral domain walls (DWs) and magnetic skyrmions, which are at the heart of a number of emerging applications in spintronics. We have shown that the DMI strength can be inferred with scanning-NV magnetometry by measuring the inner structure of DWs in ultrathin ferromagnetic wires. The method relies on quantitative stray field measurements to analyze the transition from a Bloch to a Néel DW configuration induced by the DMI. These results helped to better understand the microscopic origin of interfacial DMI in ultrathin ferromagnets and to identify magnetic samples with large DMI strength that could sustain magnetic skyrmions.
Significant publications :
- J. P. Tetienne et al., Nature Communications 6, 6733 (2015)
- I. Gross et al., Phys. Rev. B 94, 064413 (2016) - Selected as “Editor suggestion”
2 - Physics of magnetic skyrmions in ultrathin ferromagnets
Another striking phenomenon induced by the DMI is the formation of magnetic skyrmions. The goal of WP2 was to study the physics of such topological spin textures in ultrathin ferromagnets. We have first examined the impact of structural disorder and magnetic history on the morphology of skyrmions in a symmetric bilayer system, opening the way to an in-depth understanding of skyrmion dynamics in real, disordered media. In the quest to find a host ferromagnetic material providing efficient current-induced skyrmion dynamics, we then explored the possibility to use Heusler alloys, a class of ferromagnetic materials with an intrinsically low magnetic damping coefficient. We demonstrated (i) the first stabilization of magnetic skyrmions in a single ultrathin layer of Co2FeAl and (ii) an efficient current-induced nucleation process mediated by spin orbit torque, which is not specific to Heusler alloys and could be advantageous for future skyrmion-based devices. However, current-induced skyrmion motion experiments revealed that pinning effects still limit skyrmion velocity, even in a host material with low magnetic damping. Finally, we have demonstrated the stabilization of 60 nm skyrmions at zero external magnetic field in an optimized exchange-biased multilayer stack. Compared to previous studies, it corresponds to a reduction in skyrmion diameter by one-order of magnitude. These results have established exchange-biased multilayer stacks as a promising platform towards the effective realization of memory and logic devices based on the manipulation of magnetic skyrmions.
Significant publications :
- A. Hrabec et al., Nature Communications 8, 15765 (2017)
- I. Gross et al., Phys. Rev. Materials 2, 024406 (2018)
- W. Akhtar et al., Phys. Rev. Applied 11, 034066 (2019)
- Gaurav Rana et al., Phys. Rev. Applied 13, 044079 (2020) - Selected as “Editor suggestion”
3 – Imaging orbital magnetism in graphene
The goal of WP3 was to detect orbital magnetism in graphene, which has never been observed experimentally to date, and is the purpose of surprising theoretical predictions. For this specific application, we have built a scanning-NV magnetometer operating under cryogenic conditions (4 K). With this instrument we did not succeed to detect orbital magnetism in graphene yet.
