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Imaging magnetic fields at the nanoscale with a single spin microscope

Periodic Reporting for period 4 - IMAGINE (Imaging magnetic fields at the nanoscale with a single spin microscope)

Reporting period: 2020-03-01 to 2020-08-31

Detecting and imaging magnetic fields with high sensitivity and nanoscale resolution is a topic of crucial importance for a wealth of research domains, from material science, to mesoscopic physics, and life sciences. This is obviously also a key requirement for fundamental studies in nanomagnetism and the design of innovative magnetic materials with tailored properties for applications in spintronics. Although a remarkable number of magnetic microscopy techniques have been developed over the last decades, imaging magnetism at the nanoscale still remains a challenging task.

During the last years, it was realized that the experimental methods allowing for the detection of single spins in the solid-state, which were initially developed for quantum information science, open new avenues for high sensitivity magnetometry at the nanoscale. In that spirit, it was proposed to use the electronic spin of a single nitrogen-vacancy (NV) defect in diamond as a nanoscale quantum sensor for scanning probe magnetometry. This approach promises significant advances in magnetic imaging since it provides non-invasive, quantitative and vectorial magnetic field measurements, with an unprecedented combination of spatial resolution and magnetic sensitivity, even under ambient conditions.

The IMAGINE project aimed at exploiting the unique performances of scanning-NV magnetometry to achieve major breakthroughs in nanomagnetism. The first objective was to explore the physics of chiral spin textures in ultrathin ferromagnetic materials, such as homochiral Néel domain wall (WP1) and magnetic skyrmions (WP2), which both promise disruptive applications in spintronics. The second objective was to detect orbital magnetism in graphene (WP3), which has never been observed experimentally and is the purpose of surprising theoretical predictions.
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.
The field of nanomagnetism contains a wealth of opportunities for science and technology, which is being unlocked by a new generation of magnetometer. Amongst the magnetic sensors that are available to researchers today, the NV center in diamond stands alone in its ability to detect and image weak magnetic fields with high spatial resolution. The project IMAGINE has established scanning-NV magnetometry as a unique tool for probing complex magnetic order at the nanoscale. Besides ultrathin ferromagnets, we have demonstrated that scanning-NV magnetometry is ideally suited to image the magnetic order in antiferromagnetic materials. Although not initially planned in the IMAGINE project, these activities were very successful and triggered a new and unforeseen application of scanning-NV magnetometry.

Significant publication:
- I. Gross et al., Nature 549, 252 (2017)
- J. Y. Chauleau, et al., Nature Materials 19, 386 (2020)
- A. Haykal et al., Nature Communications 11, 1704 (2020)