Report
Teaser, summary, work performed and final results
Periodic Reporting for period 2 - IMAGINE (Imaging magnetic fields at the nanoscale with a single spin microscope)
Teaser
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...
Summary
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 intend to exploit the unique performances of scanning-NV magnetometry to achieve major breakthroughs in nanomagnetism. The first objective is 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 is to detect orbital magnetism in graphene (WP3), which has never been observed experimentally and is the purpose of surprising theoretical predictions.
Work performed
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 homochiral Néel domain walls (DWs) and magnetic skyrmions. Since these chiral spin textures are at the heart of a number of emerging applications in spintronics, it is crucial to quantify precisely the DMI strength in technologically relevant ferromagnetic heterostructures.
In this context, 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. We have conclusively demonstrated that Bloch-type DWs are observed in a Ta/CoFeB(1nm)/MgO trilayer stack, which indicates that interfacial DMI can be safely neglected at the Ta/CoFeB interface. By replacing Ta by W, we have shown that the DMI is significantly increased, and is even strong enough to fully stabilize the DWs into the right-handed Néel configuration. By changing the interface to Pt/Co, we have further demonstrated that the sign of DMI can be reversed and the DWs then exhibit a left-handed Néel structure. Such studies will help 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 :
- 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. Such chiral spin structures with a vortex-like configuration cannot be continuously deformed to a ferromagnetic or other magnetic state. This remarkable topological protection promises skyrmion motion at ultralow current densities. Moreover, skyrmions can be as small as a few nanometers across and could potentially provide an ultrahigh information-storage density. These properties make skyrmions attractive candidates for information storage and processing at the nanoscale. However, in technologically relevant magnetic materials, structural defects often result in considerable pinning that limits the propagation velocity. Whereas domain walls necessarily experience all the disorder landscape when propagating along a magnetic track, skyrmions are localized magnetic quasiparticles, which are predicted to move in two dimensions while avoiding strong pinning sites. Skyrmions are thus expected to display limited interaction with disorder, leading to highly efficient motion at low current densities. Surprisingly, several recent experiments have instead shown that skyrmion dynamics are in fact strongly affected by disorder, suggesting that pinning effects have been oversimplified in seminal simulations of skyrmion dynamics. These observations motivate a more precise description of disorder in magnetic materials hosting skyrmions.
We have used scanning-NV magnetometry in quenching mode as a non-invasive, high resolution tool to investigate the morphology of isolated skyrmions in ultrathin magnetic films. We have shown that the skyrmion size and shape are strongly affected b
Final results
\"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.
Towards the end of the project, we will make use of our fully functional scanning-NV magnetometer operating at room temperature to explore the spin texture of antiferromagnetic (AF) materials, with a key focus on systems relevant for the emerging field of AF spintronics. This encompasses the real-space imaging, with sub-30 nm spatial resolution, of (i) AF domains, (ii) uncompensated domain walls, (iii) periodic spin orders and (iv) topologically-protected spin structures such as skyrmions. Scanning NV-magnetometry will then be combined with \"\"operando\"\" manipulation of the AF order (e.g. by electric means) leading to the direct observation of the AF domains dynamics under external driving. Following our recent proof-of-concept experiment, the work will be focused on multiferroic materials (BiFeO3, TmFeO3) in which antiferromagnetism coexists with ferroelectricity, enabling an efficient electrical control of magnetization through magnetoelectric coupling. This work might lead to a new paradigm for antiferromagnetic spintronics.
In parallel, we will use our recently developed NV magnetometer operating at cryogenic temperature (4K) to detect orbital magnetism in graphene. Furthermore, recent advances in studies of van-der-Waals (vdW) crystals have demonstrated the existence of magnetic order down to the monolayer limit. While such 2D ferromagnets offer opportunities for radically new functionalities in vdW heterostructures, the underlying mechanisms stabilizing the magnetic order remain poorly understood. We will use NV magnetometry at low temperature to shed light on the magnetic order in 2D materials and to investigate its interactions with semiconducting materials in complex vdW heterostructures.
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