Towards oxide-integrated epitaxial graphene-based spin-orbitronics

An international team of researchers from IMDEA Nanociencia and Complutense and Autónoma universities in Madrid, the Institut Néel in Grenoble and the ALBA Synchrotron in Barcelona has elucidated a new property of Graphene/Ferromagnetic interfaces: the existence of a sizable magnetic unidirectional interaction, technically a Dzyaloshinskii–Moriya Interaction of Rashba origin, which is responsible for establishing a chiral character to magnetic domain wall structures.

A major challenge for future spintronics is to develop suitable spin transport channels with long spin lifetime and propagation length. Graphene can meet these requirements, even at room temperature. On the other side, taking advantage of the fast motion of chiral textures, that is, Néel-type domain walls and magnetic skyrmions, can satisfy the demands for high-density data storage, low power consumption, and high processing speed. The integration of graphene as an efficient spin transport channel in the chiral domain walls technology depends on the ability to fabricate graphene-based perpendicular magnetic anisotropy (PMA) systems with tailored interfacial SOC.

Studies on graphene-based magnetic systems are not abundant and, typically, make use of metallic single crystals as substrates which jeopardize the exploration of their transport properties (since the current is drained by the substrate). To solve this challenge, the IMDEA Nanociencia leading team succeeded to fabricate high-quality epitaxial asymmetric gr/Co/Pt(111) structures grown on (111)-oriented oxide substrates. The quality of the interfaces was checked by low-energy electron diffraction and also by advanced high-resolution transmission microscopy at the Universidad Complutense de Madrid (UCM) microscopy centre and resonant X-ray specular reflectivity at BOREAS beamline at ALBA (see fig.1). The magnetic anisotropy and properties were investigated by magneto-optical Kerr magnetometry in IMDEA and Universidad Autónoma de Madrid (UAM) and complemented with element resolved XMCD magnetometry also at BOREAS beamline. Finally, the chirality of the magnetic domain walls was analysed using a customized magneto-optical Kerr effect microscope and pulse field electronics in collaboration with the team at Institut Néel in Grenoble.

>Read more on the ALBA website

 

Diamond celebrates publication of its 7000th paper

A paper in PNAS by an international scientific collaboration from the UK, Germany and Switzerland is the 7000th to be published as a result of innovative research conducted at Diamond Light Source, the UK’s Synchrotron.

This new paper reveals details of the 3D spin structure of magnetic skyrmions, and will be of key importance for storing digital information in the development of next-generation devices based on spintronics.

Laurent Chapon, Diamond’s Physical Sciences Director, explains the significance of these new findings:  “A skyrmion is similar to a nanoscale magnetic vortex, made from twisted magnetic spins, but with a non-trivial topology that is ‘protecting them’. They are therefore stable, able to move, deform and interact with their environment without breaking up, which makes them very promising candidates for digital information storage in next-generation devices. For years, scientists have been trying to understand the underlying physical mechanisms that stabilise magnetic skyrmions, usually treating them as 2D objects. However, with its unique facilities and ultra-bright light, Diamond has provided researchers the tools to study skyrmions in 3D revealing significant new data.”

As spintronic devices rely on effects that occur in the surface layers of materials, the team was investigating the influence of surfaces on the twisted spin structure. It is commonly assumed that surface effects only modify the properties of stable materials within the top few atomic layers, and investigating 3D magnetic structures is a challenging task. However, using the powerful circularly polarised light produced at Diamond, the researchers were able to use resonant elastic X-ray scattering (REXS) to reconstruct the full 3D spin structure of a skyrmion below the surface of Cu2OSeO3.

>Read more on the Diamond Light Source website

Image: (extract) Illustration of a ‘Skyrmion tornado’. The skyrmion order changes from Néel-type at the surface to Bloch-type deeper in the sample. On the right hand side, the corresponding stereographic projections of these two boundary skyrmion patterns are shown. Full image and detailed article here.

Ferromagnetic and antiferromagnetic coupling of spin molecular interfaces

Researchers from the physics department of the Università “La Sapienza” in Rome, Centro S3 of Modena and ALBA, have demonstrated that magnetic coupling of metal-organic molecules to a magnetic substrate mediated by a graphene layer can be tuned in strength and direction by choosing the symmetry of the molecular orbitals that is largely preserved thanks to the graphene layer. The results have been published in the journal Nano Letters.
Paramagnetic molecules become potential building blocks in spintronics when their magnetic moments are stabilized against thermal fluctuations, for example, by a controlled interaction with a magnetic substrate. Spin molecular interfaces with preserved magnetic activity and exhibiting magnetic remanence at room temperature (RT) can open the route to engineer highly spin-polarized, nanoscale current sources. The need to fully control the organic spin interface and the tuning of ferromagnetic (FM) or antiferromagnetic (AFM) coupling to achieve a stable conductance has motivated a vast experimental interest.

Image: Figure 1: a,b) Antiferromagnetic/Ferromagnetic coupling as deduced by element-specific hysteresis loops of  a FePc and CuPc (respectively) to a Cobalt layer with perpendicular magnetic anisotropy intercalated below graphene. c,d) orbital-porjection of the spin-density for the FePc and CoPc interface reflecting the different symmetry of the molecular orbitals involved in the ferromagnetic and antiferromagnetic interaction.

Researchers obtain nanometric magnetite with full properties

When reducing materials at the nanoscale, they typically lose some of their properties. The experiments have been carried out at the CIRCE beamline of the ALBA Synchrotron.

Magnetite is a candidate material for various applications in spintronics, meaning that can be employed in devices where the spin of the electron is used to store or manipulate information. However, when it is necessary to create structures of the material at the nanometric scale, their properties get worse. A study, recently published in the scientific journal Nanoscale, has proved that, with suitable growth, magnetite could be used to create nanostructured magnetic elements without losing their properties.

“Oxides have been proposed to be used for spin waves in triangular structures for computing. And our results suggest that magnetite could be used for this purpose, “says Juan de la Figuera, scientist from the Spanish National Research Council (CSIC).

>Read more on the ALBA website

Image: Beamline involved where nanometric magnetite has been obtained, keeping its full properties.
Credit: ALBA

Maximal Rashba-like spin splitting via kinetic-energy-coupled inversion-symmetry breaking

Research collaboration led by Professor Philip King from University of St. Andrews, and comprising the researchers from Max Planck Institute for Chemical Physics of Solids in Dresden, Institute for Theoretical Physics of the University of Heidelberg and researchers from I05 beamline at Diamond Light Source and APE beamline at Elettra, described a new route to maximise the spin-splitting of surface states.
The electronic properties of surfaces are often different from those of the bulk. In particular, the intrinsically broken symmetries of the surface compared with the bulk of the material allow for appearance of the new electronic surface states. For the systems in which spin-orbit interaction is strong, a non-negligible separation of these states according to their spin takes place. The spin splitting of surface- or interface-localized two-dimensional electron gases is characterized by a locking of the electron spin perpendicular to its momentum.

Read more on the Elettra website.

(a) Bulk and surface Fermi surfaces of PtCoO2 measured by ARPES; (b) Expected spin texture of the surface states; (c) Spin-resolved ARPES measurements of an in-plane spin polarization (〈Sy〉) of the Fermi surface for the cut along kx.

Time – and spatially – resolved magnetization dynamics driven by spin-orbit torques

There is a strong correlation between the rise of a civilization and writing. The so-called Information Age developed in parallel with the ability to write, store, and process large amounts of digital data. To keep pace with the increasing demand for data of our days, not only the size but also the speed of digital memories must increase dramatically, while keeping the energy consumption at reasonable levels. In order to achieve that, we must learn to write anew.

>Read More on the PSI website

Image: Magnetisation switching of a 500 nm diameter Pt/Co/AlOx disc.