Topological magnetism emerges in amorphous materials

Single-crystals and multilayer heterostructures with global inversion symmetry breaking can promote the formation of topological solitary vector fields owing to a vector spin exchange known as the Dzyaloshinskii-Moriya interaction (DMI). - And so can disorder. - In this work, we report experimental evidence of 3D chiral spin textures, i.e., helical spins and skyrmions with different chirality and topological charge, stabilized in amorphous Fe–Ge thick films. Our results demonstrate that structurally and chemically disordered materials with a random DMI can resemble inversion symmetry broken systems with similar magnetic properties, moments, and states. Yet, disordered systems are distinct by their degenerate spin chirality that allows for forming isotropic and anisotropic topological spin textures at remanence while offering greater flexibility in materials synthesis, voltage and strain manipulation, and an enhanced spin-orbit coupling relevant to prospective microelectronics applications.

The work was carried out in collaboration with the University of California, Berkeley, and at Berkeley Lab's Advanced Light Source, National Center for Electron Microscopy, and Materials Sciences Division.

The original work was published in Adv. Mater. 33, 2004830 (2021). 🔗

Suvechhya Lamichhane and Bibek Tiwari join as graduate research assistants

We are excited to welcome Suvechhya and Bibek to the group! Suvechhya and Bibek are second-year graduate students in the Physics department. Their work will focus on numerical modeling of temperature, strain and resonance effects in exchange and dipole-couple materials in preparation of experimental studies.

Ruthi Zielinski joins as undergraduate research assistant

Welcome to the group, Ruthi! We are excited to have you on-board. Ruthi received one of UNL's First Year Research Experience (FYRE) awards. Her research concerns numerical studies of order-by-disorder phenomena in dipole-coupled macro spin systems.

2020 Emerging Leaders of the Journal of Physics: Condensed Matter

Robert Streubel is recognized as an Emerging Leader by the Journal of Physics: Condensed Matter. His special issue contribution "The effect of Cu additions in FePt-BN-SiO2 heat-assisted magnetic recording media" focuses on the impact of structural and chemical order in L10 FePt. Two copper-driven mechanisms, although competing, can lead to improvements in both structural and magnetic properties. Cu substitution on the Fe-site degrades magnetic properties due to delocalized electron orbitals originating from a larger Cu d-orbital occupancy. At the same time, Cu enhances crystallographic order and consequently magneto-crystalline anisotropy, which offsets the former effect to a great extent.

The work was carried out in collaboration with Western Digital and at Berkeley Lab's Advanced Light Source.

The work was published in J. Phys.: Condens. Matter 33, 104003 (2021). 🔗

Perspective on ferromagnetic liquids

Assembly and jamming of nanoparticles at liquid interfaces is a versatile approach to construct liquids with solid-state properties. These hierarchical systems obtain their functionality from the short-range order and properties of individual nanoparticles. Using superparamagnetic nanoparticles imparts a magnetic moment, contrary to ferrofluids, and enables heterostructuring of the interface, when using various types of nanoparticles. We discuss recent advances in synthesis and characterization, and outline future scientific and technological directions, challenges and application potential of all-liquid robotics.

The perspective was published in Materials 13, 2712 (2020). 🔗

Launching a new dimension with 3D magnetic nanostructures

Three-dimensional magnetic nanostructures emerge from bulk and 2D planar materials with exciting novel physical phenomena originating from the increased complexity in spin textures, topology, and frustration in three dimensions. They bear tremendous potential for applications in magnetic sensing and information processing technologies in terms of improved energy efficiency, processing speed, functionalities, and miniaturization of future spintronic devices. We provide an overview of recent progress and scientific challenges with regard to advances in synthesis approaches and state-of-the-art nanoscale characterization techniques that are prerequisite to understand, realize, and control the properties, behavior, and functionalities of three-dimensional magnetic nanostructures.

The review was published in APL Materials 8, 010701 (2020). 🔗

Reconfigurable ferromagnetic liquid droplets

Have you ever wondered whether a liquid could be ferromagnetic? - Ferrofluids, although reconfigurable, lose their magnetic moment and thus functionality upon removal of the external magnetic field. - In our work, we manufacture a material that is mechanically a liquid and possesses magnetic properties of a solid magnet. It can be easily reconfigured into different shapes while preserving the magnetic properties of solid ferromagnets with classic north-south dipole interactions. These novel functionalities rely on a reversible paramagnetic-to-ferromagnetic transformation of ferrofluid droplets by the jamming of a monolayer of magnetic nanoparticles assembled at the water-oil interface.

"This new liquid is [...] mesmerizing [...] and could be a boon to medicine and robotics" (New York Times).

The work was carried out in collaboration with Berkeley Lab's Materials Sciences Division.

The original work was published in Science 365, 264 (2019). 🔗

Our activities are highlighted in: Science Perspective | Science Podcast | New York Times | Berkeley Lab | Newswise | Science News | Phys.org | pro-physik.de

Special issue on 3D nano magnetism and magnetic materials

Advances in synthesis science and characterization techniques combined with novel concepts for microelectronics, magnetic storage, and sensing applications have fueled the appeal of 3D nano magnetism. This vital research area is comprised of magnetic nano structures, nano membranes, and particle assemby, as well as 3D magnetization vector fields driven by vector spin exchange or spin frustration in heterogeneous condensed matter. In these systems, new functionalities emerge owing to the nano scale features of magnetization. The emergence of topological magnetic states, on the nano scale, renders magnetic materials in the light of quantum materials, which are envisioned to serve future microelectronics based on neuromorphic computing and racetrack memory applications.

This Special Issue welcomes original research contributions and review articles highlighting recent advances and future directions in the field of 3D nano magnetism and magnetic materials. 🔗

Review on x-ray ptychography

Tailoring structural, chemical, and electronic (dis-)order in heterogeneous media is one of the transformative opportunities to enable new functionalities and sciences in energy and quantum materials. This endeavor requires elemental, chemical, and magnetic sensitivities at the nano/atomic scale in 2D and 3D space. One of the most promising methods is coherent diffraction imaging, namely, x-ray ptychography, which is envisioned to take on the dominance of electron imaging techniques offering atomic resolution in the age of diffraction limited light sources. In this review, we discuss the current research examples of far-field diffraction-based x-ray ptychography on 2D and 3D semiconductors, ferroelectrics, and ferromagnets and their blooming future as a mainstream tool for materials sciences.

The review was published in Appl. Phys. Rev. 6, 011306 (2019). 🔗

Observation of order-by-disorder phenomena in XY macro spin lattices

The appeal of geometric frustration is fueled by exotic physical behaviors originating from a macroscopic ground state degeneracy, which manifest spin ices, spin glasses, quantum spin liquids, vortices, Skyrmions, and Hopfions. In our work, we investigate magnetic phases and correlations of planar XY macro spin lattices and report the experimental observation of magnetic ground states through direct imaging, predicted three decades ago. The possibility to fabricate and investigate XY dipolar systems at the micro scale opens up a new avenue in the field of artificial spin ice toward XY spins, spin liquids, Kitaev’s XYZ honeycomb lattices, and magnonics with tunable interactions.

The work was carried out in collaboration with Berkeley Lab's Advanced Light Source, Molecular Foundry, and Materials Sciences Division.

The original work was published in Nano Lett. 18, 7428 (2018). 🔗

It is featured as a cover art. 🔗

Experimental evidence of chiral ferrimagnetism

To date, chiral spin textures, such as Skyrmions, have mainly been studied in helimagnets and thin ferromagnets with heavy-element capping. In our work, we present experimental evidence that the concept of chirality driven by interfacial Dzyaloshinskii–Moriya interaction can be generalized to complex multicomponent systems, and demonstrate it on the example of chiral ferrimagnetism in amorphous GdCo films, utilizing high-resolution Lorentz microscopy with exit wave reconstruction and quantitative x-ray magnetic circular dichroism spectroscopy. Our findings open up a new avenue towards investigating chirality effects in spin orbitronics, spin caloritronics, all-optical switching and phase transitions.

The work was carried out in collaboration with the University of California, Berkeley, and at Berkeley Lab's Advanced Light Source, National Center for Electron Microscopy, and Materials Sciences Division.

The original work was published in Adv. Mater. 30, 1800199 (2018). 🔗

It is featured as a front-is-piece cover art. 🔗

Our activities are highlighted in: Newswise | Berkeley Lab | Phys.org | EurekAlert

Robert Streubel invited to participate in 2017 CREDITS retreat

The Center for Research, Excellence and Diversity in Team Science (CREDITS) is an integrated research and training program to increase and enhance Team Science (TS) and collective intelligence capacity, effectiveness, and excellence in California. Diversity on teams is known to have positive effects on creativity, innovation, and productivity. Apart from its contribution to scientific breakthroughs and grand challenge problems, TS has beneficial impacts on individual research careers. Diverse TS projects garner more funding, and yield greater publication productivity, and higher impact publications.

Robert Streubel had the great honor to be one of 22 faculty-track attendees selected from the CSU and UC systems. The retreat has been an invaluable experience for extending perception of good leadership and networking with dedicated team players ranging from recently appointed tenure-track to senior professors.

More information on this intriguing program and retreat can be found online. 🔗

Nanosecond x-ray photon correlation spectroscopy

Coherent x-ray scattering has emerged as one of the leading tools to study microscopic fluctuations in materials. However, the temporal resolution for the so-called x-ray photon correlation spectroscopy (XPCS) is limited by coherence and acquisition, and at best a few microseconds. Using a pair of x-ray pulses generated by two individual excitation lasers at the free electron laser LCLS (SLAC National Accelerator Laboratory), we were able to probe fluctuations in magnetic Skyrmion phases on nanosecond time scales. This work was led by Joshua Turner (SLAC) and Sujoy Roy (Berkeley Lab), and carried out in close collaboration with Eric Fullerton and Sunil Sinha (UCSD), and Steve Kevan and Peter Fischer (Berkeley Lab).

The original work was published in Phys. Rev. Lett. 119, 067403 (2017). 🔗

Our activities are highlighted in: Physics | MRS Bulletin | Newswise | SLAC | Berkeley Lab | Phys.org | Nanowerk | Advantage Business Media | Long Room

Review on three-dimensional nanomagnetism

Whereas patterned nanomagnets are traditionally two-dimensional planar structures, recent work is expanding nanomagnetism into three dimensions; a move triggered by the advance of unconventional synthesis methods and the discovery of new magnetic effects. In three-dimensional nanomagnets more complex magnetic configurations become possible, many with unprecedented properties. We address the creation of these structures and their implications for the emergence of new physics, the development of instrumentation and computational methods, and exploitation in numerous applications.

The review was published in Nat. Commun. 8, 15756 (2017). 🔗

Topical review selected for inclusion in "Highlights of 2016"

Our topical review on Magnetism in curved geometries has been selected by the Editorial Board of the Journal of Physics D: Applied Physics for inclusion in the 50th anniversary's "Highlights of 2016" collection. 🔗

The citation reads:

"Magnetism in curved geometries" by Robert Streubel, Peter Fischer, Florian Kronast, Volodymyr P Kravchuk, Denis D Sheka, Yuri Gaididei, Oliver G Schmidt and Denys Makarov has been selected by the Editorial Board of Journal of Physics D: Applied Physics for inclusion in the exclusive "Highlights of 2016" collection, and were chosen on the basis of timeliness, scientific impact and broadness of appeal.

Master student Sven Velten graduates with distinction

Congratulations to Sven Velten who graduated in Physics with distinction from the University Hamburg, Germany. He had been with our group for one year working on "Magnetic Interactions in Nanodisk Arrays", and my very first directly supervised master student. We wish him all the best for his future scientific career.

ALS UEC gets a new website

Reforming nominations and elections processes to enhance engagement, efficiency and diversity of UEC members is only one aspect of current UEC's business. In order to improve the visibility and public engagement with ALS users, ALS staff and external parties, and incorporate those changes, I redesigned the UEC website from scratch hosting all UEC related activities.

The online presence was officially launched right before the ALS Users' Meeting. 🔗

Topical review on curved magnetic geometries

Bending and twisting two-dimensional structures into three-dimensional space can modify conventional or help to discover novel functionalities in electronic, photonic, plasmonic or magnetic devices. In the first review on “Magnetism in curved geometries”, we discuss the emerging peculiarities in geometrically curved magnetic thin films. A profound introduction to fundamental physics, state-of-the-art characterization and fabrication techniques as well as to perspectives and applications is given highlighting the great potential of this new research area.

The review was published in J. Phys. D: Appl. Phys. 49, 363001 (2016). 🔗

The review is highlighted in: JPhys+

It has been selected by the Editorial Board of the Journal of Physics D: Applied Physics for inclusion in the "Highlights of 2016" collection. 🔗

Founding board member of the Berkeley Lab Postdoc Association

The recently founded Berkeley Lab Postdoc Association acts to enrich the postdocs' scientific careers through various social and professional events and activities. One of our foci is set on networking among postdocs and with students, alumni and senior scientists.

More information can be found here: homepage | LinkedIn | facebook | Twitter

Robert becomes program co-chair of the ALS users' meeting 2016

Taking action as newly elected members of the ALS UEC, Fanny Rodolakis and Robert Streubel will co-chair the scientific programme of the ALS users' meeting 2016. The ALS users' meeting takes place October 3-5, 2016 and provides a platform for scientific exchange and networking across disciplines. Experienced researchers and students are invited to advance expertise attending plenary addresses and focused workshop sessions on state-of-the-art techniques and research. 🔗

Robert's dissertation honored with Ernst-Eckhard-Koch-Prize 2015

The Society of Friends of Helmholtz-Zentrum Berlin (HZB) announces each year the Ernst-Eckhard-Koch-Prize, which is bestowed for outstanding doctoral theses in the field of research with synchrotron radiation performed at German synchrotron radiation facilities. The Ernst-Eckhard-Koch-Prize includes a monetary award of 2500 €.

Robert Streubel's dissertation has been selected by the committee. He was awarded the prize in a ceremony during the Users' Meeting of the HZB on December 10, 2015. 🔗

Robert Streubel elected member of ALS UEC

The Users' Executive Committee (UEC) at the Advanced Light Source (ALS) represents the users' interest at the ALS. Among an outstanding slate of 11 scientists, three new regular members including myself were elected to join UEC for three years. Thank you everyone for your support.

In his function, Robert will help the ALS promoting the unique potential of synchrotron radiation specifically to younger students thinking about entering one of the many fields, where synchrotron radiation can have a dramatic impact, but also to the many visitors coming to the ALS on a daily base. He is also interested in helping our UEC communicating to our political representatives the importance of continued and sufficient funding of large scale facilities by working with the NUFO organization.

More information can be found here. 🔗

Dissertation by Robert Streubel

Dissertation entitled Imaging Spin Textures on Curved Magnetic Surfaces is evaluated with summa cum laude by the faculty of natural sciences of the TU Chemnitz.

The thesis summarizes pioneering research activities on visualizing spin textures on 3D curved magnetic surfaces that have been highlighted as numerous publications in high-ranked journals, including Nature Communications, Nano Letters and Advanced Materials. It represents a first milestone in 3D imaging of magnetic microtextures by reconstruction utilizing Magnetic soft x-ray Tomography (MXT), a visualization technique relying on x-ray magnetic circular dichroism (XMCD) as element-specific contrast mechanism. Two distinct 3D curved magnetic surfaces, namely magnetic cap structures and rolled-up magnetic nanomembranes with cylindrical shape are experimentally investigated in view of fundamental physics, e.g. curvature- and topology-driven modifications to magnetic domain configurations, magnetization reversal processes and spin excitations, and applications as storage devices, sensors and self-propelled targeted drug delivery.

The work is honored with:

• Ernst-Eckhard-Koch-Prize 2015 by the Society of Friends of Helmholtz-Zentrum Berlin (HZB)
• Leibniz Dissertation Award Finalist 2016 of the Leibniz Association, Germany
• Ehrenfried Walther von Tschirnhaus-Plakette 2016 by the IFW Dresden
• Condensed Matter Dissertation Prize Finalist 2016 of the German Physical Society

The dissertation is available at Qucosa. 🔗

Magnetic soft x-ray tomography

While x-ray tomography is extensively applied, its magnetic analog – magnetic x-ray tomography – although demanded for medical applications and materials sciences, had not been available. We put forth the foundation of magnetic soft x-ray tomography and demonstrate its capabilities by reconstructing 3D spin textures of microscopic tubular architectures. This work was carried out in close collaboration with partners at the Helmholtz-Zentrum Berlin, BESSY II (Dr. Florian Kronast) and Advanced Light Source at Berkeley (Dr. Peter Fischer).

The original work was published in Nat. Commun. 6, 7612 (2015). 🔗

Our activities are highlighted in: pro-physik.de | Nanowerk | Nanotechnology Now | Analytica-world | Jura-Forum | HZB, BESSY II