The trajectories of small magnetic entities referred to as skyrmions have been captured and recorded with the help of X-ray holography. Researchers gained new insight from the analysis of this motion: these nanoscale vortices possess mass. It is a familiar phenomenon: if a spinning top is bumped or is set in rotation on an inclined surface, it usually does not move in a straight line, but instead scribes a series of small arches. Researchers at Technische Universität Berlin and the Johannes Gutenberg University Mainz (JGU) together with research teams from the Netherlands and Switzerland have now succeeded in capturing and recording this pattern of movement in a magnetic thin film system – in the form of small magnetic nanovortices. In doing so, the researchers made a new discovery: the nanovortices possess mass. The article will be published in the renowned scientific journal ("Dynamics and inertia of skyrmionic spin structures"). 
The local magnetisation is depicted by small arrows; a magnetic vortex is located in the centre. A brief current pulse through this nanowire deflects the skyrmion out of its rest position; it then moves back to its initial position on a spiral trajectory. This motion can be observed with the help of X-ray holography. The skyrmion and the spiral shape of its trajectory are represented schematically above the structure. (Image: TU Berlin) Vortices of 100 nanometres diameter “With the help of magnetic fields, we can selectively create the magnetic nanovortices, then give them a shove so that they are deflected out of their equilibrium position”, explains Dr. Felix Büttner, who pursued this research as his Ph.D. project. “We were then able to very precisely track how these skyrmions, as these special nanovortices are called, return to their rest position”, Büttner explains further. The vortices are formed in a magnetic system of thin film multilayers, where alternating layers composed of a cobalt-boron alloy and platinum are stacked on one another. Each individual layer is less than one nanometre thick. This arrangement allows the researchers to very specifically tailor the magnetic properties of the system, enabling the skyrmions to exist. The diameter of these magnetic vortices is no more than 100 nanometres. X-ray holography at BESSY II Special techniques enabled the researchers to track the movements of the skyrmions with a precision of better than a few nanometres at individual time steps less than one nanosecond apart. This was facilitated by holographic recording techniques using intense X-ray pulses of the BESSY II synchrotron source at Helmholtz-Zentrum Berlin (HZB). These holographic recording techniques have been developed and improved by the TU Berlin “Nanometre Optics and X-ray Scattering” research group in conjunction with HZB over a number of years, a joint effort directed by Prof. Stefan Eisebitt from TU Berlin. No mass is no option What Büttner and his co-workers observed in the X-ray holograms was remarkable: “Similar to bumping a spinning top, the nanovortex does not move in a straight line, but instead along a spiral trajectory”, explains Büttner. “By comparing our measurements with model calculations, we were able to determine that this spiral-shaped movement can only be explained if the skyrmion has mass.” This is an important discovery, since the nanovortices observed here represent only one special type of skyrmions found in nature. "In the past, skyrmions were often described as being massless”, explains Christoforos Moutafis from the Paul Scherrer Institute, who has long been involved with the theoretical description of these kinds of structures. Now, the application of the concept of mass to such particles, as established by this work, will also contribute to the understanding of other types of skyrmions, as the researchers point out in the renowned scientific journal . Applications in information technology There could also be tangible applications for these magnetic nanovortices within thin magnetic layers – they are already being discussed today as an alternative information medium in data processing and storage. Researchers suspect that due to their “skyrmion property”, such bits (units of information) can be stored more densely and transferred more reliably than at present. The new insights into skyrmion behaviour might contribute to realising these kinds of novel concepts for information processing.
The local magnetisation is depicted by small arrows; a magnetic vortex is located in the centre. A brief current pulse through this nanowire deflects the skyrmion out of its rest position; it then moves back to its initial position on a spiral trajectory. This motion can be observed with the help of X-ray holography. The skyrmion and the spiral shape of its trajectory are represented schematically above the structure. (Image: TU Berlin) Vortices of 100 nanometres diameter “With the help of magnetic fields, we can selectively create the magnetic nanovortices, then give them a shove so that they are deflected out of their equilibrium position”, explains Dr. Felix Büttner, who pursued this research as his Ph.D. project. “We were then able to very precisely track how these skyrmions, as these special nanovortices are called, return to their rest position”, Büttner explains further. The vortices are formed in a magnetic system of thin film multilayers, where alternating layers composed of a cobalt-boron alloy and platinum are stacked on one another. Each individual layer is less than one nanometre thick. This arrangement allows the researchers to very specifically tailor the magnetic properties of the system, enabling the skyrmions to exist. The diameter of these magnetic vortices is no more than 100 nanometres. X-ray holography at BESSY II Special techniques enabled the researchers to track the movements of the skyrmions with a precision of better than a few nanometres at individual time steps less than one nanosecond apart. This was facilitated by holographic recording techniques using intense X-ray pulses of the BESSY II synchrotron source at Helmholtz-Zentrum Berlin (HZB). These holographic recording techniques have been developed and improved by the TU Berlin “Nanometre Optics and X-ray Scattering” research group in conjunction with HZB over a number of years, a joint effort directed by Prof. Stefan Eisebitt from TU Berlin. No mass is no option What Büttner and his co-workers observed in the X-ray holograms was remarkable: “Similar to bumping a spinning top, the nanovortex does not move in a straight line, but instead along a spiral trajectory”, explains Büttner. “By comparing our measurements with model calculations, we were able to determine that this spiral-shaped movement can only be explained if the skyrmion has mass.” This is an important discovery, since the nanovortices observed here represent only one special type of skyrmions found in nature. "In the past, skyrmions were often described as being massless”, explains Christoforos Moutafis from the Paul Scherrer Institute, who has long been involved with the theoretical description of these kinds of structures. Now, the application of the concept of mass to such particles, as established by this work, will also contribute to the understanding of other types of skyrmions, as the researchers point out in the renowned scientific journal . Applications in information technology There could also be tangible applications for these magnetic nanovortices within thin magnetic layers – they are already being discussed today as an alternative information medium in data processing and storage. Researchers suspect that due to their “skyrmion property”, such bits (units of information) can be stored more densely and transferred more reliably than at present. The new insights into skyrmion behaviour might contribute to realising these kinds of novel concepts for information processing.
Graham Yelton and Sandia National Laboratories colleagues have developed a single electroforming technique that tailored key factors to better thermoelectric performance: crystal orientation, crystal size and alloy uniformity. The work was the first time researchers managed to control crystal orientation, crystal size and alloy uniformity by a single process. All three factors contribute to better thermoelectric performance, Yelton said. “The three together mean a huge gain, and it’s hard to do,” he said. “It’s turning the knobs of the process to get these things to behave.” Better nanowire geometries can reduce heat conductivity and improve what’s called the thermoelectric figure of merit, a measure of a material’s electrical and thermal conductivity. The higher the electrical conductivity and the lower the thermal conductivity, the higher the figure of merit and, therefore, the more efficient the material. However, the quality of previous thermoelectric nanowires proved inadequate. Thermoelectric nanowire use in its infancy Despite their inefficiency, some thermoelectric materials are already in use. Yelton compares their stage of development to the early days of solar photovoltaic cells: Everyone saw the potential, but they were so inefficient they were used only when nothing else worked. Improved efficiency in nanowires would increase the use of thermoelectric materials. They’re already used in some sensors, and vehicle manufacturers hope they can harvest heat from exhaust systems to power vehicle sensor systems, Yelton said. Decreasing the power needed to run a vehicle’s operating system could reduce battery and alternator weight and perhaps eliminate some power-generating equipment, trimming vehicle size and weight. Sandia’s paper describes how the team created thermoelectric nanowire arrays with uniform composition along the length of the nanowire and across the spread of the nanowire array, which potentially can include hundreds of millions of nanowires. In addition, they created nanowire crystals of uniform size and orientation, or direction. Uniform composition improves efficiency, while orientation is important so electrons, the carriers of energy, flow better. The team used a cost-effective method called room-temperature electroforming, which is widespread in commercial electroplating. Electroforming deposits the material at a constant rate, which in turn allows nanowires to grow at a steady rate. The method produced wires 70-75 nanometers in diameter and many microns long. Yelton used pulses of controlled current to deposit the thermoelectric material, thereby controlling composition throughout the wire and the array. “There are little nuances in the technique that I do to allow the orientation, the crystal growth and the composition to be maintained within a fairly tight range,” he said. Technique allowed control over important facets of nanowire formation The method produced a fairly large, slightly twisted crystalline wire structure that was almost a single crystal and had the desired orientation. “Without that, you couldn’t get good efficiencies,” Yelton said. The chemistry of the material also is important. For the Sandia team, antimony salts play a major role in crystalline quality and orientation. Bismuth-antimony (Bi-Sb) alloys have some of the highest thermoelectric performance — acting both as a conductor of electricity and an insulator against heat — among many materials for near-room temperature applications. But existing Bi-Sb materials don’t produce effective solid-state cooling when power is constantly delivered to the device being cooled, such as a computer. The Sandia team wanted a compound that behaved like a metal but would not conduct heat. Alloying antimony with bismuth fit the bill, Yelton said. Bi-Sb nanowire arrays electroformed with an antimony-iodide-based chemistry lacked the needed qualities, but arrays electroformed from an antimony-chloride-based chemistry produced crystallography and orientation for maximum thermoelectric performance. “The chemistry allowed us to go from poly nano-crystalline structure to near single crystals of 2-5 micrometers,” giving better control over uniformity, Yelton said. Next step: make an electrical contact The next step is more challenging: making an electrical contact and studying the resulting thermoelectric behavior. “Thermoelectric materials readily form oxides or intermetallics, leading to poor contact connections or higher electrical contact resistance. That reduces the gains achieved in developing the materials,” Yelton said. While the Sandia team has been able to get good contact at the bottom of an array, making a connection at the top has proved difficult, he said. “To make a contact and measure array performance is not trivial,” Yelton said. He and his colleagues are seeking further funding to solve the problem of successfully making contacts, and then to characterize the thermal electric properties of arrays. “If successful at the labs, we would try to find an industry collaborator to mature the idea,” he said.