Spintronics at BESSY II: Domain walls in magnetic nanowires
Magnetic sensitive PEEM images obtained at HZB: a) XAS image of the crossed nanowires. X-ray beam and magnetic field are aligned along the nanowire(vertical) direction (green arrow). b-f) XMCD images of the cross for different applied fields. © HZB
Magnetic domains walls are known to be a source of electrical resistance due to the difficulty for transport electron spins to follow their magnetic texture. This phenomenon holds potential for utilization in spintronic devices, where the electrical resistance can vary based on the presence or absence of a domain wall. A particularly intriguing class of materials are half metals such as La2/3Sr1/3MnO3 (LSMO) which present full spin polarization, allowing their exploitation in spintronic devices. Still the resistance of a single domain wall in half metals remained unknown. Now a team from Spain, France and Germany has generated a single domain wall on a LSMO nanowire and measured resistance changes 20 times larger than for a normal ferromagnet such as Cobalt.
The magnetic domain texture inherent to magnetic domain walls holds potential for spintronic applications. The electrical resistance in ferromagnets depends on whether domain walls are or not present. This binary effect (known as domain wall magnetoresistance) could be used to encode information in spintronic memory devices. Yet, their exploitation is hindered due to the small changes in resistance observed for normal ferromagnets. A particularly interesting class of materials are manganite perovskites such as La2/3Sr1/3MnO3 (LSMO). These compounds present only one type of spin (full spin polarization) which could potentially lead to domain wall magnetoresistance effects large enough to be exploited in a new generation of spintronic sensors and injectors.
Despite this promising perspective, there exist large discrepancies in the reported values of the domain wall magnetoresistance for this system. The scientists from Spain, France and Germany have fabricated nanowire-based devices enabling the nucleation of individual magnetic domain walls. Magneto transport measurements in these devices show that the presence of a domain wall leads to an increase of the electrical resistance of up to 12%. In absolute terms, the observed resistance change is 20 times larger than that reported for Cobalt.
This work is the result of a longstanding collaboration which involves film growth and nanofabrication, transport measurements, contact microscopy (MFM) imaging, theoretical simulations and the use of advanced characterization techniques such as X-ray photoemission electron microscopy. The combination of a wide variety of different techniques provides a comprehensive multi-facet view of a complex problem which has allowed to reach new insights into a highly debated open question.
- Battery research with the HZB X-ray microscopeNew cathode materials are being developed to further increase the capacity of lithium batteries. Multilayer lithium-rich transition metal oxides (LRTMOs) offer particularly high energy density. However, their capacity decreases with each charging cycle due to structural and chemical changes. Using X-ray methods at BESSY II, teams from several Chinese research institutions have now investigated these changes for the first time with highest precision: at the unique X-ray microscope, they were able to observe morphological and structural developments on the nanometre scale and also clarify chemical changes.
- BESSY II: New procedure for better thermoplasticsBio-based thermoplastics are produced from renewable organic materials and can be recycled after use. Their resilience can be improved by blending bio-based thermoplastics with other thermoplastics. However, the interface between the materials in these blends sometimes requires enhancement to achieve optimal properties. A team from the Eindhoven University of Technology in the Netherlands has now investigated at BESSY II how a new process enables thermoplastic blends with a high interfacial strength to be made from two base materials: Images taken at the new nano station of the IRIS beamline showed that nanocrystalline layers form during the process, which increase material performance.
- Hydrogen: Breakthrough in alkaline membrane electrolysersA team from the Technical University of Berlin, HZB, IMTEK (University of Freiburg) and Siemens Energy has developed a highly efficient alkaline membrane electrolyser that approaches the performance of established PEM electrolysers. What makes this achievement remarkable is the use of inexpensive nickel compounds for the anode catalyst, replacing costly and rare iridium. At BESSY II, the team was able to elucidate the catalytic processes in detail using operando measurements, and a theory team (USA, Singapore) provided a consistent molecular description. In Freiburg, prototype cells were built using a new coating process and tested in operation. The results have been published in the prestigious journal Nature Catalysis.