Overseas Travel Grant: XMCD investigation of spin-orbit coupled heavy-metal/ferromagnet heterostructures at the Advanced Light Source, Berkeley

This Overseas Travel Grant proposal seeks support from the EPSRC to cover the expenses related to travel and accommodation costs for the final phase of our beam time at the Advanced Light Source, Lawrence Berkeley National Laboratory to study the spin-orbit coupling in heavy-metal/ferromagnet multilayers. Spin-orbit coupling has recently been under intense focus in spintronics research, offering a revolutionary way to manipulate spins without ferromagnets which has driven classical spintronic devices. Spin-orbit coupling in heavy-metals in contact with ferromagnets has also been utilised to generate novel topological objects like magnetic skyrmions that offers the possibility of next-generation of ultra-dense data storage. Furthermore, recent work has shown a fundamental connection between spin-orbit coupling in heavy-metal/ferromagnet heterostructres and superconductivity that offers the possibility to develop a superconducting analogue of spin-orbit coupling-driven spintronics.

Although the importance of spin-orbit coupled ferromagnets designed from these heterostructures is established, several materials-related questions remain. For example, the exact dependence of the strength of the spin-orbit coupling as a function of the nature of heavy-metal or ferromagnet remains poorly understood. A comprehensive study using different ferromagnets and heavy-metals will allow us to understand the microscopic origins of the spin-orbit coupling in these systems and ways to manipulate it. To investigate this, we have been awarded 24 shifts of X-ray Magnetic Circular Dichroism (XMCD) beamtime at the 6.3.1. beamline at the Advanced Light Source in Berkeley from 2017-2018. We have already attended 18 shifts and this proposal seeks funding to sponsor our last 6 shifts of beamtime (Proposal number ALS-09050, attached beamtime schedule letter). This beamline offers high accuracy XMCD measurements with a rapid magnetic field reversal minimising scan time. The highly-automated beamline also allows several samples to be measured in one loading making it ideal for our study involving a large number of samples.

The results from this beamtime will allow us in future to quantitatively relate material parameters with the strength of spin-orbit coupling, thereby providing experimentalists a list to select materials from to specifically tailor the properties of these. Not only this will benefit the spintronics community, but will also allow us to consolidate our understanding of the newly discovered role of spin-orbit coupling in superconducting spintronics.

The outcomes from this beamtime will have both short and long-term impact beyond the immediate academic beneficiaries. A continued focus in the last decade in spintronics-based industry has been the design of spin transfer torque random access memories (STT-MRAM). These have significantly higher speed, low power consumption and cheaper than the DRAMs where the information is stored and read electrically rather than using magnetic fields. STT-MRAMs are being developed by several companies like Everspin, IBM, Smasung, Crocus, Avalnache etc. and their recent focus has shifted to utilising spin-orbit coupling in heavy-metal/ferromagnet heterostructures for low-current and faster switching speeds. Evidently, a detailed understanding of the mechanism of spin-orbit coupling and its materials dependence in these structures are crucial to design optimal structures. Our results could provide a blueprint for selecting such materials. Several international conferences like Magnetism and Magnetic Materials and Intermag bring together engineers from these companies and academicians together and dissemination of our results in conferences will undoubtedly create the links. We anticipate outcomes from this proposal to make an impact in this sector in a timescale of 3-5 years as this rapidly growing industry has benefitted from academic research extensively in the last 5-10 years.

A second area of impact will result from Government's initiative to design the next generation of exascale supercomputers. USA has taken up this initiative through the C3 IARPA program to design a superconducting supercomputer. Our results, especially results from this proposa, lt will significantly enhance our understanding of superconducting spintronics driven by spin-orbit coupling. This would considerably simplify devices and realistic circuit design will be possible in this area. Superconducting spintronics will offer a significantly better solution towards the construction of the exascale supercomputers and beyond in terms of functionalities since it integrates the benefits of both superconductivity and spintronics. EPSRC has already taken a lead by investing £9 million per annum on software development with a strong focus for exascale computing. Other major international stakeholders like the G8 research Councils initiative, International Exascale Software Program and EU funded CRESTA have all been focussing on software design for the next-generation high performance computing. To maintain UK's global lead in this sector, significant investment from EPSRC has also gone towards the development of technology for the hardware required for exascale computing (recent funding in superconducting spintronics). A large-scale initiative like the C3 program is not beyond reach and in the medium term of 5 years and beyond, results from this proposal can play a significant role towards such efforts. Through the dissemination of our results in major international conferences and high-impact publications, we will communicate our findings beyond the immediate circle of researchers working in this area. From separate funding sources, we are organising three conferences in UK and India in 2018 and 2019 to bring together researchers from diverse fields including engineers from industry to engage in interdisciplinary research in this area