Researchers trying to make smaller, faster computer processors and other devices have for some time been looking to spintronics. Spintronics encodes information based not just on electrical charge, as in traditional computing, but on a property of electrons called spin, in which electrons are magnetically polarized—either “spin up” or “spin down.” In a spin current there is no net flow of electrical charge, hence no heat dissipation, which is optimal for the design of low-power microelectronics. To advance the field of spintronics, scientists need to understand exactly how spin current flows across the interfaces between materials. Using the U.S. Department of Energy’s Advanced Photon Source (APS) at Argonne National Laboratory, a group of researchers have shown a strong connection between how spin current propagates through an interface between a heavy metal and a ferromagnet, and the induced magnetic state of the heavy metal. Their results were published in the journal Physical Review B.
The researchers determined the spin current absorption via the rate of damping, in samples with varying magnetic states. They did this by building sandwiches of thin films just a few nanometers thick. One layer was a ferromagnetic material, a cobalt-iron alloy, in which magnetization could be flipped with an external magnetic field. Next was a spacer layer of a non-magnetic material, either gold or copper. The final layer was a heavy metal, in this case platinum.
Platinum is close to being a ferromagnetic material in its own right. When placed close to a magnetic layer, the metal’s spins align and the platinum becomes magnetic, a property known as proximity-induced magnetism (PIM). To test the strength of the PIM, the researchers varied the thickness of the spacer layer across its 16-millimeter length; the gold varied from 0 to 3 nanometers (nm) thick over the length of the layer, and the copper went from 0-nm to 6-nm thick.
They found that as the thickness of the spacer layer increased, both the PIM and the damping decreased rapidly. Both gold and copper are considered essentially transparent to spin current, so the difference could only come from changes in the proximity induced magnetism of the platinum layer. When damping is high, the spin current flows easily out of the magnetic layer and is absorbed in the non-magnetic layer. Such spin current propagation is a central feature of how any spintronic devices work. The direct correlation between damping and PIM demonstrates that PIM is important to spin current propagation, a conclusion that had been debated among experts.
To measure the PIM, the researchers performed x-ray magnetic circular dichroism (XMCD) and x-ray resonant magnetic reflectivity (XRMR) experiments at the X-ray Science Division Magnetic Materials Group’s beamline 4-ID-D of the APS, an Office of Science user facility at Argonne National Laboratory. In both cases, the direction of the induced magnetic moment in the platinum and gold—whether it’s majority spin up or majority spin down—affects how much a polarized x-ray beam is absorbed in the sample, so it provides a measurement of spin direction and magnitude. The XMCD measures the average magnetism across the depth of the platinum layer (Fig. 1), whereas the XRMR provides a depth-sensitive measure of the induced magnetic signal. The latter showed that most of the induced magnetism was near the interface of the layers. The researchers also added another spacer layer within the platinum layer and saw that both the number and location of the interfaces within the system affected the damping. Oddly, at the interface between the gold and the platinum, the gold shows enhanced PIM, which is hard to explain theoretically.
The results of the studies suggest there may be ways to enhance damping and improve spin transport. For instance, it may turn out that adding a thin layer of platinum to future devices will produce a better interface that in turn aids in spin transport. ― Neil Savage
See: C. Swindells1,2, H. Głowínski3, Y. Choi4, D. Haskel4, P. P. Michałowski5, T. Hase6, F. Stobiecki3, P. Kúswik3, and D. Atkinson1*, “Magnetic damping in ferromagnetic/heavy-metal systems: The role of interfaces and the relation to proximity-induced magnetism,” Phys. Rev. B 105, 094433 (2022). DOI: 10.1103/PhysRevB.105.094433
Author affiliations: 1Durham University, 2University of Sheffield, 3Polish Academy of Sciences, 4Argonne National Laboratory, 5Institute of Microelectronics and Photonics, 6University of Warwick
Correspondence: * del.atkinson@durham.ac.uk
Funding is acknowledged from EPSRC for C.S. (1771248, Ref. EP/P510476/1) and the Royal Society for D.A. (IF170030). Travel and subsistence were funded by the U.K. EPSRC XMaS facility. P.K., F.S., and H.G. acknowledge financial support from the National Science Centre Poland through the OPUS funding (Grant No. 2019/33/B/ST5/02013). This research used resources of the Advanced Photon Source, a U.S. DOE Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory under Contract No. DE-AC02-06CH11357.
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