Thermal spin-transfer torque investigated using spin-polarized scanning tunneling microscopy
Final Report Abstract
When magnetic elements are used for computing we can exploit the direction of magnetization of a small magnetic structure, such as a thin film of iron deposited on a flat substrate, as a physical analogue for bits. Ideally this magnetization would never change unless we are actively manipulating it, but external magnetic fields, increased temperatures, and unwanted injection of spin (i.e. magnetic) currents can all reduce the stability of the system. As individual elements scale down in size, they become increasingly vulnerable to such effects, and previously negligible transport effects can become limiting factors in device design, or an opportunity for efficient control and sensing. The purpose of this project was to measure one such transport effect, namely thermal spintransfer torque (TSTT), using a variant of spin-polarized scanning tunneling microscopy (SP-STM). TSTT can occur when an out-of-plane temperature difference serves to drive a spin current into a magnetic system, thereby exerting a torque on the magnetization. As with other related transport effects, such as magneto-Seebeck tunneling, this effect can be observed in tunneling junctions. These structures are especially relevant for device physics for such properties as energy efficient state switching and high on/off ratios, making the measurement of TSTT in such a system a priority. However TSTT has proven difficult to decisively measure, with a variety of approaches resulting in indirect results. SP-STM has the potential to measure TSTT while avoiding potential sources of error, such as device defects, thus moving towards an unambiguous measurement. To perform SP-STM an electrically biased, atomically sharp, and magnetic tip is brought into tunneling contact with an electrically conductive sample, and a tunneling current begins to flow. Raster scanning of the tip with a feedback loop to maintain a constant tunneling current then provides an atomically resolved image of topographic, electronic, and magnetic properties of a surface. If the tip or sample is then heated, a temperature difference is created and charge and spin can begin to flow. If the spin is injected into a sensitive magnetic system, we can then detect TSTT by the influence it has on this sensor system. These microscopes usually operate in ultra-high vacuum at liquid helium temperatures, and in high magnetic fields. For this project, we successfully constructed and commissioned such a microscope with the addition of temperature control at the tunneling junction. This was a challenging addition to the already strict requirements imposed by SP-STM, and the creation and control of a continuously variable temperature difference between tip and sample is a critical step forwards towards a high-quality measurement of TSTT with SP-STM.
Publications
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“Spin-Caloritronics using spin-polarized STM” (Invited colloquia), Poznań University of Technology, Poznań, October 2022
C. Friesen
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“STM Platform for Spin-Caloritronic Studies” (Poster), Spin- Caloritronics XI, Urbana-Champaign Illinois, May 2022
C. Friesen & R. Wiesendanger
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“Measuring thermal spin-transfer torque using spin-polarized scanning tunneling microscopy” (Invited colloquia), University of Bielefeld, Bielefeld, November 2023
C. Friesen
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“Measuring thermal spin-transfer torque using spin-polarized scanning tunneling microscopy” (Poster), Spin-caloritronics XII, Tsukuba Japan, May 2023
C. Friesen & R. Wiesendanger
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“Spin-caloritronics using spin-polarized STM” (Contributed talk), DPG Spring Meeting, Dresden, March 2023
C. Friesen & R. Wiesendanger
