Thermal spin-transfer torque investigated using spin-polarized scanning tunneling microscopy

• Applicant: Dr. Cody Friesen
• Subject Area: Experimental Condensed Matter Physics
• Term: since 2021
• Project identifier: Deutsche Forschungsgemeinschaft (DFG) - Project number 466776736

The objective of this project is to verify the existence of thermal spin-transfer torque (T-STT), and evaluate its potential use for magnetic data storage technologies. Additionally, inelastic tunneling and heat transfer effects across a magnetic tunneling junction (MTJ) will be studied. This will be done using spin-polarized scanning tunneling microscopy (SP-STM) combined with a heated tunneling tip and time-resolved measurements of the magnetic state lifetimes of superparamagnetically switching nanoislands. This directly builds on the work done during my doctoral thesis, measuring magneto-Seebeck tunneling using SP-STM. With this approach I will avoid the drawbacks of solid-state planar devices, such as in-plane temperature gradients, inhomogeneous tunneling barriers, and lack of atomic-scale control over the tunnel junction, that have so far prevented the deconvolution of T-STT from heating-induced or structural effects.To fulfill the primary objective of the project, I will attempt to measure the effect of T-STT, current-induced STT, and the effect of an external magnetic field on a single magnetic nanoisland, via a change in magnetic state lifetime asymmetry. In this way the magnitude of the T-STT can be precisely quantified by direct comparison with the other known techniques on the exact same magnetic system. For comparison to existing literature, Fe monolayer islands on W(110) will be prepared. A second target system will be Co nanoislands grown on Ir(111), which have already been extensively characterized using SP-STM. The dependence of T-STT on factors such as the symmetry of the spin-injection point relative to the island geometry, the external magnetic field, and the absolute temperature of the tip and the sample will then be systematically studied.Simultaneously, I will use my experimental approach to explore additional effects of heat transfer in an STM junction. This will be done by observing a simultaneous increase in both state lifetimes, which should vary inversely with the temperature of the nanoisland. While the heat carried across the junction due to tunneling electrons is relatively simple to calculate, there are numerous additional effects, such as radiative transfer, electrostatic interactions, evanescent electric fields, surface phonon polaritons, or phonon transport across the vacuum to consider. A key difference between these effects is their dependence on the vacuum barrier width, which can be scanned with spectroscopic accuracy on a single atomic site using SP-STM. This measurement is not possible using planar devices. Thus by using switching nanoislands as sensitive detectors of heat transfer, such effects can be studied with unprecedented spatial resolution, for both in-plane and out-of-plane geometries.

• DFG Programme: Research Grants