Local interactions of hot electron-spins with nanomagnets studied by spin-polarized scanning field emission microscopy and spectroscopy
Final Report Abstract
The project was aiming at the experimental study of the interactions of hot-electron spins with magnets on the atomic scale. Unoccupied electronic resonance states (RSs), known as Stark-shifted image-potential states and field states, evolving above the surface of magnetic nanostructures served as model systems, focusing on the investigation of local phenomena. Utilizing spin-polarized scanning tunneling microscopy (SP-STM) in the resonant tunneling mode, spin-polarized electrons have been injected into these states, resulting in a hot-electron gas in front of the surface. Understanding its local interaction with magnetic surface defects, like step edges and nanoislands, or with laterally varying surface magnetization structures was in the focus of this project. We have investigated the reflection of hot-electron spins at noncollinear magnetic surfaces. Even for energies up to 20 eV above the Fermi level, the RSs are found to be spin split, exhibiting the same local spin quantization axis as the underlying spin texture, thus serving as atomic-scale probes for surface magnetism. The observed RS spin-sensitivity is attributed to the atomic-scale nature of the spin-dependent electron reflection at the surface. Mapping the spin-dependent electron wave function phase shift upon reflection at the surface on the atomic scale demonstrates the relevance of all magnetic ground state interactions for the scattering of spin-polarized low-energy electrons. Our work shows, that the local analysis of RSs by means of SP-STM provides a novel experimental approach for the detailed investigation of electron reflection - with tunable electron energies, on the atomic scale and with spin resolution. Our SP-STM experiments on ultrathin films with non-collinear spin textures demonstrate that resonant tunneling allows for atomic-scale spin-sensitive imaging in real space at tip-sample distances of up to 8 nm. The spin-polarized RSs evolving between the foremost atom of a magnetic probe tip and the opposed magnetic surface atom are found to provide a loophole from the hitherto existing dilemma of losing spatial resolution when increasing the tip-sample distance in a scanning probe setup. Our findings show that the spin-polarized RSs act as mediators for the spin contrast across the nm-spaced vacuum gap. With technically feasible distances in the nm regime, resonant tunneling in SP-STM qualifies for a spin-sensitive read-write technique with ultimate lateral resolution in future spintronic applications. We have shown that the delimitation of a ferromagnetic material, caused by a step edge to a nonmagnetic substrate, results in the evolution of a RS that is bound to the rim. Spin-resolved experiments on monodomain nanoislands reveal a spin-polarization of this rim state causing a spatial modulation of the spin-polarization of the tunneling electrons above the uniformly magnetized nanoisland. Our studies on thermally switching nanoislands indicate, that this modification of the spin-polarization via resonant tunneling can be used to tailor the spin-transfer torque for currentinduced magnetization switching. Hence, nanostructuring a magnetic surface and injecting the current via resonant tunneling may open a new route to tune the local spin-polarization, and thus to spatially tailor the spin-transfer torque. Step edges represent a local break of lateral symmetry in ultrathin magnetic films. Though unscheduled for the project, we have investigated the spin coupling across atomic step edges on Fe/W(110) by means of SP-STM and SP-STS. Local modifications of the spin texture toward step edges separating double from single layer areas are observed, and selection rules indicate a chiral spin coupling that significantly changes with the propagation along the [1-10] or the [001] crystallographic direction. The findings are explained via anisotropic Dzyaloshinskii-Moriya interactions arising from the broken lateral symmetry at atomic step edges. The tunneling of spin-polarized electrons across a magnetic tunnel junction driven by a temperature gradient is a fundamental process for the thermal control of electron spin transport. In collaboration with the SPP1538 ”Spin Caloric Transport” we have experimentally investigated the atomic-scale details of this magneto-Seebeck tunneling in an SP-STM setup. Heating the tip and measuring the thermopower of the junction while scanning across the spin texture of the sample lead to spin-resolved Seebeck coefficients that can be mapped at atomicscale lateral resolution.
Publications
- Scanning Seebeck tunneling microscopy, J. Phys. D: Appl. Phys. 51, 324001 (2018)
C. Friesen, H. Osterhage, J. Friedlein, A. Schlenhoff, R. Wiesendanger, and S. Krause
(See online at https://doi.org/10.1088/1361-6463/aacfab) - Magneto-Seebeck tunneling on the atomic scale, Science 363, 1065 (2019)
C. Friesen, H. Osterhage, J. Friedlein, A. Schlenhoff, R. Wiesendanger, and S. Krause
(See online at https://doi.org/10.1126/science.aat7234) - Step-edge-induced anisotropic chiral spin coupling in ultrathin magnetic films, Phys. Rev. Lett. 123, 037201 (2019)
A. Schlenhoff, S. Krause and R. Wiesendanger
(See online at https://doi.org/10.1103/PhysRevLett.123.037201) - Vacuum resonance states as atomic-scale probes of noncollinear surface magnetism, Phys. Rev. Lett. 123, 087202 (2019)
A. Schlenhoff, S. Kovarik, S. Krause and R. Wiesendanger
(See online at https://doi.org/10.1103/PhysRevLett.123.087202) - Real-space imaging of atomic-scale spin textures at nanometer distances, Appl. Phys. Lett. 116, 122406 (2020)
A. Schlenhoff, S. Kovarik, S. Krause and R. Wiesendanger
(See online at https://doi.org/10.1063/1.5145363)
