Utilizing Vacuum States above Surfaces for Imaging and Manipulation of Atomic-Scale Magnetism INVITED

Abstract

Imaging and manipulation of atomic-scale magnetism for future spintronic applications demands for a spin-sensitive technique with ultimate lateral resolution. Exploiting the tunneling effect between magnetic electrodes, as between a magnetic probe tip and a magnetic sample in a spin-polarized scanning tunneling microscopy (SP-STM) set up, has proven to be capable of resolving and switching magnetic structures down to the single atomic level. However, this approach disqualifies for many technical applications, since it requires tip-sample distances in the order of a few angstroms. For practical applications, technical feasible distances, like flying distances of read-write heads in current data storage devices, are in the range of a few nanometers. Electron tunneling over this distance is very unlikely and therefor not detectable. Spin-polarized vacuum states are unoccupied electronic states located nanometers apart from the underlying magnetic surface in vacuum [1]. In an SP-STM setup, these states can be addressed by spin-polarized electrons that tunnel resonantly from the magnetic tip via a vacuum state into the surface, resulting in a magnetic image [2]. As I will show in this talk, these states exhibit the same local spin quantization axis as the spin texture of the underlying sample surface, even when the spins are rotating on the atomic-scale [2]. Our SP-STM experiments at low temperatures on ultrathin films with non-collinear spin textures demonstrate that the spin-polarized resonant tunneling via the vacuum states allows for atomic-scale spin-sensitive imaging in real space at tip-sample distances of up to 8 nm [3]. Acting as mediators for the spin contrast across the nm-spaced vacuum gap, the vacuum states provide thereby a loophole from the hitherto existing dilemma of losing spatial resolution when increasing the tip-sample distance in a scanning probe setup. Experimental results will be presented and discussed in terms of the vacuum states’ spin-splitting and the magnetic contrast as a function of bias and tip-sample distance, as well as in terms of the atomic-scale nature of the resonant tunneling condition between the probe tip and the sample. While spin-polarized resonant tunnel currents in the low nA regime are used for non-perturbative imaging, injecting high spin-polarized resonant tunnel currents (~100 nA) via vacuum states considerably affect the underlying magnetism. As I will demonstrate in this talk, atomic-scale ferromagnets can be efficiently switched via these states, with the underlying current-induced mechanism being based on thermally-assisted spin-transfer torque mediated by the vacuum states [4]. Consequently, spin-polarized vacuum states can be utilized for the readout and manipulation of atomic-scale magnetic objects from nanometer distances. Spin-polarized resonant tunneling qualifies for a spin-sensitive read-write technique with ultimate lateral resolution, potentially opening a pathway towards future technical applications.