Mapping spin structures on the atomic scale

Abstract

A fundamental understanding of magnetic and spin-dependent phenomena requires the determination of spin structures and spin excitations down to the atomic scale. The direct visualization of atomic-scale spin structures [1][2][3][4] has first been accomplished for magnetic metals by combining the atomic resolution capability of Scanning Tunnelling Microscopy (STM) [5,6] with spin sensitivity, based on vacuum tunnelling of spin-polarized electrons [7]. The resulting technique, SpinPolarized Scanning Tunnelling Microscopy (SP-STM), nowadays provides unprecedented insight into collinear and non-collinear spin structures at surfaces of magnetic nanostructures and has already led to the discovery of new types of magnetic order at the nanoscale [8]. More recently, the detection of spin-dependent exchange and correlation forces has allowed a first direct real-space observation of spin structures at surfaces of antiferromagnetic insulators [9]. This new type of scanning probe microscopy, called Magnetic Exchange Force Microscopy (MExFM), provides a powerful new tool to investigate different types of spin-spin interactions based on direct-, super-, or RKKY-type exchange down to the atomic level. By combining SP-STM with inelastic electron tunnelling spectroscopy [10] or by performing MExFM together with high-precision measurements of damping forces [9] localized or confined spin excitations in magnetic systems of reduced dimensions now become experimentally accessible. Principles of Spin-Polarized Scanning Tunnelling Microscopy (SP-STM) and Spectroscopy (SP-STS)The technique of SP-STM is based on vacuum tunnelling of spin-polarized electrons as can be observed in STM-type tunnel junctions involving a magnetic tip and a magnetic sample (Fig. 1). In this case, the tunnelling current does not only depend on the tip-surface separation and the applied bias voltage between tip and sample, but also on the spin polarization of the electronic states near the Fermi level of both electrodes as well as on the relative orientation of the local magnetization of the sample and the magnetic moment at the tip apex. Early experiments based on planar tunnel junctions involving magnetic electrodes have been performed by Jullière in the midseventies [11]. The invention of the STM by G. Binnig and H. Rohrer [5] allowed the replacement of the ill-defined tunnel barrier of planar tunnel junctions by a well defined vacuum barrier and the replacement of one of the planar electrodes by a freely movable and positionable probe tip. Magnetic tips have first been employed in scanning probe microscopy to perform spatially resolved measurements of magnetic dipole forces [12]. The corresponding technique, Magnetic Force Microscopy (MFM), has become a routine method for magnetic domain imaging [13]. However, the spatial resolution of MFM is limited to 10 -20 nm due to the long-range nature of the magnetic dipole forces [6]. In order to improve the spatial resolution of magnetic-sensitive scanning probe microscopy down to the atomic scale, two different a...

Reduction of quantum fluctuations by anisotropy fields in Heisenberg ferro- and antiferromagnets

“…From the experimental point of view, the investigation of magnetism at the atomic scale is greatly influenced by the experimental technique of the spin-polarized scanning tunneling microscopy (SP-STM) [30,31]. In the course of according experimental studies, single spins as well as two-dimensional films on substrates have been intensely studied [32][33][34][35][36][37][38][39][40].…”

Reduction of quantum fluctuations by anisotropy fields in Heisenberg ferro‐ and antiferromagnets

“…From the experimental point of view, the investigation of magnetism at the atomic scale is greatly influenced by the experimental technique of the spin-polarized scanning tunneling microscopy (SP-STM) [30,31]. In the course of according experimental studies, single spins as well as two-dimensional films on substrates have been intensely studied [32][33][34][35][36][37][38][39][40].…”

Image of dynamic local exchange interactions in the dc magnetoresistance of spin-polarized current through a dopant

“…Here we predict that the dc magnetoresistance of electrical current through the spin state of a single dopant, which can be measured e.g. using spin-polarized STM (SP-STM [24][25][26]), should directly image the coherent effects of the environment on the dopant's spin dynamics, e.g. exchange or hyperfine interactions, even at temperatures far larger than the energy scales of those interactions.…”