Magnetic imaging with scanning probe microscopy

Abstract

We review our research on the application of scanning tunnelling microscopy (STM) andnon-contact atomic force microscopy (NC-AFM) for magnetic imaging in highspatial resolution even down to the atomic scale. In the first part, we propose anew experimental scheme of spin-polarized STM (SP-STM) with a GaAs spinprobe to obtain a large contribution of spin-polarized electrons in the tunnellingcurrent. This is yielded by injecting the spin-polarized photo-excited electronsin an optically pumped GaAs tip into the spin-polarized empty states near theFermi level of a bcc-Fe(001) surface. According to the bandgap energy of GaAsand the surface state of the sample observed at 0.4 eV above the Fermi level, thespin-dependant electron injection can be achieved by applying a sample bias voltage of−1 V. The tunnel current in the positive bias region depends on the helicity of the circularpolarized pumping light, and is modified when the applied magnetic field is reversed.Mapping the current asymmetry provides a spin-dependent SP-STM image. In the secondpart, we describe the progress towards spin imaging with NC-AFM. The spinimaging can be achieved by detecting short-range magnetic interaction such asexchange interaction between a ferromagnetic tip and a magnetic sample. Wedemonstrate the capabilities of NC-AFM by imaging the spin structure of anantiferromagnetic NiO(001) surface on the atomic scale. The cross-sectional lineprofiles of the atomically resolved images obtained using several ferromagnetictips (Fe, Ni) were analysed by adding the atomic corrugation amplitude on thebasis of the periodicity of the image. The results of the analysis show that thedifference of the neighbouring maxima depends on the crystal direction. On the otherhand, no significant indication of the directional dependency can be seen on theimages obtained by using a non-magnetic Si tip. The directional dependencycoincides with the antiferromagnetic spin alignment of the NiO(001) surface.