Magnetic imaging with scanning probe microscopy

We review our research on the application of scanning tunnelling microscopy (STM) and non-contact atomic force microscopy (NC-AFM) for magnetic imaging in high spatial resolution even down to the atomic scale. In the first part, we propose a new experimental scheme of spin-polarized STM (SP-STM) with a GaAs spin probe to obtain a large contribution of spin-polarized electrons in the tunnelling current. This is yielded by injecting the spin-polarized photo-excited electrons in an optically pumped GaAs tip into the spin-polarized empty states near the Fermi level of a bcc-Fe(001) surface. According to the bandgap energy of GaAs and the surface state of the sample observed at 0.4 eV above the Fermi level, the spin-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 circular polarized 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 second part, we describe the progress towards spin imaging with NC-AFM. The spin imaging can be achieved by detecting short-range magnetic interaction such as exchange interaction between a ferromagnetic tip and a magnetic sample. We demonstrate the capabilities of NC-AFM by imaging the spin structure of an antiferromagnetic NiO(001) surface on the atomic scale. The cross-sectional line profiles of the atomically resolved images obtained using several ferromagnetic tips (Fe, Ni) were analysed by adding the atomic corrugation amplitude on the basis of the periodicity of the image. The results of the analysis show that the difference of the neighbouring maxima depends on the crystal direction. On the other hand, no significant indication of the directional dependency can be seen on the images obtained by using a non-magnetic Si tip. The directional dependency coincides with the antiferromagnetic spin alignment of the NiO(001) surface.