Previous chapters give an introduction to novel magnetic imaging methods based on the scanning tunneling microscope (STM) or on the scanning force microscope (SFM). While the STM is sensitive to the surface density of electronic states and to its spin dependence, the magnetic force microscope (MFM), as a special variant of the SFM, detects the magnetic stray field produced by a sample, or the response of the sample to the local stray field produced by the probe. The basic setup in tunneling or force microscopy establishes a unified experimental approach as the basis of all scanning probe methods (SPM). The present chapter summarizes the basic aspects underlying this approach, analyzes achievements as well as limitations, and introduces three additional SPM.
Microscopic imaging requires suitable interactions between a probe and a sample that allow one to map a physical quantity — in the present context, a magnetic property — at a certain lateral resolution. In the case of tomographic methods, a certain depth resolution is also required. Most frequently employed probe-sample interactions in magnetic imaging involve electron exchange and the analysis of the spin polarization, the reflection and transmission of light in terms of the Kerr and Faraday effects, and magneto-static field effects, as, e.g., being used in the Bitter colloid method. All these interactions, used as the basis of classical methods of magnetic imaging, can also be implemented in scanning probe strategies. The electron spin is detected by the spin-polarized STM(SPSTM). Kerr and Faraday effects are utilized in the magneto-optic scanning near-field optical microscope (MOSNOM). Near-surface stray fields produced by ferromagnetic or superconducting samples can be analyzed using scan ning SQUID (superconducting quantum interference device) microscopes (SSM) or microscopes based on other field sensors like Hall probes or magneto-resistive probes. Together with a discussion of some general aspects concerning probe-sample magnetic interactions, the MOSNOM and the SSM will be introduced in the following.
The high-resolution detection of spin resonances for imaging purposes has been the subject of considerable effort for quite some time. Also in this area classical approaches can be adapted to scanning probe strategies in order to analyze nuclear magnetic resonance (NMR), electron spin resonance (ESR), or ferromagnetic resonance (FMR) at a sub-micron scale. Some general information on the respective approaches of magnetic resonance force microscopy (MRFM) is provided as well by the following discussion.