11 - Microscopic Magnetic Imaging Techniques

Summary

Magnetic imaging techniques enable one to have a direct view of magnetic properties on a microscopic scale. One of the most well-known magnetic microstructures is the magnetic domain. The other example of magnetic microstructures is the nucleation and growth of a magnetic phase across a first-order magnetic phase transition. Such structures can be observed in real space, and their distribution as a function of material and geometric properties can be investigated in a straightforward manner. In this chapter, we will discuss three different classes of magnetic imaging techniques, namely (i) electron-optical methods, (ii) imaging with scanning probes, and (iii) imaging with X-rays from synchrotron radiation sources. There are numerous scientific papers and review articles on these subjects. Instead of going into detail about the individual techniques, this chapter will provide a general overview of the working principles of various magnetic imaging techniques. There are not many specialist books, monographs, or review articles covering all these magnetic imaging techniques under the same cover, but the present author has found the book Modern Techniques for Characterizing Magnetic Materials [1] and the article “Magnetic Imaging” [2] to be quite useful while writing this chapter.

Electron-Optical Methods

Electron-optical methods and electron microscopy encompass a large body of techniques for magnetic imaging. The advanced electron microscopy techniques today can provide images with very impressive resolutions of the order of 1 nm, and show high contrast and sensitivity to detect small changes in magnetization in a material. The particle-like classical picture of the Lorentz force acting on an electron in a magnetic field form the basis of magnetic images of materials observed in various modes of electron microscopy. Electrons are charged particles, and hence electromagnetic fields are utilized as lenses for electrons. A magnetic lens consists of copper wire coils with an iron bore. The magnetic field generated by this assembly acts as a convex lens, which can bring the off-axis electron beam back to focus. Change in the trajectory of an electron in the magnetic field of a magnetic sample results in magnetic contrast and, in turn, provides information on the local magnetization in the material. However, the correct interpretations of the results in many cases involve a wave-like quantum mechanical picture of electrons.