Magnetic domain microscopy

Microscopy electron, light (with video enhancement), scanning tunneling, atomic forces, IR microscopy, NMR imaging, thermal and magnetic domain microscopy. 

The mechanism for coercivity in the Cr—Co—Fe alloys appears to be pinning of domain walls. The magnetic domains extend through particles of both phases. The evidence from transmission electron microscopy studies and measurement of JT, and anisotropy vs T is that the walls are trapped locally by fluctuations in saturation magnetization. 

The second mode of operation is the non-contact mode, in which the distance between tip and sample is much larger, between 2 and 30 nm. In this case one describes the forces in terms of the macroscopic interaction between bodies. Magnetic force microscopy, in which the magnetic domain structure of a solid can be imaged, is an example of the non-contact mode operation. 

Several physical methods may be used to provide indirect estimates of the degree of dispersion. Sizes of particles, or of single crystallites, or of magnetic domains determined, respectively, by electron microscopy, X-ray line broadening, and magnetization measurements, can be used for this purpose but, in all cases, assumptions must be introduced into the calculations. 

Tsuno, K. (1988). Magnetic domain observation by means of Lorentz electron microscopy with scanning technique. Reviews of Solid State Science, 2,623 58. 

The first application of the SNOM for the MO studies happened in 1992 [62], when it was demonstrated that near-field MO observation can be obtained in the same manner as conventional far-field observation— that is, by using two cross-polarizers. Betzig et al. [62] visualized 100-nm magnetic domains and claimed spatial resolution of 30-50 nm. The possibility of MO domain imaging was confirmed in both the transmission regime (Faraday geometry) [63,64] and the reflection regime (Kerr microscopy) [65-67]. 

When secondary electrons are emitted from a magnetic material they become polarised and so by using a polarisation sensitive detector such as a Mott detector to collect the secondary electrons an image can be obtained that has magnetic contrast, allowing magnetic domain structures to be studied. This technique is known as scanning electron microscopy with polarization analysis (SEMPA). 

The visualization of magnetic domains by electron microscopy is limited by the thickness of the films (about 1200 A). Grundy (1977, 1980) has written two review articles in which he describes and compares the properties of magnetic materials which are able to support bubble domains. He also gave numerous details relative to the observation of magnetic domains in the electron microscope. 

Fig. 4. Magnetic domains of an amorphous GdFe film (Lorentz microscopy).

There are many different ways to image magnetic domain structures in a material. All the microscopy techniques we described in Chapter 10 can be used, although, in some cases, deviations from the usual operating procedure are necessary to obtain the desired images. It is also possible to use X-ray topography to study magnetic domains. In this section, we will briefly describe three techniques the oldest, one of the newest, and the trickiest. 

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