High Resolution Imaging of Magnetization

Abstract

In 1907 Weiss postulated that a ferro-magnet has many small regions or domains that are spontaneously magnetized below the Curie temperature. In an unmagnetized ferromagnetic specimen the domains form so as to minimize the free energy, which includes contributions from the exchange energy, crystal and shape anisotropy energy, magnetostrictive energy, and so on. The actual size and shape of domains can only be calculated for the simplest geometries and for idealized materials. To investigate magnetic microstructure and how it determines macroscopic magnetic properties, we want to be able to see domain configurations and understand how the magnetic microstructure is changed by external influences and varying material properties. The observation of microscopic domain configurations can aid in engineering magnetic materials with desired magnetic properties.Domains were first observed using the Bitter method in which fine magnetic particles collect on the surface of a specimen in the field gradients at domain walls. Domains can also be observed by electron microscopy, with scanning electron microscopy, or at higher resolution with transmission electron microscopy. Specimens must be thinned to about 100 nm for use in the TEM. This, in turn, changes their magnetic properties. The magneto-optic Kerr effect has the advantage that the signal is directly related to the magnetization of the specimen and not just to the leakage fields or internal magnetic fields as in the methods mentioned above. This technique has the disadvantage of being limited to an optical resolution of typically 1μm.