Microscopic studies of the magnetization-reversal process in Co-alloy thin films (abstract)

Abstract

A quantitative study of the microscopic magnetization-reversal process in 250-Å thick Co, Co90Pt10, and Co88Cr12 films deposited on top of a 250-Å-thick Cr layer has been carried out. The entire structure was on a SiN membrane to allow ease of observation in the transmission electron microscope (TEM). The magnetization-reversal process was studied by using an electromagnet to saturate the film in one direction; the applied field was then reversed until the sample reached a magnetization state between +Mr and -Mr; the field was then reduced to zero and the sample taken to the TEM for observation of the resulting magnetic structure using Fresnel and DPC Lorentz microscopy.1 This process was repeated for a number of reversal fields ranging from zero to the loop closure field. This study shows that as the reversal field was increased from zero towards the coercivity, the magnetization dispersion, i.e., the range of angles between the local magnetization and the original saturation direction, increased. The local magnetization direction varied markedly from one region of the film to the next, indicating that it is strongly tied to the local anisotropy of the film. Once the steep part of the B-H loop is reached, small reversed regions are nucleated. These reversed regions consume the entire film by continued growth and nucleation as the reverse field is increased. The nucleation occurs at the same site when the magnetization is cycled around the B-H loop. This indicates that the local anisotropy determines the sites in which critical nuclei are formed in a given applied field. The boundary between the switched and unswitched regions consists of a complex magnetization vortex structure. Once the entire film has been switched, the local magnetization dispersion decreases as the reverse field is increased. Comparison of the results among the Co-alloy films will be discussed.