Magnetic domain structure of micro-patterned PtMn/CoFe exchange bias bilayers

Using magnetron sputter deposition a number of glass/Ta 7nm/PtMn 20nm/CoFe 4nm/Ta 4nm samples with large exchange bias field were prepared for magnetic pattering investigations. By means of optical lithography and physical etching several patterns with decreasing lateral sizes of either the elements or the spacing between the elements were prepared. The largest square is 50 x 50 µm2 and the smallest only 1 µm2. The separating lines range from 10 µm to 2 µm width. The magnetic characterization of the samples was done by VSM and MOKE. Kerr microscopy and MFM investigations in an applied magnetic field have been performed in order to get a deeper understanding of the domain pattern. All images show a monodomain magnetization state in zero magnetic field. The shape of the structure itself dose not influence the magnetization direction. The shape anisotropy contribution is thus smaller than the unidirectional anisotropy given by the exchange bias. In addition 5 keV He+ ion irradiation was used to decrease exchange bias field value and thereby modify the ratio between unidirectional and shape anisotropy. The magnetic domain structure is investigated as a function of this ratio.