H-dependent magnetic domain structures in La_(0.67)Sr_(0.33)MnO_3 thin films

摘要

Maze- and bubble-like magnetic domain structures have been observed under ambient conditions in La_(0.67)Sr_(0.33)MnO_3 (LSMO) films by magnetic force microscopy (MFM) for films grown on a compressive (LaAlO_3) lattice-mismatched substrate. The substrate-induced stress in this soft magnetic material appears to be a necessary but not sufficient condition for the appearance of these structures, i.e. no magnetic structure has been seen for other films grown under similar conditions on the same substrate with similar distortions to the unit cell. In this study, in order to understand the stability of these structures, we have examined two films grown at 750℃ and three films grown at 800℃ by pulsed-laser deposition by scanning tunneling and magnetic force microscopy to obtain microstructure and domain structures as a function of in-plane magnetic field strength. The latter films consisted of pitted coalesced layers with some surface 3D growth. Although the films were nominally grown under the same conditions, there were subtle differences in the domain structure, wall spacing, the latter due in part to differences in film thickness, and response to an applied in-plane magnetic field. As the field was increased, the maze-like structures became stripe domains with reduced out-of-plane magnetization and decreased wall spacing, consistent with parallel rather than antiparallel alignment of in-plane spin polarization. After removal of the field, the stripe domains remained but the wall spacing and z polarization component returned to the original value. For one 800℃ film the field required for changes in and disappearance of the domains paralleled the slope in the field-dependent magnetization. In contrast, the magnetic structure for one film with a thin insulating cap layer was reversible, suggesting that the walls were pinned by the cap.