Orientation and thickness dependence of magnetization at the interfaces of highly spin-polarized manganite thin films

摘要

We have probed the nature of magnetism at the surface of (001)-, (110)-, and (111)-oriented La_(0.7)Sr_(0.3)MnO_3 thin films. The spin polarization of La_(0.7)Sr_(0.6)MnO_3 thin films is not intrinsically suppressed at all surfaces and interfaces but is highly sensitive to both the epitaxial strain state as well as the substrate orientation. Through the use of soft x-ray spectroscopy, the magnetic properties of (001)-, (110)-, and (111)-oriented La_(0.7)Sr_(0.3)MnO_3/SrTiO_3 interfaces have been investigated and compared to bulk magnetometry and resistivity measurements. The magnetization of (110)- and (111)-oriented La_(0.7)Sr_(0.3)MnO_3/SrTiO_3 interfaces is more bulk-like as a function of thickness whereas the magnetization at the (001)-oriented La_(0.7)Sr_(0.3)MnO_3/SrTiO_3 interface is suppressed significantly below a layer thickness of 20 nm. Such findings are correlated with the biaxial strain state of the La_(0.7)Sr_(0.3)MnO_3 films; for a given film thickness it is the tetragonal distortion of (001) La_(0.7)Sr_(0.3)MnO_3 that severely impacts the magnetization, whereas the trigonal distortion for (111)-oriented films and monoclinic distortion for (110)-oriented films have less of an impact. These observations provide evidence that surface magnetization and thus spin polarization depend strongly on the crystal surface orientation as well as epitaxial strain.