Transport and Magnetic Anisotropy in CMR Thin Film La1-xCaxMnO3 (x ? 1/3) Induced by a Film-Substrate Interaction

Abstract

We present a study of anisotropy of transport and magnetic properties in a La 1-x Ca x MnO 3 (x1/3) film prepared by pulsed-laser deposition onto a LaAlO 3 substrate. We found a non-monotonic dependence of magnetoresistance (MR) on magnetic field H for both H perpendicular and parallel to the film plane but perpendicular to the current. In the longitudinal geometry (when H is parallel to both the current and the film plane) the MR was negative at all fields below 20 kOe, as expected for colossal-magnetoresistance manganites. This rather complex behavior of MR manifests itself at rather low temperatures, far below the Curie temperature T c , which was close to room temperature. Two main sources of MR anisotropy in the film have been considered in the explanation of the results: (1) the existence of preferential directions of magnetization (due to strains stemming from the lattice film-substrate mismatch or other reasons); (2) dependence of resistance on the angle between current and the magnetization, which is inherent in ferromagnets. The transport and magnetic properties of the film correspond well to this view. In particular, the following angle dependence of MR is found: R(θ)/R(0) = 1 + δ an (T,H) sin 2 θ (where θ is the angle between the field and current directions in the plane normal to the film but parallel to the current). The temperature and magnetic field dependences of δ an (T,H) were recorded and analyzed. A clear magnetization anisotropy, that generally favors the magnetization in the film plane is also found. At the same time the recorded magnetization curves (as well as the MR data) indicate, that the film crystal structure should be inhomogeneous in such a way that various parts of the films have non-identical magnetic properties (with different directions of spontaneous magnetization). This hypothesis is supported by X-ray diffraction which revealed that the film is inhomogeneous in strain, lattice parameter and lattice orientation. This peculiar macroscopic-scale disorder is caused by a film-substrate interaction. The possible reasons for formation of such structure and its effect on MR anisotropy are considered.