Transport and Magnetic Anisotropy in CMR Thin Film La1xCaxMnO3 (x ≈ 1/3) Induced by a Film–Substrate Interaction

Abstract

We present a study of anisotropy of transport and magnetic properties in a La1−xCaxMnO3 (x ≈ 1/3) film prepared by pulsed-laser deposition onto a LaAlO3 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 Tc, 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 + δan(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 δan(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.

Anisotropic Magnetoresistance in Perovskite Manganites

“…Recently, giant and unusual AMR effects in several perovskite manganite thin films were discovered. [10][11][12][13][14][15][16][17][18][19][20][21][22][23][24][25][26][27] In particular, the observed AMR exhibits strong and non-monotonic temperature and field dependencies. This kind of behavior differs dramatically from what has been observed in conventional 3d transition metals or alloys, suggesting a different mechanism beyond conventional theories of AMR.…”

“…4 The AMR was measured in magnetic fields up to 0.7 T. This field is larger than that required to saturate the magnetization in LCMO thin films. 11,24 The temperature dependence of resistivities and magnetoresistivities, MR = [ρ(0) − ρ(H)/ρ(H)] are shown in Fig. 10.…”

Magnetotransport properties of La 1−x Ca x MnO 3 (0.52 ≤ x ≤ 0.75): Signature of phase coexistence

“…It was also shown that the domain rotation and magnetocrystalline anisotropy have an important influence on the AMR of manganites [37,40]. However, all our samples have the same thickness and therefore we do not expect that the strain inhomogeneities (due to the substrate-film lattice interaction) should affect the AMR for different x [44]. Our intention is to explore the possible change in the AMR with increasing x for 0.52 ≤ x ≤ 0.75 to monitor perceived phenomena of the CMR for x > 0.5.…”

Intrinsic and extrinsic inhomogeneities in mixed-valence manganites

“…The highest values of magnetoresistance were observed in thin films. But it should be taken into account that films are always in an inhomogeneous strained state due to inevitable substrate-film lattice interaction, that induces, as a rule, a considerable magnetic and magne-toresistance anisotropy [11]. Due to strained state, some other film properties (among other things, the value of T c ) can be quite different from these of bulk materials.…”