Large Magnetoresistance Anisotropy in Strained Pr 0.67 Sr 0.33 MnO 3 Thin Films
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H. S. Wang, Y. F. Hu, E. Wertz, and Qi Li Department of Physics, Pennsylvania State University, University Park, PA 16802 ABSTRACT We have studied the anisotropic magnetoresistance (AMR) of strained Pr0 .67 Sr0 .33 MnO3 thin films by measuring the MR as a function of the angle between the magnetic field direction and the substrate normal (out-of-plane). The results show that the compressive- and tensile-strained ultrathin films (5-15 nm) grown on LaA10 3 (001) (LAO) and SrTiO 3 (001) (STO) substrates show unusually large out-of-plane AMR, but with opposite signs. In contrast, the almost strainfree films on the NdGaO 3 (110) substrates show much smaller AMR over all the temperature and field ranges studied. Thick films on LAO and STO substrates also show much smaller AMR. I. INTRODUCTION The effect of the anisotropic magnetoresistance (AMR) is an important property of ferromagnetic (FM) materials. It has a wide range of practical applications in magnetic devices such as magnetic reading heads,' and magnetic sensors. By measuring the AMR effect, it is also possible to obtain information about the magneto-crystal anisotropy and spin-orbit coupling.2 3' For thin film samples, one can measure the AMR in the film plane by changing the magnetic field direction relative to the current flow direction. In this case, the measured AMR includes both the Lorentz MR 4 and the magneto-crystalline AMR. One can also measure the out-of-plane AMR, in which the magnetic field and the current can always be kept perpendicular to each other as the magnetic field rotates from in-plane to out-of-plane. The Lorentz MR can then be eliminated, and the AMR from magneto-crystalline anisotropy can be exclusively obtained. The AMR effect in single crystal samples of the colossal magnetoresistance (CMR) manganites has been found negligible. However, in epitaxial thin film samples the magnetic anisotropy can be much stronger and the AMR can be much larger than in bulk materials due to uniaxial strain effect induced by lattice mismatch between the film and substrate.5"' Recently, Eckstein et al.6 have reported relatively large in-plane AMR (-8 %) in the La0.,6Ca0 .33MnO 3 thin films. Wolfmnan et al.' have studied the out-of-plane AMR of relatively thick Pr0.7Sr 0 .3MnO1 3 films at different magnetic fields and found a correlation between the magnetization and the AMR. They reported a small (< 1 %)magneto-crystalline AMR. Very recently,"" 2 we have shown that the uniaxial strain can strongly affect the low-filed MR anisotropy in Pr0 .6 7Sr0 .3 3MnO 3 (PSMO) thin films. We attributed the effect primarily to spin dependent scattering and magnetic domain rotation process. In this work, we report on the out-of-plane AMR effect of differently strained PSMO thin films measured at high magnetic field where the magnetization has reached its saturation and the intrinsic AMR can be measured. II. EXPERIMENTAL High quality PSMO thin films with thickness of 4-400 nm have been epitaxially grown on LaA10 3 (001) (LAO), SrTiO 3 (001) (STO), and NdGaO3 (110) (N
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