Lattice Distortions and Domain Structure in Epitaxial Manganite Thin Films
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ABSTRACT Lattice distortions, be they in the form of chemical and hydrostatic pressure in bulk or lattice mismatch between film and substrate, have significant effects on the transport as well as the magnetic properties of colossal magnetoresistance (CMR) materials. We summarize here our results on tensilely and compressively strained La 0 .7Sr 0 .3MnO 3 (LSMO) thin films that indicate the important role of lattice distortions due to the lattice mismatch between the film and substrate. The strain due to lattice distortions can be used to tune the magnetic domain structure, magnetization, magnetic anisotropy and magnetotransport of LSMO thin films. INTRODUCTION The magnetism and magnetotransport of colossal magnetoresistance materials has been shown to be highly sensitive to lattice distortions both in bulk and in thin films. Magnetic domain structure may also lead to distinctive magnetotransport effects in thin films. Several groups have shown that properties such as Curie temperature, resistivity and magnetoresistance effect are extremely sensitive to chemical as well as hydrostatic pressure. Others have identified domain wall scattering as an additional source of magnetoresistance. [1-11]. Studies of bulk polycrystalline pellets, thin films of varying polycrystallinity and isolated grain boundaries have shown that the magnetoresistance is profoundly affected by transport across grain boundaries [2,12,13]. In epitaxial manganite films, the substrate imposes a strain on the film. Such strain is manifest in lattice distortions measured by diffraction techniques. These distortions, in turn, affect the magnetic anisotropy, magnetic domain formation and anisotropy of magnetoresistance. We have conducted a detailed investigation of the evolution of structure, magnetic anisotropy, magnetic domain configurations and magnetoresistance (MR) of smooth epitaxial La0 .7Sr 0.3MnO 3 (LSMO) thin films. Films grown on (100) and (110) SrTiO 3 substrates are tensilely strained while (100) oriented films grown on LaA10 3 substrates are compressively strained and exhibit a strong perpendicular anisotropy. In films grown on SrTiO 3 , the magnetic anisotropy and the anisotropic magnetoresistance are dominated by tensile strain effects. In thin films on LaAIO 3, the magnetic anisotropy is more than sufficient to overcome the film demagnetization factors and results in perpendicularly magnetized domains with fine scale - 200 nm domain subdivision, which we image directly at room temperature using magnetic force microscopy. The main MR effects can be understood in terms of bulk colossal MR and anisotropic MR. We also find evidence for a small but measurable domain wall (DW) contribution to the MR, which is significantly larger than that expected based purely on a double exchange picture. 207
Mat. Res. Soc. Symp. Proc. Vol. 602 © 2000 Materials Research Society
EXPERIMENT SETUP The epitaxial CMR thin films and heterostructures were deposited in a pulsed laser deposition (PLD) system with a KrF excimer laser (248nm) operating at 10 Hz wit
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