Low Field Magnetoresistance

The strain effect on the LFMR was first studied on poly-crystalline Laa67Sr033MnO3 [90,91]. It was also extensively studied by Wang et al. in Pra67Sra33MnO3 [131]. Films with compressive strains (on LaAlO3) show a large LFMR [132] when the field is applied perpendicularly to the substrate plane (Fig. 12), while they exhibit a positive MR when the film is under tensile stress (on SrTiO3) [131]. Almost no LFMR is observed when the film is stress-free (on NdGaO3). O'Donnel et al. confirm that the LFMR depends on the strains and the orientation of the field, by studying highly crystallized Laa7Caa3MnO3 thin films made by molecular beam epitaxy [33]. It was also shown that the LFMR is dominated by the grain boundaries [90,91] and its sign can be explained by a simple atomic d-state model [133]. This idea of anisotropic MR [134] was evidenced by La07Sr03MnO3 deposited on (001)-SrTi03, (110)-SrTi03 [135], and (110)-LaGaO3. Magnetic anisotropy was also seen recently on La07Ca03MnO3 films grown on (001)-NdGa03 [128].

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