The role of strain in low-field magnetotransport properties of manganite thin films
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ABSTRACT Strain-induced large low-field magnetoresistance has been observed in very thin Pr 0.67Sr0.33MnO 3 films'. To better understand the role of strain in the low-field magnetotransport properties of manganite thin films, we have studied and compared very thin (3-20 nm) Pr0o67Sr0 .33 MnO 3 (PSMO), La0.67Ba 0.33 MnO 3 (LBMO), La0 .67Sr 0.33MnO 3 (LSMO) and La0.67Ca0.33MnO 3 (LCMO) films grown on different substrates, such as LaAIO 3(001) (LAO), NdGaO 3(110) (NGO), and SrTiO 3(O01) (STO). Due to the lattice mismatch between the films and the substrates ranging from -2.6% to +1%, different strains can be imposed to the films. We have found that: (1) large low-field magnetoresistance(LFMR) behaviors are observed in PSMO, LCMO and LSMO thin films on LAO substrates when a magnetic field is applied perpendicular to the film plane, but the maximum LFMR is the largest in PSMO and LCMO samples; (2) most of the films grown on STO substrates show positive MR when a magnetic field is applied perpendicular to the film plane, and when the field is parallel to the film plane all films show negative MR regardless of the substrates; (3) the large low-field MR is strongly dependent on the film thickness and the composition of the manganites. The anomalous low-field MR effect will be discussed based on strain-induced magnetic anisotropy and domain rotation and movement.
INTRODUCTION During the last decade, tremendous efforts have been devoted to heterogeneous ferromagnetic materials, such as thin-film multilayers and cluster-alloy compounds which display so-called giant magnetoresistance effect (GMR). The discovery of "colossal" magnetoresistance (CMR) in perovskites has renewed interest in these materials. It launched a frenetic scientific race to understand the cause of the effect and raised expectations on potential applications. The crucial problem for developing practical devices based on colossal magnetoresistance is that a large response is only observed in relatively large magnetic fields -typically a few tesla. However, for magnetic field-sensoring applications, low-field responsivity is necessary. On the other hand, one important issue both for physics and for any possible application is the strain dependence of material properties. In particular, many proposed applications involve films, and films typically have large biaxial strains since lattice distortion can be easily introduced in thin films due to lattice mismatch between the film and the substrate. Therefore, the thin film samples provide good candidates for the study of the biaxial strain effect on the electrical and magnetic properties of the materials. We have recently reported the strain-induced large low-field magnetoresistance in very thin Pr0 .67 Sr 0.33 MnO 3 (PSMO) films grown on LaA1O 3 (001) (LAO) substrates'. The films are under compressive strain imposed by the lattice mismatch with the substrate. We obtained MR •: -92% at H=800 Oe (MR is defined as (R(0)-R(H))/R(0)) and T=70 K. We also observed large low-field 63 Mat. Res. Soc. Symp. Proc. Vol. 602
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