Anomalous magnetoresistance effect in strained manganite ultrathin films

Magnetotransport properties of strained epitaxial Pr_2/3Sr_1/3MnO_z (PSMO) ultrathin films have been studied. The strains are controlled by growing PSMO films on different lattice-mismatched substrates which impose biaxial compressive-, tensile-, and very little strain in the films, respectively. Distinctive magnetoresistancc effects in low magnetic field (LFMR) have been observed in films with different types of strain. The films with compressive strain show very large negative LFMR (> 1000% at 2500 Oe) and significant magnetoresistance hysteresis when a magnetic field is applied perpendicular to the film plane. In a parallel field, the LFMR is much smaller with almost no hysteresis. In contrast, the tensile-strained ultrathin films show positive LFMR at low temperatures in a perpendicular field and a negative LFMR in a parallel field. In comparison, the almost strain-free ultrathin films show very small LFMR (< 2 %) as in single crystal samples at similar temperatures and magnetic fields. In the compressive-strained samples, the large LFMR decreases rapidly with increasing film thickness. These results are interpreted based on spin dependent scattering at the domain walls and domain-rotation.