Stress-induced competing ferromagnetic and antiferromagnetic orders in epitaxial films of A-type antiferromagnet La0.45Sr0.55MnO3

Thin films of La 0.45 Sr 0.55 MnO 3 grown epitaxially on single-crystal surfaces which offer a tensile lattice mismatch of 1.7% and 0.47% show properties similar to that of bulk La 0.45 Sr 0.55 MnO 3 and Nd 0.45 Sr 0.55 MnO 3 respectively. These results establish a direct correlation between a reduction in lattice expansion and contraction of the lattice by the smaller rare earth ion (Nd). The antiferromagnetic-to-ferromagnetic transition in films with the lower shifts to higher temperatures as the strain is relieved with increasing film thickness. The electrical resistivity of these films at T > T N has distinct signatures of polaronic transport whose activation energy drops from ∼43 to ∼31 meV on releasing the in-plane strain. The polaron activation energy is higher by a factor of two in films of similar thickness grown on the substrate which offers the larger tensile mismatch. These results show a direct correlation between polaron binding energy ( E 0 ) and the lattice strain. The release of strain manifested by lengthening of the out-of-plane lattice parameter also makes the Néel state robust. However, an external magnetic field ( H ) suppresses T N . The resistivity of the sample at this magnetic transition shows a large negative magneto-resistance and a re-entrant metallic state at T > T N in large fields (≥3 T). The drop in polaron activation energy seen with increasing field (d E 0 /d H ∼−3.82 meV T −1 ) suggests a magnetic character of the polarons. The metallic antiferromagnetic state realized in epitaxial thin films of La 0.45 Sr 0.55 MnO 3 makes it a candidate material for exchange biasing of the manganite-based magnetic tunnel junctions.