Stress-induced competing ferromagnetic and antiferromagnetic orders in epitaxial films of A-type antiferromagnet La_0.45Sr_0.55MnO₃

Abstract

Thin films of La_0.45Sr_0.55MnO₃ 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_0.45Sr_0.55MnO₃ and Nd_0.45Sr_0.55MnO₃ 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 ε shifts to higher temperatures as the strain is relieved with increasing film thickness. The electrical resistivity of these films at Tgt;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 (E₀) and the lattice strain. The release of strain manifested by lengthening of the out-of-plane lattice parameter also makes the Neel 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 Tgt;TN in large fields (≥3 T). The drop in polaron activation energy seen with increasing field (dE₀/dH~-3.82 meV T^-1) suggests a magnetic character of the polarons. The metallic antiferromagnetic state realized in epitaxial thin films of La_0.45Sr_0.55MnO₃ makes it a candidate material for exchange biasing of the manganite-based magnetic tunnel junctions.