Thin films of La0.45Sr0.55MnO3 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
La0.45Sr0.55MnO3 and Nd0.45Sr0.55MnO3 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 shifts to higher temperatures as the strain is relieved with
increasing film thickness. The electrical resistivity of these films at
T>TN 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 Néel state robust. However, an external magnetic field
(H)
suppresses TN. The resistivity of the sample at this magnetic transition shows a
large negative magneto-resistance and a re-entrant metallic state at
T>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 La0.45Sr0.55MnO3 makes it a candidate material for exchange biasing of the manganite-based magnetic tunnel
junctions.
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