Strain-enhanced phase separation affecting electro- and magnetotransport in La0.67Ca0.33MnO3 films

Biaxial strain during nucleation influences phase separation into ferromagnetic (metallic) and nonferromagnetic (insulating) regions and that, in turn, markedly affects the electric transport of a manganite film. A 40-nm-thick La0.67Ca0.33MnO3 film, coherently constrained by a (001)LaAlO3 substrate, possesses a noticeably contracted unit cell volume (Veff≈56.70 Å3) as compared with that of a stoichiometric bulk sample. It corresponds to a higher relative concentration (45%) of tetravalent manganese ions in the manganite layer than that in the target (33%). The resistivity ρ(T) curve of the strained film peaks twice in the range 4.2–300 K. The charge transport of strained La0.67Ca0.33MnO3 films is non-ohmic at T<130 K. A magnetic field H linearizes the current–voltage characteristic, but its impact on ρ(T) and I–V decreases at low temperature. The unusual features in the electro- and magnetotransport properties of thin La0.67Ca0.33MnO3/(001)LaAlO3 films are ascribed to a strain-enhanced phase separation, which is also responsible for the large magnetoresistance (up to 90%) at 5 T within a broad temperature range. Thicker films experience a relaxation, a smaller resistivity, and less non-linear properties.