Extrinsic Magnetotransport in Manganites and its Dependence on Mechanical Strain

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The extrinsic electrical transport in manganites of the type (La,A)(Mn,B)O3
(where A = Ca2+, Sr2+ or Pb2+ and B = Ru+, Ta+ or W+) has been studied on
bulk ceramic and thin film samples. In particular, its dependence on reversible
strain applied with a piezoelectric substrate has been investigated. The first
part discusses results on bulk polycrystalline samples, (La0.7Ca0.3MnO3)/LaAlO3
(001) films forming step edge junctions (SEJ) on substrates containing lithographically
defined parallel steps and La0.7Sr0.3MnO3/SrTiO3 (001) films on a bicrystal
substrate containing one grain boundary. Magnetoresistance measurements have
been carried out in static and in pulsed magnetic fields of μ0H = 50 T. The
magnetoconductance G(H) depends on the susceptibility of a magnetically less
ordered layer around the grain boundaries. In particular, the second-order grain
boundary tunnelling model of Lee et al. [Lee 99] is employed to discuss the magnetoconductance
data. The bulk ceramic samples of various compositions and a
polycrystalline La0.7Sr0.3MnO3 film show dominating antiferromagnetic interactions
in the grain boundary layer, whereas spin-glass-like disorder is concluded
from transport data for the (La0.7Ca0.3MnO3)/LaAlO3 (001) SEJ. The bicrystal
transport behaviour appears dominated by a small number of magnetic domains
next to the bicrystal grain boundary. The second part of this work addresses the
influence of a reversible strain on electrical grain boundary transport. The strain
is applied through a piezoelectric pseudocubic substrate of PMN-PT (001) to the
films. In detail, polycrystalline films, films grown on substrates with parallel steps
of several 100 nm height and a film on a substrate containing an intentionally
applied scratch were investigated. Large strain response of the junction resistance
and the magnetoresistance has been observed for the SEJ films, with strong dependence
on the film thickness, and is discussed within the indirect tunnelling
model (mentioned above). Polycrystalline films showed a more moderate strain
effect.