Strain and disorder effects on inhomogeneous states of manganites
Carmine Antonio Perroni, Vittorio Cataudella, Giulio De
Filippis, and Vincenzo Marigliano Ramaglia
Coherentia-INFM and Dipartimento di Scienze Fisiche, Università Federico II, Napoli
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Manganites are characterized by strong tendencies toward phase
separation and intrinsic inhomogeneities. These properties are
strongly affected by long-range strain and intrinsic disorder.
Therefore the study of manganite films is interesting not only for
eventual technological applications but also from a basic point of
view: in fact it is possible to grow optimized films with
different amounts of disorder and substrates allowing to change
the strain. Thus the thickness dependence, strain and disorder
effects in films of La1-xAxMnO3 perovskites have been
analyzed in the colossal magnetoresistance regime.
A reduction in the thickness of the film causes a decrease of
critical temperature and an increase of resistivity at low
temperatures. The strain effects are in good agreement with
experimental data only if the dependence of the hopping matrix
elements on the Mn-O-Mn bond angle is properly taken into account.
Finally variations of the electron-phonon coupling linked to the
presence of strain turn out to be important in influencing the
balance of coexisting phases in the film.
In collaboration with experimental groups, the resistivity has
been investigated finding that in the low temperature state the
prominent contribution to the resistivity scales as T2.5
supporting the role of single magnon scattering in presence of
minority spin states localized by the disorder. In the high
temperature phase the resistivity shows the activated behavior
characteristic of polaronic carriers. Finally in the whole range
of temperatures the experimental data are found to be consistent
with a phase separation scenario also in films doped with
strontium. Recently we have analyzed the STM data of the films
finding direct evidence of multiphase modulations.The effect of
the disorder and electron-phonon coupling is able to trigger the
metal-insulator transition with polaronic nanoscale correlations.
However, with increasing strain, the mesoscopic scale emerges with
phase coexistence.
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