The dynamics of the charge ordered (CO) state under non-equilibrium
conditions created by strong dc-electric field (~106 V/cm) and
photo-illumination with short (~ 6 ns) laser pulses is investigated in
Bi1-xCaxMnO3 (x > 0.5) epitaxial films. A pulsed laser deposition method was
used to synthesize films on (100) LaAlO3 (LAO) and (100) SrTiO3 (STO)
substrates. The crystallographic structure, temperature dependence of
electrical resistivity and magnetization of the samples of different
composition prepared under different oxygen partial pressure (pO2) and
deposition temperature (TD) are studied. For the x = 0.6 sample grown on LAO, a
clear signature of charge ordering at ~275 K is seen in the magnetization and
at ~ 260 K in the resistivity data. The same sample grown on STO revealed a
complex behavior, which entails charge ordering at ~300 K, a Neel order at ~150
K and finally a weak ferromagnetic phase below 50 K. A strong correlation
between charge ordering temperature (TCO) and the c-axis lattice parameter (c)
of the type (dTCO/dc ~-350 K/A) imerges from measurements on films deposited
under different growth conditions. Since the out of plane lattice parameter (c)
increases with in plane compressive strain, this effect directly show a
compressive strain induced suppression of the TCO. The current (I)- voltage (V)
characteristics of the samples at T < TCO show hysteresis due to a compound
effect of Joule heating and collapse of the CO state. Transient changes in
conductivity of lifetime ranging from nano to microseconds are seen at T < TCO
on illumination with pulsed UV (355 nm) radiation. These observations are
explained on the basis of the topological and electronic changes in the charge
ordered phase.Comment: 19 figures, 34 page