Studies of structural, magnetic, electrical, and photoconducting properties of Bi1-xCaxMnO3 epitaxial thin films

The dynamics of the charge ordered (CO) state under nonequilibrium conditions created by strong dc-electric fields (<= 10(6) V/cm) and photoillumination with short (approximate to 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 (pO(2)) and deposition temperature (T-D) are studied. For the x=0.6 sample grown on LAO, a clear signature of charge ordering at approximate to 275 K is seen in the magnetization and at approximate to 260 K in the resistivity data. The same sample grown on STO revealed a complex behavior, which entails charge ordering at approximate to 300 K, a Neel order at approximate to 150 K, and finally a weak ferromagnetic phase below 50 K. A strong correlation between charge ordering temperature(T-CO) and the c-axis lattice parameter (c) of the type (dT(CO)/dc approximate to-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 shows a compressive strain induced suppression of the T-CO. The current (I)-voltage (V) characteristics of the samples at T < T-CO show hysteresis due to a compound effect of joule heating and collapse of the CO state. Transient changes in conductivity of lifetime ranging from nanoseconds to microseconds are seen at T < T-CO 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.