Studies of structural, magnetic, electrical and photoconducting properties of Bi$_{1-x}$Ca$_{x}$MnO$_{3}$ epitaxial thin films

Abstract: 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.