Studies of structural, magnetic, electrical, and photoconducting properties of Bi_1-xCa_xMnO₃ epitaxial thin films

Abstract

The dynamics of the charge ordered (CO) state under nonequilibrium conditions created by strong dc-electric fields (≤10⁶ V/cm) and photoillumination with short (≈6 ns) laser pulses is investigated in Bi_1-xCa_xMnO₃ (x gt;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 (T_D) 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(T_CO) and the c-axis lattice parameter (c) of the type (dT_CO/dc≈-350/λ) 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 lt;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.