Low-temperature transition to a metallic state in (La0.5Pr0.5)(0.7)Ca0.3MnO3 films

The electrical resistivity of epitaxial (La0.5Pr0.5)(0.7)Ca0.3MnO3 films de posited on single crystalline LaAlO3 substrates was studied at temperature and magnetic fields ranging from 4.2 to 300 K and from 0 to 3 T, respective ly. On cooling from room temperature at zero magnetic field, the films demo nstrate at first the behavior typical of the charge-ordered (CO) insulating state, whereas below 40 K they undergo the transition to a metal-like stat e with slowly decreasing resistivity. On heating from 4.2 K, the films rema in metallic and their resistivity rho(T) coincides with the cooling curve o nly at T>80 K. This hysteretic behavior fully reproduces itself at repeated cooling-heating cycles. Near the low-temperature transition to a metal-lik e state the charge ordering (CO) is metastable and the resistivity exhibits the relaxation phenomena. The applied magnetic field as low as H=1 T suppr esses CO, and the temperature hysteresis gradually disappears. The rho(T) m easurements at nonzero fields reveal a pronounced colossal magnetoresistanc e effect with the resistivity drop by a factor exceeding 10(6) at H=3 T. It was also found that relatively small de voltages (<3 V) can cause the swit ching from CO to a metal-like state within the metastability range in the v icinity of 40 K when the charge ordering can be rather easily suppressed. W ithin this range, the current-voltage characteristics are highly nonlinear, with a memory effect: after switching the sample remains metallic even if the voltage is lowered. The observed effects are interpreted in terms of st rong competition between charge ordering and ferromagnetic spin ordering. T his competition can give rise to different kinds of spatial inhomogeneities involving the charge-ordered state, which should manifest themselves most clearly in the vicinity of the low-temperature transition to the metal-like state. The behavior of resistivity before the transition gives indications of the two-phase state. The transition itself can be a manifestation of a percolative nature of conductivity in this regime. [S0163-1829(99)06209-8].