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

The electrical resistivity of epitaxial (La0.5Pr0.5)0.7Ca0.3MnO3 films deposited 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, respectively. On cooling from room temperature at zero magnetic field, the films demonstrate at first the behavior typical of the charge-ordered (CO) insulating state, whereas below 40 K they undergo the transition to a metal-like state with slowly decreasing resistivity. On heating from 4.2 K, the films remain metallic and their resistivity ρ(T) coincides with the cooling curve only at T>80K. This hysteretic behavior fully reproduces itself at repeated cooling-heating cycles. Near the low-temperature transition to a metal-like state the charge ordering (CO) is metastable and the resistivity exhibits the relaxation phenomena. The applied magnetic field as low as H=1T suppresses CO, and the temperature hysteresis gradually disappears. The ρ(T) measurements at nonzero fields reveal a pronounced colossal magnetoresistance effect with the resistivity drop by a factor exceeding 106 at H=3T. It was also found that relatively small dc voltages (<3V) can cause the switching from CO to a metal-like state within the metastability range in the vicinity of 40 K when the charge ordering can be rather easily suppressed. Within 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 strong competition between charge ordering and ferromagnetic spin ordering. This 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