Low-temperature transition to a metallic state in „ La 0.5 Pr 0.5 … 0.7 Ca 0.3 MnO 3 films N. A. Babushkina and L. M. Belova * Institute of Molecular Physics, Russian Research Center ‘‘Kurchatov Institute,’’ Kurchatov Square 1, Moscow, 123182 Russia D. I. Khomskii Materials Science Center, University of Groningen, Nijenborgh 4, 9747 AG Groningen, The Netherlands and Lebedev Physics Institute, Leninskii Prospect 53, Moscow, 117924 Russia K. I. Kugel Scientific Center for Applied Problems in Electrodynamics, Izhorskaya Street 13/19, Moscow, 127412 Russia O. Yu. Gorbenko and A. R. Kaul Chemistry Department, Moscow State University, Vorobievy Gory, Moscow, 119899 Russia Received 15 July 1998 The electrical resistivity of epitaxial (La 0.5 Pr 0.5 ) 0.7 Ca 0.3 MnO 3 films deposited on single crystalline LaAlO 3 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  ( T ) coincides with the cooling curve only at T 80 K. This hysteretic behavior fully repro- duces 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=1 T suppresses CO, and the temperature hysteresis gradually disappears. The  ( T ) mea- surements at nonzero fields reveal a pronounced colossal magnetoresistance effect with the resistivity drop by a factor exceeding 10 6 at H=3 T. It was also found that relatively small dc voltages ( 3 V) 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. S0163-18299906209-8 I. INTRODUCTION Magnetic oxides with metallic conductivity, especially those characterized by the colossal magnetoresistance CMR exhibit a whole wealth of interrelated phenomena such as metal-insulator transitions, orbital or Jahn-Teller ordering, charge ordering, double exchange, lattice and mag- netic polarons, etc. see, e.g., Ref. 1. This interplay provides a better insight into the origin of magnetism, the nature of electronic states, and transport phenomena in these materials. The compounds where the transition points corresponding to different types of ordering are close to each other are of special interest because small composition changes and rela- tively weak applied fields can produce rather pronounced effects. A good example of such materials is presented by the doped rare-earth manganites (La 1 -y R y ) 1 -x A x MnO 3 ( R =Pr, Nd; A =Ca, Sr. These manganites are very sensitive to composition variations and can be switched, for example, from the insulator to the metallic state under effect of pres- sure, magnetic and electric fields, and even x-ray irradiation. 2–4 A dramatic manifestation of these phenomena is the metal-insulator transition induced by 16 O → 18 O iso- tope substitution. 5–7 The unusual behavior of these manganites stems from the strong competition between two possible types of states: a charge-ordered CO state where Mn 3 + and Mn 4 + are local- ized at separate sublattices this state is insulating and usu- ally antiferromagnetic and a charge-delocalized CD ferro- magnetic state with a metal-like conductivity. Note that Mn 3 + is the Jahn-Teller ion, and the corresponding distor- tions of MnO 6 octahedra can give rise to the orbital ordering and to the additional stabilization of the CO state. Above a certain characteristic temperature both CO and CD states transform to a charge-localized CL paramagnetic phase. The CD-CL transformation is accompanied by a sig- nificant increase in the electrical resistivity, and it is rather sensitive to the applied magnetic field giving rise to the CMR. For the CL state, it is usually assumed that charge carriers are polarons localized due to lattice distortions of the PHYSICAL REVIEW B 1 MARCH 1999-II VOLUME 59, NUMBER 10 PRB 59 0163-1829/99/5910/69947/$15.00 6994 ©1999 The American Physical Society