Ž . Thin Solid Films 373 2000 9497 Intergranular Coulomb barriers in thin films of magnetoresistive manganites M. Garcıa-Hernandez  , A. de Andres, F. Guinea, J.L. Martınez, C. Prieto, A. Munoz, ´ ´ ´ ´ ˜ L. Vazquez Instituto de Ciencia de Materiales de Madrid, Consejo Superior de In estigaciones Cientıficas, Cantoblanco s  n, ´ E-28049, Madrid, Spain Abstract The low-temperature behavior of the resistance of polycrystalline thin films of magnetoresistive manganites has been investigated for grain sizes in the nanometric scale. We have optimized the dc-sputtering deposition and annealing procedures so as to meet the grain size requirements while keeping the basic features of the magnetic and electronic transport of the bulk Ž . material. A systematic upturn of the resistance at approximately 50 K is observed in all films 12  80 nm grain diameter . These results point to the existence of a Coulomb gap affecting the conduction of electrons at least in the smallest grain sizes explored. The evolution of this effect with the magnetic field is also investigated.  2000 Elsevier Science S.A. All rights reserved. Keywords: Electrical properties and measurements; Magnetic properties and measurements; Manganese; Oxides 1. Introduction The study of the low-temperature electrical transport in granular metals and metal  insulator composites have been the focus of a great deal of studies since the pioneering works of Abeles and Sheng in the early   1970s 1  3 . The existence of a soft Coulomb gap in the density of states near the Fermi level was also dis- cussed by Efros and Shklovskii in amorphous and doped  crystalline semiconductors 4 . In granular metals, it is well established that the electrical conduction results from the transport of electrons and holes from charged to neutral grains. A carrier has, therefore, to be cre- ated by removing one electron from a neutral grain and placing it in a neighboring neutral grain. This process involves an electrostatic energy, Ec. The charging en- ergy, Ec, can be written as  Corresponding author. Ž . E-mail address: marmar@icmm.csic.es M. Garcıa-Hernandez . ´ ´ Ec  e 2 2 C  e 2 4   d o where e stands for the electronic charge and C is the capacitance of a grain,  and  are the permeability of o the vacuum and the relative permeability, and d is the grain diameter. From this equation it is obvious that Ž Ec becomes very large for very small grain sizes in the . nanometer scale . Consequently, for small grains, as the temperature decreases it is increasingly difficult to activate the transport process and this situation may evolve to a point in which the transport could be effectively blocked. In this case an increase in the Ž . resistance, RT , of the system should be observed at low temperatures where the electron localization ef- fects take place. This phenomenon has been reported  for all granular metal films explored 2 . Recently, a similar behavior in the low-temperature   resistivity of some oxides has been observed 5  7 . This phenomenology has been explained either in terms of the existence of an intergranular Coulomb gap also in these materials or related to the divergent behavior 0040-609000$ - see front matter  2000 Elsevier Science S.A. All rights reserved. Ž . PII: S 0 0 4 0 - 6 0 9 0 00 01111-1