Abstract
The magnetic and transport properties of La0.5Ca0.5MnO3 and La0.4Ca0.6MnO3 films with different thickness, prepared by rf-magnetron sputtering with the use of a so-called “soft” (or powder) target on a LaAlO3 substrate, are investigated. Electron-diffraction and high-resolution electron microscopy (HREM) studies show that the charge-ordered phase is observed at room temperature for all films. Both the paramagnetic-to-ferromagnetic transition at TC≈250K upon cooling and the appearance of an antiferromagnetic (AFM) phase at TN≲140K are observed in the La0.5Ca0.5MnO3 films, while the La0.4Ca0.6MnO3 films exhibit the AFM transition only, at the same temperature, except for a small ferromagnetic (FM) response from a “dead” layer. It is shown that the volume fraction of the FM phase in the La0.5Ca0.5MnO3 film does not exceed 30% and that the FM phase coexists with the AFM phase at low temperature. All films manifest an exponential temperature dependence of the resistance, with no evidence of the metal-insulator transition. This is explained by the scarcity of the FM phase for the formation of an infinite percolating cluster and by the existence of a charge-ordered phase. The field-dependent magnetoresistance at low temperature is described in terms of the spin-assisted polaron-hopping model.
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