Magnetotransport properties of nanometric La2/3Sr1/3MnO3 granular perovskites

The electrical resistivity of granular nanometric La2/3Sr1/3MnO3 ceramics is studied as a function of temperature and magnetic field. Both low-field (LFMR) and high-held (HFMR) magnetoresistance response have been measured as a function of the grain size (diameter). The behavior of the LFMR and HFMR responses turns out to be substantially different: whereas LFMR saturates at a value of about 30% for grain sizes diameter approximate to 400 nm, with no increase with further reduction of the grain size, the HFMR progressively increases as the grain size is reduced; Experimental data indicates that HFMR originates from the existence of a non-collinear ferromagnetic surface layer with a thickness that increases as diameter decreases. The results can be analyzed within the framework of a model involving the existence of a Coulomb gap E-C, superimposed to a tunneling barrier between individual grains, which produces a significant blockade at temperatures below similar to 100 K. Detailed data analysis allows to separate the contributions from tunneling and Coulomb gap and to determine its value and the grain size and magnetic field dependence. (C) 2000 Elsevier Science B.V. All rights reserved.