Resistivity minimum in granular composites and thin metallic films

Abstract

Ramat Aviv, 69978 Tel Aviv, IsraelWe analyze the temperature dependence of conductivity in thick granular ferromagnetic compounds NiSiO2 and in thin weakly coupled films of Fe, Ni and Py in vicinity of metal-insulator transition. Development of resistivity minimum followed by a logarithmic variation of conductivity at lower temperatures is attributed to granular structure of compounds and thin films fabricated by conventional deposition techniques. Resistivity minimum is identified as a transition between temperature dependent intra-granular metallic conductance and thermally activated intergranular tunneling. PACS: 71.30.+h, 72.80.Tm, 73.50 Introduction.There is a number of universal features demonstrated by variety of materials in vicinity of metalto-insulator transition. One of them is existence of the resistivity minimum at temperature , usually followed by a logarithmic increase of resistivity with decreasing temperature. The effect was observed in numerous thin, considered as two-dimensional, and thick, considered as threedimensional, crystalline and amorphous normal metals [1-3], paramagnetic and ferromagnetic materials [4 -8] and superconductors in their normal state [9,10]. Historically, the phenomenon of resistivity minimum associated with very small amount of paramagnetic impurities in crystalline and amorphous alloys was understood as arising from the spin flip scattering of conduction electrons off the local magnetic moments, randomly distributed in the alloys [11]. Within the accepted Kondo theory, the minimum is suppressed with increasing impurity concentration and finally washed out as magnetic ordering sets in, since magnetic ordering of the local spins destroys the freedom of the spin to flip, the basic mechanism needed to have a resistivity minimum. Thus, observation of the effect in ferromagnetic materials below the magnetic ordering temperature [4 -8] raised a question of fundamental conceptual importance: since magnetic ordering destroys the Kondo explanation, what then is the source of the observed resistivity minimum? While much work was done in attempts to settle Kondo model with macroscopic ferromagnetism [12, 13] it was noted by Cochrane et al [6] that the logarithmic dependence of resistivity below in different ferromagnetic materials fabricated by different methods by different groups was totally unaltered by application of high magnetic field and thus might be not related to magnetism at all. The next round of interest to the effect came with prediction and observation of quantum corrections to resistivity of disordered materials: weak localization and electron-electron interactions [14, 15]. Both mechanisms predict similar logarithmic temperature dependence of conductivity, but in two-dimensional cases only. Experimentally, not only thin disordered films but also thick three-dimensional granular materials in vicinity of the metal-insulator transition were found to demonstrate the logarithmic temperature variation of resistivity. It was suggested by Deutscher et al. [16] that althou...

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