Spin-dependent transport in granular films with mixed length-scale magnetic coherence

We investigated the magnetoresistance effect in presence of mixed length-scale magnetic coherence in systems displaying coexistence of ferromagnetic- and superparamagnetic-like behaviors. In spite of their different microstructure, all the samples investigated show magnetoresistance curves vs. sample magnetization with a characteristic feature: the magnetoresistance remains unchanged (viz., equal to its value for M = 0) over a wide range of magnetization values. The results are interpreted considering the interplay between the different magnetic ordering scales probed by the spin-dependent scattering and the magnetization reorientation processes and the presence in the films of magnetic coherence at different length-scales. From the comparison of the results with a phenomenological model, developed for dealing with local variations of magnetic ordering, we determined the field evolution of the ratio between the characteristic length-scale for magnetic coherence and spin diffusion length in the different samples.