Magnetic interactions as a source of temperature and field dependent magnetic grain size

In systems made of magnetic (M) and non-magnetic (NM) species, where the composition is not homogeneous on the nanometric scale, the magnetic size distribution of M regions, f(V), directly affects the magnetic (m) and the giant magnetoresistive (mr) response to temperature (T) and to external field (H) changes. In particular, if the whole f(V) falls within the superparamagnetic (SP) regime, mr(H) and m(H) can be properly described by Langevin formal-ism, i.e. they should scale with the H/T ratio, and mr proportional to m2. If due to a broad size distribution or to interparticle interactions the system leaves the SP behaviour, as m and mr sensitivity to magnetic size are different, a progressive decoupling between magnetic and resistive response is observed. However, specifically for high applied fields, this should not affect the H/T in-variance. In this work, we study this topic on FeAg samples and we compare mr data collected as a function of Fe concentration using different techniques, ultra-short pulsed laser ablation and plasma co-sputtering deposition. Our results indicate that, irrespective of the technique and of iron concentra-tion, the systems mr behaviour displays the presence, at high fields, of an al-most T independent component. This result has been interpreted in terms of the presence of an effective f(V,T,H) governed by magnetic interactions [1]. In particular, f(V,T,H--∞)-- f∞(V), i.e. a temperature independent distribution is observed. These results will be presented along with the samples magnetic characterization. [1] P. Allia et al., Phys. Rev. B 67 (2003) 174412