The Fe_1-xAl_x system exhibits unusual magnetic properties around x~3 where ferromagnetism (FM) first sets in (for x<xc). In particular the magnetization under low applied fields (H_a<100 Oe) decreases with temperature below ~150K, as the FM state goes into a superparamagnetic (super-PM) structure. The understanding of the different phases below T_C still continues to present problems [1].

We report a magnetoresistance (MR) study on Fe_1-xAl_x samples with x=0.295, 0.300 and 0.305, prepared by melt spinning and exhibiting a polycrystalline A2 structure (from X-ray diffraction). A four-point probe dc technique was used for the MR measurements, with the magnetic field applied parallel to the electrical current. The residual resistivity (ρ₀) increases dramatically with x in our samples (dρ₀/dx~181μΩcm per 1%Al addition), reaching 353 μΩcm for x=0.305. This reflects a drastic increase in structural and magnetic disorder within a narrow range of composition, in a scale of the order of the electron mean free path.

Magnetoresistance and coercive field measurements clearly indicate three magnetic regimes below room temperature, with transition points around 50K and 150K. In fact from T_Cinv ~150K down to T_iso~50K, the MR behavior indicates the presence of super-PM. Below T_iso the samples exhibit full super-PM, but the increase of the coercive field as T decreases indicates enhancement of local magnetic anisotropy.

The MR behavior can be understood using a dynamical model based on FM clusters, whose sizes vary with temperature within a PM matrix. From T_C to T_Cinv we have overlapping FM clusters leading to the highest degree of FM behavior. Below T_Cinv such coalescence disappears (smaller FM clusters) but the separate clusters are still magnetically interacting. Below T_iso~50K, inter-cluster magnetic interactions fade away and full super-PM emerges.

[1] D.Schmool et al J. Mag. Mag .Mat. 272-276 (2004) 1342

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