Transition from ferro to mictomagnetism in rich FexAl1-x alloys. A magnetoresistivity study.

M M. Amado 1M. E. Braga 1D. S. Schmool 1Andre M. Pereira 1J. M. Moreira 1Joao P. Araujo 1J. B. Sousa 1J. S. Garitaonandia 2F. Plazaola 3

1. Departamento de Fisica and IFIMUP, Universidade do Porto (UP), Rua do Campo Alegre, 687, Porto 4169-007, Portugal
2. Elektrika eta elektronica saila, Euskal Herriko Unibertsitatea, Bilbao, Spain
3. Fisika Aplikatua II saila, Euskal Herriko Unibertsitatea, Spain


The Fe1-xAlx 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 (Ha<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 TC still continues to present problems [1].

We report a magnetoresistance (MR) study on Fe1-xAlx 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 TCinv ~150K down to Tiso~50K, the MR behavior indicates the presence of super-PM. Below Tiso 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 TC to TCinv we have overlapping FM clusters leading to the highest degree of FM behavior. Below TCinv such coalescence disappears (smaller FM clusters) but the separate clusters are still magnetically interacting. Below Tiso~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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Presentation: poster at E-MRS Fall Meeting 2005, Symposium B, by M M. Amado
See On-line Journal of E-MRS Fall Meeting 2005

Submitted: 2005-04-26 11:06
Revised:   2009-06-07 00:44
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