Onset of ferromagnetism in 3d-substituted FeAl alloys. I. Ti, V and Cr substitutions

Abstract

For pt.I see ibid., vol.15, p.681 (1985). Magnetic and structural studies using X-ray, neutron diffraction, magnetisation and small-angle neutron scattering (SANS) techniques are reported for FeAl1-xTx alloys in which partial substitution of Al by Mn or Co has been effected. The results of the neutron diffraction studies are consistent with a B2 model in which the Co atoms exercise a site preference for Fe sites, whereas Mn atoms appear to enter both Fe and Al sites. Consequently, for both systems displacement of Fe atoms into Al sites creates 'antistructure' (AS) Fe atoms. The susceptibilities of both alloy systems were observed to increase with the concentration of the transition metal solute. An apparent onset of spontaneous magnetisation was observed beyond certain 'critical' concentrations: x=0.3 and x=0.2 for Mn and Co substitutions respectively. However, it was not until a Co concentration with x=0.3 was reached that a Curie temperature could be established in the Arrott plots. No TC could be established at all in similar plots for the Mn series having x<or=0.4. Furthermore, the SANS measurements revealed no sharp peaking in the intensity contours of the critical magnetic scattering at temperatures where Arrott plots (based on high-field magnetic data only) had indicated a Curie temperature. Possible mechanisms for these observations are discussed in terms of current theories of the onset of magnetic order in magnetic transition intermetallic compounds.