Magnetic and structural studies of Fe and Ni substituted CoAl alloys

• Published in: Journal of magnetism and magnetic materials 1995-01, Vol.139 (3), p.299-311
• Main Authors: Al-Kanani, H.J., Booth, J.G.
• Format: Article
• Language: English
• ISSN: 0304-8853
• DOI: 10.1016/0304-8853(95)90008-X

Previous work has shown that the substitution of a 3d element (Ti, V, Cr or Mn) for Al in the equiatomic intermetallic compound CoAl results in the onset of ferromagnetic order beyond a critical concentration. The systems appear magnetically and structurally similar to the equivalent CoGa substituted compounds for which the critical concentration appears to be correlated with an average electron-atom ratio rather than woth any obvious change in the crystallographic order. The present paper completes the investigation of the CoAl series by presenting results for the systems CoAl 1− x Fe x and CoAl 1− x Ni x . X-ray and pulsed neutron diffraction techniques showed that the ordered B2 (CsCl) structure is retained for both these series but for the Ni series a second phase fcc structure was also observed. The neutron diffraction data was consistent with Fe atoms occupying Al sites alone and Ni atoms occupying Co sites with a consequent displacement of Co atoms to Al sites. A striking feature is that although CoAl is paramagnetic only a small amount of Fe ( x≥0.03) produces ferromagnetic order with a subsequent rapid increase in Curie temperature (to ≈ 900 K for x=0.2). The disparity between the moments obtained from the high temperature paramagnetic and low temperature ferromagnetic regimes is suggestive of an itinerant magnetism and an analysis is attempted using spin fluctuation models of magnetism. Results for the entire series of CoAl substituted alloys are reviewed and compared with those for the related compounds of CoGa.