Mössbauer study of Fe-Cr-Co magnetic alloys

Abstract

Mössbauer spectroscopy has been used to investigate the hyperfine fields in an Fe-27.5% Cr-17.5% Co-0.5% Al alloy for different heat treatments. Spectra for the as-cast condition consist of two superimposed 6 line spectra and a central peak and reveal the three distinct iron environments ofα ₁,α ₂, andγ phases respectively. Following homogenization a broad distribution of hyperfine fields associated with broad isomer shift distribution is indicative of randomized atomic distribution within a single phase. Upon isothermal heat treatment at 620° C theα phase spinodally decomposes into 2 ferromagnetic environments: a Fe-Co richα ₁ phase withH _eff ≈ 350 kOe and a Cr-richα ₂ phase withH _eff ≈ 200 kOe. Evidence of a third ferromagnetic iron rich environment (α ₃ phase withH _eff ≈ 325 kOe) is also found as a transition phase which forms prior to subsequent ageing. The progressive reduction in ageing temperature from 600 to 540° C to improve coercivity transforms theα ₁ andα ₂ modulated structure into a paramagnetic Cr-rich precipitate giving a single line spectrum and a highly ferromagnetic Fe-Co rich matrix givingH _eff ≈ 350 kOe. Anisotropy of the Fe-Co matrix introduced by isothermal decomposition under the influence of the external magnetic field is clearly observable after the ageing process through changes to the Mössbauer spectrum.