EXAFS studies of enhancement of L21-B2 chemical disorder induced by ball milling in martensitic Ni50Mn36Sn14 pseudo-Heusler alloy

Abstract

Chemical disorders in L21-B2-type structure, naturally occurred in 57Fe-doped Ni50Mn36Sn14 pseudo-Heusler alloy or enhanced in milled material, were systematically investigated using local probe methods: 57Fe Mössbauer spectroscopy (MS) and extended X-ray absorption fine structure (EXAFS). 57Fe Mössbauer studies show two Fe configurations: paramagnetic (singlet) and ferromagnetic (magnetic distribution) with approximately a 1:1 ratio. Fe atoms in singlet configuration are replacing Ni or Sn sites, where a near zero or magnetically frustrated state occurs. The magnetic distribution component is due to Fe magnetically coupled to Mn atoms. The singlet fraction is 100% after 220 s of milling. Low temperature thermal annealing easily recovers the original magnetic component and its initial ratio. Ni and Mn K-edges EXAFS spectra suggest that the milled material still displays the long range crystalline ordering, but with an enhancement of the local disorder due to Mn atoms occupying Sn sites and Mn-distance changes (D-sites of the L21 structure: Mn–Sn). While Mn–Mn distances seem to reduce slightly from 2.91 Å to 2.89 Å after the milling, the chemical disorder parameter (σ2) enhanced 29%. No change in Mn–Ni distances was observed; a condition necessary for observation of structural transition inside of crystalline grain. The structural transition properties change due to the increase of σ2-parameters of Mn–Ni (enhanced to 38%) and Mn–Sn (increased to 29%). EXAFS spectra of annealed sample indicate an improvement of the D-site symmetry, which is confirmed by a distance value of 2.92 Å for the Mn–Mn and Mn–Sn in the L21 structure as well as a large reduction (between 40% and 80%) of the σ2 parameter value. The chemical order improvement of the L21-structure favors Mn–Mn ferromagnetic order, indirectly measured by 57Fe MS that probed a magnetic hyperfine field component.