Self-consistent embedded-cluster model for magnetic impurities: Fe, Co, and Ni inβ′--NiAl

Mössbauer spectroscopic study of a low temperature magnetic transition in ordered Fe-doped NiAl

“…The results shown thus far confirmed that Mössbauer effect measurement can unambiguously detect the magnetic transition accompanying the Fe-doping of NiAl intermetallic materials as were the cases by Wiley et al [12] and Ellis et al [13]. In the first case with Wiley and coauthors, they noted a difference in the isomer shifts recorded via MS on Al 50 Ni 48 Fe 2 and (NiAl) 98 Fe 2 with the Ni deficient alloy having the higher isomer shift, δ.…”

“…For the Al rich alloys (>50% Al), vacancies instead of Al anti-sites were formed on the Ni sublattice, which constituted a more effective hardening agent than the Ni Al anti-site defects. Thus, the softening behavior observed in the Fe doped material can be investigated and discussed in the light of theoretical calculations [13] and Mössbauer effect measurements. According to Ellis et al [13], transition metal impurities on Ni sites have reduced moments compared to free atom, with small polarizations on nearest-neighbor Al atoms, while on the Al sites.…”

“…Thus, the softening behavior observed in the Fe doped material can be investigated and discussed in the light of theoretical calculations [13] and Mössbauer effect measurements. According to Ellis et al [13], transition metal impurities on Ni sites have reduced moments compared to free atom, with small polarizations on nearest-neighbor Al atoms, while on the Al sites. On the other hand, they generally showed a sizable magnetic moment with significant polarization of the nearest Ni shell.…”

“…According to them, this implied that Fe substituted on the Ni lattice should have a charge density lower than if it were substituted on the Al lattice. In the nomenclature of Ellis et al [13], MX 8 referred to a central atom, M, with eight nearest X neighbors, whereas MX 8 Y 6 referred to a central atom with eight nearest and six next nearest neighbors respectively. Their calculations showed that in the case of Fe impurities, a nonzero hyperfine field at either substitutional site, H s × −30 kG for Fe Ni (FeAl 8 ) (i.e., Fe substituting Ni resulting in Fe with eight nearest Al neighbors), and H s × +20 kG for Fe Al (FeNi 8 ) (i.e., Fe substituting Al resulting in Fe with eight nearest Ni atoms) would arise.…”

Theory of positrons in solids and on solid surfaces

“…In this calculation the embedded-cluster model (Ellis, 1979;Umrigar and Ellis, 1980) One of the important tasks of the positron theories for solids has been the prediction of the positron lifetimes for vacancy clusters as a function of the cluster size and possible decoration by impurities. This information has a direct bearing on the interpretation of lifetime measurements when the evolution of defect structures is investigated, for example, during annealing after particle irradiation or plastic deformation.…”