Magnetism In Alloys

Magnetism in transition metal materials can be described in quantitative detail by spin-density functional theory (SDFT). At low temperatures, the magnetic properties of a material are characterized in terms of its spin-polarized electronic structure. It is on this aspect of magnetic alloys that is focused on here. From this basis, the early Stoner-Wohlfarth picture of rigidly exchange-split, spin-polarized bands is shown to be peculiar to the elemental ferromagnets only. Shown are the origins of two commonly occurring features of ferromagnetic alloy electronic structures, and the simple structure of the Slater-Pauling curve for these materials (average magnetic moment versus electron per atom ratio), can be traced back to the spin-polarized electronic structure. The details of the electronic basis of the theory can, with care, be compared to results from modern spectroscopic experiment. Although SDFT is “first-principled,” most applications resort to the local approximation (LSDA) for the many electron exchange and correlation effects. This approximation is widely used and delivers good results in many calculations. The LDA in magnetic materials fails when it is straightforwardly adapted to high temperatures. This failure can be redressed by a theory that includes the effects of thermally induced magnetic excitations, but which still maintains the spin-polarized electronic structure basis of standard SDFT. Although not spin-polarized “globally”—i.e., when averaged over all orientational configurations—the electronic structure is modified by the local-moment fluctuations, so that “local spin-polarization” is evident. A mean field theory of this approach and its successes for the elemental ferromagnetic metals and for some iron alloys are discussed. The dynamical effects of these spin fluctuations in a first-principles theory remain to be included. Also emphasized is how the state of magnetic order of an alloy can have a major effect on various other properties of the system, and effect upon atomic short-range order by describing case studies of NiFe, FeV, and AuFe alloys is dealt with at length. The results of our calculations with details of “globally” and “locally” spin-polarized electronic structure are linked. The relativistic generalization of SDFT and covered its implication for the magnetocrystalline anisotropy of disordered alloys, with specific illustrations for CoPt alloys are discussed. In summary, the magnetic properties of transition metal alloys are fundamentally tied up with the behavior of their electronic “glues.” As factors like composition and temperature are varied, the underlying electronic structure can change and thus modify an alloy's magnetic properties. Likewise, as the magnetic order transforms, the electronic structure is affected and this, in turn, leads to changes in other properties. Here the focussed is the effect on ASRO, but much could also have been written about the fascinating link between magnetism and elastic properties—“Invar” phenomena being a particularly dramatic example. Keywords: alloys; magnetism; principles; solid solution alloys; alloy electronic structure; slater pauling curves; finite temperature; first principles; local exchange splitting magnetic anisotropy; iron; nickel; cobalt; data analysis; interpretation; atomic long range order; atomic short range order; problems