Non-collinear magnetism in iron at high pressures.pdf

a r X i v : c o n d - m a t / 0 3 0 1 6 1 5 v 1 [ c o n d - m a t .m t r l - s c i ] 3 1 J a n 2 0 0 3 Non-collinear magnetism in iron at high pressures R. E. Cohen Geophysical Laboratory, Carnegie Institution of Washington 5251 Broad Branch Rd., N.W., Washington, D.C. 20015 Abstract Using a first principles based, magnetic tight-binding total energy model, the magnetization energy and moments are computed for various ordered spin configu- rations in the high pressure polymorphs of iron (fcc, or γ-Fe, and hcp, or ?-Fe), as well ferromagnetic bcc iron (α-Fe). For hcp, a non-collinear, antiferromagnetic, spin configuration that minimizes unfavorable ferromagnetic nearest neighbor ordering is the lowest energy state and is more stable than non-magnetic ? iron up to about 75 GPa. Accounting for non-collinear magnetism yields better agreement with the experimental equation of state, in contrast to the non-magnetic equation of state, which is in poor agreement with experiment below 50 GPa. Key words: electronic structure, iron, Fe, high-pressure, magnetism, tight-binding PACS: 71.55.Ak, 64.30.+t 1 Introduction Magnetism is known to be important in the phase stability, structure and elastic properties of iron. For example, α-Fe, the ground state at ambient con- ditions, would be mechanically unstable if it were not magnetic. Even above the Curie temperature, Tc there are local magnetic moments in α-Fe. Face- centered cubic iron (fcc or γ-Fe) has incommensurate magnetic correlations which change rapidly with volume, and give rise to the anti-Invar effect (large thermal expansivity) (Mryasov et al., 1992). The magnetic behavior of hcp iron is important for high pressure materials research, and for interpreting high pressure experiments aimed at understanding the Earth’s inner core. Hcp iron is not quenchable to zero pressure, so magnetic studies must be made in Email address: cohen@ (R. E. Cohen). Preprint submitted to Elsevier Science 2 February 2008 situ at high pr