EFFECTS OF MAGNETISM ON THE THERMAL EQUATION OF STATE OF IRON

Magnetism has large effects on the static equation of state of iron and on its thermal equation of state. For example, the bulk modulus for the high pressure phase of iron, hexagonal close packed (hcp) is about 100% too high when magnetism is not taken into account [1]. Magnetism in face centered cubic (fcc) iron is known to lead to a huge thermal expansivity, known as the anti-Invar effect [2]. In order to simulate the properties of iron as a function of temperature we are using a tight-binding model fit to first-principles calculations [3], modified with a parametrized exchange interaction to model non-collinear magnetism [4,5]. The model is in good agreement with self-consistent Linearized augmented plane wave (LAPW) computations. An effective Hamiltonian (augmented Heisenberg model) was fit to the tight-binding results, and used with Monte Carlo simulations to simulate the behavior of magnetic iron as a function of temperature. The behavior of magnetic iron is important because many experiments on iron are at pressures below 50 GPA, where magnetism is important. It is also important to understand how magnetism interacts with thermal equations of state since iron in most minerals is also magnetic at pressures throughout most of the mantle.

[1] G. Steinle-Neumann, L. Stixrude, and R.E. Cohen, First-principles elastic constants for the hcp transition metals Fe, Co, and Re at high pressure, Phys. Rev. B 60, 791-799 (1999).

[2] O.N. Mryasov, V.A. Gubanov, and A.I. Liechtenstein, Spiral-spin-density-wave states in fcc iron: Linear-muffin-tin-orbitals band-structure approach, Phys. Rev. B 45, 12330-12336 (1992).

[3] R.E. Cohen, L. Stixrude, and E. Wasserman, Tight-binding computations of elastic anisotropy of Fe, Xe, and Si under compression, Phys. Rev. B 56, 8575-8589 (1997).

[4] S. Mukherjee and R.E. Cohen, Tight binding based non-collinear spin model and magnetic correlations in Iron, Journal of Computer Aided Materials Design 8, 107-115 (2002).

[5]R.E. Cohen and S. Mukherjee, Non-collinear magnetism in iron at high pressures, Phys. Earth Planet. Inter. in press, (2003).