Ab initioelectron theory for magnetism in Fe: Pressure dependence of spin-wave energies, exchange parameters, and Curie temperature

The spin-wave spectra for bcc Fe at various lattice constants are calculated by the ab initio density functional electron theory for various lattice constants, and the exchange parameters of a Heisenberg model are determined from the data. The spin-wave energies increase with decreasing lattice constant, and the behavior of the exchange parameters can by no means be described by the Bethe-Slater curve for an effective nearest-neighbor Heisenberg model. From the spin-wave frequencies the pressure dependence of the Curie temperature is derived in mean-field and random-phase approximation, yielding ∂TcMF/∂p≈1.6K/kbar and ∂TcRPA/∂p≈1.8K/kbar, in contrast to the experimental result ∂Tc/∂p≈0. Possible reasons for this discrepancy are discussed.