Spin-wave method for the total energy of paramagnetic state

Spin-wave formalism provides a convenient alternative way of modeling the high-temperature paramagnetic state for a certain type of magnets within the framework of Hamiltonian-type electronic-structure methods. For Heisenberg systems, it is formally equivalent to the so-called disordered local moment approach, which is usually used in the methods based on the coherent potential approximation within the Green's function or multiple-scattering techniques. In this paper, we demonstrate that the spin-wave method has certain advantages when it comes to the calculation of forces and relaxations. It also allows one to take magnetic short-range-order effects into consideration. As examples of the application of the spin-wave method, we calculate the energy of the paramagnetic state in fcc Co and bcc Fe, the vacancy formation energy, elastic constants, and phonon spectrum in bcc paramagnetic Fe. We demonstrate that magnetic short-range-order effects play a crucial role in the mechanical stabilization of the bcc Fe at high temperature in the paramagnetic state.