Marjana Lezaic, Phivos Mavropoulos, Gustav Bihlmayer, Stefan Blügel
We present a method for calculating interatomic exchange interactions within density-functional theory (DFT) in the full-potential linearized augmented plane wave (FLAPW) scheme. Our approach is based on total-energy calculations of spin-spiral states in reciprocal space employing the generalized Bloch theorem and a Fourier transform in order to obtain the real-space interactions, including the case of more than one atom in the unit cell. We discuss applicable symmetry relations that allow for a reduction of the computational time. We examine under which conditions the force theorem holds in close-packed and open systems, in particular regarding the treatment of the magnetization in the interstitial region. By including the exchange interactions into a classical Heisenberg Hamiltonian, we study the thermodynamics at finite temperatures within a Monte Carlo method. We specifically discuss necessary corrections in the case that the magnetic moments of one of the atomic species are soft or induced by strong moments of the neighboring atoms. We argue that a quadratic dependence of the energy on the soft local moment is a good approximation in many such cases, and for this situation we derive exact results which allow an elimination of the soft moments from the thermodynamic calculations by considering a renormalization of the strong-moment interactions. We prove that these renormalized interactions are temperature-independent.
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http://arxiv.org/abs/1210.2375
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