Berlin 2012 – wissenschaftliches Programm
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KR: Fachgruppe Kristallographie
KR 5: Poster I – including Multiferroics (MA with DF, DS, KR, TT)
KR 5.81: Poster
Dienstag, 27. März 2012, 12:15–15:15, Poster A
Temperature dependence of the magnon dispersion relation in low-dimensional transition-metal systems: A first-principles investigation — •Waldemar Töws and Gustavo M. Pastor — Universität Kassel, Heinrich-Plett-Str. 40, 34132 Kassel
The influence of Stoner excitations on the spin-wave spectrum of one- and two-dimensional 3d transition metals has been investigated. The physical situations represented by the Stoner excitations correspond to extreme nonequilibrium states, which can be induced by strong ultrashort laser pulses. In this work we quantify to what extent an important increase of the electronic temperature Te describing Stoner excitations affects the stability of magnetism within the metal. For this purpose, we perform numerical calculations in the framework of ab initio density-functional theory with a generalized gradient approximation to the exchange and correlation energy. The free energy of frozen-magnon states as a function of spin-wave vector q and Te have been systematically studied for V, Fe, Co and Ni wires and monolayers with various nearest-neighbor distances. First of all, we demonstrate that the local magnetic moments are extremely stable even at temperatures Te much larger than the Curie temperature TC. The Te-dependence of the magnetic couplings between the local moments is quantified by fitting the effective exchange couplings Jij to the free-energy dispersion relation in the framework of a classical spin model. One actually finds that electronic temperatures Te well above TC are needed to change the nature of the magnetic order within the metal. The consequences for the theory of laser-induced magnetization dynamics are discussed.