A spin-lattice dynamics study of effects of magnon excitations on physical properties of BCC iron

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A spin-lattice dynamics study of effects of magnon excitations on physical properties of BCC iron

 

Author: Wen, Haohua
Title: A spin-lattice dynamics study of effects of magnon excitations on physical properties of BCC iron
Degree: Ph.D.
Year: 2013
Subject: Spin-lattice relaxation.
Molecular dynamics.
Iron alloys.
Hong Kong Polytechnic University -- Dissertations
Department: Dept. of Electronic and Information Engineering
Pages: xxv, 233 leaves : ill. ; 30 cm.
Language: English
InnoPac Record: http://library.polyu.edu.hk/record=b2616051
URI: http://theses.lib.polyu.edu.hk/handle/200/7022
Abstract: Spin-lattice dynamics (SLD) study of the effects of spin vibrations on physical properties in body-centred-cubic (BCC) iron has been performed in the thesis. Since the exchange integral governing spin-spin correlation depends on the atomic distance, the degrees of freedom of spin and lattice in BCC iron are coupled, so that the participation of spins leads to the change of anharmonicity of the crystal potential of BCC iron for both lattice dynamics and spin dynamics. Consequently, through the spin-lattice coupling, the enhanced phonon- and magnon scattering give rise to the more softening and shorter lifetimes for both elementary thermal excitations, which are revealed from the dispersion curves for phonons and magnons, respectively. For instance, the spin-lattice coupling results in the shift of Curie temperature from ~1100K to ~1000K. In addition, the spin stiffness confirms such effects in spin dynamics. By using the thermodynamic integration method, we calculated the free energy and other thermodynamic quantities, e.g. entropy and heat capacity are calculated. The results show that the spin vibrations give rise to not only the energetic contribution, but also entropic contributions, which leads to the anomalous temperature dependence of the thermal, magnetic and mechanical properties in BCC iron, especially near FM/PM phase boundary. Examples are the thermal expansion coefficient, Gruneisen parameter, specific heat, as well as the isothermal elastic constants. Contributions from spin vibrations are particularly large compared with the effects of multi-phonon interactions. The effects of spin vibrations are studied in the self- and mono-vacancy diffusion in BCC iron. Based on the scheme of SLD and modified thermodynamics integration (TI) method, the free energies of vacancy migration and formation are calculated over a wide range of temperature, across FM/PM phase boundary in BCC iron, from which the attempt frequency of vacancy migration is calculated for the first time atomistically. The non-Arrhenius behavior of vacancy activation is also first time found from the atomistic simulation, from which the migration enthalpy increases ~0.15eV attributed to the long-range magnetic order. The results are in good quantitative agreement with other calculations and experimental data. Furthermore, the effects of spin vibration on vacancy formation and migration entropies and enthalpies are investigated by using the modified conjugated gradient (MCG) and TI methods. It is found that the change of spin configuration is the principal origin to the energetic change in vacancy migration and formation. Otherwise, the dynamical relaxation of spin vibrations near Curie temperature gives arising to the extra heat dissipation during the vacancy formation and migration processes, which result in the cusps of temperature dependence of entropies of vacancy migration and formation. Our calculation results are in principle consistent with the theoretic predictions.

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