Spin Dynamics and Magnetic Multilayers

Theoretical studies based on first-principles theory are presented for a number of different magnetic systems. The first part of the thesis concerns spin dynamics and the second part concerns properties of magnetic multilayers. The theoretical treatment is based on electronic structure calculations performed by means of density functional theory.A method is developed for simulating atomistic spin dynamics at finite temperatures, which is based on solving the equations of motion for the atomic spins by means of Langevin dynamics. The method relies on a mapping of the inter atomic exchange interactions from density functional theory to a Heisenberg Hamiltonian. Simulations are performed for various magnetic systems and processes beyond the reach of conventional micro magnetism. As an example, magnetization dynamics in the limit of large magnetic and anisotropy fields is explored. Moreover, the method is applied to studying the dynamics of systems with complex atomic order such as the diluted magnetic semiconductor MnGaAs and the spin glass alloy CuMn. The method is also applied to a Fe thin film and a Fe/Cr/Fe trilayer system, where the limits of ultrafast switching are explored. Current induced magnetization dynamics is investigated by calculating the current induced spin-transfer torque by means of density functional theory combined with the relaxation time approximation and semi-classical Boltzmann theory. The current induced torque is calculated for the helical spin-density waves in Er and fcc Fe, where the current is found to promote a rigid rotation of the magnetic order.
Properties of magnetic multilayers composed of magnetic and nonmagnetic layers are investigated by means of the Korringa-Kohn-Rostocker interface Green’s function method. Multilayer properties such as magnetic moments, interlayer exchange coupling and ordering temperatures are calculated and compared with experiments, with focus on understanding the influence of interface quality. Moreover, the influence on the interlayer exchange coupling of alloying the nonmagnetic spacer layers with small amounts of a magnetic impurity is investigated.


1 Introduction
2 Electronic structure theory
2.1 Schrödingerequation
2.2 Density functional theory
2.3 The KKR-ASA Green’s function method
2.4 The coherent potential approximation
2.5 Layered systems
2.6 Heisenberg exchange parameters
3 Spin dynamics
3.1 Introduction
3.2 Length scales and excitations
3.3 Equations of motion
3.3.1 Fast variables
3.3.2 Slow variables
3.3.3 Parametrization
3.4 Relaxation of magnetic systems
3.4.1 Thermodynamic reservoirs
3.4.2 Angular momentum transfer
3.4.3 Dissipation of angular momentum
3.5 Finite temperature modeling
3.5.1 One thermal reservoir
3.5.2 Several thermal reservoirs
3.6 Extracting information
3.6.1 Trajectories
3.6.2 Average magnetic moment
3.6.3 Correlations between magnetic moments
3.6.4 Energy distributions
3.7 Areas of application
4 Current driven magnetization dynamics
4.1 Spin-transfer torque
4.2 Spin-transfer torque in helical spin density waves
5 Magnetism in layered materials
5.1 Technological applications
5.2 Interlayer exchange coupling
5.2.1 Quantumwell model
5.2.2 Atomistic model
5.3 Alloying in magnetic multilayers
5.4 Interface structure
5.5 Coupling between an AFM and a FM
6 Magnetic switching
6.1 Switching in a ferro magnet
6.2 Ultrafast switching by a relativistic electron beam
6.3 Ultrafastswitchinginantiferromagnets
7 Outlook
Summary in Swedish

Author: Skubic, Björn

Source: Uppsala University Library

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