The topic of this Thesis is magnetization dynamics on atomic length scales. A computational scheme, Atomistic Spin Dynamics, based on density functional theory, the adiabatic approximation and the atomic moment approximation is presented. Simulations are performed for chemically disordered systems, antiferromagnets and ferrimagnets and also systems with reduced dimensionality The autocorrelation function of the archetypical spin glass alloy CuMn is sampled in simulations following a quenching protocol. The aging regime can be clearly identified and the dependence of the relaxation on the damping parameter is investigated. The time evolution of pair correlation and autocorrelation functions has been studied in simulations of the dilute magnetic semiconductor GaMnAs. The dynamics reveal a substantial short ranged magnetic order even at temperatures at or above the ordering temperature. The dynamics for different concentrations of As antisites are discussed. Antiferromagnets offer opportunities for ultrafast switching, this is studied in simulations of an artificial antiferromagnet. For the right conditions, the cooperative effect of applied field torque and and the torque from the other sublattice enables very fast switching. The dynamics of bcc Fe precessing in a strong uniaxial anisotropy are investigated. It is demonstrated that the magnetization can shrink substantially due to a spin wave instability. The dynamics of a two-component model ferrimagnet at finite temperature are investigated. At temperatures where the magnetic system is close to the magnetic and angular momentum compensations points of the ferrimagnet, the relaxation in a uniaxial easy exis anisotropy resembles results in recent experiments on ferrimagnetic resonance. The different cases of uniaxial or colossal magnetic anisotropy in nanowires at different temperatures are compared. The magnon softening in a ferromagnetic monolayer is investigated, giving results that compare well with recent experiments. The effect of lattice relaxation can be treated in first principles calculations. Subsequent simulations captures the softening of magnons caused by reduced dimensionality and temperature. / Felaktigt tryckt som Digital Comprehensive Summaries of Uppsala Dissertations from the Faculty of Science and Technology 706

Выполнено систематическое исследование магнитных свойств аморфных плёнок (РЗМ)xCo100-x, содержащих редкоземельные металлы РЗМ различного типа: La не имеет магнитного момента; Gd обладает сферической электронной оболочкой; Tb характеризуется анизотропией электронной структуры. На основе анализа спонтанной намагниченности плёнок определены концентрационные зависимости средних атомных магнитных моментов кобальта (mCo), гадолиния (mGd) и тербия (mTb). Показано, что в интервале 0 < x < 50 % mCo уменьшается от 1,7 µB до нуля, mGd не меняется и практически совпадает с магнитным моментом свободного атома (7 µB), а mTb монотонно снижается, причем скорость уменьшения зависит от способа получения образцов. Найденные закономерности связываются с концентрационным изменением электронной структуры Со и спецификой магнитной структуры плёнок, которая имеет ферромагнитный, ферримагнитный или сперимагнитный характер для образцов, содержащих La, Gd или Tb соответственно. Установлено, что в пленках систем Gd-Co и Tb-Co может присутствовать одноосная магнитная анизотропия, сильно различающаяся по своей величине. Механизм её формирования может быть связан с упорядочением пар атомов в GdxCo100-x и магнитострикционной деформацией или проявлением локальной анизотропии атомами тербия в TbxCo100-x. / A systematic study of magnetic properties of (RE)xCo100-x amorphous films, containing rare earth metals of various types, was conducted: La has no magnetic moment; Gd has a spherical electron shell; Tb is characterized by anisotropy of the electron structure. Concentration dependences of average atomic magnetic moments of cobalt (mCo), gadolinium (mGd), and terbium (mTb) were determined based on the films spontaneous magnetization analysis. It was shown that mCo decreases in the range of 0 < x < 50 % from 1.7 μB to zero, mGd does not change and it is almost identical to the magnetic moment of a free atom (7 μB), and mTb decreases monotonically, and the reduction rate depends on the method of samples preparation. The observed behavior is associated with the change in the electron shell of Co and specificity of the films magnetic structure, which is ferromagnetic, ferrimagnetic or sperimagnetic for the samples, containing La, Gd, and Tb, respectively. It was found that Gd-Co and Tb-Co systems films can demonstrate uniaxial anisotropy of considerably different magnitude. The mechanism of its formation may be associated with the atoms pairs ordering in GdxCo100-x and magnetostrictive deformation or manifestation of the local anisotropy of terbium atoms in TbxCo100-x.