Ultrafast magnetization dynamics in ferromagnetic materials
نویسنده
چکیده
In recent magneto optical experiments (TR-MOKE) the timescale of laser induced demagnetization was found to be shorter than 0.5 ps. However, doubts have been raised if the measured signal reflects genuine loss of magnetic moment at all. Also, for these experiments no broadly accepted theory exists. Although a few models are able to fit the data, they generally have in common that they violate or disregard conservation of angular momentum. In this thesis both issues are addressed. First, a macroscopie model is presented to calculate the genuine magnetization dynamics from measurements of the magneto-optical Kerr effect at two different ambient temperatures. Secondly, a phonon based microscopie model is introduced to describe the laser induced demagnetization in terms of its microscopie origin, taking into account conservation of angular momentum. This model consists of a Fermi sea of spinless electrons, and an ensemble of two level oscillators describing the phonon system. The magnetic degree of freedom of the system is described by an ensemble of two level oscillators (magnons), and treated in a mean field approximation (Weiss model). The interactions included in the model are electron-electron scattering, electron-phonon scattering, and electron-phonon scattering with spin-flip, and these are described by Boltzmann equations. The ratio of the spin-flip and non spin-flip events is determined by the spin-flip parameter a 8 J. Although in literature phonon based mechanisms are generally considered to be too slow to account for the rapid demagnetization, and would take necessarily longer than electron-phonon equilibration, it was shown that the demagnetization achieved with this mechanism can be faster than e-p equilibration. Furthermore, it was shown that the demagnetization can even be faster than the thermalization time of the electron system.
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تاریخ انتشار 2016