Study Of Femtosecond Magnetism In Ferromagnetic Metals

Ultrafast demagnetization induced by femtosecond laser pulse in ferromagnetic metals is a new frontier of condensed matter physics.However,there is some debate about the underlying physical mechanism.Although the classic LLG equation describes magnetization dynamics well at picosecond time scale,but as time scale decreases to femtosecond,the magnetic moment change must relate to new physical process and theory.Therefore,based on first-principles,we systematic studied femtosec-ond magnetism in ferromagnetic metals,using a program which can calculate the spin dynamic properties at femtosecond time scale.Besides,this method can also analysize the magnetization change in momentum space,predict experimental results and provide theoretical guidance.The studies are not only include the femtosenond magnetism of fcc Ni,fcc Co and bcc Fe,but also involve the pressure effect to magnetic moment change behaviour at femtosecond time scale.The physical mechanism of ultrafast demagnetization and theoretical origin of pressure effect are also discussed.The major finding and innovations are shown as follow:1.We develop a credible method to calculate femtosecond magnetism by combining first-principles calculation and spin dynamics.2.We propose a new concept of spin surface to describe the distribution of magnetic moment change in momentum space.Spin surface,together with Fermi surface,constitutes a full description of electron.Through the concept of spin surface,we can easily demonstrate the distribution of magnetic moment change in the first Brillouin zone,and find the hot spin spots rapidly.3.Two,two and three hot spin spots are detected in fcc Ni,fcc Co and bcc Fe,respectively.The hot spin spots'positions of fcc Co are consist with experimental results.4.We propose a simple method called SD to calculate the magnetic moment change induced by femtosecond laser pulse.A corresponding new concept of optical spin generator is also proposed.The results of optical spin generator are similar as the results of spin dynamics,so we believe that the physics origin of ultrafast demag-netization is the competition between photon and spin via spin-orbital coupling.5.Pressure can affect the strength of system's magnetic moment change.This is due to that the pressure make the valence band of fcc Ni shift down,so the resonance absorption of photon turns into non-resonance absorption.