Pulsed x rays based techniques are used to accede to time resolved (100 fs) and space resolved (100 nm) magnetization dynamics. Synchronized synchrotron polarized x rays (at BESSY – SOLEIL) and pulsed x rays from free electron linear accelerators, X-FEL’s are used to resolve the dynamics of the systems ( L, S and magnetic domains in the nm range). The x-ray magnetic circular dichroïsm (XMCD) effect is used to be sensitive to the spin and to the orbital magnetic moment addressing specific core levels transitions.
Ultrafast dynamics of spin and orbital moments
Ultra-short optical laser pulses were used to induce changes of the magnetization in ferromagnetic CoPd films. The capability of x rays to differentiate between the electronic structure, spins (S) and the orbital (L) moments has led to new insight into the ultrafast demagnetization processes. At HZB-BESSY –UE56/1 we could evidence the differences in the ultrafast changes of the magnetic moments S and L and show that ultrafast manipulation of the spin-orbit coupling is possible using femtosecond laser pulses. [Nature 465, 468 (2010)].
Space resolved fs dynamics
Photon flux at present fs-slicing X-ray sources is just large enough to observe a change in spectroscopic contrast preventing more detailed in the femto second time scale, especially spatially resolved insight into the demagnetization process itself. Due to the significantly higher femtosecond X-ray intensity of X-FEL’s (LCLS Stanford – FLASH- Hamburg), this limitation can now be overcome. Actual goal is snapshot imaging of the temporal evolution of the magnetic domain structure upon the non-thermal excitation from a femtosecond optical laser pulse. The perfectly suited imaging technique for this purpose is Fourier Transform Holography. This technique allows snapshot imaging with sub 100 nm spatial resolution using a single x-ray pulse of X-FEL’s. Demagnetization process of single magnetic domains will describe new aspects of ultra-fast processes. This work is performed in collaboration with SOLEIL, UPMC-Paris, CSNSM-Paris, SLAC, LCLS-Stanford and T.U. Berlin –BESSY.