Time resolved scanning transmission X-ray microscopy on asymmetric rings
Andrea Krone (Institut für Physik)

Scanning transmission X-ray microscopy is an imaging technique of magnetization. Polarized light of a synchrotron is used for the X-ray magnetic circular dichroism (XMCD) providing the magnetic contrast of the microscope. Combining STXM and a pump-and-probe mechanism results in time resolved scanning transmission X-ray microscopy. The discussed experimental setup was performed at the MAXYMUS Beamline, BESSY II at HZB (Helmoltz Zentrum Berlin) in Berlin. Provided spatial resolution there is 30nm to 40nm and the time resolution about 30ps.

The investigated ferromagnetic rings are of dimensions in nm to µm range. Asymmetry provides an inhomogeneous potential landscape for the domain walls (DWs) resulting in automotion of the DWs as well as influence of the motion of the walls driven by a rotating magnetic field. A precise measurement of the magnetization dynamics is necessary to understand the processes during movement of the walls.

Nucleation and rotation of DWs is achieved through current pulses sent through crossed striplines right above the sample. The circuit for excitation is combined with the pump-and-probe mechanism due to synchronization of pump signal and probe signal. Pump corresponds to the excitation signal and probe refers to light flashes from synchrotron electron buckets taking pictures every 2ns. A high signal to noise-ratio as well as an averaging process is gained through a whole number of excitation events per pixel. Hence, a resulting movie not only gives information on position of the walls, structure of the walls and speed of the walls but also on reproducibility of the visible effects.