Transmission x-ray miscroscopy
- imaging of the magnetization dynamics -
Fresnel zone plates serve as focusing and imaging elements in a transmission x-ray microscope. They consist of concentric rings, which serve as circular diffraction grids, and constructively focus the soft x-rays to a focal-point. State of the art zone plates are manufactured with modern lithographic procedures and can achieve very high lateral resolutions down to 20 nanometers.
There are two different types of transmission x-ray microscopes. The imaging transmission x-ray microscope (ITXM) illuminates the whole sample and the magnified picture is recorded in transmission by means of a CCD-Camera. In the case of scanning transmission x-ray microscope (STXM) the sample is scanned with a small spot of focussed x-rays and the transmitted intensity is measured by a small photodetector.
Soft x-rays are produced by a synchrontron radiation source. Our experiments are being performed by the „Advanced Light Source“ in Berkeley, USA http://www-als.lbl.gov In an electron storage ring, the so-called synchrotron, bunches of electrons are accelerated to very high velocities close to the speed of light. At certain places along the storage ring, the electrons are deflected by strong magnets and emit very intensive, bundled and polarized x-rays. The polarization-property of synchrontron radiation is the essential requirement for the magnetic x-ray microscopy. As a magnetic contrast mechanism, the x-ray magnetic circular dichroism (XMCD) is used. Due to this effect, the absorption of circularly polarized x-rays depends on the orientation of the magnetization relative to the propagation direction of the x-rays. Magnetic regions oriented in the direction of propagating x-rays will appear bright; the regions oriented against the propagation direction of x-rays will appear dark. In this way the XMCD effect leads to a light-dark contrast of the transmitted intensity. The x-ray circular dichroism appears, when the energy of x-rays corresponds to a certain absorption edge of a magnetic atom. Local magnetization distribution and the spin dynamics in magnetic multilayers, can be resolved layer-by-layer and element-selectively by adjusting the energy of the x-rays to the appropriate element-specific absorption edge. The inherently pulsed temporal structure of synchrotron radiation is used for the visualization of dynamic processes. Temporal resolution is given by short x-ray flashes with duration of less than 100 picoseconds. A common method for the imaging of magnetization dynamics is a pump-probe“ experiment. The magnetic equilibrium state is excited by periodic magnetic pump-pulses and afterwards stroboscopically probed with x-ray flashes.