Abstract:
The thesis deals with Fourier transform holography at third and fourth generation x-ray sources. This lensless imaging technique exploits the coherence of the light sources to couple a reference wave into a transmission scattering experiment. As a result, the complete wavefront information of the object wave is recorded in the detection process. In this way Fourier transform holography solves the phase problem of lensless imaging methods and provides an image of the object. Fourier transform holography in combination with x-rays features a high spatial resolution and special contrast mechanisms. Especially x-ray magnetic circular dichroism makes the technique suited for the investigation of magnetic samples with perpendicular anisotropy. Today, the element-specific imaging of magnetic domain patterns is routinely performed at synchrotrons with a spatial resolution below 50nm in a variable magnetic field and temperature environment. By exploiting all of these parameters it becomes possible to investigate the duplication of domains in an exchange-decoupled system. The magnetic configuration of a magnetically hard Co/Pd multilayer is successfully replicated through a 10-nm-thick Pd layer into a magnetically soft CoNi/Pd multilayer. This duplication can be triggered either by the variation of an external magnetic field or the environmental temperature. Fourier transform holography reduces the field of view on a specimen to typically less than 1 μm^2. In the case of magnetic Co/Pd and Co/Pt multilayers this area corresponds to the width of few domains. Due to this confinement it is possible to find sudden irreproducible magnetization jumps in local hysteresis loops measured in transmission at the Co-L_3 edge. It is shown that such steps in the local hysteresis loop are observed when using increased sputter pressure during samples deposition or by introducing an antiferromagnetically coupled interlayer. In addition, holography images on the identical sample region allow detailed investigation of the magnetic configuration. For the antiferromagnetically coupled sample system an antiferromagnetic stripe domain state could be directly imaged. Free-electron lasers provide femtosecond x-ray pulses with unprecedented peak brightness and promise a high spatial and temporal resolution. At the same time, the source’s statistic character and energy deposition above the ablation level of many materials require the development of new experimental techniques. Fourier transform holography supports multiple references which, in conjunction with a split-and-delay line, make this technique capable to superimpose two holograms on the same detector and deliver two images of a specimen within a femtosecond time delay. The concept maps the temporal information of the pulse arrival to a spatial information in the hologram reconstruction. The resulting twoimage sequence constitutes a first step towards ultrafast x-ray movies at the nanoscale.