Soft X-ray holographic microscopy

04-03-2010

Magnetic imaging with nanoscale domain resolution is now possible thanks to a new way of carrying out soft X-ray holography. The key feature of this microscopy-like imaging technique is a movable field-of-view that can be freely positioned on the sample. The microscope allows imaging of extended samples with some tens of nanometres resolution and provides chemical as well as magnetic contrast.

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Scientists working in the field of nanoscale magnetism have developed a new microscopy-like imaging technique that utilises coherent and circularly polarised soft X-rays. The holographic microscope offers new insights into the magnetic properties of nanostructured materials.

X-ray holography [1,2] is a most interesting alternative to the well-established zone-plate based methods of transmission soft X-ray microscopy. Due to its entirely lensless operation it promises to be capable of higher lateral resolution for lower technological effort. In contrast to plain coherent diffraction imaging, the phases of the waves are recorded directly, so the problem of numeric phase retrieval is completely absent.

In conventional “Fourier transform holography” imaging, the scattered wave from a confined semi-transparent object is superimposed with a coherent spherical wave originating close to the object and recorded with a CCD camera. To reconstruct the object, a simple numeric Fourier transform of the hologram is performed. The established setup [2,3] aims at maximum rigidity by preparing the sample, the field of view defining aperture, and a nanometre-size aperture from which the reference wave originates, all on a single silicon nitride membrane. However, in such a setup, the maximum field of view is limited to a few micrometres in diameter and locked to one position on the sample, this puts a severe constraint on possible applications.

The new microscope, developed by a team from Hamburg University and DESY, in close collaboration with staff at beamline ID08, effectively eliminates this constraint. The trick is to use two membranes instead of one (Figure 1). The first membrane has been metalised to make it opaque and structured to create both apertures. The second membrane carries only the sample; it can be moved laterally with nanometre precision and stability.

 

Setup for holographic microscopy

Figure 1. Beamline sample enviroment setup for holographic microscopy.

 Figure 2 gives an example of the extended capabilities of the instrument. A Co/Pt multilayer film magnetised out-of-plane was covered with a wedge-shaped iron cap-layer, of thickness ranging from zero to 4 nm shown from left to right in the image. The image was assembled from individually reconstructed holograms acquired at consecutive positions along the iron wedge. Magnetic contrast is obtained from difference holograms at opposite helicities using the magnetic circular dichroism at the cobalt L-absorption edge. As the additional magnetic moments of the iron layer change the energetics of the coupled system, a change of the domain size is clearly observed in the image. An example for non-magnetic imaging can be found in [4].

Domain size evolution of an out-of-plane magnetised Co/Pt-multilayer film with an iron cap-layer of continuously increasing thickness

Figure 2. Domain size evolution of an out-of-plane magnetised Co/Pt-multilayer film with an iron cap-layer of continuously increasing thickness from left to right.

 Only now can magnetic structures on such an extended length scale be investigated by holography. This system's use of separate membranes offers an additional benefit in that the membrane carrying the apertures can be used multiple times. It can be well characterised and optimised. Sample preparation is possible without any restriction due to technological processes. All these features are constituents of a real microscope – they can now be used in a holographic experiment. This combination is especially promising for time-resolved measurements at free-electron laser sources.

 

References
[1] I. McNulty et al., Science 256, 1009 (1992).
[2] S. Eisebitt et al., Nature 432, 885 (2004).
[3] S. Streit-Nierobisch et al., J. Appl. Phys. 106, 083909 (2009).
[4] C. Tieg et al., J. Phys.: Conf. Ser. 211, 012024 (2010).

 

Principal publication and authors
D. Stickler (a), R. Frömter (a), H. Stillrich (a), C. Menk (a), C. Tieg (b), S. Streit-Nierobisch (c), M. Sprung (c), C. Gutt (c), L.-M. Stadler (c), O. Leupold (c), G. Grübel (c), H.P. Oepen (a), Soft X-ray holographic microscopy, Appl. Phys. Lett. 96, 042501 (2010).
(a) Institut für Angewandte Physik, Universität Hamburg (Germany)
(b) ESRF. Now at Helmholtz-Zentrum Berlin (Germany)
(c) HASYLAB at DESY, Hamburg (Germany)