Dynamic x-ray speckle patterns of the magnetic superstructure peak of an ultrathin Ho film
If coherent light is scattered from a disordered sample it will produce a random diffraction pattern (speckle pattern)
which is related to the exact spatial arrangement of the sample.
When the spatial arrangement of the sample changes, also the speckle pattern will change.
In this way it is possible to extract the time scales of the fluctuating disorder (intensity fluctuation spectroscopy).
In this study we use this technique to extract the temperature-dependent time scales
of critical magnetic fluctuations with resonant coherent soft x-rays.
The scattering experiments have been performed at the Berlin synchrotron
BESSY, in cooperation with the Universität zu Köln
and the Freie Universität Berlin.
Poster: Magnetic fluctuations in a thin holmium film measured with soft x-ray photon correlation spectroscopy April 2007
Click here to download the pdf document (471 kB)
Text and images by Stan Konings:
Magnetic speckle pattern of a 3-nm thin Holmium film
Click
here
to see the fluctuating magnetic speckle at three different temperatures [10 MB].
Sketch of the experimental setup
Sketch of the scattering experiment (not to scale) and the helical antiferromagnetic structure. The soft x-ray scattering
pattern is recorded after reflection from the holmium film with a charged coupled device (CCD detector). To select the spatially
coherent part of the undulator radiation, two pinholes were placed in the beam before it is reflected from the sample.
The helical magnetic structure is described below.
Helical magnetic structure
Holmium has a helical magnetic structure which is ferromagnetically aligned in the plane of the atomic layer
but the spin direction is rotated by a specific turning angle between atomic layers.
Magnetic sattelites arise left and right from the magnetic Bragg peaks along the sample normal with a distance in reciprocal space that
is determined by the length of the helix period τ.
Scattering geometry
Scattering geometry of the X-ray photon correlation experiment.
The CCD intensity contains a mix of both correlations in the plane of the sample (magnetic domain structure)
as well as correlations out of the film plane (length of the magnetic helix period).
At the experiment the sample was placed in the vertical plane and the picture should be rotated 90 degrees.
Ewald sphere
The magnetic satellite peak has an elliptical shape which comes from the way the peak is cut in q space.
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