Fourier Transform Holography for Magnetic Imaging
Duckworth, Thomas A.
Thesis or dissertation
University of Exeter
State-of-the art Fourier transform holography (FTH) techniques use x-ray magnetic circular dichroism (XMCD) as a contrast mechanism for element-specfi c imaging of magnetic domains. With the soft x-ray Nanoscience beamline at Diamond Light Source in the UK, and the Dragon beamline at the European Synchrotron Radiation Facility (ESRF) in France, the possibility of new methods to study nanostructured magnetic systems has been demonstrated. The ability to record images without the use of lenses, in varying magnetic fi elds and with high spatial resolution down to 30 nm has been used to study in-plane magnetism of 50 nm thin permalloy (NiFe alloy) nanoelements. The holographic technique used extended reference objects rather than conventional pinhole references, which allowed a high flexibility on the direction of magnetisation that is probed. The element specific nature of the imaging, with the additional choice in the directions of magnetisation that are probed has been used to study dipolar interactions in a hard/Ta/soft [Co/Pt]30/Ta/Py multi-layered system. Images of the out-of-plane magnetised domains of [Co/Pt]30 were found to bare strong spatial resemblance to the in-plane domains of the permalloy. The domain structure is thought to be magnetostatically imprinted into permalloy during the growth stage of the lm, where stray elds generated by the adjacent Co/Pt multilayer influence the formation of domains in the permalloy. Strong resemblance between the two layers could be found at remanence within a pristine sample, however the similarities disappear after the sample was exposed to a saturating magnetic field. This disagreed with micromagnetic simulations performed in The Object Oriented MicroMagnetic Framework (OOMMF) program, and an explanation for the observations has been sought in the growth process of the multi-layered fi lm, with conditions that are diffi cult to recreate in the model. Optical holography has been used for preliminary insight into implementing a method of FTH in a reflective geometry at soft x-rays wavelength. With scattering chambers at BESSY II in Germany and at the Stanford Synchrotron Radiation Lightsource (SSRL) in California the possibility of reducing scattered noise in a hologram recorded in a reflective geometry has been investigated. Studies into specular and dif use reflections have been performed optically however the use of extended references alone may alleviate the current problem at x-ray wavelengths which lie in the weak signal given by a reflective point-like reference source.
The Centre for Material Physics and Chemistry (CMPC) of the Science and Technology Facilities Council (STFC)
The Engineering and Physical Sciences Research Council (EPSRC)
Optics Express, Vol. 19, Issue 17, pp. 16223-16228 (2011) http://dx.doi.org/10.1364/OE.19.016223
New Journal of Physics, 15, Issue 2, 023045 (2013)
Holographic imaging of magnetic nanostructures using extended references, Diamond Light Source Annual Review 2011/12
PhD in Physics