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Analysis of coherent resonant x-ray scattering and reconstruction of magnetic domains Rahmim, Arman

Abstract

We have explored the use of coherent resonant x-ray scattering as a powerful technique to study, characterize and reconstruct magnetic domains for antiferromagnetic (AFM) and ferromagnetic (FM) thin films. This method is capable of high-resolution imaging (as it is not limited by optical aberrations), is able to probe buried interfaces and is operational in the presence of other fields. Here we report the first experimental observation of x-ray speckle patterns generated by AFM domains. Resonant x-ray scattering was performed on LaFe0₃ thin films possessing two types of domains with their AFM orientations perpendicular to each other. X-ray magnetic linear dichroism (XMLD) at the FeL₃ absorption edge has been exploited in order to give rise to modulations of the scattering amplitudes according to domain distributions, resulting in magnetic speckle. We also report resonant x-ray scatterng in the transmission geometry from FM domains of Co/Pt multilayers. Magnetic x-ray circular dichroism (MXCD) has been utilized with the contrast arising from the dependence of scattering amplitude on magnetization direction of FM domains, which are oriented normal to the surface (i.e. parallel of antiparallel to photon helicity) due to the perpendicular interfacial anisotropy provided by the broken symmetry at the Co-Pt interface. By tuning the energy to the CoL₃ edge, magnetic speckle is very clearly demonstrated. We have analytically shown that upon reversal of magnetic contrast (tuning of the scattering energy to either of the two crystal field split peaks of the FeL₃ edge in the first experiment, and changing the photon helicity in the second experiment) changes in speckle patterns will be observed solely arising from the interference between roughness and/or pinhole scattering with magnetic scattering. We have developed a new reconstruction technique, upon extension of Fourier transform iterative algorithms previously utilized in other reconstruction tasks, capable of reconstructing AFM and FM magnetic structure from resonant x-ray scattering intensity measurements. This technique is shown to be very successful upon application to noisy simulated data. Using this method, experimental speckle data from the F M domains of the Co/Pt multilayer have been inverted resulting in magnetic domains showing a remarkable similarity to the worm-domain structure of the actual domain distribution imaged using magnetic force microscopy (MFM). This, to our knowledge, has been the first reconstruction of magnetic domains from experimental data. Moreover, direct (non-iterative) reconstruction of FM domains has been shown to be possible upon using small pinholes and/or rough samples with roughness scale comparable to the size of domains.

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