We have recorded magnetic speckle patterns of unprecedented quality on a tiny sample using soft-x-ray resonant magnetic scattering in reflection geometry. This geometry is well suited to the study of epitaxial thin films. The microscopic object consisted of an FePd wire with transversal stripe domains of alternatingly up and down magnetization. Sharp magnetic superstructure peaks reflect the domain periodicity, whereas the magnetic speckles give access to the domain morphology. A high degree of coherence has been obtained (ϭ0.85) as evidenced from the strong observed intensity fluctuations.
We report on the dynamics of phason modes in the i-Al-Pd-Mn icosahedral quasicrystal, measured between room temperature and 650°C, using the Xray Intensity Fluctuation Spectroscopy technique (XIFS). Up to 500°C, the auto-correlation function ℑ(q, t) displays almost no time evolution as expected for frozen-in phason fluctuations at low temperature. At higher temperatures, ℑ(q, t) follows a single exponential time decay from which the characteristic time τ c (q) is extracted. These results are compared with the expected shape of ℑ(q, t) as derived from the expressions of the eigenvalues and eigenvectors of the C ⊥⊥ (q) phason dynamical matrix. In agreement with the hydrodynamic theory of quasicrystals which predicts phasons with diffusive character, we find that τ c (q) varies linearly with q-2 at 650°C. The corresponding diffusion coefficient is 2.2(± 0.5)×10-18 m².s-1 and the activation energy is estimated around 2.3(± 1) eV.
International audienceWe have used coherent soft x-ray resonant magnetic scattering to locally track reversal processes in magnetic nanostructures. Coherent illumination of a limited number of nano-objects in a CoPt nanoline grating produces a specific speckle pattern, whose evolution under in situ magnetic field reveals the true local magnetic ordering. While each nanoline behaves as a single macrospin whose direction depends on the dipolar coupling with neighbors, the global reversal of the line array is successively governed by two effects: first, by the random distribution of defects, followed by the dipolar coupling favoring antiferromagnetic ordering
Frozen-in phason fluctuations in single grains of icosahedral Al-Pd-Mn quasicrystals have been studied by high-resolution coherent X-ray scattering. Bragg peak widths scale as expected for a distribution of uniform phason strains. Strong relaxation of the uniform phason strain is observed after sample annealing. Large intensity fluctuations or speckle patterns are observed in the Bragg peaks due to the uniform phason strain distribution in the sample. Speckle patterns are also observed in the diffuse scattering located close to the Bragg reflections and are related to long-wavelength phason fluctuations taking place in the sample.
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