A STUDY OF THE X-RAY FOCUSING EFFECT OF A MAGNETOACOUSTICALLY EXCITEDFEBO3 CRYSTAL USING SYNCHROTRON-RADIATION DIFFRACTION IMAGING

Ultrasonic standing waves, excited in an FeBO3(111) crystal through ma gnetoelastic coupling, are visualized using synchrotron radiation diff raction imaging ('topography'). This observation relies on the possibi lity at the ESRF of recording images with long sample-to-film propagat ion distances (up to 1.5 m) without substantial loss of resolution. Th e crystal acts as an x-ray focusing lens periodically in time, an effe ct that strongly depends both on the amplitude of the magnetic field e xcitation and on the propagation distance. The standing-wave-related c ontrast is hardly visible close to the sample, very sharp for a focusi ng distance (typically similar or equal to 0.4 m) whereas beyond this position the intensity maxima broaden due to the defocusing effect. We propose a model which accounts for the presented results, allows the understanding of the details of the resonance patterns and gives acces s to the main characteristics of the vibration: amplitude, shape, pola rization, wavelength and sound velocity. This effect could be used to simultaneously monochromatize and focus the x-ray beam.