Double-crystal diffraction imaging with a small effective divergence source: application to the magneto-acoustic vibrations in FeBO3

Ultrasonic standing waves, excited in FeBO3 (111) crystal plates through ma gneto-elastic coupling, were visualized using monochromatic Bragg diffracti on imaging (topography) with synchrotron radiation. The images depend stron gly on whether diffraction by the sample occurs in the same plane as in the double-crystal monochromator, or in the perpendicular plane. The observati ons are explained by taking into account (a) the strong spatial dispersion which prevails because of the small effective divergence (angular size of t he source as seen from a point in the specimen), which is less than one mic roradian in this experiment, and (b) the sample vibration and curvature.