Etude de la polarisation circulaire du rayonnement synchrotron dans la gamme des rayons X par diffraction sur un composé ferrimagnétique

The reported experiment is based upon two phenomena: (1) synchrotron radiation, when observed at some tenths of milliradians from the orbital plane, is elliptically polarized; (2) the Bragg intensity diffracted by a ferro- or ferrimagnetic sample depends on the orientation of the magnetization relative to the incident and diffracted beams, by a term which is proportional to the circular polarization rate. A vertical-axis X-ray diffractometer was placed at the beam port D11 at LURE-DCI, the wavelength being 1.804 Å. It can be moved vertically, so as to receive radiation with variable circular polarization. The sample, powdered Zn_0.5Fe_2.5O₄, was magnetized in a horizontal direction by an electromagnet whose field is reversed every 20 s: the difference between the intensities diffracted with the field in the two directions was measured for three Bragg peaks and three polarization rates. The vertical position of the beam center, as well as the width of the source, were determined from the variation of the linear polarization versus the height; this polarization is obtained by the measurement of two Bragg peaks. The results agree with the theory as regards their signs and their absolute values, in the limits of the errors, which are about 20%.