X-ray optical activity and the Faraday effect in cobalt and its compounds

Rotation of the plane of polarization following transmission of a synchrotron x-ray beam through a thin sample has been directly observed. Within a few eV of the cobalt K-absorption edge, rotations of up to 2 mrad were obtained in cubic ${\mathrm{Co}}_{0.9}$ ${\mathrm{Fe}}_{0.1}$, and somewhat less in a cobalt metallic glass and in chiral organometallic samples. These effects were observed using a high-extinction tunable perfect-crystal Bragg-reflection x-ray polarimeter of novel design which can detect optical rotations as small as 70 μrad.