X-ray linear birefringence and linear dichroism in a cobalt crystal measured with a tunable X-ray polarimeter

X-ray linear birefringence and linear dichroism spectra have been measured simultaneously and quantitatively for the first time. The sample was a hexagonal cobalt single-crystal foil. The apparatus used was an energy-tunable X-ray polarimeter, consisting of a polarizer, a phase retarder and an analyser. The X-­ray energy was scanned over a range of 350 eV near the K-absorption edge. The Kramers–Kronig relation between the refraction and absorption anisotropy has been confirmed on an absolute scale. The phase-difference sensitivity of the polarimeter as a polarization inferometer is as small as 2π/10000, which is about 100 times smaller than that of the Bonse–Hart inferometer.