X-Ray Magnetochiral Dichroism: A New Spectroscopic Probe of Parity Nonconserving Magnetic Solids

We report the first experimental detection of x-ray magnetochiral dichroism in magnetoelectric ${\mathrm{Cr}}_2{\mathrm{O}}_3$. This dichroism, which does not require any polarized x-ray beam, is related to the time-reversal odd part of the optical activity tensor dominated by electric dipole-electric quadrupole $E1E2\mathrm{}$ interference terms. The experiments were carried out using either a single crystal or a powdered pellet required to grow a single antiferromagnetic domain by magnetoelectric annealing. This new element (edge) specific spectroscopy offers unique access to the atomic orbital anapole moment ${\Omega }_z^{-}$.