Magneto-electric interactions probed by x-ray optical activity

Xray Optical Activity (XOA) is caused mainly by E1E2 interference terms in systems with odd space Parity (P). Extra emphasis is put below on nonreciprocal XOA effects in Magneto-Electric (ME) solids in which both Parity and Time-reversal (?) symmetries are broken whereas the magnetic structure remains invariant in the product P?. Two types of dichroism related to non-reciprocal XOA have been observed experimentally: (i) The x-ray Magnetochiral dichroism (XM?D) detected in the antiferromagnetic (AFM) phase of CrO is associated with the (unpolarized) Stokes component S; (ii) A non-reciprocal x-ray Magnetic Linear Dichroism (XMLD) was also observed in the low temperature AFM phase a (V Cr)O single crystal and is associated with the Stokes components S or S. Edge selective XOA sum rules were derived recently which give a unique access to a whole family of spherical ME operators mixing orbitals of different parities in what is assumed to be the ground state. It is proved that the orbital anapole moment ? is primarily responsible for the XM?D signal measured both with a single crystal or a powdered sample of CrO. It is suggested that the nonreciprocal XOA in both CrO and (VCr) O is due to partially unquenched angular moments that had a different quantization axis from the spins. © Physica Scripta 2005.