Magneto-Electric Interactions Probed by X-ray Optical Activity

X-ray 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 (ð) symmetries are broken whereas the magnetic structure remains invariant in the product Pð. Two types of dichroism related to non-reciprocal XOA have been observed experimentally: (i) The X-ray Magnetochiral dichroism (XMðD) detected in the antiferromagnetic (AFM) phase of Cr2O3 is associated with the (unpolarized) Stokes component S0 ; (ii) A non-reciprocal X-ray Magnetic Linear Dichroism (XMLD) was also observed in the low temperature AFM phase a (V1−xCrx)2O3 single crystal and is associated with the Stokes components S1 or S2. 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 ð(1) is primarily responsible for the XMðD signal measured both 0 with a single crystal or a powdered sample of Cr2O3. It is suggested that the non- reciprocal XOA in both Cr2O3 and (V1−xCrx)2 O3 is due to partially unquenched angular moments that had a different quantization axis from the spins.