THE 3D-4F EXCHANGE INTERACTION, X-RAY 2ND-ORDER OPTICAL PROCESSES ANDTHE MAGNETIC CIRCULAR-DICHROISM (MCD) SPIN SUM-RULE IN RARE-EARTHS

We discuss the effect of the 3d-4f exchange interaction in rare earths by choosing two subjects on the basis of an atomic model including th e full multiplet. One subject is the selection rules in resonant 4f ph otoemission spectroscopy (PES), 4f --> 3d X-ray emission spectroscopy (XES) and inverse photoemission spectroscopy (IPES) at the 3d threshol d. The other is the applicability of the magnetic circular dichroism ( MCD) spin sum rule in 3d --> 4f X-ray absorption (XAS). The interplay between the diagonal term of the exchange interaction and the 3d core spin-orbital interaction is shown to cause the contribution of the hig her-energy final states of PES and XES, which are not accessible at of f-resonance, to the process passing through the intermediate state in the higher-energy region at each of the 3d(5/2) and 3d(3/2) thresholds . Furthermore it is shown to cause a suppression of the contribution o f the higher-energy final states of IPES seen at off-resonance for the process passing through the intermediate state with the lower-energy region at each of the 3d(5/2) and 3d(3/2) thresholds. We show, on the other hand, that the off-diagonal term of the exchange interaction cau ses a large amount of 3d(5/2)-3d(3/2) mixing in less-than-half filled 4f shells, but not in the more-than-half filled cases, in the Hund's r ule ground state. This means that the use of the so-called M-4 and M-5 edge MCD intensities in the MCD spin sum rule formula, which is the o nly possible choice in its application, is meaningless in less-than-ha lf filled 4f shells at least, although the Mg and Mg edges are apparen tly separated from each other. (C) 1997 Elsevier Science B.V.