H. Ebert, Inst. for Phys. Chemistry, Univ. Munich, Germany
he@gaia.phys.chemie.uni-muenchen.de

With the increasing availability of high intensity radiation of well-defined polarisation supplied by synchrotrons and storage rings many spectroscopies could be more and more refined during the last decades. One of the most exciting examples for this is the observation of magnetic circular and linear dichroism for magnetic materials; i.e. the dependency of the measured signal on the magnetic state of the sample and the polarisation of the radiation. In particular, magnetic dichroism in X-ray absorption could be developed to a new and powerful tool to probe a great variety of magnetic properties of materials. This development was strongly supported by a close interaction of experiment and theory triggering new theoretical techniques and schemes that in turn provided a very sound basis for the interpretation of complex dichroic spectra. Here one should mention the so-called sum-rules introduced by various authors [1,2,3,4] as well as the fully relativistic treatment of X-ray absorption for magnetic materials developed by the author and his coworkers [5,6].

Inspite of the high technical level achieved nowadays in calculating magnetic X-ray absorption spectra in the XANES- as well as in the EXAFS-regime, one has to note that there is still space left for improvements concerning the basic formalism as well as techniques and also that more refined experiments demand for more refined calculations. Some of these necessary developments are listed in the following together with references to some recent corresponding contributions of our group.

Full potential mode calculations [7,8] are expected to be important in XANES-region if highly anisotropic systems are studied. This applies in particular if surfaces are involved. Concerning magnetic dichroism systems showing a non-collinear spin structure have to be accessible. Furthermore, the ground state Hamiltonian has to be improved compared to the present standard SDFT-formalism [9,10,11]. This also applies to the self-energy that represents dynamical effects and for which its spin-dependency is still an open problem. Finally, description of chemical and spatial disorder within the calculation of absorption spectra has to be improved [12].

All these desirable or necessary developments are not only useful for dealing with X-ray absorption but also the theoretical treatment of other kind of spectroscopies will benefit from these. Some examples for these are: core-level photoemission, valence-band photoemission in the UPS- and XPS-regime, spin-resolved appearance potential and Auger-electron spectroscopy.

Bibliography

1

R. Wienke, G. Schütz, and H. Ebert, J. Appl. Physics 69, 6147 (1991).

2

G. Schütz, M. Knülle, and H. Ebert, Physica Scripta T49, 302 (1993).

3

B. T. Thole, P. Carra, F. Sette, and G. van der Laan, Phys. Rev. Letters 68, 1943 (1992).

4

P. Carra, B. T. Thole, M. Altarelli, and X. Wang, Phys. Rev. Letters 70, 694 (1993).

5

H. Ebert, P. Strange, and B. L. Gyorffy, J. Appl. Physics 63, 3055 (1988).

6

H. Ebert, Rep. Prog. Phys. 59, 1665 (1996).

7

T. Huhne et al., Phys. Rev. B in press, 1 (1998).

8

T. Huhne and H. Ebert, Solid State Commun. to be submitted, 1 (1998).

9

H. Ebert, M. Battocletti, and E. K. U. Gross, Europhys. Lett. 40, 545 (1997).

10

H. Ebert and M. Battocletti, Solid State Commun. 98, 785 (1996).

11

H. Ebert, Solid State Commun. 100, 677 (1996).

12

H. Ebert et al., Int. J. Mod. Phys. B 7, 750 (1993).