Atom Specific Surface Magnetometry with Linear Magnetic Dichroism in Directional Photoemission
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2 2 GIORGIO ROSSI,1, FAUSTO SIROTTI, and GIANCARLO PANACCIONE fir Festkdrperphysik, ETH-Ziirich, CH-8093 2Laboratoire pour l'Utilisation du Rayonnement Electromagnetique, CNRS, CEA, MESR F-94305 Orsay
ABSTRACT A practical atom specific surface magnetometry can be based on the measure of magnetic dichroism in the angular distribution of core photoelectrons using linearly polarized synchrotron radiation. The magnetic dichroism effect on the photoemission intensity of 3p core levels of the ferromagnetic transition elements is as large as 46% in the case of Fe(100). The most efficient scheme for measuring the magnetic dichroism in photoemission requires two mirror experiments in chiral geometry, i.e. only two times more experiments than standard core level photoemission for surface chemical analysis. We describe the dichroism magnetometry and show examples for Fe, Co, Ni and Cr surfaces and interfaces, including the measurement of the temperature dependence of the Fe(100) surface magnetization and of the effect of S-segregation on the surface magnetic moment of iron. INTRODUCTION The understanding of the magnetic properties of surfaces and low dimensional solids requires accurate measurements of all the structural, chemical, and magnetic parameters of the surface atoms. The field of surface magnetism has been opened when the measurement of the magnetiza-
tion of surface and near-surface atoms became practical by the application of spin-polarimetry to the photoelectrons and secondary electrons ejected from surfaces. 1 The measure of spin polarization (SP) of secondaries is intrinsically surface sensitive due to the short escape depth for low energy photoelectrons in ferromagnets, and can be understood semi-quantitatively, but gives an average magnetization values, i.e. does not resolve contributions from individual atomic species constituting the magnetic surface. On the other hand the basic surface science spectroscopies, Auger electron spectroscopy and photoelectron spectroscopy, which allow a full chemical characterization and sensitivity to local order in the diffraction of the ejected electrons, are not directly sensitive to magnetism. To gain magnetic sensitivity AES and PES have been measured in the spin-resolved mode, i.e. by measuring the number of Auger electrons or core photoelectrons with their spins aligned or counteraligned to a macroscopic magnetization direction.4 These are close to ideal tools for surface magnetism, but suffer for a great technical handicap: the low efficiency of spin-detection which is only some 10-3 and severely reduces the applications of these techniques. An alternative is to exploit the fact that polarized atoms, like the atoms in a magnetically ordered ferromagnetic material, can be recognized by their directional anisotropies in the photoionization matrix elements. Angular photoemission experiments on core levels about the magnetization direction (vector) show dichroism and therefore allow to probe magnetism in a highly efficient way. A large magnetic dichroism effect is shown
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