Regensburg 2002 – wissenschaftliches Programm

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O: Oberflächenphysik

O 16: Elektronische Struktur (III)

O 16.4: Vortrag

Dienstag, 12. März 2002, 12:00–12:15, H36

Structural origin of ferromagnetism in ultrathin Fe films grown on Cu(100) and Cu(111) single crystals — •Albert Biedermann, Rupert Tscheließnig, Werner Rupp, Michael Schmid, and Peter Varga — Institut für Allgemeine Physik, Technische Universität Wien, A-1040 Wien, Österreich

For many years, ferromagnetic ultrathin Fe films (less than 10 monolayers thick) grown on FCC Cu single crystal substrates have been cited as prototypical systems for epitaxially stabilized ferromagnetic FCC iron. This exotic structure and its stability (bulk iron is ferromagnetic BCC iron below and paramagnetic FCC above 1184 K) is of substantial interest for the solid state physics of Fe, surface magnetism, and ultrathin magnetic films. Our measurements by scanning tunneling microscopy, however, show that this picture is fundamentally incorrect. Both systems studied, Fe/Cu(100) and Fe/Cu(111), show significant reconstructions of the FCC structure in those temperature and thickness ranges, where ferromagnetism is reported in literature. The reason for these “reconstructions” is most obvious for the 2-4 monolayer Fe/Cu(100) films, which actually show a narrowly twinned and about 9% strained BCC structure (nanomartensite)[1]. These results suggest that ferromagnetic FCC Fe cannot be stabilized in simple films on single crystal surfaces. The origin of ferromagnetism is the transformation of the FCC lattice to an intermediate structure, which is as close to perfect BCC as the respective matching conditions at the film-substrate interface allow.

[1] A. Biedermann, R. Tscheließnig, M. Schmid, P. Varga, Phys. Rev. Lett. 87, 086103 (2001)