Association

2. Theoretical background
. . . The hero is a metal atom with strong personal magnetism,
whose special properties may vary strongly with the
environment.
(Prof. P.-O. Löwdin in the ceremony speech for the Nobel Prize
for P. W. Anderson, Sir N. F. Mott, and J. H. van Vleck 28 )
Europium oxide is one of the very few magnetic insulators, whose ferromagnetism arises
from localized spin-only moments. Moreover, the ferromagnetic oxide EuO is perfectly suited
as spin-functional tunnel contact in the field of silicon spintronics. 10 In this thesis, we aim
towards an understanding of the fundamental magnetic and electronics properties of EuO thin
films, and towards a successful integration with silicon technology. As a fundament for our investigations,
we discuss the basic properties of EuO as well as principles of advanced probing
techniques used in this thesis.
For the present experimental work on EuO including its meticulous preparation (see Ch. 3),
it is obligatory to point out its chemical characteristics which feature a high affinity to oxygen
and water. Therefore, at first we provide an introduction to the thermodynamic analysis of
reactions which may appear during the MBE-synthesis of EuO (see Ch. 4), and in particular
at the EuO/Si interface (see Ch. 5).
We proceed with a discussion of the electronic structure of EuO with focus on valence and
conductions bands, as well as the open shell 4f , which is the origin of the magnetic order
in EuO. Subsequently, we describe the ferromagnetism of EuO: the Heisenberg model and
couplings to nearest and next-nearest neighbors are briefly introduced, as well as the tuning
of T C and possible applications of EuO as a spin filter oxide.
In order to investigate the electronic structure and to quantify chemical phases of EuO thin
films, photoemission spectroscopy is the method of choice. We discuss the physical mechanism
of the photoemission process, the interpretation of characteristic spectral features, and
finally the specificities of hard X-ray excitation (HAXPES). Finally, we introduce the magnetic
circular dichroism (MCD) effect in photoemission, which provides insight into magnetic
properties of EuO in terms of the intra-atomic coupling of core-level final states with
the magnetic open shell.
2.1. The challenge of stabilizing stoichiometric europium oxide
EuO crystallizes in the face-centered cubic rock-salt structure (fcc) with a lattice parameter
of 5.144 Å 31 at room temperature. † In order to produce bulk EuO single crystals, very
high temperatures of about 1800 ◦ C under an exact stoichiometric supply of Eu and oxygen
Indeed, ferromagnetism in an insulator was even severely disputed up to the mid-1950s. 29
† An overview of structural data of EuO and possible substrates as epitaxial seed is given in Appendix A.1.
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