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5.1. Chemical stabilization of bulk-like EuO directly on silicon 87
are depicted in Fig. 5.1. In stoichiometric EuO (i), M(T ) roughly follows a Brillouin function,
as expected for a Heisenberg ferromagnet with spin angular momentum S = 7/2. 18 We can
observe a magnetic saturation moment of M S = 6.7µ B , close to the bulk value of 7µ B per Eu 2+
as expected for a 4f 7 : 8 S7/2 system. Likewise, the normalized M(H) characteristics taken at
2 K (see inset of Fig. 5.1) displays a clear square-like ferromagnetic hysteresis with a coercive
field of H c ≈ 100 Oe of the (i) stoichiometric EuO/Si (001) heterostructure. This magnitude
of coercive field is characteristic for polycrystalline EuO.
The complementary EuO/Si heterostructure (ii) comprising oxygen-rich EuO, in contrast,
reveals an M(T ) curve in Fig. 5.1 which is almost completely suppressed. Here, a reduced
magnetic saturation moment M S = 1.5µ B is detected at 2 K, caused by a dominant fraction of
paramagnetic Eu 3+ cations in Eu 3 O 4 or Eu 2 O 3 phases, which instantly form under any oxygen
excess. This mainly paramagnetic behavior is also reflected in the M(H) curve (inset) of
oxygen-rich EuO, in which a hysteretic behavior of remaining EuO is hardly identified.
The complete alteration of the magnetic behavior due to an oxygen partial pressure change
of only 2 × 10 −9 Torr underlines that the oxygen supply is an extremely important parameter
during EuO synthesis, in order to obtain EuO/Si structures of stoichiometric type (i) instead
of Eu 1 O 1+x (ii).
HAXPES: Analysis of the Eu 4f valence level
Eu 3d
Eu 4f
e −
e − z EuO = 50..95 Å
Eu 4d
e −
Eu 4s
e −
photoelectron
yield
Al
Figure 5.2.: Photoemission information
depths z, as calculated
for the particular EuO/Si heterostructures
and core-levels
investigated in this study
under hard X-ray excitation of
hν = 4.2 keV.
hard x-ray
excitation
10.7 nm
19 nm
z
d EuO = 45 Å
18 nm
17 nm
information depth calculated
for electrons from Eu orbitals
Si
(100)
EuO
A straightforward analysis of the EuO chemical state is feasible by the analysis of the harddray
photoemission spectra of the Eu 4f orbital. Due to the highly localized character of the
4f valence band with an energy dispersion limited to 0.3 eV, 41,171 hybridization with other
ligand states is weak and photoemission from the deeper bound oxygen 2p valence band becomes
well distinguishable, as indicated † by a recent LDA+U band structure of EuO 41 . In
contrast to photoemission spectroscopy of deep core-levels, final state screening effects of the
4f photo-hole are negligible here. Depending on the Eu 1 O 1+x stoichiometry, Eu cations will
be either in a divalent Eu 2+ initial state with half-filled 4d 10 4f 7 shell and a ferromagnetic
moment of 7µ B , or exist as trivalent Eu 3+ occupying a 4d 10 4f 6 level, which is chemically
One may want to compare this oxygen paramater with the synthesis of stoichiometric EuO on cubic oxides
in Ch. 4.1 on p. 60.
† as depicted in Fig. 2.4 on p. 10.