Association

2.3. Magnetic properties of EuO 15
order to maintain the insulating nature of EuO tunnel contacts. An exotic method to provide
itinerant 5d electrons in EuO 1−x is by light doping using a laser irradiation of hv ≈ 2eV. 23
Finally, if chemical constituents with different ionic radii are incorporated into the crystal,
this gives rise to isotropic strain (chemical pressure). This chemical pressure is supposed to
vary the f –f and d–f exchange interactions. 69
2.3.2. Thin film effects in EuO
d EuO = 5.1 nm
n=20
z
S
d EuO = 2.6 nm
4
n=7
3
n=5
2
n=4
1
n=3
n=2
0
n=1
0 10 20 30 40 50 60 70
Temperature (K)
n=10
n=15
surface Layer
center Layer
Figure 2.8.: Ferromagnetism in ultrathin EuO films. In (a), the Curie temperature is calculated by
a ferromagnetic Kondo-lattice model and seen to be reduced with decreasing number of EuO
monolayers. Adapted from Schiller and Nolting (2001). 70 Polycrystalline EuO thin films show
a decrease in T C (b), that roughly matches the calculation depicted in (a). Adapted from Müller
et al. (2009). 48
Since we investigate mainly ultrathin EuO films (d 5 nm) in this thesis, the behavior of the
magnetic moment and T C in dependence on the thickness is of fundamental interest. The
ferromagnetism in fcc EuO depends on the inter-atomic exchange coupling of the 4f orbitals.
This coupling will be affected by reduced dimensions, e. g. in ultrathin EuO films. Schiller
and Nolting (2001) calculated the electronic structure of one up to 20 monolayers of singlecrystalline
EuO as a function of temperature and thickness in a Kondo-lattice model. 70 In
their study, the averaged spin moment of the 4f begins to reduce when the EuO thickness
drops below 2.5 nm, as depicted in Fig. 2.8a. For one monolayer EuO, they found a T C of 15 K.
Remarkably, the temperature-dependent magnetization curves of center layers follow a Brillouin
function, while surface layers show a lower magnetization. This can be explained by the
Heisenberg model: Eu 2+ ions in center layers can couple to 12 neighbors, while surface ions
only to 8 nearest neighbors. Experimental evidence of the reduction of the Curie temperature
is shown in Fig. 2.8b, where the T C values of polycrystalline EuO match Schiller’s model very