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4.1. Coherent growth: EuO on YSZ (100) 65
1
(a) ultrathin single-crystalline EuO/YSZ (001)
M(T) / M(0)
0
Brillouin fn
(S=7/2, T C =69 K)
EuO layer thickness:
d=30 nm
d=20 nm
d= 4 nm
d= 3 nm
d=12 nm
d=11 nm
0
20
40
60
80
temperature (K)
100
120
140
(b)
coercive field (Oe)
200
150
100
50
0
0
10
20
30
(c)
Curie temperature (K)
80
60
40
20
0
0
5
10
T C of ultrathin EuO films
calculated T C by meanfield
(after Jensen 1992)
15
20
T C of bulk EuO
calculated T C for S=7/2
Heisenberg ferromagnets
(Schiller and Nolting 1999)
25
30
35
EuO thickness d (nm)
EuO thickness d (nm)
Figure 4.6.: Thickness dependent magnetic properties in single-crystalline EuO thin films. (a) depicts
the in-plane magnetization under applied saturation field. (b) shows the coercive field for every
thickness of EuO, with the original hysteresis loop as insets. (c) summarizes the Curie temperature
for every EuO thickness, together with model predictions.
Starting with bulk-like EuO, for the EuO thickness d = 20–30 nm no change in the magnetization
curves is observable, as depicted in Fig. 4.6, and a bulk-like saturation and Curie
temperature is observed. If the EuO thickness is reduced, the coercive field increases from an
optimum value of 23 Oe of the bulk-like EuO film up to ∼130 Oe for an ultrathin (d = 2 nm)
EuO film (Fig. 4.6b). This variation of coercivity with thickness also has been found in
Suran et al. (1995) for soft ferromagnetic thin films. 153 A reduction of the EuO thickness
to d = 15 nm does not affect the Curie temperature, but the magnetization curve is altered
from the shape of a Brillouin function to a reduced magnetization (Fig. 4.6a). The field dependence
changes towards a mixture of ferromagnetic (rectangular-shaped hysteresis) and
paramagnetic (linear response) behavior. This trend is continued in the course of a further
reduction of EuO thickness down to d = 2 nm. Bulk-near magnetic ordering temperatures
(T C = 64 K) are maintained down to d = 2 nm and the saturation magnetization is still approximately
half the bulk EuO value for this thickness. Finally for 1 nm EuO, the magnetization
curve strongly deviates from the Brillouin shape and we estimate T C as 18 K. In this
case, the remanence is near zero and the field dependence is mainly paramagnetic.
We conclude, that the threshold for establishing sizable ferromagnetic properties of coher-