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Magnetic Oxide Heterostructures: EuO on Cubic Oxides ... - JuSER

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4.1. Coherent growth: EuO on YSZ (100) 65<br />

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Figure 4.6.: Thickness dependent magnetic properties in single-crystalline EuO thin films. (a) depicts<br />

the in-plane magnetization under applied saturation field. (b) shows the coercive field for every<br />

thickness of EuO, with the original hysteresis loop as insets. (c) summarizes the Curie temperature<br />

for every EuO thickness, together with model predictions.<br />

Starting with bulk-like EuO, for the EuO thickness d = 20–30 nm no change in the magnetization<br />

curves is observable, as depicted in Fig. 4.6, and a bulk-like saturation and Curie<br />

temperature is observed. If the EuO thickness is reduced, the coercive field increases from an<br />

optimum value of 23 Oe of the bulk-like EuO film up to ∼130 Oe for an ultrathin (d = 2 nm)<br />

EuO film (Fig. 4.6b). This variation of coercivity with thickness also has been found in<br />

Suran et al. (1995) for soft ferromagnetic thin films. 153 A reduction of the EuO thickness<br />

to d = 15 nm does not affect the Curie temperature, but the magnetization curve is altered<br />

from the shape of a Brillouin function to a reduced magnetization (Fig. 4.6a). The field dependence<br />

changes towards a mixture of ferromagnetic (rectangular-shaped hysteresis) and<br />

paramagnetic (linear response) behavior. This trend is continued in the course of a further<br />

reduction of EuO thickness down to d = 2 nm. Bulk-near magnetic ordering temperatures<br />

(T C = 64 K) are maintained down to d = 2 nm and the saturation magnetization is still approximately<br />

half the bulk EuO value for this thickness. Finally for 1 nm EuO, the magnetization<br />

curve strongly deviates from the Brillouin shape and we estimate T C as 18 K. In this<br />

case, the remanence is near zero and the field dependence is mainly paramagnetic.<br />

We conclude, that the threshold for establishing sizable ferromagnetic properties of coher-

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