Association

4.1. Coherent growth: EuO on YSZ (100) 63
Figure 4.4.: Magnetic properties of a 20 nm single-crystalline EuO (100) film with M measured inplane
and out-of-plane. The measurement was conducted by B. Zijlstra.
All EuO lattice parameters agree well with bulk EuO. In conclusion, we confirm a seamless
coherent growth of EuO on conductive YSZ (100) by HR-TEM.
A key property of EuO is its ferromagnetic behavior. In Fig. 4.4, the temperature dependent
magnetization curves and the hysteresis curves are depicted for a 20 nm EuO thin film coherently
grown on YSZ (100). Both in-plane and out-of-plane magnetization curves follow the
shape of a Brillouin function. A large difference is observed in the magnetic switching behavior:
the coercive field for in-plane magnetic switching along the [100] direction shows a low
value of H c = 43 Oe which is indicative for a good crystallinity of EuO. For the out-of-plane direction,
in contrast, the coercive field exhibits an eight times larger value of H c = 329 Oe, and
the saturation magnetization cannot be reached. The reduced magnetization in out-of-plane
direction can be explained by the significant fraction of interface and surface layers of EuO
with reduced nearest neighbor coordination, as predicted by Schiller and Nolting (2001).
Mainly three different anisotropy contributions determine the magnetic switching: crystalline
anisotropy, shape anisotropy and pinning by defects. The crystalline anisotropy is
weak in EuO, and the shape anisotropy is dominant. We distinguish between in-plane and
out-of-plane anisotropy. A measure for the magnetic anisotropy is the anisotropy constant in
first order K 1 , expressed as 54
K 1 = −1/2H an σ sat . (4.1)
Here, H an denotes the anisotropy field which is necessary to saturate the magnetic sample
to the saturation moment σ sat in the magnetic easy direction, to which H an is parallel.
For our single-crystalline EuO thin film, we determine the out-of-plane anisotropy as
K1 ⊥ (cryst. film) = −0.851 × 105 erg/g. We compare this out-of-plane anisotropy with a polycrystalline
EuO thin film (d = 100 nm) from literature, 150,151 for which K1 ⊥ (poly. film) =
−9.3 × 10 5 erg/g was found. The large difference of magnetic anisotropy between polycrystalline
and single-crystalline EuO thin films can be explained by magnetic pinning: while
in polycrystalline EuO the pinning of magnetic moments due to crystalline defects is omnipresent,
in single-crystalline EuO without defects, one would expect this pinning effect to
vanish. Indeed, in single-crystalline EuO the shape anisotropy is smaller by a factor of ten
than for the polycrystalline EuO film.
For EuO thin film effects, please see Fig. 2.8 on p. 15.