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Magnetismus Poster: Do., 13:00–15:30 D-P235
Magnetic coupling of NiO and CoO to Fe3O4 investigated by X-PEEM
Ingo Krug 1 , Ulrich Hillebrecht 1 , Claus M. Schneider 1
1 Forschungszentrum Jülich, Institut für Festkörperforschung (IFF)
Exchange coupling across ferromagnet-antiferromagnet-interfaces has gained great importance
in magnetic storage and readout technology. Despite of widespread experimental
and theoretical efforts, none of the present models provides an accurate description
for a wider range of material systems. In order to separate and understand the
microscopic details that lead to macroscopic effects like spin-flop coupling, coercivity
enhancement and exchange bias, it is necessary to study quasi-ideal model systems.
Therefore we investigated the exchange coupling of ultrathin films of NiO on CoO to
high quality synthetic single crystals of magnetite. The films were grown epitaxially
by in-situ Ox-MBE onto Fe3O4(111) substrates. We use Photoelectron Emission Microscopy
(PEEM) in combination with polarized soft x-rays to simultaneously measure
magnetization direction and spin-axis orientations in the ferrimagnet and the antiferromagnet,
respectively, on a microscopic level(∆x = 50 − 100 nm). For CoO, we find
the coupling to be of parallel (collinear) type (see Fig.1), as has also been observed for
metallic (Co) ferromagnets in contact with NiO. In contrast, we observe 90 ◦ -coupling
for NiO. This difference in behaviour is astonishing, since both substances have the
same crystalline and similar magnetic structures. To detect small magnetic moments
induced in the antiferromagnet by exchange coupling to the ferrimagnet, we employ
a differential measurement technique based on 180 ◦ -domains present in the sample,
achieving a magnetic signal to noise ratio better than 10 −4 . From this approach we
can further differentiate between the two AF materials: for NiO, there is hardly any
induced moment present at the samples, whereas the CoO adlayers show a large induced
ferromagnetic signal. These differences are analyzed within a model which takes
into account magnetostrictive effects.
Fig. 1: Line profiles along the
colored bars in the PEEM images
of CoO/Fe3O4(111): (A) XMCDcontrast
of the substrate. (B) Co-
XMCD, (coupling-induced moment
at the interface), (C) CoO-XMLD
contrast with p-polarized light,
(D) CoO-XMLD-contrast with spolarized
light. By the phase relation
of XMCD and XMLD profiles, the
coupling-type can be determined (in
this case: parallel).