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Magnetismus Poster: Do., 13:00–15:30 D-P220
Investigation of Dilute Magnetic Semiconductor Multilayers with Polarized
Neutron Reflectivity
Heiko Braak 1 , Amitesh Paul 1 , Diana Rata 1 , Reinert Schreiber 1 , Daniel
Buergler 1 , Peter Grünberg 1 , Claus Schneider 1 , Thomas Brückel 1
1 Institut für Festkoerperforschung, Forschungszentrum Jülich GmbH, D-52425 Jülich,
Germany
In recent years, one of the remarkable achievements in spintronics was the observation
of hole-mediated ferromagnetism in diluted III-V compounds. However, the growth
of group-IV diluted ferromagnetic semiconductors (DMS) such as MnxGe1−x has also
triggered strong interest in searching for higher ordering temperatures TC in Ge-based
compounds [1]. Recently, we have reported [2] on the successful synthesis of a new
DMS compound: Ge co-doped with Mn and Fe. The addition of Fe to Ge(Mn) enables
to conserve epitaxial growth for higher total concentrations of the magnetic species,
which results in an increase of TC. The alloy films are prepared by repeatedly growing
Ge/Mn/Fe trilayers with various thicknesses of the Ge base layers. When the Ge thickness
becomes comparable with the diffusion length of Mn and Fe in Ge, a material without
detectable interfaces is formed as confirmed by X-ray reflectivity [3]. Magnetization
and magnetotransport measurements reveal ferromagnetic ordering, but show two
different TC (210 and above 300 K), which we relate to two magnetic phases [3]. Comparison
of the temperature dependence of the anomalous Hall effect and the saturation
magnetization for a [Ge(40 ˚A)/Fe(2 ˚A)/Mn(4 ˚A)]22 sample shows that the contribution
from the phase with TC > 300 K does not exceed ≈ 10 % of the total magnetization.
Transmission electron microscopy shows the formation of precipitates depending on the
preparation condition and the content of magnetic species. In order to have an unequivocal
assignment of the magnetic phases to the precipitates and the matrix, respectively,
more knowledge about the magnetic structure is required. Preliminary measurements
were done on our polarized neutron beamline HADAS at FRJ-2. Nonspin-flip and
spin-flip reflectivity spectra for the two samples, [Ge(40 ˚A)/Fe(4 ˚A)/Mn(2 ˚A)]11 and
[Ge(40 ˚A)/Fe(2 ˚A)/Mn(4 ˚A)]11, were taken at 50 and 250 K for various applied fields
(Ha). Significant contrast between magnetic moment parallel and antiparallel to Ha is
visible only at 50 K and above 1.0 kOe, which corresponds to the saturation state of
the sample with 4 ˚A Fe. The sample with 4 ˚A Mn shows similar contrast at somewhat
lower fields when measured at 50 K, but this net magnetization is observable at higher
fields even when measured at 250 K. So far, we could not obtain any element-specific
insight in order to investigate the origin of the carrier-induced ferromagnetism, but we
hope that this can be achieved by measuring at neutron sources with higher flux in
the future. The present results are valuable guidelines for the optimized preparation of
single-phase, homogenous Ge(Mn,Fe) DMS films with high TC and also provide useful
feedback for future neutron measurements. [1] Y.D. Park et al., Science 295 (2002)
651. [2] H. Braak et al., Magn. Magn. Mater. 286 (2005) 46. [3] H. Braak, PhD thesis,
Universität zu Köln (2006).