Dilute Magnetic Semiconductor-Ferromagnet Hybrids
S. Halm*, F. Seifert*, T. Kümmell*, Ellen Schuster*, W. Keune*,
G. Bacher*, Yang-Hsiung Fan, J. Puls, and F. Henneberger
*Universität Duisburg-Essen
(DFG Projekt im Schwerpunktprogramm SPP 1133 ‚Ultraschnelle Magnetisierung’)
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Nanostructured ferromagnets (FMs) provide the opportunity to
manipulate spins in an underlying diluted magnetic semiconductor (DMS) heterostructure.
Fringe fields from Fe/Tb FMs with a remanent out-of-plane magnetization induce
a local out-of-plane magnetization in a (Zn,Cd,Mn)Se/ZnSe DMS quantum well. Due to the
sp-d exchange interaction, optically generated electron-hole pairs align their
spin along the DMS magnetization which is probed by both spatially resolved,
polarization sensitive magneto-photoluminescence spectroscopy. Faraday rotation
uncovers the alignment of the spins of the magnetic Mn ions.
S. Halm et al., Proc. E-MRS 2006 Fall Meeting, Warzawa,
to appear in phys. stat. sol. (c)
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The strongest vertical fringe fields can be achieved by using a planar FM with a
small aperture (called an "antidot" ) To prove the spin alignment,
a single 10µm x 10 µm Fe/Tb antidot structure with a 2µm x 2µm aperture is studied.
The circular optical polarization degree reflecting directly the electron-hole pair
(exciton) spin polarization is measured inside the aperture as a function of
an externally applied magnetic field. For comparison, a DMS region without FM is
investigated as a reference. The field was swept from - 5T to + 5T (open symbols)
and backwards (filled symbols) and the polarization degree was measured in the relevant
range from - 1T to + 1T. While there is no difference in between the up sweep and
the down sweep on a DMS area without FMs, we observe a clear hysteresis in the
center of the antidot with a 12% spin polarization degree at zero external field.
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DMS structures on transparent ZnSe substrates allows us to perform Faraday rotation
(FR) experiments on hybrid structures. FR measures the spin polarization of the
magnetic Mn ions. Via the sp-d interaction, the circularly polarized exciton transitions
are split energetically resulting in different phase velocity for right crcularly and left
circularly polarized light. Using linearly polarized ligth, a rotation of polarization plane
occurs in transmission. We studied the FR signal on a 200 µm x 200 µm array of FM antidots with
1 µm apertures and a period of 2 µm. Here, the FR signal is plotted for the antidot field
at zero field (upper part) and a DMS reference area without FMs (lower part) with varying
external fields at 1.5 K. Pronounced magnetic field dependent features are found
at the photon energies of the heavy-hole exciton groundstate as well as the heavy-hole
subband edge. The features found on the hybrid structure prove that a remanent spin
polarization without external field can be realized for the magnetic Mn ions.
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Previous results: H. Schömig et al., Appl. Phys. Lett. 84, 2826 (2004)
