Optical Investigation on Hybrids of Nano-Ferromagnets and Diluted Magnetic Semiconductors

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The main aspect of this work is to study a hybrid structure made of metallic
ferromagnet (FM) on top of a semiconductor to prove possibilities of spin
control via the stray field of a FM.
To achieve an effective sensitivity, a diluted magnetic II-VI semiconductor
quantum well (DMSQW) is used, where a film ZnCdMnSe well is used to
prove the stray field at a fixed distance from the FM. The s,p-d exchange
interaction between magnetic Mn ions and band electrons leads to the giant
Zeeman effect with g-factors in the order of 500 at low temperature (T ≈
1.5 K). In typical DMSQW with zinc blende symmetry and compressive
strain, the exchange interaction of the exciton ground-state is dominated by
the p-d coupling of the heavy-hole which has a negligible in-plane magnetic
moment and interacts thus mainly with a field component along the quantum
well growth direction z.
The sample is especially designed with transparent ZnSe substrate to
allow photoluminescence (PL) and Faraday rotation (FR) studies. Mn concentration
is chosen to obtain an optimal g-factor. The thickness of the cap
layer of 25 nm is an empirical value to ensure the stable formation of a type
I quantum well structure as well as a strong enough magnetic fringe field
in the well layer in the case of hybrid structure. Since the Fe/Tb thin film
exhibits perpendicular magnetic anisotropy (PMA), it obeys a remanent outof-
plane magnetization which creates the necessary vertical magnetic fringe
field component at the position of the DMS well. Patterned FMs (”wire” and
”anti-dot” arrays) are fabricated at the Duisburg-Essen University by electron
beam lithography of a mask, thermal evaporation of the FM constituents
in ultrahigh vacuum and lift-off technique. Wire arrays consist of 1200 nm
wide slender bands separated by 800 nm, anti-dot arrays have 1000 nm ×
1000 nm square apertures in the FM. The total height of the FM is smaller
than the lateral dimensions. The ratio of FM height to cap layer thickness is
designed to optimize the out-of-plane component of magnetic fringe field at
the DMS layer. Therefore, this hybrid structure satisfies well the necessary
properties for the studies.
At the beginning of this work, molecular beam epitaxy (MBE) grown
ZnCdMnSe/ZnSe DMSQWs on transparent substrate are characterized and
compared with properties of DMSQWs on GaAs substrate. The excitation
intensity dependence of the giant Zeeman splitting shows that the Mn spin
temperature is equalized with the lattice temperature via spin-lattice relaxation.
The FM acts as shadow mask for the optical fields and, apart from a
minor diffraction contribution, only uncovered sample regions are optically
accessed. For photoluminescence (PL), a laser spot with a diameter below
100 μm was carefully adjusted to the center of the patterned fields. A photon
energy of 2.75 eV and an excitation intensity 1W/cm2 ensure negligible spin
heating. The giant Zeeman splitting associated with the FM fringe field is
documented by the shift of the PL maximum in different circular polarization
detection. An polarization degree of exciton PL | ρ |≈ 30% is found. On the
reference DMSQW, an external magnetic field of Bext ≈ 40 mT is required
to achieve the same | ρ |.
The Faraday rotation (FR) technique is used to verify the fringe field by
detecting the splitting of exciton states directly. The resulting difference of
phase velocity of right and left circularly polarized light yields the rotation
of polarization plane of incident linearly polarized light. A rotation angle
ΘFR ≈ 0.3◦ is found corresponding to Bext ≈ 40 mT for reference sample.
The values for FR are fully in agreement with the data obtained from the
PL polarization degree studied. Both for PL and FR, the opposite signal
signs prove that the FR in the hybrid structure is indeed caused by the
reversed fringe field in the regions between FMs.
To perform the spin manipulation of both FM and DMSQW by optical
pulse, the Faraday rotation technique is used. A single laser pulse with a
photon energy of Eex ≈ 2.1 eV below the DMS bandgap is chosen to avoid
heating of DMSQW as well as substrate and ensures thus dominant interaction
with the metallic FMs. FR spectra demonstrate the complete erasing
of the average FM magnetization at a pulse energy density at 16mJ/cm2.
After application of an external field of again Bext = +5 T, both FM and
DMSQW magnetization appear to be fully restored. Physically, the dominant
role of heating in the present experiments is related to the pulses with
nanosecond duration used. Nonthermal demagnetization of metallic FMs can
be achieved only on a much shorter time scale. To reverse the FM magnetisation,
a single laser pulse with again 16mJ/cm2 is applied at a reversed
field Bext = 0.5 T. The heating pulse enhances the FM temperature reducing
coercive field strength Bc below Bext. The momentary narrowing of magnetic
hysteresis by laser pulse results in the reversing of FM magnetization under
the impact of a laser pulse at a biased field.
Finally, Zn1−xMnxO-epilayers with x ≤ 0.01 are studied to determine
the s,p-d exchange integrals Noα and Noβ. The data are extracted from the
splitting of A- and B-exciton resonance in reflection spectra. To calculate the
exchange interaction constants, the usual Zeeman splitting and the electronhole
exchange interaction should be considered. The likely value Noβ is
derived to be approximately 0.50 eV for Δso < 0.