Published:August 17, 2011
r2011 American Chemical Society
dx.doi.org/10.1021/nl2021653|Nano Lett. 2011, 11, 3935–3940
A New Route toward Semiconductor Nanospintronics: Highly
Mn-Doped GaAs Nanowires Realized by Ion-Implantation under
Dynamic Annealing Conditions
Christian Borschel,*,†Maria E. Messing,‡Magnus T. Borgstr€ om,‡Waldomiro Paschoal, Jr.,‡
Jesper Wallentin,‡Sandeep Kumar,‡Kilian Mergenthaler,‡Knut Deppert,‡Carlo M. Canali,§
H?akan Pettersson,‡,||Lars Samuelson,‡and Carsten Ronning†
†Institute for Solid State Physics, Jena University, Max-Wien-Platz 1, 07743 Jena, Germany
‡Solid State Physics/The Nanometer Structure Consortium, Lund University, Box 118, SE-221 00 Lund, Sweden
§Division of Physics, School of Computer Science, Physics and Mathematics, Linnæus University, 39233 Kalmar-Sweden
Department of Mathematics, Physics, and Electrical Engineering, Halmstad University, Box 823, SE-301 18, Halmstad, Sweden
S Supporting Information
pensated spins as well as p-doping, allowing hole-mediated
ferromagnetism. Transition-metal solubility limits in III?V
semiconductors are low, but the necessary Mn fractions for
ferromagnetic dilute magnetic semiconductors (DMS) can be
achieved, for example, by nonequilibrium growth using low
temperature MBE at 250 ?C.1Higher growth temperatures or
semiconductor, but also segregation into MnAs clusters.2,3
to be of interest as versatile building blocks of high functionality
for various devices, such as field-effect transistors,4sensors,5and
facilitates direct growth on silicon wafers. Dilute magnetic
of future spintronic devices with mainstream silicon technology,
which is one of the main goals of “More than Moore”.
Self-organized growth of epitaxial GaAs NWs of high-crystal-
line quality is possible via metal organic vapor phase epitaxy
(MOVPE).7?9The relatively high growth temperatures for
nanowire growth, typically 400?500 ?C, inhibit the homoge-
neous incorporation of high Mn-concentrations during growth
due to the low solubility limit.
in bulk or as thin films, where Mn provides the uncom-
Over the years, several attempts have been made to fabricate
successful growth of InGaMnAs NWs by migration enhanced
epitaxy.10Later, they established growth of Ga1?xMnxAs NWs
using MnAs nanoclusters as growth seeds.11The obtained NWs
contained up to 7% of Mn and were strongly tapered with
irregular side facets. The high Mn content led to a branching of
the NWs.12Martelli et al. performed doping of GaAs NWs using
Mn-assisted growth.13Recently, Kim et al. synthesized gold-
seeded Ga1?xMnxAs NWs using a vapor transport method with
low Mn content (<5%) exhibiting room-temperature ferro-
magnetism;14however, the magnetic properties are unclear
due to lack of any mechanism which could explain the high TC
values.15An additional complication is that the Mn may not
incorporate homogeneously in the NWs. In fact, it might be that
the Mn accumulates in the form of a shell, as indicated by
tomographic atom probe measurements for Ge NWs doped
during VLS growth.16Jeon et al. reported on room-temperature
ferromagnetism of GaMnAs NWs with a Mn content of 20%,17
which is a metallic alloy rather than diluted Mn ina GaAs matrix.
June 27, 2011
August 12, 2011
ABSTRACT: We report on highly Mn-dopedGaAs nanowires (NWs) of high
crystalline quality fabricated by ion beam implantation, a technique that allows
doping concentrations beyond the equilibriumsolubilitylimit.Westudied two
approaches for the preparation of Mn-doped GaAs NWs: First, ion implanta-
NWs and phase segregation of MnAs and GaAs. The second approach was ion
obtained. The electrical resistance of such NWs dropped with increasing temperature (activation energy about 70 meV).
Corresponding magnetoresistance measurements showed a decrease at low temperatures, indicating paramagnetism. Our findings
suggest possibilities for future applications where dense arrays of GaMnAs nanowires may be used as a new kind of magnetic
KEYWORDS: Nanowires, GaAs, doping, DMS, ion implantation, dynamic annealing
dx.doi.org/10.1021/nl2021653 |Nano Lett. 2011, 11, 3935–3940
Rudolph et al. exploited the possibility to change growth mode
from axial to radial nanowire growth and fabricated ferromag-
netic GaAs/GaMnAs core/shellNWs.15However, thegrowth of
the GaMnAs shell leads to a morphological change from the
perfect hexagonal GaAs core NWs toward roughened core/shell
GaAs/GaMnAs NWs. A completely different approach was
performed by Wolff et al.18who decorated GaAs NWs with
Mn nanoparticles, which were then transformed into MnAs
nanoparticles by thermal annealing in hydrogen and arsine.
Most of the reported attempts suffer from segregation of the
MnAs phase during growth leading to nonideal nanowire mor-
phologies. This can easily be exemplified by the morphological
change from the perfect hexagonal GaAs core NWs toward
totally roughened core/shell NWs after growth of the Ga1?x-
MnxAs shells around them.15Furthermore, the detection of
segregated MnAs nanoclusters in GaAs with X-ray diffraction
(XRD) or high-resolution transmission electron microscopy
(HR-TEM) is reported to be rather challenging19and these
clusters can easily be mistaken for a DMS. In conclusion, in situ
Mn-doping of GaAs NWs turns out to be rather difficult to
achieve by established growth methods. This observation is well
in agreement with the phase diagram determined by Ohno for
Ga1?xMnxAs layer growth via MBE:20For x in the order of
0.5?6%, MnAs forms above about 300 ?C.
independent from the growth method of the crystal. It is a
standard method for doping of large scale wafers in industry, but
it has also been successfully applied for doping of semiconductor
NWs.21The successful p-doping of GaAs NWs via ion beam
implantation of Zn and subsequent annealing at 800 ?C under
tertiarybutylarsine atmosphere has been reported.22Ion beam
implantation introduces defects in the target, making postim-
plantation annealing a necessity. For Mn-implanted GaAs thin
films, standard thermal annealing (650 ?C) has been reported to
lead to MnAs cluster formation.23Only pulsed laser melting24,25
and ion beam-induced epitaxial crystallization annealing26have
shown promising results regarding the incorporation of Mn in
GaAs to create DMS systems.
The annealing methods from bulk cannot directly be trans-
volume ratio, the equilibrium phases strongly differ from those
compared to their bulk counterparts.
In this Letter, we compare two approaches for preparation of
Mn-doped GaAs NWs by (a) ion implantation at room tem-
perature with subsequent annealing and (b) ion implantation at
elevated temperatures making use of in situ dynamic annealing.
Furthermore, we report on magnetotransport measurements
that were done in order to study the magnetic properties of
the implanted Ga1?xMnxAs NWs.
Single crystalline, epitaxial GaAs NWs were grown via
MOVPE using monodisperse Au particles as catalyst. Details
have been published elsewhere.27In order to optimize the
implantation angle, the NWs were grown on GaAs(001) sub-
strates leading to an inclined growth direction of 35? toward the
substrate.28NWs were implanted with Mn ions using a general
purpose implanter (High Voltage Engineering Europa). Implan-
tation energies of typically 40?60 keV were selected. Results
from computer simulations of the ion beam implantation show
that these energies lead to a reasonably homogeneous concen-
tration of Mn in the GaAs NWs. Frequently, these simulations
are carried out with the widely used SRIM program.29However,
SRIM assumes flat sample geometry and cannot accurately
describe the distribution of implanted ions in NWs. Therefore,
we used the new in-house developed code iradina,30which
correctly takes into account the nanowire geometry.
NWs were implanted with Mn doses of 2 ? 1015to 1 ? 1016
ions/cm2resulting in total Mn concentrations from 0.5 to 2.9%
(corresponding to a stoichiometry of Ga1?xMnxAs with x from
x = 0.01 to x = 0.058) as calculated with the iradina code.
Detailed simulation results comparing SRIM and iradina for the
The first set of samples (a) was implanted at room temperature
and subsequently annealed for 30 min either under vacuum at
temperatures up to 500 ?C, or under AsH3atmosphere (molar
fraction of 2.20 ? 10?3) from 350 to 650 ?C. The second set of
samples (b) was implanted at elevated temperatures ranging
from 100 to 350 ?C. The NWs were characterized by transmis-
sion electron microscopy (TEM) using a Jeol 3010 and a Jeol
3000F microscope. The Mn concentration was monitored via
X-ray energy dispersive spectroscopy (XEDS).
For magnetotransport measurements, NWs were mechani-
cally transferred onto a silicon substrate covered by a 210 nm
thick silicon dioxide layer on which reference markers and
macroscopic metal pads were predefined. Prior to transferring
of the NWs, trenches were etched in the SiO2layer to align the
wires for magnetotransport studies. Electron beam lithography
was used to define contacts connecting individual NWs to the
macroscopic contact pads. The samples were treated in HCl/
heated (40 ?C) NH4Sx/H2O solution. To investigate the influ-
ence of the contact resistance, 4 Pd(10 nm)/Zn(10 nm)/Pd-
(35 nm) contacts were evaporated on each NW after passi-
The magnetotransport measurements were performed in a
Janis VariTemp superconducting cryomagnet system (Model
Figure 1a shows an HR-TEM micrograph of a typical non-
implanted, as-grown, NW for comparison. The single-crystalline
NWs of zinc blende (ZB) structure were grown in the Æ111æ
direction and exhibit twin planes perpendicular to the growth
direction, as commonly observed for III?V semiconductor
NWs.32Resulting from the implantation parameters at room
temperature, the NWs were mostly amorphized, which is illu-
strated in Figure 1b. Depending on the Mn dose, the ion energy,
and the exact diameter of the NWs, some parts that face away
Figure 1. (a) HR-TEM image of an as-grown GaAs nanowire, with the
FFT shown in the inset. (b) TEM micrograph of an implanted,
nonannealed nanowire (with approx 2.5% Mn), that is partly amor-
phized and partly crystalline. The two insets show FFTs of the
amorphous and crystalline parts, respectively.
dx.doi.org/10.1021/nl2021653 |Nano Lett. 2011, 11, 3935–3940
from the ion beam however remained crystalline as illustrated by
the fast Fourier transform (FFT) diffraction pattern (inset in
The NWs recrystallized under annealing in vacuum up to
400 ?C, however not to single crystals. A typical polycrystalline
in Figure 2a. After annealing in vacuum at 500 ?C or higher
temperatures, we observed that the NWs disappeared, which we
interpret as a temperature-induced decomposition of the GaAs
NWs, mainly due to the high equilibrium As vapor pressure over
GaAs. Therefore, annealing in AsH3atmosphere, as previously
used to recrystallize Zn-implanted GaAs NWs,22was attempted.
Within the investigated parameter space, the present Mn-doped
NWs survived the treatment but although some recrystallization
occurred, they did not become single crystalline (see Figure 2b).
Additionally, they featured “bumps” on the sides, as illustrated in
Figure 2c. Analysis of the FFT patterns and XEDS shows that
these bumps consist of crystalline GaAs without measurable Mn
content; the postimplantation annealing with arsine background
on the amorphized NW.
At this point, we can conclude that room-temperature im-
plantation and subsequent annealing is not suitable to create
the parameter conditions investigated, neither in vacuum, nor in
an arsine atmosphere.
During ion beam implantation, so-called dynamic annealing
can occur.33Each ion induces a collision cascade in the material,
energyis deposited by the incident ion as lattice vibrations in the
cascade region. Within this so-called “thermal spike”, immediate
annihilation of defects can occur. When collision cascades from
subsequent ions overlap, defects that were created in the earlier
cascade can be annihilated by the following ones. If this dynamic
annealing is weak, the damage created by subsequent collision
cascades is accumulated and ultimately amorphization of the
material will occur, if a large fluence of ions is implanted, as
observed in this study. Actually, dynamic annealing is enhanced
in NWs compared to bulk34,35due to the confined geometry
this annealing effect is not strong enough at room-temperature
implantation of Mn into GaAs NWs, and is thus not able to
prevent amorphization: the NWs became amorphous when
implanted with x = 1% of Mn. Thus, the crystal orientation
was lost in the amorphous regions, and subsequent annealing
lead to polycrystalline recrystallization, as was described above.
Dynamic annealing can further be enhanced by heating the
target during ion implantation33due to increased phonon?pho-
non scattering resulting also into a slower dissipation of the
impact energy. In order to make use of this enhanced dynamic
annealing, samples were implanted at elevated temperatures. A
maximum of 400 ?C was used, because decomposition of the
GaAs NWs was observed in vacuum at higher temperatures.
Figure 3a,b showsTEM micrographsofananowire implanted
withMnat atemperature of100 ?C. Alarge partofthe nanowire
remained crystalline during implantation, and the crystalline
structure appears similar to the nonimplanted nanowire
(Figure 1a). This is in contrast to room-temperature implanta-
tion at which the NWs were mostly amorphized. However, an
amorphous “neck” close to the catalyst particle appeared after
implantation at 100 ?C, as illustrated in Figure 3b. This latter
observation can be attributed to different efficiency of the
dynamic annealing in the proximity of the catalyst particle. For
most part of the nanowire, the energy within the thermal spike is
sufficient to cause in situ removal of most defects, the crystal
orientation is never lost and the material remains single-crystal-
line in ZB phase and oriented in the (111)B direction. However,
of the thermal spike dissipates much faster, because the thermal
of the material occurs in this area. Such a partial amorphization
Figure2. TEMmicrographs ofimplantedandannealedNWs.(a)Afterannealinginvacuumat400?C,theFFTintheinsetshowsthatthenanowireis
not single crystalline. (b,c) After annealing in As atmosphere at 550 ?C. (b) Nanowire is polycrystalline. (c) Bumps appear on nanowire, consisting
Figure 3. (a) GaAs nanowire implanted with x = 1% Mn at 100 ?C.
(b) HR image of the same wire showing the amorphous gap between
crystalline nanowire coreandAutip.(c)GaAsnanowireimplanted with
x = 1% Mn at 200 ?C.
dx.doi.org/10.1021/nl2021653 |Nano Lett. 2011, 11, 3935–3940
could be avoided by further increasing the implantation tem-
perature to above 100 ?C. Then, the dynamic annealing was
sufficiently enhanced to maintain crystallinity of the complete
nanowire, as illustrated for a nanowire implanted at 200 ?C in
Figure 3c. It should be noted that apart from dynamic annealing
temperature in the annealing.
From thin film growth of Ga1?xMnxAs it is known that phase
Mn concentration.20However, due to their small size this limit
may be lower for NWs. Therefore, we also examined implanta-
Figure 4a, showing an HR-TEM image of a nanowire implanted
with 1016ions/cm2, corresponding to a total Mn concentration
of 2.9% (x = 0.058). Even at these high concentrations, the NWs
were observed to remain single-crystalline and no secondary
phases were observed. XEDS measurements were performed on
several areas on these NWs (Supporting Information Figure 2).
They confirmed a Mn concentration of about 2?3%, showing
that Mn has not diffused out during implantation at elevated
It should be noted at this point, that computer simulations of
the ion beam implantation require that the correct nanowire
geometry is taken into account. Simulations using SRIM29over-
a factor of 1.7, while simulations using the iradina code30predict
is that SRIM can only simulate flat targets and therefore cannot
take into account ions leaving the nanowire from the sides.
A recovery stage for the electronic properties of GaAs around
250 ?C has been reportedbyThommenfor high-energy electron-
beam irradiated GaAs and subsequent annealing.36Thus, it seems
useful to use at least 250 ?C during implantation, although single-
crystallinity was already observed at 150 ?C. We further increased
the implantation temperature to 350 ?C and still obtained single
crystalline NWs; however, the optimum temperature has to be
balanced, since, as mentioned in the introduction, at higher
temperatures the precipitation of MnAs nanoclusters becomes
more likely.37We did not observe MnAs precipitates in the
implanted NWs up to 350 ?C, but nanometric inclusions of this
phase are not easily detectable19and cannot be excluded.
Considering the upper and lower limitations, 250 ?C was
identified as the suitable temperature for creating GaMnAs
nanowire with high Mn content via ion beam implantation. This
is consistent with the temperature at which Ohno et al. have
grown LT-MBE magnetic GaMnAs thin films.1Furthermore,
250 ?C is also the same temperature Chen et al. used for ion-
beam annealing of Mn implanted GaAs thin films.26
Transport measurements were carried out between 1.6 and
300 K. To investigate possible influence of contact resistance,
2-point and 4-point measurements were compared down to
about 70 K where 4-point measurements became difficult to
carry out due to a very high resistance. From these measure-
ments, we conclude that the contact resistance is negligible
compared to the NW resistance. The IV-characteristics are in
general fairly linear down to about 100 K (Supporting Informa-
tion Figure 3). At lower temperatures, a symmetrical nonlinear
behavior becomes increasingly dominant. Figure 5 displays the
resistance of a typical implanted NW versus reciprocal tempera-
We have investigated several NWs that all show the same
behavior. A similar behavior has previously been reported for
from a Mn impurity band to the valence band.38The activation
energy is smaller than the typical 100?110 meV observed
for weakly doped GaAs:Mn, where the emission originates from
isolated Mn impurities.39Interestingly, the room-temperature
resistance is in the MΩ regime, which is unexpectedly high,
considering the nominal x = 5.8% Mn incorporation. A
plausible explanation for the apparent low hole concentration
(∼1017cm?3assuming a reduced mobility of 60 cm2/(V s)38
and 40 nm wire diameter) is a Fermi-level pinning at the
GaAs surface, which effectively creates a radially depleted semi-
conductor wire. Also, most likely compensation occurs due to
incorporation of interstitial Mn and substitutional AsGaantisites,
too low to create the necessary effective ferromagnetic coupling
between Mn spins, which results in a paramagnetic state rather
than in ferromagnetic ordering. The absence of the onset of any
any singularity in the derivative of the resistance as a function of
temperature in the range of temperatures (40?190 K), which is
expected to occur at the critical temperature.40Further under-
standing of the magnetic state of our implanted wires is obtained
from magnetoresistance (MR) studies. In Figure 6, we plot the
MR as a function of an external magnetic field applied in the
direction parallel and perpendicular to the wire. The data readily
Figure 4. (a) HR-TEM micrograph of GaAs nanowire implanted with
x = 5% Mn at 250 ?C and FFT of the complete image as inset
demonstrating excellent crystalline quality of the Mn-implanted wires.
(b) HR-TEM micrograph with higher magnification.
Figure 5. Plot of resistance versus reciprocal temperature with indi-
cated activation energy. The inset shows an SEM micrograph of a
nanowire supplied with contacts for 4-point measurements.
dx.doi.org/10.1021/nl2021653 |Nano Lett. 2011, 11, 3935–3940
hysteresis effects was found in our measurements and addition-
ally, the MR does not seem to saturate (up to 5 T). There is
also no clear indication of any anisotropic MR changes at small
magneticfieldsthattypicallyoccur inferromagnetic DMS dueto
reorientation of the magnetization relative to the current direc-
tion. While these MR results support the conclusion that our
a paramagnetic state of the system due to the presence of Mn
impurities. Indeed, nonimplanted NWs display dramatically
different electrical properties with very high resistance (∼200
GΩ) already at room temperature, which makes magnetotran-
normally attributed to spin-disorder scattering in metallic
samples20and to suppression of Anderson localization of holes
in insulating or semiconducting samples.41Possibly, in the
regime of low conductance of our sample, both mechanisms
are contributing to some extent in reducing the resistance. The
of a Zeeman shift of the Fermi energy toward the Anderson
mobility edge by the applied magnetic field, which further-
more reduces spin-disorder scattering by successively aligning
the randomly oriented individual Mn spins along its direction.
The Zeeman energy for a spin 5/2 is of the order of 20 K at 5 T.
If we assume that the magnetization of the sample at a given
temperature and magnetic field is proportional to the Brillouin
function, the magnetization and consequently the MR should
reach saturation at fields on the order of 8 T when the tem-
perature is 1.6 K. On the other hand, as shown in Figure 6, at
temperatures above 70 K and at a magnetic field of 5 T, the
thermal energy totally randomizes the spin orientation which
consequently quenches the MR.
The MR curves further indicate that the Mn is mainly diluted
in the GaAs host and not incorporated in the form of ferromag-
netic MnAs clusters; Michel et al. have theoretically studied the
influence of ferromagnetic MnAs clusters in a paramagnetic
GaMnAs host matrix on the MR behavior.42They find that a
high density of MnAs clusters leads to a strong negative MR
below 30 K and a very strong positive MR of several hundred
percent between 30 and 100 K. The Mn-doped nanowires only
show a weak negative MR below 70 K and no MR at higher
at least a very low density of them.
In conclusion, we have realized high-crystalline quality
Mn-doped GaAs NWs via ion beam implantation, which allows
incorporation of dopants into the target material far beyond the
solubility limit. In order to minimize ion beam-induced defects
different annealing routes were investigated. Postannealing was
not successful for NWs, because the NWs either decomposed or
became polycrystalline within the investigated parameter space.
However, heating the sample to higher temperatures during
implantation (250 ?C) enabled increased dynamic annealing in
addition to thermal healing, resulting in single-crystalline
Ga1?xMnxAs NWs with high Mn content.
We also report on magnetotransport measurements on the
Mn-implanted NWs, where astrong-temperaturedependence of
the resistance was observed in addition to a clear negative MR at
and, in combination with the observed high resistance of the
NWs, support the hypothesis that the implanted NWs are
paramagnetic due to a strong reduction in free hole concentra-
tion that stems from surface pinning effects and, possibly, from
Mn interstitials and AsGaantisites.
This study provides the groundwork to pursue further ion-
implantation experiments, for example, in thicker NWs, and to
develop novel technology for tuning the hole concentration in
and thus ferromagnetism. We believe that the work presented
here demonstrates a promising route for future fabrication of
complex integrated DMS nanostructures, where arrays of Mn-
doped NWs may be functioning as a novel kind of material
system for science as well as for applications.
figures. This material is available free of charge via the Internet
Additional information and
We acknowledge support by the Swedish Research Council
under Grant Number: 621-2010-3761.
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