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ABSTRACT: Single planar arrays of Ga(x)Fe(4-x)N magnetic nanocrystals embedded in GaN
have been fabricated in an epitaxial process. The phase of the nanocrystals and
their epitaxial relationship with the host matrix are studied $via$
high-resolution transmission electron microscopy and high-resolution x-ray
diffraction. By changing the growth parameters and mode, the crystallographic
phase and chemical composition of the nanocrystals can be varied on demand. In
view of the different magnetic properties of the various phases, applications
in room-temperature ferromagnetic as well as antiferromagnetic spintronic
devices are envisaged.
08/2012;
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G Kunert,
S Dobkowska,
Tian Li,
H Reuther,
C Kruse,
S. Figge,
R Jakieła, A. Bonanni,
J. Grenzer,
W. Stefanowicz,
J. von Borany,
M. Sawicki,
T. Dietl,
D. Hommel
[show abstract]
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ABSTRACT: Sub-micron mapping of GHz magnetic susceptibility using scanning transmission x-ray microscopy Appl. Phys. Lett. 101, 182407 (2012) Structural, magnetic, and optical properties of Pr and Zr codoped BiFeO3 multiferroic ceramics J. Appl. Phys. 112, 094102 (2012) Controlling the current flux in magnetic-barrier induced graphene waveguide Appl. Phys. Lett. 101, 163104 (2012) Hard magnetic properties of Mn-Ga melt-spun ribbons
01/2012; 101(112):22413-83901.
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M. Sawicki,
T. Devillers,
S. Gałȩski,
C. Simserides,
S. Dobkowska,
B. Faina,
A. Grois,
A. Navarro-Quezada,
K. N. Trohidou,
J. A. Majewski,
T. Dietl, A. Bonanni
[show abstract]
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ABSTRACT: By employing highly sensitive millikelvin SQUID magnetometry, the magnitude of the Curie
temperature as a function of the Mn concentration x is determined for thoroughly characterized
Ga1−xMnxN. The interpretation of the results in the frame of tight binding theory and of Monte
Carlo simulations, allows us to assign the spin interaction to ferromagnetic superexchange and to
benchmark the accuracy of state-of-the-art ab initio methods in predicting the magnetic characteristics
of dilute magnetic insulators.
arXiv:1202.6233v2 [cond-mat.mtrl-sci] 29 Feb 2012. 01/2012;
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M Sawicki,
T Devillers,
S Gał˛,
C Simserides,
S Dobkowska,
B Faina,
A Grois,
A Navarro-Quezada,
K N Trohidou,
J A Majewski,
T Dietl, A Bonanni
[show abstract]
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ABSTRACT: By using highly sensitive millikelvin superconducting quantum interference device magnetometry, the magnitude of the Curie temperature as a function of the Mn concentration x is determined for thoroughly characterized Ga 1−x Mn x N. The interpretation of the results in the frame of tight-binding theory and of Monte Carlo simulations allows us to assign the spin interaction to ferromagnetic superexchange and to point out the limited accuracy of state-of-the-art ab initio methods in predicting the magnetic characteristics of dilute magnetic insulators.
Physical Review B 01/2012; 85:205204. · 3.69 Impact Factor
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A. Navarro-Quezada,
N. Gonzalez Szwacki,
W. Stefanowicz,
Tian Li,
A. Grois,
T. Devillers,
M. Rovezzi,
R. Jakieła,
B. Faina,
J. A. Majewski,
M. Sawicki,
T. Dietl, A. Bonanni
[show abstract]
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ABSTRACT: The effect of Mg codoping and Mg deposition mode on the Fe distribution in (Ga,Fe)N layers grown by metal-organic vapor phase epitaxy is investigated. Both homogeneously and digitally Mg-codoped samples are considered and contrasted to the case of (Ga,Fe)N layers obtained without any codoping by shallow impurities. The structural analysis of the layers by high-resolution transmission electron microscopy and by high-resolution and synchrotron x-ray diffraction gives evidence of the fact that, in the case of homogeneous Mg doping, Mg and Fe competitively occupy the Ga-substitutional cation sites, reducing the efficiency of Fe incorporation. Accordingly, the character of the magnetization is modified from ferromagneticlike in the non-codoped films to paramagnetic in the case of homogeneous Mg codoping. The findings are discussed vis-à-vis theoretical results obtained by ab initio computations, showing only a weak effect of codoping on the pairing energy of two Fe cations in bulk GaN. However, according to these computations, codoping reverses the sign of the pairing energy of Fe cations at the Ga-rich surface, substantiating the view that the Fe aggregation occurs at the growth surface. In contrast to the homogeneous deposition mode, the digital one is found to remarkably promote the aggregation of the magnetic ions. The Fe-rich nanocrystals formed in this way are distributed nonuniformly, giving reason for the observed deviation from a standard superparamagnetic behavior.
Phys. Rev. B. 07/2011; 84(15).
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ABSTRACT: Near band-gap photoluminescence and reflectivity in a magnetic field are employed to determine the exchange-induced splitting of free exciton states in paramagnetic wurtzite Ga1-xMnxN, x≲1%, grown on sapphire substrates by metal-organic vapor phase epitaxy. The band gap is found to increase with x. The giant Zeeman splitting of all three excitons A, B, and C is resolved, enabling the determination of the apparent exchange integrals N0α(app)=0.0±0.1 eV and N0β(app)=+0.8±0.2 eV. These nonstandard values and signs of the s-d and p-d exchange energies are explained in terms of recent theories that suggest a contribution of the electron-hole exchange to the spin splitting of the conduction band and a renormalization of the free hole spin splitting by a large p-d hybridization. According to these models, in the limit of a strong p-d coupling, the band gap of (Ga,Mn)N increases with x, and the order of hole spin subbands is reversed, as observed.
Phys. Rev. B. 03/2011; 83(9).
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ABSTRACT: Ga1�xMnxN (0rxr0.18) filmsgrownontoamorphoussilicasubstratebyreactivesputteringare
characterisedbyhighresolutiontransmissionelectronmicroscopy,energydispersivespectroscopy,and
energy filteredtransmissionelectronmicroscopy.Theelectrontransmissionimagesandtheelectron
diffractionpatternsevidencethepresence,atthesubstrate–filminterface,ofafewtensofnmthick
intermediatelayerwithahighdensityofnon-orientednanocrystals(NCs).Thisintermediatelayer
representsthenucleationsiteforthesubsequentgrowthofacompactGa1�xMnxN columnar
nanostructure,whosethickness(600–900nm)isonlylimitedbythedepositiontime.Thecolumnar
region showsafibretexturewiththe c axisofthewurtzitenanocrystalscorrespondingtothecolumn
axis,bothdisposedperpendiculartothefilmsurface.ThethicknessoftheinitialNC-richlayerandthe
coalescenceofthenanocolumnsarefoundtohaveasystematicdependenceontheMnconcentration.
No evidenceofMnsegregationorofMnrichphasesisobservedevenforthesampleswiththehighest
Mn concentration.Thecorrelationbetweentheobservedfilmmicrostructureandthereactive
sputteringdepositionparametersisdiscussed.
Journal of Crystal Growth 03/2011; 327:209. · 1.73 Impact Factor
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ABSTRACT: We demonstrate the control of the hole concentration in Ga 1−x Mn x P over a wide range by introducing compensating vacancies. The resulting evolution of the Curie temperature from 51 K to 7.5 K is remarkably similar to that observed in Ga 1−x Mn x As despite the dramatically different character of hole transport between the two material systems. The highly localized nature of holes in Ga 1−x Mn x P is reflected in the accompanying increase in resistivity by many orders of magnitude. Based on variable-temperature resistivity data we present a general picture for hole conduction in which variable-range hopping is the dominant transport mechanism in the presence of compensation.
01/2011;
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ABSTRACT: We employ x-ray spectroscopy to characterize the distribution and magnetism of particular alloy constituents in (Ga,Fe)N films grown by metal organic vapor phase epitaxy. Furthermore, photoelectron microscopy gives direct evidence for the aggregation of Fe ions, leading to the formation of Fe-rich nanoregions adjacent to the samples surface. A sizable x-ray magnetic circular dichroism (XMCD) signal at the Fe L-edges in remanence and at moderate magnetic fields at 300 K links the high temperature ferromagnetism with the Fe(3d) states. The XMCD response at the N K-edge highlights that the N(2p) states carry considerable spin polarization. We conclude that FeN{\delta} nanocrystals, with \delta > 0.25, stabilize the ferromagnetic response of the films. Comment: 4 pages, 3 figures, 1 table
11/2010;
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[show abstract]
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ABSTRACT: We demonstrate the control of the hole concentration in Ga1-xMnxP over a wide
range by introducing compensating vacancies. The resulting evolution of the
Curie temperature from 51 K to 7.5 K is remarkably similar to that observed in
Ga1-xMnxAs despite the dramatically different character of hole transport
between the two material systems. The highly localized nature of holes in
Ga1-xMnxP is reflected in the accompanying increase in resistivity by many
orders of magnitude. Based on variable-temperature resistivity data we present
a general picture for hole conduction in which variable-range hopping is the
dominant transport mechanism in the presence of compensation.
10/2010;
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A. Bonanni,
M. Sawicki,
T. Devillers,
W. Stefanowicz,
B. Faina,
Tian Li,
T. E. Winkler,
D. Sztenkiel,
A. Navarro-Quezada,
M. Rovezzi,
R. Jakiela,
A. Grois,
M. Wegscheider,
W. Jantsch,
J. Suffczynski,
F. D'Acapito,
A. Meingast,
G. Kothleitner,
T. Dietl
[show abstract]
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ABSTRACT: The sign, magnitude, and range of the exchange couplings between pairs of Mn
ions is determined for (Ga,Mn)N and (Ga,Mn)N:Si with x < 3%. The samples have
been grown by metalorganic vapor phase epitaxy and characterized by
secondary-ion mass spectroscopy; high-resolution transmission electron
microscopy with capabilities allowing for chemical analysis, including the
annular dark-field mode and electron energy loss spectroscopy; high-resolution
and synchrotron x-ray diffraction; synchrotron extended x-ray absorption
fine-structure; synchrotron x-ray absorption near-edge structure; infra-red
optics and electron spin resonance. The results of high resolution magnetic
measurements and their quantitative interpretation have allowed to verify a
series of ab initio predictions on the possibility of ferromagnetism in dilute
magnetic insulators and to demonstrate that the interaction changes from
ferromagnetic to antiferromagnetic when the charge state of the Mn ions is
reduced from 3+ to 2+.
08/2010;
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A. Navarro-Quezada,
W. Stefanowicz,
Tian Li,
B. Faina,
M. Rovezzi,
R.T. Lechner,
T. Devillers,
F D'Acapito,
G Bauer,
M. Sawicki,
T. Dietl, A. Bonanni
[show abstract]
[hide abstract]
ABSTRACT: The local chemistry, structure, and magnetism of (Ga,Fe)N nanocomposites
grown by metal organic vapor phase epitaxy is studied by high resolution
synchrotron x-ray diffraction and absorption, transmission electron microscopy,
and superconducting quantum interference device magnetometry as a function of
the growth temperature $T_{\mathrm{g}}$. Three contributions to the
magnetization are identified: i) paramagnetic -- originating from dilute and
non-interacting Fe$^{3+}$ ions substitutional of Ga, and dominating in layers
obtained at the lowest considered $T_{\mathrm{g}}$ (800$^{\circ}$C); ii)
superparamagnetic-like -- brought about mainly by ferromagnetic nanocrystals of
$\epsilon-$Fe$_3$N but also by $\gamma'$-Fe$_4$N and by inclusions of elemental
$\alpha$- and $\gamma$-Fe, and prevalent in films obtained in the intermediate
$T_{\mathrm{g}}$ range; iii) component linear in the magnetic field and
associated with antiferromagnetic interactions -- found to originate from
highly nitridated Fe$_x$N ($x \leq$ 2) phases, like $\zeta$-Fe$_2$N, and
detected in samples deposited at the highest employed temperature,
$T_{\mathrm{g}}$ = 950$^{\circ}$C. Furthermore, depending on $T_{\mathrm{g}}$,
the Fe-rich nanocrystals segregate towards the sample surface or occupy
two-dimensional planes perpendicular to the growth direction.
01/2010;
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ABSTRACT: Because of strong exchange interactions between localized spins and effective mass carriers, transition metal impurities in semiconductors lead to giant magneto-optical effects. Furthermore, band-gap levels derived from open d shells of magnetic impurities act as efficient recombination centers for photo-carriers. This paper reviews studies of excitonic magneto-reflectivity performed on (Ga, Fe)N epilayers, and shows how hybridization between d levels and band states, particularly strong in nitrides and oxides, renormalizes the exchange splitting of the valence band states in these systems. Photoluminescence measurements on the same structures demonstrate an increase of infrared Fe-related emission at the expense of ultraviolet near band-gap luminescence. This sensitivity of luminescence to the presence of Fe impurities is exploited to monitor the aggregation of FexN nanocrystals that account for the room temperature ferromagnetism of (Ga, Fe)N, but do not act as inhibitors of excitonic luminescence.
Journal of Physics Condensed Matter 10/2008; 20(45):454222. · 2.55 Impact Factor
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A Bonanni,
A Navarro-Quezada,
Tian Li,
M Wegscheider,
Z Matĕj,
V Holý,
R T Lechner,
G Bauer,
M Rovezzi,
F D'Acapito,
M Kiecana,
M Sawicki,
T Dietl
[show abstract]
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ABSTRACT: The control on the distribution of magnetic ions into a semiconducting host is crucial for the functionality of magnetically doped semiconductors. Through a structural analysis at the nanoscale, we give experimental evidence that the aggregation of Fe ions in (Ga,Fe)N and consequently the magnetic response of the material are affected by the growth rate and doping with shallow impurities.
Physical Review Letters 10/2008; 101(13):135502. · 7.37 Impact Factor
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J. Liday,
I. Hotový,
H. Sitter,
P. Vogrinčič,
A. Vincze,
I. Vávra,
A. Šatka,
G. Ecke, A. Bonanni,
J. Breza,
C. Simbrunner,
B. Plochberger
[show abstract]
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ABSTRACT: In this study we investigated the effect of a NiO
x
layer on the electrical properties of oxidized Au/NiO
x
/p-GaN ohmic contacts. Au/NiO
x
layers with a small concentration of oxygen in NiO
x
were deposited on p-GaN by reactive DC magnetron sputtering and annealed in a mixture of O2+N2, and in N2. Auger electron spectroscopy (AES) and time-of-flight secondary ion mass spectrometry (TOF-SIMS) depth profiling in combination
with transmission electron microscopy (TEM), field-emission scanning electron microscopy (FE SEM) and the circular transmission
line method (CTLM) of contact resistance measurements of the contact structure with low content of oxygen in the NiO
x
layer have been used to explain the reduction of the contact resistance as a result of its anneal treatment. It has been
found that creation of a metal/p-NiO/p-GaN contact structure due to annealing of the Au/NiO
x
/p-GaN structure in either N2 or O2+N2 is the main mechanism that is responsible for the ohmic nature of the system. However, lowering of the contact resistance
is similarly affected also by Ga atoms leaving the vacancies at the metal/p-GaN interface after diffusion of Ga into the metallic
layer. The effect of various ways of cleaning the p-GaN surface prior to metallization on the contact resistance has also
been investigated.
Journal of Materials Science Materials in Electronics 08/2008; 19(8):855-862. · 1.08 Impact Factor
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W Pacuski,
P Kossacki,
D Ferrand,
A Golnik,
J Cibert,
M Wegscheider,
A Navarro-Quezada, A Bonanni,
M Kiecana,
M Sawicki,
T Dietl
[show abstract]
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ABSTRACT: The giant Zeeman splitting of free excitons is measured in Ga(1-x)Fe(x)N. Magneto-optical and magnetization data imply the ferromagnetic sign and a reduced magnitude of the effective p-d exchange energy governing the interaction between Fe(3+) ions and holes in GaN, N_{0}beta(app)=+0.5+/-0.2 eV. This finding corroborates the recent suggestion that the strong p-d hybridization specific to nitrides and oxides leads to significant renormalization of the valence band exchange splitting.
Physical Review Letters 02/2008; 100(3):037204. · 7.37 Impact Factor
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ABSTRACT: We report on the structural and chemical analysis of (Ga,Fe)N films grown by metal-organic chemical vapour deposition using
transmission electron microscopy and energy dispersive X-ray spectroscopy. In homogeneously Fe-doped GaN, we have found Fe-rich
nanoclusters to be assigned to α-Fe and ε-Fe3N. In GaN δ-doped with Fe and co-doped with 5Mg, we have observed a significant reduction of the threading dislocations to
be associated with the growth interruption, while the Fe ions tend to float to the surface.
12/2007: pages 77-80;
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W. Pacuski,
P. Kossacki,
D. Ferrand,
A. Golnik,
J. Cibert,
M. Wegscheider,
A. Navarro-Quezada, A. Bonanni,
M. Kiecana,
M. Sawicki,
T. Dietl
[show abstract]
[hide abstract]
ABSTRACT: A direct observation of the giant Zeeman splitting of the free excitons in (Ga,Fe)N is reported. The magnetooptical and magnetization data imply the ferromagnetic sign and a reduced magnitude of the effective p-d exchange energy governing the interaction between Fe^{3+} ions and holes in GaN, N_0 beta^(app) = +0.5 +/- 0.2 eV. This finding corroborates the recent suggestion that the strong p-d hybridization specific to nitrides and oxides leads to significant renormalization of the valence band exchange splitting.
09/2007;
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ABSTRACT: Nowadays Metal Organic Chemical Vapor Deposition (MOCVD) is the most frequently used fabrication process for growing III–V-nitrides. The missing ultra high vacuum (UHV) conditions narrow the window of possible in situ characterization techniques to only optical methods like spectroscopic ellipsometry (SE) and X-ray diffraction. We are able to observe the growing surface simultaneously with a multi wavelength ellipsometer and a X-ray diffraction (XRD) system mounted on an AIXTRON 200 RF-S reactor. Properties like crystal quality, composition, superlattice periodicity and strain relaxation of hexagonal GaN/AlGaN heterostructures are determined in situ using XRD. (© 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
Physica Status Solidi (A) Applications and Materials 07/2007; 204(8):2798 - 2803. · 1.46 Impact Factor
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[show abstract]
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ABSTRACT: In this article we present the Fourier analysis of kinetic reflectometry spectra acquired during metalorganic chemical vapor deposition. We can show that offset errors due to background radiation can be completely removed by the method itself without using filters or lock-in amplifiers. Additionally, calibration of the reflected intensity is needed as long as the response of the detector is linear to the reflected intensity of the sample. By analyzing the time dependent part of the signal growth rate, layer thickness and the refractive index of the growing layer can be deduced. We demonstrate that, by applying the method to the GaN:Mg δ -doping process, it is possible to obtain accurate information about the time, optical properties, and thickness of the grown multilayers with a resolution down to the monolayer range.
Journal of Applied Physics 06/2007; · 2.17 Impact Factor
