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Wiktor Stefanowicz,
Dariusz Sztenkiel,
Bogdan Faina,
Andreas Grois,
Mauro Rovezzi,
Thibaut Devillers,
Andrea Navarro-Quezada,
Tian Li,
Jakieła Rafał,
Maciej Sawicki, Tomasz Dietl,
Alberta Bonanni
[show abstract]
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ABSTRACT: Systematic investigations of the structural and magnetic properties of single crystal GaxMn1−xN films grown by metal organic vapor phase epitaxy are presented. High resolution transmission electron microscopy, synchrotron x-ray diffraction, and extended x-ray absorption fine structure studies do not reveal any crystallographic phase separation and indicate that Mn occupies Ga-substitutional sites in the Mn concentration range up to 1%. The magnetic properties as a function of temperature, magnetic field and its orientation with respect to the c-axis of the wurtzite structure can be quantitatively described by the paramagnetic theory of an ensemble of non-interacting Mn 3+ ions in the relevant crystal field, a conclusion consistent with the x-ray absorption near edge structure analysis. A negligible contribution of Mn in the 2+ charge state points to a low concentration of residual donors in the studied films. Studies on modulation doped p-type GaxMn1−xN/(Ga,Al)N:Mg heterostructures do not reproduce the high temperature robust ferromagnetism reported recently for this system.
arXiv.org. 02/2013; arXiv:0912.4216.
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Gerd Kunert,
Sylwia Dobkowska,
Tian Li,
Helfried Reuther,
Carsten Kruse,
Stephan Figge,
Rafal Jakiela,
Alberta Bonanni,
Jörg Grenzer,
Wiktor Stefanowicz,
Maciej Sawicki, Tomasz Dietl,
Detlef Hommel
[show abstract]
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ABSTRACT: We report on the fabrication of pseudomorphic wurtzite GaMnN grown on GaN
with Mn concentrations up to 10% using molecular beam epitaxy. According to
Rutherford backscattering the Mn ions are mainly at the Ga-substitutional
positions, and they are homogeneously distributed according to depth-resolved
Auger-electron spectroscopy and secondary-ion mass-spectroscopy measurements. A
random Mn distribution is indicated by transmission electron microscopy, no
Mn-rich clusters are present for optimized growth conditions. A linear increase
of the c-lattice parameter with increasing Mn concentration is found using
x-ray diffraction. The ferromagnetic behavior is confirmed by superconducting
quantum-interference measurements showing saturation magnetizations of up to
150 emu/cm^3.
05/2012;
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Thibaut Devillers,
Mauro Rovezzi,
Nevill Gonzalez Szwacki,
Sylwia Dobkowska,
Wiktor Stefanowicz,
Dariusz Sztenkiel,
Andreas Grois,
Jan Suffczyński,
Andrea Navarro-Quezada,
Bogdan Faina,
Tian Li,
Pieter Glatzel,
Francesco d'Acapito,
Rafał Jakieła,
Maciej Sawicki,
Jacek A Majewski, Tomasz Dietl,
Alberta Bonanni
[show abstract]
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ABSTRACT: Owing to the variety of possible charge and spin states and to the different ways of coupling to the environment, paramagnetic centres in wide band-gap semiconductors and insulators exhibit a strikingly rich spectrum of properties and functionalities, exploited in commercial light emitters and proposed for applications in quantum information. Here we demonstrate, by combining synchrotron techniques with magnetic, optical and ab initio studies, that the codoping of GaN:Mn with Mg allows to control the Mn(n) (+) charge and spin state in the range 3≤n≤5 and 2≥S≥1. According to our results, this outstanding degree of tunability arises from the formation of hitherto concealed cation complexes Mn-Mg(k), where the number of ligands k is pre-defined by fabrication conditions. The properties of these complexes allow to extend towards the infrared the already remarkable optical capabilities of nitrides, open to solotronics functionalities, and generally represent a fresh perspective for magnetic semiconductors.
Scientific Reports 01/2012; 2:722.
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[show abstract]
[hide abstract]
ABSTRACT: Colossal negative magnetoresistance and the associated field-induced
insulator-to-metal transition, the most characteristic features of magnetic
semiconductors, are observed in n-type rare earth oxides and chalcogenides,
p-type manganites, n-type and p-type diluted magnetic semiconductors (DMS) as
well as in quantum wells of n-type DMS. Here, we report on magnetostransport
studies of Mn modulation-doped InAs quantum wells, which reveal a magnetic
field driven and bias voltage dependent insulator-to-metal transition with
abrupt and hysteretic changes of resistance over several orders of magnitude.
These phenomena coexist with the quantised Hall effect in high magnetic fields.
We show that the exchange coupling between a hole and the parent Mn acceptor
produces a magnetic anisotropy barrier that shifts the spin relaxation time of
the bound hole to a 100 s range in compressively strained quantum wells. This
bistability of the individual Mn acceptors explains the hysteretic behaviour
while opening prospects for information storing and processing. At high bias
voltage another bistability, caused by the overheating of electrons10, gives
rise to abrupt resistance jumps.
06/2011;
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[show abstract]
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ABSTRACT: A theoretical model is presented which allows to reconcile findings of
scanning tunnelling spectroscopy for (Ga,Mn)As [Richardella et al. Science 327,
66 (2010)] with results for tunneling across (Ga,Mn)As thin layers [Ohya et al.
Nature Phys. 7, 342 (2011); Phys. Rev. Lett. 104, 167204 (2010)]. According to
the proposed model, supported by a self-consistent solution of the Poisson and
Schroedinger equations, a nonmonotonic behaviour of differential tunnel
conductance as a function of bias is associated with the appearance of
two-dimensional hole subbands rather in the GaAs:Be electrode than in the
(Ga,Mn)As layer.
02/2011;
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[show abstract]
[hide abstract]
ABSTRACT: The comprehensive search for multifunctional materials has resulted in the
discovery of semiconductors and oxides showing ferromagnetic features
persisting to room temperature. In this tutorial review the methods of
synthesis of these materials, as well as the application of element-specific
nano-analytic tools, particularly involving synchrotron radiation and electron
microscopy, are described and shown to reveal the presence of nano-scale phase
separations. Various means to control the aggregation of magnetic cations are
discussed together with the mechanisms accounting for ferromagnetism of either
condensed or diluted magnetic semiconductors. Finally, the question of whether
high temperature ferromagnetism is possible in semiconductors not containing
magnetic ions is touched upon.
01/2011;
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Wiktor Stefanowicz,
Dariusz Sztenkiel,
Bogdan Faina,
Andreas Grois,
Mauro Rovezzi,
Thibaut Devillers,
Francesco d’Acapito,
Andrea Navarro-Quezada,
Tian Li,
Rafał Jakieła,
Maciej Sawicki, Tomasz Dietl,
Alberta Bonanni
[show abstract]
[hide abstract]
ABSTRACT: Systematic investigations of the structural and magnetic properties of single crystal Ga1−xMnxN films grown by metal organic vapor phase epitaxy are presented. High-resolution transmission electron microscopy, synchrotron x-ray diffraction, and extended x-ray absorption fine structure studies do not reveal any crystallographic phase separation and indicate that Mn occupies Ga-substitutional sites in the Mn concentration range up to 1%. The magnetic properties as a function of temperature, magnetic field and its orientation with respect to the c axis of the wurtzite structure can be quantitatively described by the paramagnetic theory of an ensemble of noninteracting Mn3+ ions in the relevant crystal field, a conclusion consistent with the x-ray absorption near edge structure analysis. A negligible contribution of Mn in the 2+ charge state points to a low concentration of residual donors in the studied films. Studies on modulation-doped p-type Ga1−xMnxN/(Ga,Al)N:Mg heterostructures do not reproduce the high-temperature robust ferromagnetism reported recently for this system.
Physical Review B 06/2010; 81(23). · 3.69 Impact Factor
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[show abstract]
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ABSTRACT: The temperature dependence of magnetic anisotropy in (113)A (Ga,Mn)As layers grown by molecular beam epitaxy is studied by means of superconducting quantum interference device (SQUID) magnetometry as well as by ferromagnetic resonance (FMR) and magnetooptical effects. Experimental results are described considering cubic and two kinds of uniaxial magnetic anisotropy. The magnitude of cubic and uniaxial anisotropy constants is found to be proportional to the fourth and second power of saturation magnetization, respectively. Similarly to the case of (001) samples, the spin reorientation transition from uniaxial anisotropy with the easy along the [-1, 1, 0] direction at high temperatures to the biaxial <100> anisotropy at low temperatures is observed around 25 K. The determined values of the anisotropy constants have been confirmed by FMR studies. As evidenced by investigations of the polar magnetooptical Kerr effect, the particular combination of magnetic anisotropies allows the out-of-plane component of magnetization to be reversed by an in-plane magnetic field. Theoretical calculations within the p-d Zener model explain the magnitude of the out-of-plane uniaxial anisotropy constant caused by epitaxial strain, but do not explain satisfactorily the cubic anisotropy constant. At the same time the findings point to the presence of an additional uniaxial anisotropy of unknown origin. Similarly to the case of (001) films, this additional anisotropy can be explained by assuming the existence of a shear strain. However, in contrast to the (001) samples, this additional strain has an out-of-the-(001)-plane character. Comment: 13 pages, 9 figures
02/2010;
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[show abstract]
[hide abstract]
ABSTRACT: The comprehensive search for multifunctional materials has resulted in the discovery of semiconductors and oxides showing ferromagnetic features persisting to room temperature. In this tutorial review the methods of synthesis of these materials, as well as the application of element-specific nano-analytic tools, particularly involving synchrotron radiation and electron microscopy, are described and shown to reveal the presence of nano-scale phase separations. Various means to control the aggregation of magnetic cations are discussed together with the mechanisms accounting for ferromagnetism of either condensed or diluted magnetic semiconductors. Finally, the question of whether high temperature ferromagnetism is possible in semiconductors not containing magnetic ions is touched upon.
Chemical Society Reviews 02/2010; 39(2):528-39. · 28.76 Impact Factor
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Wiktor Stefanowicz,
Dariusz Sztenkiel,
Bogdan Faina,
Andreas Grois,
Mauro Rovezzi,
Thibaut Devillers,
Francesco D'Acapito,
Andrea Navarro-Quezada,
Tian Li,
Rafal Jakiela,
Maciej Sawicki, Tomasz Dietl,
Alberta Bonanni
[show abstract]
[hide abstract]
ABSTRACT: Systematic investigations of the structural and magnetic properties of single
crystal (Ga,Mn)N films grown by metal organic vapor phase epitaxy are
presented. High resolution transmission electron microscopy, synchrotron x-ray
diffraction, and extended x-ray absorption fine structure studies do not reveal
any crystallographic phase separation and indicate that Mn occupies
Ga-substitutional sites in the Mn concentration range up to 1%. The magnetic
properties as a function of temperature, magnetic field and its orientation
with respect to the c-axis of the wurtzite structure can be quantitatively
described by the paramagnetic theory of an ensemble of non-interacting
Mn$^{3+}$ ions in the relevant crystal field, a conclusion consistent with the
x-ray absorption near edge structure analysis. A negligible contribution of Mn
in the 2+ charge state points to a low concentration of residual donors in the
studied films. Studies on modulation doped p-type (Ga,Mn)N/(Ga,Al)N:Mg
heterostructures do not reproduce the high temperature robust ferromagnetism
reported recently for this system.
12/2009;
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[show abstract]
[hide abstract]
ABSTRACT: The question whether the Anderson-Mott localisation enhances or reduces magnetic correlations is central to the physics of magnetic alloys. Particularly intriguing is the case of (Ga,Mn)As and related magnetic semiconductors, for which diverging theoretical scenarios have been proposed. Here, by direct magnetisation measurements we demonstrate how magnetism evolves when the density of carriers mediating the spin-spin coupling is diminished by the gate electric field in metal/insulator/semiconductor structures of (Ga,Mn)As. Our findings show that the channel depletion results in a monotonic decrease of the Curie temperature, with no evidence for the maximum expected within the impurity-band models. We find that the transition from the ferromagnetic to the paramagnetic state proceeds via the emergence of a superparamagnetic-like spin arrangement. This implies that carrier localisation leads to a phase separation into ferromagnetic and nonmagnetic regions, which we attribute to critical fluctuations in the local density of states, specific to the Anderson-Mott quantum transition. Comment: 8 pages, 3 figures
09/2009;
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[show abstract]
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ABSTRACT: X-ray absorption fine-structure (XAFS) measurements supported by {\em ab
initio} computations within the density functional theory (DFT) are employed to
systematically characterize Fe-doped as well as Fe and Si-co-doped films grown
by metalorganic vapour phase epitaxy. The analysis of extended-XAFS data shows
that depending on the growth conditions, Fe atoms either occupy Ga
substitutional sites in GaN or precipitate in the form of $\epsilon$-Fe$_3$N
nanocrystals, which are ferromagnetic and metallic according to the DFT
results. Precipitation can be hampered by reducing the Fe content, or by
increasing the growth rate or by co-doping with Si. The near-edge region of the
XAFS spectra provides information on the Fe charge state and shows its partial
reduction from Fe$^{+3}$ to Fe$^{+2}$ upon Si co-doping, in agreement with the
Fe electronic configurations expected within various implementations of DFT.
02/2009;
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[show abstract]
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ABSTRACT: We present results of numerical calculations of domain-wall resistance in the ferromagnetic semiconductor (Ga,Mn)As. We employ Landauer-Buttiker formalism and the tight binding method. Taking into account the full valence band structure we predict the magnitude of the domain-wall resistance without disorder and compare it to experimental values. Next we add disorder to the model and study numerically both small and large disorder regime. Comment: Comments accepted to: Proceedings of the 28th International Conference on the Physics of Semiconductors (AIP CP)
01/2007;
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[show abstract]
[hide abstract]
ABSTRACT: Recent works aiming at understanding magnetotransport phenomena in ferromagnetic III-V and II-VI semiconductors are described. Theory of the anomalous Hall effect in p-type magnetic semiconductors is discussed, and the relative role of side-jump and skew-scattering mechanisms assessed for (Ga,Mn)As and (Zn,Mn)Te. It is emphasized that magnetotransport studies of ferromagnetic semiconductors in high magnetic fields make it possible to separate the contributions of the ordinary and anomalous Hall effects, to evaluate the role of the spins in carrier scattering and localization as well as to determine the participation ratio of the ferromagnetic phase near the metal-insulator transition. A sizable negative magnetoresistance in the regime of strong magnetic fields is assigned to the weak localization effect.
07/2003;
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ABSTRACT: We describe recent progress and open questions in the physics of current-induced domain-wall displacement and creep in (Ga,Mn)As. Furthermore, the reasons are recalled why, despite strong disorder and localization, the p–d Zener model is suitable for the description of this system.
Journal of Magnetism and Magnetic Materials.
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Alberta Bonanni,
Michał Kiecana,
Clemens Simbrunner,
Tian Li,
Maciej Sawicki,
Matthias Wegscheider,
Martin Quast,
Hanka Przybylińska,
Andrea Navarro-Quezada,
Rafał Jakieła,
Agnieszka Wolos,
Wolfgang Jantsch, Tomasz Dietl
[show abstract]
[hide abstract]
ABSTRACT: We report on the metalorganic chemical vapor deposition of GaN:Fe and (Ga,Fe)N layers on c-sapphire substrates and their thorough characterization via high-resolution x-ray diffraction, transmission electron microscopy (TEM), spatially resolved energy dispersive x-ray spectroscopy (EDS), secondary-ion mass spectroscopy (SIMS), photoluminescence (PL), Hall-effect, electron-paramagnetic resonance (EPR), and magnetometry employing a superconducting quantum interference device (SQUID). A combination of TEM and EDS reveals the presence of coherent nanocrystals presumably FexN with the composition and lattice parameter imposed by the host. From both TEM and SIMS studies, it is stated that the density of nanocrystals and, thus the Fe concentration increases towards the surface. According to Hall effect measurements, electrons from residual donors are trapped by midgap Fe acceptor states in the limit of low iron content x≲0.4%, indicating that the concentration of Fe2+ ions increases at the expense of Fe ions in the 3+ charge state. This effect is witnessed by PL measurements as changes in the intensity of the Fe3+-related intraionic transition, which can be controlled by codoping with Si donors and Mg acceptors. In this regime, EPR of Fe3+ ions and Curie-like magnetic susceptibility are observed. As a result of the spin-orbit interaction, Fe2+ does not produce any EPR response. However, the presence of Fe ions in the 2+ charge state may account for a temperature-independent Van Vleck–type paramagnetic signal that we observe by SQUID magnetometry. Surprisingly, at higher Fe concentrations, the electron density is found to increase substantially with the Fe content. The coexistence of electrons in the conduction band and Fe in the 3+ charge state is linked to the gradient in the Fe concentration. In layers with iron content x≳0.4% the presence of ferromagnetic signatures, such as magnetization hysteresis and spontaneous magnetization, have been detected. A set of precautions has been undertaken in order to rule out possible sources of spurious ferromagnetic contributions. Under these conditions, a ferromagneticlike response is shown to arise from the (Ga,Fe)N epilayers, it increases with the iron concentration, it persists up to room temperature, and it is anisotropic—i.e., the saturation value of the magnetization is higher for in-plane magnetic field. We link the presence of ferromagnetic signatures to the formation of Fe-rich nanocrystals, as evidenced by TEM and EDS studies. This interpretation is supported by magnetization measurements after cooling in and without an external magnetic field, pointing to superparamagnetic properties of the system. It is argued that the high temperature ferromagnetic response due to spinodal decomposition into regions with small and large concentration of the magnetic component is a generic property of diluted magnetic semiconductors and diluted magnetic oxides showing high apparent Curie temperature.
Phys. Rev. B. 75(12).
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ABSTRACT: Various mechanisms that control the magnitude of the ferromagnetic interaction in semiconductors and their quantum structures are described. Two alternative approaches, the Ruderman–Kittel–Kasuya–Yosida (RKKY) and self-consistent models are recalled and their equivalence is demonstrated in the mean field approximation (MFA). It is shown how the long-range nature of the RKKY interactions in semiconductors, by making the thermal fluctuations of magnetization irrelevant, stabilizes an ordered phase and results in the validity of the MFA, even in the disordered magnetic systems of reduced dimensionality. The role of confinement and of the associated modifications in the density-of-states is examined and shown to be important. It is pointed out that disorder and carrier–carrier interactions may have a profound influence on the spin–spin coupling, particularly near the metal-to-insulator transition (MIT). The corresponding effects are analyzed in terms of the disordered Fermiliquid and Hubbard models, developed for the description of doped semiconductors on the metallic side of the MIT. Outstanding properties of 1D systems, resulting from peculiarities of their DOS, tendency toward spin-density formation and possible charge-spin separation are mentioned. The question concerning the role of non-scalar spin–spin interactions driven by spin-orbit coupling, notably in strained systems, is addressed. Finally, the important issue of control of magnetic properties by electrostatic gate potentials or illumination is presented.
Materials Science and Engineering: B.
