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  1. 45 Twardowski A.

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1
Content available remote

Diluted Magnetic III-V Semiconductors

100%
Twardowski A.
Acta Physica Polonica A
|
2000
|
vol. 98
|
issue 3
203-216
EN
During recent years diluted magnetic semiconductors based on III-V compounds have been of considerable interest. In this respect we review the basic properties of these materials, which are nearly exclusively Mn-based systems, such as GaMnAs, InMnAs, GaMnSb, and GaN:Mn. We discuss the nature of Mn impurity. Different Mn centers are considered and experimental pieces of evidence suggesting the dominating role of Mn (d^{5}) configuration are given. Then we analyze s, p-d exchange, together with resulting magnetooptical properties (in particular absorption edge slitting for heavily p-type GaMnAs). The coupling between Mn ions (d-d exchange) and ferromagnetic ordering observed in InMnAs and GaMnAs is the next subject. Some mechanisms responsible for this ordering are presented. Finally we discuss transport properties and some selected problems of quantum structures based on III-V diluted magnetic semiconductors.
2
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Cr-Based II-VI Semimagnetic Semiconductors

100%
Twardowski A.
Acta Physica Polonica A
|
1995
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vol. 87
|
issue 1
85-93
EN
We review the magnetic and optical properties of recently grown Cr­-based II-VI semimagnetic semiconductors. We focus on two features of these materials which distinguish them from other semimagnetic semiconductors: the particular magnetic behaviour of these crystals, resulting from the Cr^{++} ion energy structure determined by a strong, static Jahn-Teller effect, and a ferromagnetic p-d exchange interaction, which is unique for II-VI semimag­netic semiconductors.
3
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Magnetic Anisotropy of V^{++} in CdS

52%
Herbich M., Mac W., Twardowski A., Demianiuk M.
Acta Physica Polonica A
|
1996
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vol. 90
|
issue 4
809-812
EN
The magnetization measurements at magnetic field up to 6 T obtained from newly grown hexagonal Cd_{1-x}V_{x}S (x ≈ 0.0004) are presented. The strong anisotropy of magnetization is observed at low temperatures (1.6 < T < 20 K). The data are well described by the crystal field model calculations taking into account static trigonal Jahn-Teller distortion and spin-orbit coupling.
4
Content available remote

s,p-d Exchange Constants of CdFeSe Semimagnetic Semiconductor

52%
Arciszewska M., Pakuła K., Perez I., Twardowski A.
Acta Physica Polonica A
|
1991
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vol. 79
|
issue 2-3
345-348
EN
We report determination of s, p-d exchange constants for hexagonal CdFeSe combining exciton splitting and magnetization measurements performed on the same samples.
5
Content available remote

Submilimeter Magnetospectroscopy on ZnFeSe

52%
Krevet R., Kempf P., Von Ortenberg M., Twardowski A.
Acta Physica Polonica A
|
1991
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vol. 79
|
issue 2-3
381-383
EN
We report FIR laser spectroscopy study of Zn_{1-x}Fe_{x}Se (x < 0.06) Semimagnetic Semiconductor at the temperature range of 2-26 K and magnetic fields up to 18T.
6
Content available remote

The Influence of Jahn-Teller Effect on Magnetic Properties of Fe-Based Semimagnetic Semiconductors

52%
Lebeckl K., Twardowski A., Liro Z.
Acta Physica Polonica A
|
1991
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vol. 79
|
issue 2-3
373-375
EN
We report results of calculations of magnetic properties (specific heat, magnetization) of Fe-based Semimagnetic Semiconductors including Jahn-Teller coupling between Fe electronic states and phonons.
7
Content available remote

Magnetization of Pb_{1-x}Cr_{x}Te Semimagnetic Semiconductor

52%
Mac W., Story T., Twardowski A.
Acta Physica Polonica A
|
1995
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vol. 87
|
issue 2
492-494
EN
We present experimental studies of magnetization of Pb_{1-x}Cr_{x}Te (x ≤ 0.01) crystals. The reasonable description of the data is obtained for a composition of x ≤ 0.001 using Cr^{+++} model (Brillouin type paramagnetism S = 3/2).
8
Content available remote

Specific Heat of Cr-Based Semimagnetic Semiconductors

46%
Twardowski A., Eggenkamp P., Mac W., Swagten H., Demianiuk M.
Acta Physica Polonica A
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1993
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vol. 84
|
issue 3
563-565
EN
Specific heat of ZnCrSe and ZnCrS was measured for 1.5 < T < 10 K and B < 3 T. The data were interpreted using "crystal field model" taking into account tetrahedral crystal field, tetragonal Jahn-Teller distortion and spin-orbit interaction.
9
Content available remote

Exchange Parameters of CdCrS Semimagnetic Semiconductor

46%
Herbich M., Mac W., Twardowski A., Ando K., Demianiuk M.
Acta Physica Polonica A
|
1997
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vol. 92
|
issue 4
774-776
EN
Free exciton magnetoreflectance and magnetization of Cd_{1-x}Cr_{x}S (0.0024 < x < 0.0031) were measured at T = 2.0 K and magnetic field up to 5 T. Combining the heavy hole exciton splitting with the magnetization data the exchange integral N_{0}β = +0.46±0.04 eV was evaluated within the framework of mean field approximation.
10
Content available remote

Superconductivity Study of GaN Highly Doped by Transition Metals

46%
Gosk J., Boćkowski M., Tokarczyk M., Kowalski G., Twardowski A.
Acta Physica Polonica A
|
2013
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vol. 124
|
issue 5
877-880
EN
GaN:Cr and GaN:Fe single crystals as well as GaN:Mn micropowders highly doped by transition metals were grown to investigate low temperature superconductivity. Magnetic measurements revealed type I superconductivity with T_{C} ≈ 6 K and H_{C} ≈ 600 Oe, identical for all compounds and also identical to that observed before in GaP:Cr and GaAs:Cr. The presence of amorphous inclusions of gallium may explain existing superconductivity as a result of a phase transition leading to β-Ga during cooling down of the sample. Since the observed parameters are close to those characteristic for superconducting Ga(II) this possibility could not be ruled out.
11
Content available remote

High-Field EPR of Zn_{1-x}Cr_{x}Te

46%
Boonman M. E. J., Mac W., Wittlin A., Twardowski A., Demianiuk M.
Acta Physica Polonica A
|
1995
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vol. 88
|
issue 5
829-832
EN
High magnetic field electron paramagnetic resonance experiments have been performed on Zn_{1-x}Cr_{x}Te covering the energy range 1-7 cm^{-1} in fields up to 20 T at T = 1.2 K. The static magnetic field was oriented along the (100), (110) and (111) crystallographic axes of the sample. Pronounced absorption lines for intra-chromium transitions have been observed for these different orientations, revealing a strong anisotropy due to a static Jahn-Teller distortion. The measured low energy level structure of the Cr^{++} ion can be described by a cubic crystal field model including this distortion of the Cr centers.
12
Publication available in full text mode
Content available

Asymmetric Magnetization Reversal in the Exchange-Biased MnO/(Ga,Mn)As Heterostructure Studied by Ferromagnetic Resonance

46%
Dziatkowski K., Ge Z., Liu X., Furdyna J., Twardowski A.
Acta Physica Polonica A
|
2006
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vol. 110
|
issue 3
319-324
EN
We report ferromagnetic resonance study of the magnetization reversal in the exchange-coupled MnO/(Ga,Mn)As system. The low-field parts of ferromagnetic resonance spectra measured along [1/10] and [100] directions of (Ga,Mn)As were combined into hysteresis loops, which under field-cooling conditions similarly to SQUID loops are shifted toward negative magnetic fields. The magnetization reversal process revealed by the loops occurred remarkably asymmetric for both sample configurations.
13
Content available remote

Exchange Interaction in ZnCrSe Semimagnetic Semiconductor

46%
Mac W., Nguyen K., Twardowski A., Gaj J., Demianiuk M.
Acta Physica Polonica A
|
1992
|
vol. 82
|
issue 5
902-904
EN
Magnetoreflectance and magnetization of ZnCrSe were measured for B ≤ 5 T and T = 2 K. A linear dependence between exciton splitting and magnetization was found. The s, p-d exchange parameter was estimated: N_{0}α-N_{0}β=0.67eV.
14
Content available remote

Influence of Local Potential on Exciton Splitting in Highly Diluted Cd_{1-x}Mn_{x}Te

46%
Kłopotowski L., Herbich M., Mac W., Stachow-Wójcik A., Twardowski A.
Acta Physica Polonica A
|
1997
|
vol. 92
|
issue 4
837-840
EN
We investigated magnetoreflectance and magnetization of highly diluted bulk Cd_{1-x}Mn_{x}Te crystals 0.2% ≤ x ≤ 10%. The exchange constant in terms of mean field approximation and virtual crystal approximation (the ratio of the heavy hole exciton splitting to mean spin per unit cell) was evaluated and found x-dependent. This deviation from the mean field approximation and virtual crystal approximation prediction is caused by the local potential introduced by Mn ions. We discuss the problem within a Wigner-Seitz approach and within a model of magnetic and chemical disorder based on the alloy theory.
15
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Time-Resolved Studies of Gallium Nitride Doped with Gadolinium

40%
Witek B., Wysmołek A., Kamińska M., Twardowski A., Potemski M., Boćkowski M., Grzegory I.
Acta Physica Polonica A
|
2008
|
vol. 114
|
issue 5
1425-1430
EN
Time-resolved photoluminescence experiments on high quality bulk GaN doped with Gd are presented. It was found that the decay time of Gd-related transitions observed for 4.2 K around 1.78 eV is of about 3 ms. Such a long decay time strongly supports the identification of this emission band as due to transitions between Gd³+(4f⁷) levels. The decay time measured for Gd-related transitions observed in the UV spectral range, close to the GaN band-gap, was found to be much faster than 1 μs. This suggests that these emission lines could hardly be correlated with internal transitions within Gd³+(4f⁷). Possible origin of the Gd-related UV luminescence is discussed.
16
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Faraday Rotation and Photoionization of ZnCrSe Semimagnetic Semiconductor

40%
Mac W., Nguyen K., Gaj J., Twardowski A., Demianiuk M., Stachow-Uciakowska A.
Acta Physica Polonica A
|
1993
|
vol. 84
|
issue 3
567-570
EN
The magnetoabsorption and the Faraday rotation of ZnCrSe were measured for B ≤ 5 T and T = 2 K in the region of photon energies lower than the fundamental absorption edge. The circular dichroism of the photoionization absorption bane was observed and analyzed. The Faraday rotation was interpreted as due to the sum of contributions from interband and photoionization optical transitions.
17
Content available remote

On the Question of Ferromagnetism in Proton and He-Irradiated Carbon

40%
Szczytko J., Juszyński P., Teliga L., Twardowski A., Stonert A., Ratajczak R., Korman A.
Acta Physica Polonica A
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2008
|
vol. 114
|
issue 5
1387-1390
EN
We tried to repeat the observation of the ferromagnetic response in proton and He-irradiated carbon made by the group of Esquinazi et al. We used He^+ and H^+ beams focused on graphite sample. The amount of charge deposited in the sample was comparable to the amount of charge used by Esquinazi. Magnetic measurements were performed in SQUID magnetometer. The magnetization of the samples before and after irradiation was compared. We did not observe any ferromagnetic enhancement of magnetization of our irradiated samples. Even if experiment was not the same as Esqinazi's one, we can exclude some of the mechanisms of ferromagnetism proposed by Esquinazi.
18
Content available remote

Magnetization of GaMnN Ceramics Prepared from Nanopowders by an Anaerobic Synthesis and High-Pressure High-Temperature Sintering

40%
Gosk J., Drygaś M., Janik J., Gierlotka S., Pałosz B., Twardowski A.
Acta Physica Polonica A
|
2016
|
vol. 129
|
issue 1a
A-103-A-106
EN
Herein, we report a study on magnetic properties of GaMnN ceramics prepared by no additive high-pressure high-temperature sintering of a range of nanopowders, the latter made via an anaerobic synthesis method in the Ga/Mn bimetallic system at various nitridation temperatures and different levels of initial Mn concentration. Measurements of the magnetization as a function of temperature and magnetic field for the ceramics and parent nanopowders showed a typical paramagnetic behavior. Antiferromagnetic interactions between Mn-ions incorporated in the GaN lattice, GaMnN, were revealed and shown to be much stronger in the ceramics than in the respective nanopowders. In addition, in all of these materials an antiferromagnetic contribution originating from a residual Mn₂SiO₄ by-product was also observed. The highest calculated Mn concentration in the nanopowders reached 3.4 at.%. Complex mixtures of gallium nitride polytypes with multimodal particle size distributions in the nanosized range (small nano: 2-8 nm, large nano: 35-60 nm) were converted upon sintering to the single hexagonal GaN phase with average crystallite sizes of 40-80 nm and higher. For the optimal 700°C-treated materials, the Mn concentration in the parent GaMnN nanopowder was 3.2 at.% whereas in the derived ceramics it amounted to 5.5 at.%. At the same time, contributions of the adventitious Mn₂SiO₄ by-product significantly decreased upon sintering.
19
Content available remote

Effects of the Spin-Dependent Potential fluctuations on Free Exciton Zeeman Splitting of Zn_{1-x}Mn_{x}Se

40%
Herbich M., Mac W., Stachow A., Twardowski A., Tworzydło J., Demianiuk M.
Acta Physica Polonica A
|
1996
|
vol. 90
|
issue 4
813-816
EN
Magnetoreflectance of free exciton and magnetization in Zn_{1-x}Mn_{x}Se (0.0007 < x < 0.12) were measured. The evaluated proportionality factor between the exciton splitting and the magnetization showed a strong concentration dependence. This effect cannot be explained within the mean field approximation and the virtual crystal approximation and it was described within the model including chemical and magnetic disorder.
20
Publication available in full text mode
Content available

Photoluminescence Study of Bulk GaN Doped with Beryllium

40%
Jaworek M., Wysmołek A., Kamińska M., Twardowski A., Boćkowski M., Grzegory I.
Acta Physica Polonica A
|
2005
|
vol. 108
|
issue 4
705-710
EN
Photoluminescence of bulk GaN:Be grown by high pressure method is presented. The investigated crystals show well-resolved photoluminescence due to free and bound excitons similar to that observed for homoeptitaxial GaN layers. In addition to the excitonic transitions, pronounced luminescence band at 3.38 eV, due to Be acceptor, is observed. It was found that temperature behavior of this emission is typical of donor- and conduction band-acceptor transitions. The optical activation energy of Be acceptor is obtained to be of 60±15 meV.
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