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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
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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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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).
4
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
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1991
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vol. 79
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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.
5
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Submilimeter Magnetospectroscopy on ZnFeSe

52%
Krevet R., Kempf P., Von Ortenberg M., Twardowski A.
Acta Physica Polonica A
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1991
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vol. 79
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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

Magnetic Anisotropy of V^{++} in CdS

52%
Herbich M., Mac W., Twardowski A., Demianiuk M.
Acta Physica Polonica A
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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.
7
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.
8
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.
9
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
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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.
10
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
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2006
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vol. 110
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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.
11
Content available remote

Exchange Interaction in ZnCrSe Semimagnetic Semiconductor

46%
Mac W., Nguyen K., Twardowski A., Gaj J., Demianiuk M.
Acta Physica Polonica A
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1992
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vol. 82
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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.
12
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
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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.
13
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
|
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.
14
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
|
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.
15
Content available remote

Magnetooptical Properties of GaAs:Mn

40%
Szczytko J., Stachow A., Mac W., Twardowski A., Becla P., Tworzydło J.
Acta Physica Polonica A
|
1996
|
vol. 90
|
issue 5
951-954
EN
Magnetoreflectance from GaAs:Mn was measured in the free exciton range for B ≤ 5 T and T = 2.0 K. Combining optical results with magnetization data the value for the exchange parameter N_{0}α-N_{0}β =-(2.3 ± 0.6) eV was obtained. Assuming that N_{0}α ≈ +0.2 eV, the ferromagnetic p-d exchange interaction was found with the magnitude N_{0}β ≈ +2.5 eV.
16
Publication available in full text mode
Content available

Observation of Magnetic Anisotropy in Bulk GaMnN:Mg Crystals

40%
Gosk J., Zając M., Kamińska M., Twardowski A., Grzegory I., Boć M., Porowski S.
Acta Physica Polonica A
|
2003
|
vol. 103
|
issue 6
665-669
EN
Magnetic properties of bulk wurtzite n-type GaMnN and highly resistive GaMnN:Mg monocrystals were studied for the magnetic field applied parallel and perpendicular to the crystal hexagonal c-axis. Magnetization of both types of samples reveals paramagnetic behavior. However, for n-type GaMnN isotropic magnetization was observed which is in agreement with Mn d^5 configuration. On the other hand, GaMnN co-doped with Mg shows large magnetic anisotropy which suggests Mn to be in nonspherical d^4 or d^3 configuration.
17
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EPR Spectra of Multispin Systems. Application to Poly-m-p-Aniline

40%
Juszyński P., Twardowski A., Gosk J., Kulszewicz-Bajer I., Pawłowski M., Dziatkowski K., Szczytko J.
Acta Physica Polonica A
|
2011
|
vol. 119
|
issue 5
717-719
EN
In this communication we present NiceMagnetic, software for simulations and analysis of EPR spectrum and SQUID measurements. Systems of arbitrary number N of spin-only magnetic moments (S ≤ 0 and L = 0) were considered within the lines of the Heisenberg and Zeeman Hamiltonians. Effective algorithms used for the computations are discussed. This includes modified secant method adapted to calculate resonance fields with demanded precision. Finally, application of NiceMagnetic to experimental SQUID magnetization and EPR spectra of poly-m-p-aniline, one of recently synthesized high spin polymers containing unpaired electrons will be presented. As a result, exchange integral between electron and nitrogen nuclei spins (ligand hyperfine interaction) J = 6.3 × 10^{-4} cm^{-1} was estimated.
18
Content available remote

MnAs Nanocrystals Embedded in GaAs

40%
Kwiatkowski A., Wasik D., Kamińska M., Twardowski A., Dłużewski P., Borysiuk J., Sadowski J.
Acta Physica Polonica A
|
2008
|
vol. 114
|
issue 5
1207-1211
EN
Magnetic properties of MnAs nanocrystals embedded in GaAs are analyzed in the frame of phenomenological model proposed by Sasaki for ferritin superparamagnets. Our calculations explain qualitatively experimental data of magnetization versus temperature, obtained according to zero-field-cooled and field-cooled protocols. They show dynamics of magnetization of MnAs nanocrystals in range of temperature from 10 K to 320 K. There is transition from state in which very slow dynamics is observed (frozen state) to state in which dynamics is fast (quasi-superparamagnetic state).
19
Content available remote

Magnetic Properties of EuS/PbS Semiconducting Structures

40%
Stachow-Wójcik A., Twardowski A., Story T., Dobrowolski W., Grodzicka E., Sipatow A.
Acta Physica Polonica A
|
1997
|
vol. 92
|
issue 5
985-988
EN
We report results of magnetization study of EuS/PbS superstructures with different thicknesses of magnetic and nonmagnetic layers. Reduction of ferromagnetic phase transition temperature was found with decreasing EuS thickness. Reasonable description of this effect is obtained within the model based on the mean field approximation.
20
Publication available in full text mode
Content available

Observation of Superconductivity in Highly Cr-Doped GaP Single Crystals

40%
Gosk J., Strzelecka G., Kowalski G., Hruban A., Kamińska M., Twardowski A., Gronkowski J.
Acta Physica Polonica A
|
2006
|
vol. 110
|
issue 2
189-193
EN
Studying bulk GaP, highly doped with Cr, and searching for possible ferromagnetic semiconductor in view of spintronic applications, we found superconducting behavior of this material unexpectedly. Magnetization techniques and X-ray diffraction were applied to study these crystals. Magnetization revealed superconducting features up to about 6 K and X-ray studies showed that superconductivity might be related to small size Cr precipitates.
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