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1
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Correlation between Magnetic and Electronic Properties of Sn_{1-x}Gd_{x}Te

100%
Łusakowski A.
Acta Physica Polonica A
|
1995
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vol. 87
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issue 1
197-200
EN
Magnetic susceptibility, electron paramagnetic resonance and transport properties of Sn_{1-x}Gd_{x}Te with 0.04 < x < 0.07 and hole concentrations in the range from 0.7 × 10^{20} to 16 × 10^{20} cm^{-3} were investigated. After annealing of the Sn_{1-x}Gd_{x}Te samples with x < 0.05 in Sn vapor their hole concentration decreased from 5 × 10^{20} cm^{-3} to about 3 × 10^{20} cm^{-3} and their paramagnetic Curie temperature increased a few times. In samples with x > 0.05 no significant change in the magnetic properties was observed after annealing, even at lower hole concentrations. The results can be explained by assuming that an indirect exchange interaction, 4f-5d-band electrons, is responsible for the coupling among Gd ions.
2
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On the Possible Analogy between Spin-Glasses and Systems

100%
Łusakowski A.
Acta Physica Polonica A
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1992
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vol. 82
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issue 4
620-622
EN
An analogy between a spin-glass and a material containing DX centers is postulated. The basis of the analogy is the formal identity of the Hamiltonians of these two systems. A correspondence between parameters of the spin-glass and of the DX system is established. Experiments which could support the above idea are proposed.
3
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Free Carrier Concentration Dependence of Gd-Gd Exchange Constant in SnTe:Gd

100%
Łusakowski A.
Acta Physica Polonica A
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1995
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vol. 88
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issue 5
1018-1022
EN
In some samples of SnTe:Gd the measured paramagnetic Curie temperature strongly depends on the Fermi level position. The semi-quantitative theory which explains this effect is presented. The theory is based on Kasuya's model of exchange interaction between two 4f spins of rare earth ions. In this model the interaction between 4f electrons and the valence band goes via Gd atomic 5d orbitals which, contrary to the 4f orbitals are much more extended in space. The exchange integral and the paramagnetic Curie temperature dependence on the Fermi level position are calculated.
4
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Possible Long Time Tails in Current-Current Correlation Function for Two-Dimensional Electron Gas in Random Magnetic Field

64%
Łusakowski A., Turski Ł. A.
Acta Physica Polonica A
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1997
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vol. 92
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issue 6
1109-1115
EN
We consider a two-dimensional degenerate electron gas in the presence of perpendicular random magnetic field. The magnetic field disorder which is assumed to be Gaussian is characterized by two parameters. The first is proportional to the amplitude of local magnetic field fluctuations. The second one characterizes the disorder on the longer scale - we call it the screening length. Using the Kubo formula for the conductivity we have found a class of diagrams which leads to the long time tails in the current-current correlation function. For short times in comparison with the diffusion time corresponding to the screening length this function behaves like logarithm of time, for longer times it decays like t^{-1} which of course brings into question the diffusive character of the behavior of the charged particle in random magnetic field widely assumed in the literature.
5
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Influence of Spin-Orbit Interaction on Band Structure and Elastic Properties of PbTe

64%
Radzyński T., Łusakowski A.
Acta Physica Polonica A
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2009
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vol. 116
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issue 5
954-955
EN
Inclusion of spin-orbit interaction in density functional theory calculations of PbTe band structure leads to wrong results concerning the symmetry of valence and conduction band wave functions. Using two ab initio codes: OpenMX and Quantum Espresso we show that changing the spin-orbit coupling strength in pseudopotentials for Pb it is possible to obtain proper energy gap and wave functions of proper symmetries.
6
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Magnetic Anisotropy in GeMnTe - ab initio Calculations

64%
Łusakowski A., Bogusławski P.
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vol. 126
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issue 5
1177-1179
EN
Density functional theory calculations of the energy of magnetic anisotropy in GeMnTe were performed using OpenMX package with fully relativistic pseudopotentials. The discussion of microscopic origin of magnetic anisotropy was conducted. The main conclusion is that the magnetic anisotropy is caused by interplaty between spin-orbit and Coulomb interactions. The discussion includes also the influence of the free carriers concentration on the amplitude of energy of magnetic anisotropy. The important role of the chemical disorder is also pointed out.
7
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Distance Dependence of the Mn-Mn Exchange Interaction in IV-VI Semimagnetic Semiconductors

64%
Łusakowski A., Górska M.
|
EN
In the present paper we calculate the exchange interaction between two manganese ions in IV-VI semiconductors with the rocksalt structure. The method of calculations is based on the fourth order perturbation theory with respect to hybridization between band states and localized d orbitals of Mn ions. This hybridization is described by three Harrison integrals: V_{pdσ},V_{pdπ}, and V_{sdσ}. The band states of IV-VI semiconductor are obtained from the semiempirical tight binding model built from s and p orbitals of cations and anions. The resulting exchange term in the Hamiltonian is of the form -Σ_{i,j=xyz}J_{ ij}Ŝ^1_iŜ^2_j, however nondiagonal terms of the exchange integral tensor J_{i≠j} are very small. The dependence of J_{ij} on the Mn-Mn distance is non-monotonic. We also discuss the influence of the local crystal deformations on the exchange integral.
8
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Analysis of Chemical Disorder in Pb_{1-x}Ge_{x}Te

64%
Radzyński T., Łusakowski A.
Acta Physica Polonica A
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2008
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vol. 114
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issue 5
1317-1322
EN
The PbGeTe crystal lattice is locally deformed due to different ionic radii of cations and their random placement in the lattice. It is important to quantitatively characterize such microscopic disorder because local deformations have significant influence on the properties of magnetic ions introduced to such crystal. The simples method to study microscopic disorder is to consider purely classical model of the lattice in which neighboring atoms are connected by springs with properly chosen spring constant and equilibrium lengths. In the present work, using ab initio methods we discuss applicability of the springs-atoms model to the real crystal. We also explain results of extended X-ray absorption fine structure experiments performed on PbGeTe mixed crystals.
9
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On the Possible Origin of Large Value of Magnetic Anisotropy Constant in PbSnMnTe Mixed Crystals

51%
Łusakowski A., Story T., Gałązka R.
Acta Physica Polonica A
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1993
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vol. 84
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issue 4
829-831
EN
In PbSnMnTe crystals with the concentration of conducting holes of the order of 10^{21} cm^{-3}, the magnetic anisotropy constant K measured in ferromagnetic resonance experiments is nearly one order of magnitude larger than expected. In the present paper we investigate a contribution ΔK to the anisotropy constant, origin of which is the s-d interaction and the spin-orbit coupling.
10
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Magnetic Anisotropy in (Ge,Mn)Te Layers

51%
Knoff W., Łusakowski A., Wołoś A., Story T.
Acta Physica Polonica A
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2015
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vol. 127
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issue 2
404-406
EN
Ferromagnetic resonance study of magnetic anisotropy is presented for thin layers of IV-VI diluted magnetic semiconductor (Ge,Mn)Te with Mn content of 12 and 21 at.% grown by molecular beam epitaxy on BaF₂ (111) substrates. The layers with low Mn content grow in the rhombohedral crystal structure and exhibit perpendicular magnetic anisotropy whereas the layers with Mn content higher than about 20 at.% are of cubic (rock-salt) structure and show regular easy-plane type magnetic anisotropy. The quantitative analysis of the angular dependence of the ferromagnetic resonant field is performed taking into account the magnetic energy contributions due to rhombohedral distortion (axial term along the (111) growth direction of the layer) and the crystal field terms allowed for ferromagnetic systems of rhombohedral symmetry.
11
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Magnetic Specific Heat of Pb_{1-x}Eu_xTe

39%
Górska M., Łusakowski A., Jędrzejczak A., Gołacki Z., Anderson J., Balci H.
Acta Physica Polonica A
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2004
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vol. 105
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issue 6
631-636
EN
The temperature dependence of the magnetic specific heat was studied experimentally and theoretically in the semimagnetic semiconductor Pb_{1-x}Eu_xTe for x=0.027 and x=0.073, over the temperature range from 0.5 K to 15 K, in magnetic fields up to 2 T. In zero magnetic field at about 2 K there was a broad maximum in the magnetic specific heat, which was much higher than that predicted by the model of superexchange interaction between nearest neighbors; the maximum values increased with magnetic field. The experimental data were analyzed in the framework of a model which takes into account the spin splitting of the ground state of a single Eu2+ ion in the presence of local lattice distortions in the Pb_{1-x}Eu_xTe mixed crystal. The model describes well the experimental data, especially for lower x-values, where the contribution from singlets dominates.
12
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Raman Spectroscopy of Co-Doped ZnO Bulk Crystals

39%
Szuszkiewicz W., Morhange J., Gołacki Z., Łusakowski A., Schumm M., Geurts J.
Acta Physica Polonica A
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2007
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vol. 112
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issue 2
363-368
EN
Low-temperature Raman spectra of a bulk mixed crystal Zn_{1-x}Co_xO (x = 0.016) are shown and analyzed. Besides the common modes of the ZnO host lattice, electronic transitions related to the presence of Co^{2+} ions at the Zn sites are observed. In spite of the low Co concentration the presence of collective excitations corresponding to Co and CoO precipitates is also revealed.
13
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High Field Magnetization of Sn_{1-x}Gd_{x}Te

39%
Górska M., Anderson J., Wolters C., Łusakowski A., Story T., Gołacki Z.
Acta Physica Polonica A
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1998
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vol. 94
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issue 2
347-350
EN
The magnetization of p-type Sn_{1-x}Gd_{x}Te with x up to 0.045 and the hole concentration, p, varying from 2.7 to 8.3×10^{20} cm^{-3} has been measured in magnetic fields up to 27 T, at the temperatures 4.2 and 1.3 K. The data were fitted to a magnetization equation with single-ion and pair terms. From comparison of the exchange parameters determined from the high-field magnetization with those previously obtained from the high-temperature magnetic susceptibility it was found that in samples with p>5×10^{20} cm^{-3} the exchange was of a short-range type, while in samples with a lower carrier concentration the long-range exchange mechanism was observed.
14
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Is the Magnetism of HgSe:Fe Clearly Understood?

33%
Dybko K., Łusakowski A., Kossut J., Arciszewska M., Linden P. J. E. M. v., Wittlin A., Perenboom J. A. A. J., Mycielski A.
Acta Physica Polonica A
|
1995
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vol. 87
|
issue 1
193-196
EN
The high magnetic field magnetization measurements of low composition Hg_{1-x}Fe_{x}Se (0.0002 < x < 0.003) at temperatures ranging from 1.6 K to 40 K and in magnetic fields up to 20 T are reported. The magnetization in a sample containing only Fe^{3+} ions (x = 0.0002) is described by the Brillouin function (j = 5/2). The magnetization in the sample codoped with Ga, containing only Fe^{2+} is well described in the model of isolated Fe^{2+} ions. The latter model takes into account the tetrahedral crystal field, the spin-orbit interaction and the Zeeman term. On the other hand, the magnetization of sample with both Fe^{3+} and Fe^{2+} present simultaneously (x = 0.003) is shown to be different from a simple additive superposition of the contributions due to Fe^{3+} and Fe^{2+} subsystems. The possible origin of this discrepancy is discussed.
15
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Transport and Magnetic Study of Gd Ions in Pb_{1-y}Sn_{y}Te

33%
Story T., Arciszewska M., Łazarczyk P., Łusakowski A., Górska M., Dobrowolski W., Witkowska B., Grodzicka E., Gałązka R.
Acta Physica Polonica A
|
1997
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vol. 92
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issue 5
997-1000
EN
Electric conductivity, Hall effect and magnetic susceptibility of Pb_{1-x-y} Sn_{y}Gd_{x}Te mixed crystals with 0.13 ≤ y ≤ 0.93 and 0.001 ≤ x ≤ 0.04 were experimentally studied over the temperature range 4K ≤ T ≤ 300 K. The incorporation of Gd ions into the Pb_{1-y}Sn_{y}Te matrix results in semi-metallic n-type conductivity of the crystals with y < 0.6. For crystals with y > 0.6 one observes only semi-metallic p-type conductivity. We present a model explaining these results in terms of the Sn composition dependence of the location of Gd^{2+/3+} level with respect to the band edges of PbSnGdTe.
16
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Magnetic Properties of Sn_{1-x}Gd_{x}Te (0.002 ≤ x ≤ 0.09)

33%
Story T., Arciszewska M., Dobrowolski W., Gołacki Z., Górska M., Grodzicka E., Łusakowski A., Dynowska E., Witkowska B.
Acta Physica Polonica A
|
1996
|
vol. 90
|
issue 5
935-938
EN
Magnetic, transport and structural properties of bulk crystals of Sn_{1-x}Gd_{x}Te with Gd content 0.002 < x < 0.09 and varying carrier concentrations obtained by an isothermal annealing were studied in the temperature range T = 1.5 - 80 K. We found the effect of resonant increase in antiferromagnetic spin-spin exchange interactions in the crystals with 0.025 ≤ x ≤ 0.05. No effect was found in crystals either with higher (x > 0.05) or with lower (x < 0.025) Gd concentration. The observed Gd composition dependence of the magnetic and transport properties of SnGdTe can be explained in a proposed model relating these experimental properties to the Gd composition induced shift of the position of Gd^{3+/2+} level with respect to the top of the valence band of SnGdTe.
17
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Low-Frequency Raman Spectrum οf Bulk Zn_{0.984}Co_{0.016}O Crystal

33%
Szuszkiewicz W., Łusakowski A., Morhange J., Gołacki Z., Arciszewska M., Brodowska B., Kanehisa M., Klepka M., Dobrowolski W.
Acta Physica Polonica A
|
2009
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vol. 116
|
issue 1
103-106
EN
The influence of possible presence of Co^{2+} ion pairs in a bulk Zn_{1-x}Co_{x}O mixed crystal on the low-frequency part of the Raman spectrum is discussed. Two effects can be taken into account in the theoretical considerations when analyzing the energy level scheme corresponding to Co ions. The first is a local lattice deformation in the vicinity of Co^{2+} ion due to a presence of the second ion, smaller than the host ZnO lattice cation. Such deformation creates a trigonal field, which can only slightly modify the energy levels of Co^{2+} ion. The second effect, which results from an antiferromagnetic superexchange interaction between two Co^{2+} ions is responsible for a new set of energy levels. The Raman data taken at low temperature on the sample corresponding to the composition x = 0.016 demonstrated the presence of two structures at about 6 cm^{-1} and 13 cm^{-1}. These structures may be interpreted as electronic transitions between the ground state and the first excited state of a single Co^{2+} ion in the substitution site of ZnO lattice and as a similar transition for Co^{2+} ion pair, respectively.
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