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1
Content available remote

Temperature Dependence of Magnetization in Transition Metal Ultrathin Films of Various Thicknesses

100%
Świrkowicz R.
|
|
issue 5
689-697
EN
Temperature dependence of the local magnetization in the spin-wave regime is calculated within the framework of the multiband model for ultrathin films consisting of 5, 7 and 9 monolayers. The temperature range in which the calculated results can be fitted to the Bloch T^{3/2} law is found in all cases. The Bloch coefficient Bay corresponding to temperature dependence of the average film magnetization is found to be proportional to 1/D, where D is the thickness of the film. The spatial distribution of the local magnetization is obtained. The Bloch coefficient corresponding to the surface layer appears to be greater than the one corresponding to the central layer, namely B_{s} > B_{c}. The ratio B_{s}/B_{c} is increasing with an increase of the film thickness. The calculated results are well consistent with experimental ones obtained for ultrathin films of various thicknesses.
2
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Ground-State Properties and Spin Waves in Ultrathin Fe Films Covered with Magnetic and Nonmagnetic Materials

100%
Świrkowicz R.
Acta Physica Polonica A
|
1995
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vol. 88
|
issue 2
381-388
EN
Properties of ultrathin films consisted of 5 and 7 atomic layers of Fe and covered with magnetic (Co) or nonmagnetic (Cu, Ag) materials are investigated within the framework of the multiband model. Ground-state results consistent with those known from ab initio approaches are obtained. Spin waves are studied in the random phase approximation with the use of the susceptibility method. Amplitudes and dispersion relations are calculated. Two acoustic modes with amplitudes enhanced at the interface are found.
3
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Dynamic Susceptibility in Thin Films with Antiferromagnetic Coupling between Layers

100%
Świrkowicz R.
Acta Physica Polonica A
|
1996
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vol. 90
|
issue 6
1215-1226
EN
The dynamic susceptibility of the system with antiferromagnetic coupling between layers is investigated within the framework of the multiband model using the equation of motion with random phase approximation. Calculations are performed in the mixed Bloch-Wannier representation and a general form for χ is found. The susceptibility can be written in terms of two-particle Green's functions expressed in the local coordinate system with the z axis aligned along the local magnetization. The expression depends on an angle between the magnetization direction in a given layer and the crystal axis. Preliminary numerical calculations are performed for two systems: ultrathin Cr film and Fe/Cr multilayer structure. Imaginary part of the susceptibility corresponding to different layers is calculated and spin waves are discussed.
4
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Spin Waves and Temperature Dependence of Magnetization in Nickel Thin Films Covered with Iron and Copper

100%
Świrkowicz R.
Acta Physica Polonica A
|
1993
|
vol. 84
|
issue 2
353-363
EN
Energy spectrum and amplitudes of spin waves, as well as the temperature dependence of the local magnetization in Ni thin films covered with Fe and Cu overlayers are investigated within the framework of the itinerant electron approach. Two acoustic surface modes strongly localized in Fe over-layers are found. Energies of these modes are very close to each other but they are considerably lower than in Ni films with free surfaces. The presence of the strongly localized low-energy modes leads to fast decrease in the surface magnetization as compared to the central layer one. The quasi-linear temperature dependence of the surface magnetization is found whereas the central layer magnetization follows a quasi T^{3/2} law. On the other hand, in films covered with Cu the temperature dependence of the magnetization can be fitted to the Bloch law for all layers. Inside the film the Bloch coefficient weakly depends on the layer index but it rapidly decreases at the surface in the Cu layers.
5
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Electron Transport through Double Quantum Dot System with Inter-Dot Coulomb Interaction

64%
Sztenkiel D., Świrkowicz R.
Acta Physica Polonica A
|
2007
|
vol. 111
|
issue 3
361-372
EN
A theoretical approach to a problem of electron transport through double quantum dot systems based on non-equilibrium Green function formalism using equation of motion method is presented. I-V characteristics and differential conductance are calculated and discussed in detail in the intermediate regime with tunneling rate between the quantum dots comparable to coupling constants with external electrodes. Effects of inter-dot Coulomb correlations are studied for various values of interaction parameter U. It is shown that the interaction influences transport properties in a pronounced way and apart from the simple Coulomb blockade additional effects can be obtained. When energy levels of two quantum dots are not aligned, the asymmetry in conductance characteristics is closely related to a voltage dependence of population numbers in both quantum dots. For a one bias polarization electrons are well localized in quantum dots in a low voltage region, whereas for the opposite one they are partly delocalized.
6
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Electrical and Heat Currents in Nanoscopic System with Ferromagnetic Electrodes of Non-Collinear Magnetic Moments

64%
Wierzbicki M., Świrkowicz R.
Acta Physica Polonica A
|
2012
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vol. 121
|
issue 5-6
1073-1075
EN
Thermoelectric properties of nanoscopic system composed of a single-level quantum dot attached to ferromagnetic electrodes are studied with the use of the non-equilibrium Green function formalism. Electrical and heat currents flowing through the system under temperature gradient and bias voltage are calculated in the non-linear regime. Both charge and heat currents strongly depend on configuration of magnetic moments in the leads. TMR ratio is mostly positive, but in a region of small bias voltages, where the electron flow is determined by the temperature gradient, electrical current in parallel and antiparallel configurations flows in opposite directions leading to considerable negative TMR.
7
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Electronic Properties and Dynamic Susceptibility of MnTe Systems

64%
Wilczyński M., Świrkowicz R.
Acta Physica Polonica A
|
1997
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vol. 92
|
issue 5
1048-1050
EN
Thin MnTe films are investigated. Electronic structure, local density of states and local magnetic moments are calculated for systems with ferromagnetic and antiferromagnetic order. Spin-dependent modifications of the density of states near the surface are results of the changes in p-d hybridization. Magnetic longitudinal and dielectric susceptibilities are calculated.
8
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Electron Transport through Double Quantum Dot: from SU(4) Kondo to SU(2) Symmetry

64%
Sztenkiel D., Świrkowicz R.
Acta Physica Polonica A
|
2007
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vol. 112
|
issue 2
387-393
EN
Electron transport across two capacitively coupled quantum dots in a parallel geometry is theoretically studied in the non-linear response regime with spin and orbital degrees of freedom taken into account and the Kondo effect induced by on-site and inter-dot Coulomb correlations is analyzed. For a system with each dot symmetrically coupled to a separate set of electrodes a well-defined spin and orbital contributions to zero-bias Kondo resonance are obtained. The Kondo peak splits if spin and/or orbital degeneracies are removed. A suppression of the orbital Kondo anomaly due to orbital asymmetry and channel mixing effects is discussed.
9
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Electron Transport through Double Quantum Dots with Interdot Coulomb Repulsion

64%
Sztenkiel D., Świrkowicz R.
Acta Physica Polonica A
|
2006
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vol. 110
|
issue 3
389-394
EN
Electron transport through a system of two quantum dots connected in series is studied theoretically with the use of non-equilibrium Green function formalism based on the equation of motion method. Each dot is described by the one-level Anderson Hamiltonian and interdot Coulomb interactions in the form of the Hubbard-like term are taken into account. The electric current and occupation numbers are calculated with the use of two different approaches. The results of the methods are compared and discussed in detail. Strong asymmetry of I-V characteristics with respect to bias voltage reversal are obtained when energy levels of the dots are not aligned.
10
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Electronic Transport through the Double-Dot System

64%
Świrkowicz R., Sztenkiel D.
Acta Physica Polonica A
|
2004
|
vol. 105
|
issue 6
523-528
EN
Electron tunnelling through two quantum dots in series is theoretically studied. A limit of intermediate coupling between the dots is considered. The non-equilibrium Green function formalism is used to calculate electric current and mean number of electrons accumulated on the dots. Lesser and retarded Green functions are calculated in the Hartree-Fock approximation with the use of the equation of motion method. Current flowing through the system calculated in dependency on gate voltages shows two resonant peaks, each peak with two additional shoulders. I-V characteristics and differential conductance in a resonance and out of resonance cases are calculated and discussed.
11
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Spin Polarized Transport through the Double-Dot System

64%
Sztenkiel D., Świrkowicz R.
Acta Physica Polonica A
|
2005
|
vol. 108
|
issue 5
885-890
EN
Spin-dependent electron transport through two quantum dots in series attached to ferromagnetic electrodes is analyzed within the framework of the non-equilibrium Green function formalism. Regime of a weak coupling between the dots is investigated. I-V characteristics and tunnel magnetoresistance are calculated and discussed in detail.
12
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Non-Linear Thermal Current through Multilevel Quantum Dot Coupled to Ferromagnetic Electrodes

64%
Wierzbicki M., Świrkowicz R.
Acta Physica Polonica A
|
2012
|
vol. 121
|
issue 5-6
1204-1206
EN
We present studies of spin-dependent phenomena in the non-linear energy transport through a multilevel quantum dot/molecule in the Coulomb blockade regime. Calculations are performed within the framework of non-equilibrium Green function formalism based on the equation of motion. Thermal current flowing through the system due to temperature gradient significantly varies with gate voltage. It depends on relative orientation of magnetic moments in the electrodes, so in analogy to tunneling magnetoresistance the magnetothermal conductance is introduced, which describes the effect.
13
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Spin Torque in Double Planar Tunnel Junctions

51%
Wilczyński M., Świrkowicz R., Barnaś J.
Acta Physica Polonica A
|
2009
|
vol. 115
|
issue 1
269-271
EN
Transport in a double planar tunnel junction with ferromagnetic electrodes is analyzed theoretically in the zero-temperature limit. The in-plane and out-of-plane components of the spin torque exerted on magnetic moment of the central layer are determined as a function of the angle θ between magnetic moments of one of the external electrodes and of the central layer. Two configurations of the magnetic moments of external electrodes are considered, i.e. parallel and antiparallel ones. It is found that both torque components depend strongly on the thickness of the central layer, especially in junctions with a relatively thin central layer, where enhancement of the torque or a change of its sign can be observed for specific layer thicknesses. In junctions with thick central layer and in the limit of small bias voltage applied to the junction, the in-plane torque is generally smaller in the parallel configuration than in the antiparallel one. The opposite relation is observed for the normal torque.
14
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Phonon-Assisted Kondo Resonance in Spin-Dependent Transport through a Quantum Dot

51%
Świrkowicz R., Wilczyński M., Barnaś J.
Acta Physica Polonica A
|
2009
|
vol. 115
|
issue 1
272-274
EN
Effects of local vibrational modes on electron transport through a quantum dot attached to ferromagnetic electrodes are studied in the Kondo regime by the non-equilibrium Green function formalism based on the equation of motion method. Differential conductance is calculated for parallel and antiparallel configurations of the leads' magnetic moments, and well defined Kondo resonance peaks and their phonon satellites are found. The influence of a compensating magnetic field on the peak positions is also discussed.
15
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Spin Torque in Semiconductor Single Planar Tunnel Junctions

51%
Wilczyński M., Barnaś J., Świrkowicz R.
Acta Physica Polonica A
|
2008
|
vol. 113
|
issue 1
35-38
EN
Transport in a single planar tunnel junction with electrodes made of a ferromagnetic semiconductor is analyzed theoretically in the zero- -temperature limit. Tunneling current and both (in-plane and out-of-plane) components of the spin torque exerted on one of the ferromagnetic electrodes are determined as a function of the angle\thet a between magnetic moments of the electrodes. The influence of the bias voltage and spin splitting of the electron band (in both electrodes) on the spin torque components is analyzed numerically.
16
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Content available

Nonequilibrium Kondo Effect in a Single-Channel Quantum Dot Asymmetrically Coupled to Two Ferromagnetic Reservoirs

51%
Świrkowicz R., Wilczyński M., Barnaś J.
Acta Physica Polonica A
|
2004
|
vol. 105
|
issue 1-2
149-154
EN
Nonequilibrium Kondo effect in a quantum dot asymmetrically coupled to two ferromagnetic metallic leads is analyzed theoretically. The nonequilibrium Green function technique is used to calculate density of states and electric current. The lesser and retarded (advanced) Green functions are calculated by the equation of motion method within a consistent approximation scheme. The case where one electrode is half-metallic is analyzed numerically in details.
17
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Poor Man's Scaling and Green Function Analysis of the Kondo Anomaly in Single-Level Quantum Dots

51%
Wawrzyniak M., Świrkowicz R., Wilczyński M., Barnaś J.
Acta Physica Polonica A
|
2008
|
vol. 113
|
issue 1
553-556
EN
Kondo effect in a single-level quantum dot attached to magnetic leads is studied theoretically by the "poor man's scaling" and non-equilibrium Green function methods. From the scaling equations we derive the Kondo temperature as a function of the model parameters - in particular as a function of the angle between magnetic moments. Transport characteristics, i.e. differential conductance and tunnel magnetoresistance associated with magnetization rotation, were calculated within the non-equilibrium Green function formalism based on the equation of motion method.
18
Content available remote

Electronic Transport through a Quantum Dot Coupled to Non-Collinear Ferromagnetic Electrodes: The Kondo Regime

51%
Świrkowicz R., Wilczyński M., Wawrzyniak M., Barnaś J.
Acta Physica Polonica A
|
2008
|
vol. 113
|
issue 1
565-568
EN
Linear and non-linear conductance of quantum dots attached to magnetic leads is considered theoretically in the Kondo regime. The considerations are based on the non-equilibrium Green function formalism and the relevant equation of motion method. Splitting of the Kondo anomaly due to coupling of the dot to ferromagnetic electrodes, and its dependence on magnetic configuration of the system and on the lead's spin polarization is studied numerically.
19
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Content available

Spin Polarized Transport through Quantum Dots: Coulomb Blockade and Kondo Effect

39%
Barnaś J., Świrkowicz R., Wilczyński M., Weymann I., Martinek J., Dugaev V.
Acta Physica Polonica A
|
2003
|
vol. 104
|
issue 2
165-177
EN
Spin related effects in electronic transport through quantum dots, coupled via tunneling barriers to two metallic leads, are discussed from the point of view of fundamental physics and possible applications in spin electronics. The effects follow either from long spin relaxation time in the dots or from spin dependent tunneling through the barriers when the external leads are ferromagnetic. In the former case large nonequilibrium spin fluctuations in the dot can be induced by flowing current. These fluctuations modify transport characteristics, particularly the shape of the Coulomb steps. In the latter case electric current depends on magnetic configuration of the system, and tunnel magnetoresistance effect due to magnetization rotation can occur. Transport properties in the weak coupling regime are described perturbatively in the first (sequential) and second (cotunneling) orders. In the strong coupling regime, on the other hand, the equation of motion for nonequilibrium Green functions is used to calculate electric current at low temperatures, where the Kondo peak in conductance is formed in the zero bias regime. In symmetrical systems the Kondo peak is split in the parallel magnetic configuration, whereas no splitting occurs for the antiparallel alignment. Theoretical results are discussed in view of available experimental data.
20
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Temperature Dependence of Raman Scattering by Magnons in Bulk-Like MBE-Gwn MnTe Films

33%
Szuszkiewicz W., Mohrange J., Jouanne M., Kanehisa M., Świrkowicz R., Dynowska E., Janik E., Wojtowicz T., Kossut J.
Acta Physica Polonica A
|
1997
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vol. 92
|
issue 5
1025-1028
EN
Temperature dependence of the magnon frequency was studied for cubic MnTe epilayers by the Raman scattering measurements. Experimental data are compared to the results of theoretical calculations performed within the framework of the Heisenberg model using Green's function formalism.
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