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  1. 35 Acta Physica Polonica A

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  1. 35 Dietl T.

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1
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Influence of Magnetic Impurities upon Universal conductance Fluctuations

100%
Dietl T.
Acta Physica Polonica A
|
1997
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vol. 91
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issue 1
161-171
EN
Mesoscopic phenomena in quantum structures which incorporate magnetic impurities with localized spins may exhibit a number of novel features driven by spin-disorder scattering, exchange spin-splitting of electron bands, and the formation of bound magnetic polarons. After brief information on these effects, their influence on universal conductance fluctuations as well as on low frequency noise and quantum localization is presented. Millikelvin investigations of diffusive charge transport, which have been carried out for submicron wires of n^{+}-Cd_{1-x}Mn_{x}Te epilayers, are reviewed in some details. These studies have provided information on the significance of spin-disorder scattering in semiconductors and put into the evidence a new driving mechanism of the magnetoconductance fluctuations - the redistribution of the electrons between energy levels of the system, induced by the giant s-d exchange spin-splitting. Important implications of these findings for previous interpretations of spin effects in semiconductor and metal nanostructures are discussed.
2
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Why Ferromagnetic Semiconductors?

100%
Dietl T.
Acta Physica Polonica A
|
2001
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vol. 100
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issue Supplement
139-151
EN
Rapid development of information technologies originates from the exponential increase in the density of information that can be processed, stored, and transferred by the unit area of relevant devices. There is, however, a growing amount of evidences that the progress achieved in this way approaches its limits. Various novel ideas put forward to circumvent barriers ahead are described. Particular attention is paid to those concepts which propose to exploit electron or nuclear spins as the information carriers. Here, ferromagnetic semiconductors of III-V or II-VI compounds containing a sizable concentration of transition metals appear as outstanding spintronic materials.
3
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High Temperature Ferromagnetism and Nano-Scale Phase Separations in Diluted Magnetic Semiconductors and Oxides

100%
Dietl T.
Acta Physica Polonica A
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2007
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vol. 111
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issue 1
27-46
EN
In this paper the present understanding of the origin of ferromagnetic response that has been detected in a number of diluted magnetic semiconductors and diluted magnetic oxides at room temperature is outlined. It is argued that in these systems, owing to a typically small solubility of magnetic ions, crystallographic or chemical phase separation into regions with a large and a small concentration of magnetic component takes place. The ferromagnetic signatures then come from the regions with a large concentration of magnetic ions, which show non-zero spontaneous magnetization up to the blocking temperature, whose magnitude is proportional to the nanocrystal volume and magnetic anisotropy. Novel methods enabling a control of nano-assembling of magnetic nanocrystals in non-conducting matrices as well as possible functionalities of these spatially modulated magnetic systems are described. We also discuss phase separations into paramagnetic and ferromagnetic regions, which are driven by the Anderson-Mott localization and/or competing ferromagnetic and antiferromagnetic interactions. Finally, the question whether the high temperature ferromagnetism is possible in materials without magnetic ions is addressed.
4
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From Magnetic Polarons to Ferromagnetism

100%
Dietl T.
Acta Physica Polonica A
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1998
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vol. 94
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issue 2
111-123
EN
A brief overview is given on selected effects of the exchange coupling between effective mass electrons and localized spins in II-VI semiconductors containing Mn ions. In the case of a carrier or exciton trapped by an impurity or defect, the exchange interaction leads to zero-field spin-splitting. The current theory of such complexes, known as bound magnetic polarons, describes correctly their spectroscopic and thermodynamic properties as well as their formation dynamics. At the same time, a free magnetic polaron - a delocalized carrier accompanied by a traveling cloud of polarized spins - is not expected to exist for the actual values of the coupling constants. However, hole scattering by thermodynamic and static fluctuations is shown to affect significantly its energy. For a strong coupling, the corresponding renormalization has to be described by a non-perturbative approach. Finally, the influence of the carrier liquid upon the Mn spins is discussed. Here, either optical pumping or p-type doping may lead to a ferromagnetic order, both in bulk and layered structures. Because of a long-range character of the carrier mediated interactions, this ordering is not destroyed by the fluctuations, even in the reduced dimensionality systems.
5
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s-p-d-f Exchange Interactions in Mn and Eu Based IV-VI Semimagnetic Semiconductors

64%
Dietl T., Bauer G.
Acta Physica Polonica A
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1992
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vol. 82
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issue 5
729-732
EN
We evaluated exchange integrals which describe the spin dependent coupling between effective mass electrons and localized spins in the IV-VI semi-magnetic semiconductors taking into account contributions from the direct Coulomb exchange as well as from the hybridization between band states and magnetic shells. The obtained constants compare favorably with the experimental values for both the Mn- and the Eu-based compounds. We also discuss some peculiarities of IV-VI semimagnetic semiconductors, which may render the virtual-crystal approximation invalid.
6
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Formation Time of Magnetic Polarons

52%
Dietl T., Peyla P., Grieshaber W., Merle d'Aubigné Y.
Acta Physica Polonica A
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1995
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vol. 87
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issue 2
361-364
EN
It is shown that non-scalar spin-spin interactions rather than spin-lattice coupling or spin diffusion control the dynamics of the magnetization formation visible in time-resolved luminescence and SQUID magnetometry.
7
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The Ferromagnetic p-d Exchange in Diluted Magnetic Semiconductors

52%
Blinowski J., Dietl T., Kacman P.
Acta Physica Polonica A
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1992
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vol. 82
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issue 4
641-644
EN
The antiferromagnetic type of the interaction between the Γ_{8}-band electrons and the Mn^{2+} ions in A^{II}B^{IV} diluted magnetic semiconductors has been explained by invoking the p-d hybridization mechanism. In this paper it is shown that for particular electronic configurations of the magnetic ions in diluted magnetic semiconductors the p-d hybridization can lead to a ferromagnetic interaction between the valence-band electrons and the localized spins. This happens when all the one-electron d-orbitals which hybridize with p-band states are empty (e.g., as in Sc^{2+} and Ti^{2+} ions). The appropriate p-d Hamiltonian is derived and the ferromagnetic exchange constants are evaluated.
8
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Spatial Correlation of Impurity Charges Induced by Coulomb Interactions - Application to DX Centers in GaAs

52%
Kossut J., Wilamowski Z., Dietl T., Świątek K.
Acta Physica Polonica A
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1991
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vol. 79
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issue 1
49-58
EN
The electron mobility enhancement observed in heavily doped GaAs under hydrostatic pressure is interpreted in terms of spatial correlation between the donor charges within partially occupied system of impurities induced by strong inter-donor Coulomb interaction. A simple analytic theory is given for both DX^{0} and DX^{-} models of the impurity state. The mobility is shown to increase together with pressure in both models. Estimates of the energy of the DX level are strongly perturbed by the inter-donor Coulomb interactions.
9
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Conductivity Near the Metal-to-Insulator Transition in Cd_{1-x}Mn_{x}Se:Sc

46%
Głód P., Sawicki M., Lenard A., Dietl T., Plesiewicz W.
Acta Physica Polonica A
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1991
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vol. 79
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issue 2-3
389-392
EN
Earlier studies of transition metal impurities in II-VI compounds suggest that Sc acts as a resonant donor. We performed Hall effect and conductivity measurements of CdSe:Sc and Cd_{0.95}Mn_{0.05}Se:Sc. The results, particularly the critical concentration of the metal-to-insulator transition, turned out to be similar to those obtained previously for Cd_{1-x}Mn_{x}Se doped with hydrogenic-like impurities, such as In and Ga. Therefore, if the ground state of Sc impurity is indeed located above the bottom of the conduction band, our data demonstrate that the metal-to-insulator transition is primarily driven by the scattering, i.e. it corresponds to the Anderson localization.
10
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Observation of Two-Dimensional Electron Gas in LPE-Grown GaInAs-InP Heterostructures

46%
Fronc K., Grabecki G., Wróbel J., Dietl T., Gajewska M.
Acta Physica Polonica A
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1991
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vol. 80
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issue 3
449-452
EN
A series of GaInAs/InP heterostructures was grown by liquid phase epitaxy. The heterostructures were characterized by magnetotransport measurements carried out down to 1.8 K and up 10 T. The results demonstrate the existence of the high-mobility two-dimensional electron gas in the narrow-gap GaInAs as well as the presence of residual conductance through the InP buffer layer.
11
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Light-Controlled Transport in Doped Diluted Magnetic Semiconductors near Localization Boundary

46%
Głód P., Dietl T., Wojtowicz T., Sawicki M., Miotkowski I.
Acta Physica Polonica A
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1993
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vol. 84
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issue 4
657-660
EN
We report milikelvin studies of light induced metastable changes of the conductivity of the In doped Cd_{0.95}Mn_{0.05}Te_{0.97}Se_{0.03} crystals in the vicinity of the metal-insulator transition.
12
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Influence of Bound Magnetic Polarons on Magnetic and Electrical Properties in Metallic n-Cd_{0.95}Mn_{0.05}Se

40%
Głód P., Sawicki M., Dietl T., Skośkiewicz T., Lenard A., Plesiewicz W.
Acta Physica Polonica A
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1995
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vol. 87
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issue 1
173-175
EN
Doping-induced contribution to the millikelvin magnetic susceptibility of Cd_{0.95}Mn_{0.05}Se:In has been found to undergo a maximum at n ≈ 2n_{c}, and to vanish for n ≥ 8n_{c}, where n_{c} is the electron concentration corre­sponding to the metal-insulator transition. This confirms the presence, also in the metallic phase, of bound magnetic polarons. Their slow dynamics may account for hysteresis visible in our magnetoresistance data.
13
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Megagauss Cyclotron Resonance and Quantum Hall Effect of 2D Electron Gas in HgCdMnTe

40%
Grabecki G., Takeyama S., Dietl T., Takamasu T., Shimamotο Y., Miura N.
Acta Physica Polonica A
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1995
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vol. 88
|
issue 4
731-734
EN
Two-dimensional electron gas adjacent to a grain boundary in bicrystal of narrow-gap semiconductor p- Hg_{0.79}Cd_{0.19}Mn_{0.02}Te has been studied under ultra strong impulse magnetic fields (up to 140 T). Both cyclotron resonance and quantum Hall effect are measured for the same samples. The values of the resonance fields point to strong nonparabolicity. A broadening of the line is interpreted in terms of an intersubband mixing that occurs for the upper Landau level. A steep increase in the linewidth in the field range 20-30 T, which coincides with a strong decrease in the Hall resistance is assigned to the field-induced metal-insulator transition in our system.
14
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Search for Dimensionality Crossover of Spin-Glass Freezing in Superlattices of Cd_{0.50}Mn_{0.50}Te/CdTe

40%
Sawicki M., Dietl T., Karczewski G., Wojtowicz T., Kossut J., Skośkiewicz T.
Acta Physica Polonica A
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1996
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vol. 90
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issue 5
919-922
EN
SQUID measurements of the time decay of the thermoremanent magnetization (field-cooled in 1000 Oe) at long time scale, 10^{2} < t < 10^{5} s, are presented for MBE grown Cd_{0.50}Mn_{0.50}Te. We found that for both thin (32 Å) and thick (2500 Å) layers the spin-glass dynamics is characterized by a similar value of k = -(1/T_{f})(dT_{f}/dlogt) > 0.05, indicating the absence of the phase transition at nonzero temperature under the experimental conditions.
15
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Search for Dimensional Effects in Spin-Glass Transition in Thin Cd_{1-x}Mn_{x}Te Multilayers

40%
Sawicki M., Dietl T., Skośkiewicz T., Karczewski G., Wojtowicz T., Kossut J.
Acta Physica Polonica A
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1995
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vol. 88
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issue 5
1038-1042
EN
SQUID magnetic measurements of CdTe/Cd_{1-x}Mn_{x}Te, x ≃ 0.5, multi-layers with different CdMnTe layer thickness w reveal the persistence of the spin-glass irreversibilities down to ≃ 16 Å thin layers, i.e., well beyond the previously postulated 40 Å as a quasi-2D threshold. The freezing temperature T_{f} is found to be a monotonic function of w, and it obeys a scaling law T_{f} ∝ w^{a}, with a 0.8 similar to that for canonical spin-glasses. Magnetic properties of all studied structures are found to be independent of the orientation of the magnetic field.
16
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Ferromagnetism in II-VI Based Semiconductor Structures

40%
Cibert J., Ferrand D., Tatarenko S., Wasiela A., Kossacki P., Dietl T.
Acta Physica Polonica A
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2001
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vol. 100
|
issue 2
227-236
EN
The experimental results on carrier-induced ferromagnetic interaction in II-VI diluted magnetic semiconductors are shortly reviewed and analyzed in the light of the mean field approximation. We particularly take the point of view of the experimentalist to emphasize (i) points which are easily understood within this simple model (as the role of the detailed structure of the valence band) and hence should be incorporated into more sophisticated models, and (ii) points which are not taken into account in the mean field model (e.g., disorder effects) and hence call for more elaborate descriptions. We particularly discuss the case of a low carrier density, and show that the situation appears as experimentally very different in the highly disordered 3D layers and in the modulation doped 2D quantum wells.
17
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Precision of the Hall Quantization in Naturally Occurring Two-Dimensional System - HgCdMnTe Bicrystals

40%
Grabecki G., Wittlin A., Dietl T., Teunissen P., Wiegers S., Perenboom J.
Acta Physica Polonica A
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1992
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vol. 82
|
issue 5
733-736
EN
New magnetoresistance measurements on naturally occurring inversion layers adjacent to grain boundaries in narrow gap diluted magnetic semiconductor HgCdMnTe (Eg ≤ 200 meV) have been performed. The magnetic fields up to 20 T simultaneously with millikelvine temperatures have been applied. Possible experimental factors affecting the quantum Hall effect in our system are discussed in detail.
18
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Magnetic Characterization of Molecular Beam Epitaxy Grown Cd_{1_{x}}Mn_{x}Te Structures

33%
Sawicki M., Kolesnik S., Wojtowicz T., Karczewski G., Janik E., Kutrowski M., Żakrzewski A., Dynowska E., Dietl T., Kossut J.
Acta Physica Polonica A
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1995
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vol. 87
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issue 1
169-172
EN
The paper reports on the application of SQUID magnetometry to probe magnetic ion distribution in epilayers and at interfaces of diluted magnetic semiconductors. We present also new results on the possible influence of the magnetic confinement on the formation of the spin-glass phase, and on antiferromagnetic phase transition in zinc-blende MnTe.
19
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Magnetization Steps in Cd_{1-x}Mn_{x}Te Observed in Coherent Transport

33%
Jaroszyński J., Dietl T., Wróbel J., Karczewski G., Wojtowicz T., Kossut J., Maude D. K., van der Linden P., Portal J. C.
Acta Physica Polonica A
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1997
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vol. 92
|
issue 4
797-800
EN
Magnetoconductance measurements on submicron wires of n^{+}-Cd_{1-x}Mn_{x}Te were carried out up to 27 T and down to 100 mK. The inverse correlation field of the universal conductance fluctuations is found to increase abruptly in the vicinity of the magnetization steps due to Mn pairs in CdMnTe. No such effect is observed in similar wires of CdTe. These findings support a recent model, according to which the correlation field of the universal conductance fluctuations in magnetic systems is inversely proportional to the magnetic susceptibility of the localized spins.
20
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Effects of Ballistic Transport in Wires of n-PbTe

33%
Grabecki G., Wróbel J., Dietl T., Papis E., Kamińska E., Piotrowska A., Ueta Y., Sprinholtz G., Bauer G.
Acta Physica Polonica A
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1997
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vol. 92
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issue 4
789-792
EN
We present results of magnetotransport studies on quantum wires of submicron PbTe epilayers, fabricated by means of electron beam lithography and dry etching. When the wire width is reduced down to 1 μm, the transition from diffusive to ballistic regime is observed. Effects associated with collimation and boundary scattering are found in the Hall, longitudinal, and van der Pauw magnetoresistance for wires and junctions in the shape of a cross.
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