IX WORKSHOP ON SEMIMAGNETIC SEMICONDUCTORS MATERIALS FOR SPINTRONICS.

Transcription

1 IX Workshop on Semimagnetic Semiconductors Materials for Spintronics. Obory, February 28, 2004 IX WORKSHOP ON SEMIMAGNETIC SEMICONDUCTORS MATERIALS FOR SPINTRONICS. OBORY, FEBRUARY 28, 2004 Center of Excellence CEMOS Physics and Technology of Semiconductor Materials and Structures for Optoelectronics and Spintronics Institute of Experimental Physics, Warsaw University Workshop Chairmen: Michał Nawrocki Workshop Secretary: Paulina Płochocka Address: Dom Pracy Twórczej im. B.Prusa Obory k/konstancina ul. Literatów 2 tel: 48220) Arrival: see

2 PROGRAM Timing: Contribution title: Presented by: Chairman: 9:00 Registration 9:25 Opening 9:30 20 paper+10 10:00 10:20 10:40 11:00 Coffee break 11:30 20 paper+10 12:00 12:20 20 paper+10 12:50 13:15 Lunch 14:30 14:50 15:10 15:30 15:50 16:15 Coffee break 16:45 - ~18:00 Panel Interplay of excitons, biexcitons, and charged excitons in pump-probe absorption experiments on a (Cd,Mn)Te quantum well Quantum dots CdTe/ZnTe grown by new method X+ dissociation influenced by magnetic field and temperature in CdTe-based quantum wells The magnetooptical study of exciton transitions in Zn 1-x Co x O Magnetic anisotropy in bulk GaN:Mn,Mg Ferromagnetic resonance in epitaxial InGaMnAs Magnetic order in GaMnAs EuGdTe -growth and investigation Spin dependent interactions of free carriers and manganese ions in crystals and low dimensional structures of wide band gap ii-mn-vi semiconductors mechanism of lifetime reduction II-VI diluted magnetic p-n junctions Preparation of superconductor-semiconductor structures for detection of spin-polarization Anomalous Hall Effect in IV-VI (Sn 1-x Mn x Te) and III-V (In 1-x Mn x Sb) semimagnetic semiconductors Properties of heterostructures: ferromagnetic islands placed over DMS quantum wells Foreseeing trends in semimagnetic semiconductors research and applications (P.Bogusławski, J.Cibert, J.Gaj, D.Scalbert, D.Yakovlev) P. Płochocka P. Wojnar J. Suffczyński W. Pacuski M. Zając K. Dziatkowski O.M. Fedorych P. Dziawa S. Yatsunenko V. Kolkovsky M. Bastjan B. Brodowska K.M. Lebecki A.Twardowski T.Story Z.Wilamowski J.Kossut 2

3 List of Participants. Andrearczyk Tomasz Czernyszowa Maryna Baj Michał Fedorych Oleh Boguslawski Piotr Goryca Mateusz Dietl Tomasz Grzęda Agnieszka Dobrowolski Witold Jakubas Paweł Gaj Jan Jaworek Malgorzata Godlewski Marek Kiecana Michal Golnik Andrzej Kolkowski Vladimir Gosk Jacek Kowalik Katarzyna Grabecki Grzegorz Kudelska Izabela Grynberg Marian Lebecki Krzysztof Kacman Perła Marcinkowska Agnieszka Kamińska Maria Michaluk Estera Karczewski Grzegorz. Osinny Wiktor Kłopotowski Łukasz Pacuski Wojciech Kossacki Piotr Pawłowski Mariusz Kossut Jacek Piechal Bernard Kowalczyk Leszek Sakowicz Maciej Kudelski Arkadiusz Sankowski Piotr Majewski Jacek Skierkowski Andrzej Molenkamp Laurens Suffczyński Jan Nawrocki Michał Trajnerowicz Artur Potemski Marek Werner Krzysztof Sawicki Maciej Wojnar Piotr Scalbert Denis Wołoś Agnieszka Skośkiewicz Tadeusz Wójcik Aleksandra Story Tomasz Zagrajek Przemysław Twardowski Andrzej Zając Marcin Vladimirova Masha Wilamowski Zbyslaw Wojtowicz Tomasz 3

4 Interplay of excitons, biexcitons, and charged excitons in pump-probe absorption experiments on a (Cd,Mn)Te quantum well P. Płochocka *, P. Kossacki, W. Maślana, C. Radzewicz, and J.A. Gaj Institute of Experimental Physics, Warsaw University, 69 Hoża, Warszawa, Poland J. Cibert and S. Tatarenko Laboratoire de Spectrométrie Physique, BP87, Saint Martin d Hères cedex, France We report time-resolved absorption studies of the influence of carriers on neutral (X), charged (X+) and biexciton (XX) transitions in a semimagnetic II-VI quantum well (QW). We find that the X and XX intensity dynamics are directly related to screening by the 2D hole gas. The spin dependent X-X interactions are responsible for the change in the binding energies resulting in a blue shift of the transition lines. The experiments were performed on a modulation-doped Cd 1-x Mn x Te/Cd 1-y-z Zn y Mg z Te (x ) quantum well (QW) with variable hole concentration. The pump pulse was circularly polarized and tuned to resonance with the X transition creating a population of neutral excitons up to cm -2. A spectrally broad probe 100 fs pulse was used to record the absorption. Both polarization helicities of probe pulse were recorded separately. The hole concentration in the QW was controlled by an additional cw illumination in the range up to cm -2. In order to tune the spin polarization of the hole gas, magnetic field up to 1T was applied in Faraday configuration. In a QW depleted by a strong cw illumination i.e. without carriers, no X+ transitions were observed. The X population created by the pump pulse caused a significant bleaching of the X absorption line and appearance of the XX transition in opposite polarization to that of the pump. The decrease of X intensity is equal to increase of XX intensity and thus the total absorption oscillator strength is conserved at all the pump-probe delays. On the other hand, in the presence of carriers in the QW, the increase of XX intensity is smaller and the total intensity change is negative. This is probably due to screening of the XX transitions leading to oscillator strength stealing as reported previously for neutral excitons. Moreover, we observed a blue shift of the X transition in both polarizations, but much stronger for the same polarizations of the pump and probe pulses than for opposite. Its magnitude depends strongly on the pump intensity, i.e. on the X population. It is not influenced significantly neither by the presence of carriers nor by the magnetic field within the applied range. We therefore conclude that it is a consequence of spin-dependent carrier-carrier interaction leading to a decrease of (renormalizing) the X binding energy. The obtained results were analyzed in terms of spin-dependent interactions in the system containing carriers, X, XX, and X+. The behavior of the studied systems was found to be significantly different from that of typical III-V quantum structures. In particular, we observed much smaller influence of phase space filling effects due to smaller X Bohr radius in a II-VI QW. * corresponding author; plochock@fuw.edu.pl 4

5 Quantum dots CdTe/ZnTe grown by new method P. Wojnar, M. Aleszkiewicz, J. Kossut, and G. Karczewski Institute of Physics, Polish Academy of Sciences Self organized quantum dots CdTe/ZnTe were grown by a new method developed recently [1]. The large lattice mismatch between ZnTe and CdTe (6.1%) only is not sufficient for spontaneous 2D-3D growth mode transition and the formation of large quantum dots in this material system. It has been proposed to induce the quantum dot formation by lowering the surface energy of the strained CdTe layer [1].This is achieved by covering the CdTe layer with an amorphous tellurium layer, which is subsequently desorbed. The advantage of this method is a possibility of in situ observation of quantum dots formation using Reflection High Energy Electron Diffraction (RHEED). We can clearly observe 2D-3D transition in RHEED image, which is a fingerprint of quantum dots formation. The main part of our research are photoluminescence (PL) and µ-photoluminescence (µ-pl) measurements. Typical PL spectrum of CdTe/ZnTe quantum dots consists of a broad line in the spectral region of ev. In µ-pl, this line splits into a large number of sharp spikes with FWHM of about 150 µev. Employing the Atomic Force Microscopy (AFM), we obtain the density of quantum dots and their average sizes. We estimate the density to cm -2, the average in plane diameter to 40 nm and the height to 7 nm. One of the most important advantages of such large quantum dots is that they could give us possibility to insert magnetic ions into the bulk of the dots. We expect, that in contrast to the previous study [2}, in this case we would be able to tune their energy levels applying magnetic field. The value of minimal lattice-mismatch, which is necessary for quantum dots formation, was determined studying CdTe quantum dots in CdZnTe barrier with different Zn compositions. Quantum dots can form even when the lattice mismatch is 2.8 %. The zinc amount is then 48 %. We have also studied superlattices of quantum dots by µ-photoluminescence. CdTe quantum dots were separated by different ZnTe spacers. The spectral density of spikes decreases in samples with thinner spacer. This could be caused by strain induced ordering of quantum dots in upper layers. [1] Tinjod F, Gilles B, Moehl S, Kheng K, Mariette H. Applied Physics Letters, vol.82, no.24, 16 June 2003, pp [2] G. Karczewski, S. Maćkowski, M. Kutrowski, T. Wojtowicz, J. Kossut, A. Heiss, Applied Physics Letters 74 (1999)

6 X+ dissociation influenced by magnetic field and temperature in CdTe-based quantum wells J. Suffczyński 1, Ł. Kłopotowski 1, M. Nawrocki 1, E. Janik 2 1 Institute of Experimental Physics, Warsaw University, Hoża 69, Warsaw, Poland 2 Polish Academy of Sciences, al. Lotników 32/64, Warsaw, Poland Dissociation of positively charged exciton (X+) to neutral exciton (X) and free heavy hole is investigated by photoluminescence (PL) in asymmetric double quantum well (QW) for various conditions of temperature and magnetic field. The investigated structures consist of CdTe wells of different width coupled by barrier made of CdMnTe wide for 50A. Formation of X+ is due to an amount of excess holes in the undoped sample. Due to giant Zeeman splitting in DMS, magnetic field changes the height of the barrier what results in large energy splitting of spin polarized X and X+ states confined in the well. Measurements were carried in Faraday configuration, with magnetic field up to 5T and at several temperatures from 1.6K to 20K. Linearly polarized HeNe laser beam was used for above the barrier excitation. Detection was done in two circular polarizations of light. Intensities of PL lines arising from recombination of X and X+ are analyzed. As the total PL intensity is approximately constant, efficiency of nonradiative processes is found to be independent on temperature and magnetic field. Increasing magnetic field changes the relative distance between X and X+ energy levels what leads to level crossing (Figure 1). Drop of X+ PL intensity in σ+ and σ polarization when X+ is no longer a ground state of the system indicates that dissociation of X+ accompanied by the spin-flip of the electron takes place (Figure 2). Model taking into account also spin and energy relaxation between spin split states of X and X+ and spin polarization of holes describes obtained results well. Energy [ev] X exp. X+ exp. σ σ σ+ σ+ T= 1.6K B [T] Figure 1. Energies of X and X+ levels in magnetic field up to 5T for T = 1.6K. Line Intensity [arb. u.] T= 1.6K σ X+ B [T] σ+ X Figure 2. Intensity of photoluminescence from X and X+ levels in magnetic up to 6

7 The magnetooptical study of exciton transitions in Zn 1-x Co x O W. Pacuski 1,2, P. Kossacki 1,2, D. Ferrand 2, J. Cibert 3, C. Morhain 4, C. Deparis 4. 1 Institute of Experimental Physics, Warsaw University, Hoża 69, PL Warszawa, Poland. 2 Laboratoire de Spectrométrie Physique, Groupe «Nanophysique et Semiconducteurs» (CEA- CNRS-Université Joseph Fourier Grenoble), Boîte Postale 87, F Saint Martin d'hères Cedex, France. 3 Laboratoire Louis Néel, CNRS, BP 166, F Grenoble Cedex 9, France 4 Centre de Recherches sur l'hétéroépitaxie et ses Applications, CNRS, Valbonne-Sophia- Antipolis, France We present a study of ZnO layers doped by Co and grown on sapphire substrates by molecular beam epitaxy. The magnetospectroscopy was performed at low temperatures, in the Faraday configuration, in a magnetic field up to 6T. We investigated how the presence of Co 2+ ions modifies reflectivity, transmission and photoluminescence spectra of the host semiconductor (ZnO). We observe intra-ionic lines and absorption bands with energies smaller than the energy gap of ZnO. This absorption was calibrated and used as a very sensitive tool to determine the Co content in ZnO: Co doped layers. We demonstrate its applicability in the range of contents from 0.01% to 17%. However, the more exciting is the influence of the magnetic ions on the band carriers. Cobalt ions are expected to enhance the influence of the magnetic field on the exciton lines due to the s,p-d exchange interactions. Actually we observe a dependence on the magnetic field of the Zeeman splitting and the broadening of the excitonic structure. The study was performed at different temperatures in samples with various Co concentrations. 7

8 Magnetic anisotropy in bulk GaN:Mn,Mg Marcin Zając a), Jacek Gosk a,b), a) Institute of Experimental Physics, Warsaw University, Hoża 69, Warsaw b Faculty of Physics, Warsaw University of Technology, Koszykowa 75, Warsaw, Poland The theoretical predictions that GaN doped with a few molar percent of Mn and highly p- type should reveal room temperature ferromagnetism (FM) has led to an increased interest in synthesis and magnetic properties of GaMnN semimagnetic semiconductor. In order to understand the magnetic properties of GaMnN the nature of Mn impurity in GaN should be examined. Since more and more studies show that the Mn 2+/3+ acceptor level is most likely located in the energy gap of GaN one may expect that the nature of Mn impurity results from mutual position of Mn 2+/3+ level and the Fermi level. In n-type samples Mn should enter GaN in d 5 configuration. Contrary, codoping with Mg, which is an acceptor, should lower the Fermi level leaving Mn ions in d 4 configuration. In order to verify this hypothesis we studied magnetic properties of bulk n-type GaMnN and highly resistive GaMnN:Mg samples grown by high pressure method. In this communication we compare the results of magnetic measurements performed on these two types of samples. 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 using a SQUID magnetometer. Magnetization of both types of samples reveals paramagnetic beheviour. However for n-type GaMnN isotropic Brillouin-type magnetization was observed which is in agreement with Mn d 5 configuration. On the other hand GaMnN codoped with Mg shows large magnetic anisotropy which strongly suggests Mn to be in nonspherical d 4 or d 3 configuration. The data were analysed within a crystal field model taking into account cubic field of tetrahedral symmetry, tetragonal Jahn-Teller distortion, the spin-orbit interaction and the trigonal field along the c-axis. A very good description of the experimental data was obtained assuming the presence of Mn ions in d 4 configuration (Fig. 1). Such centers suffer strong Jahn Teller distortion along one of three <100> axes. The observed anisotropy originates from different distributions of non-equivalent Jahn-Teller centers in two orientations of magnetic field. M(emu/g) K B c BIIc 10 K B c BIIc 50 K B(T) B(T) B(T) Fig. 1. Magnetization versus magnetic field for GaN:Mn,Mg sample for two orientations of magnetic field to the c-axis. Solid lines represent results of the crystal field model. 8

9 Ferromagnetic resonance in epitaxial InGaMnAs K. Dziatkowski Institute of Experimental Physics, Warsaw University, Hoża 69, Warsaw, Poland Ferromagnetic resonance was investigated in MBE grown InGaMnAs epilayers. Strong azimuthal (in-plane) and polar (out-of-plane) anisotropy was observed, and described reasonably with Magnetic-Anisotropy-Energy (MAE) model taking into account both magnetocrystalline and demagnetization contributions. The different (biaxial and uniaxial) symmetries of anisotropies, revealed by nominally the same epilayers, were successfully interpreted in terms of MAE model parameters. This difference was attributed to the possible different ordering in distribution of magnetic ions, originating from subtle changes of the growth process. 9

10 Magnetic order in GaMnAs. O.M. Fedorych 1, Z. Wilamowski 1, and J. Sadowski 1,2 1 Instytut of Physics PAS, Warszawa, Al. Lotników 32/46, Poland 2 MAX-Lab, Lund University, PO. Box 118, Lund, Sweden Magnetic order in GaMnAs is already well proven. Theoretical models predict the critical temperature very well. But at the same time a lot of questions are still open. The most interesting questions are spin structure and role of fluctuation, spin waves observed in GaMnAs and peculiarities of the magnetic anisotropy. The investigation of the magnetic resonances in GaMnAs for Mn concentration less then 0.01 improves that in GaAs Mn impurity has a Mn 2+ charged state. The values of cubic and hyperfine constants were obtained. The dependence of the crystal field parameter D as a function Mn concentration showed that carriers are responsible for the appearance of the strong magnetic anisotropy in ferromagnetic GaMnAs. For higher Mn concentration our investigations show that in GaMnAs the change of the conductivity type from Mott insulator to semimetal accompanied with change of the magnetic order. The insulator samples are characterized by the ferromagnetic order but the semimetal samples exhibit ferrimagnetic properties. The single isotropic resonance line which is typical for the ferromagnetic samples changes on the complex and strongly anisotropic structure characteristic for the ferrimagnetic order where the systems of the localized magnetic moments of Mn and magnetic moments of the holes are ordered. The complex structure observed for the ferrimagnetic samples corresponds to spin wave (SW) resonance. But in contrast to the spin waves observed for the classical ferromagnetic materials the structure of SW in GaMnAs is different. It was noticed that the length of exchange interaction obtained from SW dispersion is long. The long range of the exchange means that a single Mn ion interacts with large number of the neighboring spins, and in spite of random distribution of Mn spins in the sample volume the alloy disorder is perfectly averaged. The value of saturation energy corresponds to the exchange energy between Mn ions in two orders of magnitude smaller than the energy expected from the critical temperature. The analysis of angular dependence of the resonance field let us to make a conclusion that the correlation length of longitudinal spin component is larger then the correlation length of the transverse component. The value of the correlation length is limited by the pseudo-dipole component of exchange coupling. In conclusion, the magnetic structure of insulating GaMnAs differ from them semimetal one. In the semimetal samples the long range of exchange interaction leads to well averaging of alloy disorder. The alloy fluctuations can be responsible for weak correlation of the transverse spin component. 10

11 EuGdTe - growth and investigation Piotr Dziawa Institute of Physics, Polish Academy of Sciences, Warsaw, Poland Recent progress in electronics area requires more investigations of different materials as well as synthesis of novel systems. We study low-dimensional structures based on EuTe, namely Eu 1- xgd x Te with x up to about 0.05, being interesting for its ferromagnetic properties. This compound is expected to be good material for spin polarized carrier injection into non-magnetic semiconductors. Europium chalcogenides are ionic compounds with rock salt structure. They are wide band gap semiconductors, i.e. with E g value changing from 1.12 ev in EuO to 2.00 ev for EuTe. Eu 2+ ion in such compounds has 5s 2 p 6 electronic configuration and half-filled 6f 7 shell. According to Hund s rule, electrons on half-filled shell have parallel spin orientation, which gives value of magnetic moment equal 7/2. This moment is strongly localized on the Eu 2+ ions and interacts with charge carriers spins. Because of dependence of the exchange interaction on distance between magnetic ions, the magnetic ordering varies for different chalcogenides anions, namely, EuO, EuS and EuSe are ferromagnets at T=0, while EuTe is a II-type antiferromagnet. We are interested in EuTe system. In order to provide charge carriers we dope that compound with gadolinium, which has an additional electron on the 5d shell. This doping gives one electron per one Gd 3+ ion. In particular, carriers make possible an additional magnetic interaction between magnetic moments localized on Eu 2+. It is the source of positive contribution to the sum of magnetic interactions and causes change magnetic ordering from antiferromagnetic to ferromagnetic. The samples are grown by MBE technique. Next, we carry out structural studies like x-ray and AFM investigations as well as measurements of high and low temperature ac susceptibility. In this presentation technique of preparation will be discussed and results of the above mentioned studies will be shown. Collaborators: Tomasz Story, Andrzej Nadolny, Badri Taliashvili (MBE); Viktor Domukhowski (x-ray); Elżbieta Łusakowska (AFM); Daniel Dobrowolski, Monika Arciszewska (magnetic measurements). 11

12 Spin dependent interactions of free carriers and manganese ions in crystals and low dimensional structures of wide band gap ii-mn-vi semiconductors mechanism of lifetime reduction S. Yatsunenko *, A. Khachapuridze *, V.Yu. Ivanov *, M. Godlewski *,** and Z. Gołacki * * Institute of Physics, Polish Acad. of Sciences, Al. Lotnikow 32/46, Warsaw, Poland ** College of Science, Cardinal S. Wyszyński University, Warsaw, Poland E.M. Goldys Division of Information and Communication Sciences, Macquarie Univ., Sydney, Australia M. Phillips Microstructural Analysis Unit, UTS, Sydney, Australia In mid 80s it was reported that manganese doped semiconductor nanostructures show high luminescence yield and short decay times, making them suitable for a range of applications, such as fluorescent labels in biology and medicine, phosphors, sensors, lasers and displays. The most unusual feature of such structures was a rapid decrease of the lifetime of Mn 2+ intra-shell emission reported for nanoparticles of ZnMnS. This reduction was first observed for Mn containing nanostructures, but is also found by us for quantum well structures and bulk samples, thus it cannot be nanosize related only. Several conflicting models were proposed to explain this observation, including the ones in which observed shortening was related to an overlap of another emission band. In this presentation we discuss the results of extensive photoluminescence (PL), time-resolved PL, structural and magnetic resonance studies of a range of wide band gap materials doped with manganese. Bulk samples, layers, quantum well and quantum dot structures of ZnMnS, CdMnS, ZnMnTe, CdMnTe and ZnMnSe were studied. Based on the results of optically detected magnetic resonance and timeresolved investigations we relate the observed lifetime decrease to spin dependent magnetic interactions between localised spins of Mn 2+ ions and spins/magnetic moments of free carriers. This mechanism is active in both bulk and in low dimensional structures, but seems to be enhanced in nanostructure samples. 12

13 II-VI diluted magnetic p-n junctions V. Kolkovsky, R. Butkute, E. Janik, and G. Karczewski Institute of Physics, Polish Academy of Science, Warsaw It is known that the active control of spins in semiconductors (spintronics) leads to significant technological advances which are very important in digital information storage, magnetic recording and sensing and in quantum computing. In this regard, spintronics is an emerging field promising device applications with more functionality and better integrability with traditional semiconductor technology. Recently, Zutic and Fabian have theoretically considered a magnetic p-n junction [1-3]. One side of the device was made of magnetic material exhibiting a giant spin splitting and the other one of a nonmagnetic semiconductor. The authors predict that in the presence of an external magnetic field the spin-polarized p-n junction will allow for spin-polarized current generation, spin amplification, voltage control of spin polarization, and a significant extension of spin diffusion range. They predict novel phenomena of exponential and giant magnetoresistance, spin-voltaic effect, spin-valve effect, if a non-equilibrium spin is added into the junction. They have demonstrated that no spin injection or extraction is possible under low bias and that only nonequilibrium spin can be injected across the space-charge region of a p-n junction. Also that nonequilibrium spin injected in a magnetic p-n junction gives rise to the spin-voltaic effect, in which the non-equilibrium spin-induced charge current is very sensitive to the spin relaxation time (T) and can flow even at no applied bias. It is proposed that T can be determine by measuring the I-V characteristics. The goal of this study is to verify experimentally the theoretical predictions of Zutic and Fabian. For that purpose a set of p-n junctions with magnetic region n-type region (CdMnTe) and nonmagnetic p-type region (CdZnTe) have been grown by Molecular Beam Epitaxy (MBE). The devices contain different contents of Mn and Zn (for Mn 1.4%, 2.4%, 4.2% and Zn 1%, 2.3%, 7.4%). We employ electrical and optical measurements for preliminary characterization of these structures. As the next step, we plan is to study the spin-polarized bipolar transport in nonhomogeneously magnetic and non-magnetic p-n junctions in the presence of magnetic field. [1] Zutic I, Das Sarma, Phys. Rev. B 66,165301(2002) [2] Zutic I, Fabian J, Das Sarma, Phys. Rev. Lett 88, (2002) [3] Zutic I, Fabian J, Das Sarma, Phys. Rev. B 64, (2001) 13

14 Preparation of superconductor-semiconductor structures for detection of spinpolarization M. Bastjan 1, G. Grabecki 2,3, E. Michaluk 1, J. Wróbel 2,3, K. Fronc 2, M. Aleszkiewicz 2 and T. Dietl 2,3 1College of Sciences UKSW, Warsaw 2Institute of Physics, Polish Academy of Sciences,, al. Lotników 32/46, PL Warsaw 3ERATO Semiconductor Spintronics Project, Japan Science and Technology Agency, al. Lotników 32/46, PL Warszawa, Poland We report on fabrication procedures and preliminary electron transport measurements performed on hybrid superconductor-semiconductor microstructures. The goal of our studies is searching for a tool enabling electrical detection of spin polarization in future semiconductor spintronic devices. It is well known that process of supercurrent conversion at a superconductor normal metal (S-N) interface, called Andreev reflection, is limited by the minority spin population near the Fermi surface. Therefore, the differential conductance of the S-N junction can reveal the spin polarization in the metal. This method has been successfully applied for many ferromagnetic metals and metallic compounds [1], however, to apply it for semiconductors [2] one has to solve the difficult problem of the S-N interface transparency. We have chosen for our studies narrow-gap semiconductor PbTe, because in this material the Schottky barrier effects are much suppressed for many metals. Additionally, PbTe has excellent transport properties enabling observations of quantum ballistic effects [3]. As a first step, we have developed methods of deposition the superconducting metal on top of our material. We have used both niobium (Nb) and indium (In). In the former case, the layers were obtained by low-power magnetosputtering. For the latter, the conventional vacuum evaporation was applied. Electron beam lithography and lift-off process were used to pattern the layers into various microstructure shapes. In particular, we have found that Nb wires deposited on PbTe layers (grown by MBE on BaF 2 substrates) conserve their superconducting properties down to the width of 1 µm. Moreover, they remained unchanged after multiple coolings to cryogenic temperatures. Similar stability was found for the In structures. In order to test the interface quality, we used a simple method of passing electric current along the PbTe layer on top of which several hundred micrometers wide, single square of the studied metal was deposited. The current contacts were placed away from the square and did not influenced the measurement. For such an arrangement, we have observed that the current easily penetrates PbTe/In interface but almost doesn t penetrate that of PbTe/Nb. Therefore, we have focused our attention on the former element and prepared more complicated structures, in the form of the Hall bridges, patterned from PbTe single quantum well (n = cm -2 ) Å thick and 60 µm wide In-stripe was deposited above the current lines. In several places small gaps of spacing between 10 to 100 µm were made. They were designed to enable the Andreev reflectionmediated transport through PbTe which could be detected in conductance measurements. However, the first prepared sample has shown unexpectedly large resistance increase when cooled down to helium temperatures. Probably, this is a result of In layer-induced thermal stress causing depletion of the PbTe quantum well from mobile carriers, in the gap region. In order to solve this problem, in the next try we plan to use the PbTe layer with significantly higher electron concentration n. This work was supported in part by Polish KBN grant (PBZ-044/P03/2001). [1] R.J. Soulen et al., Science, 82, 85, 2 nd Oct [2] I. Zutic, S. Das Sarma, Phys. Rev. B 60, (1999). [3] G. Grabecki et. al., Phys. Rev. B60, R5133 (1999); Physica E 20, 236 (2004). 14

15 Anomalous Hall Effect in IV-VI (Sn 1-x Mn x Te) and III-V (In 1-x Mn x Sb) semimagnetic semiconductors1 Beata Brodowska2 Institute of Physics, Polish Academy of Sciences, Warsaw There is a growing interest in the Anomalous Hall Effect (AHE) due to the importance of the spin polarization and spin-orbit interaction for transport properties of materials and structures of spin electronics. In ferromagnetic metals and in magnetic semiconductors, the AHE is known phenomenon, in which the transverse resistivity ( ρ H ) contains a contribution proportional to magnetization (M) in addition to the usual Hall effect: ρ = R B + R M, where B is the H 0 µ 0 magnetic field, R 0 and R S are normal and anomalous Hall coefficient, and µ 0 is magnetic constant. The purpose of our work is to study and compare the Anomalous Hall Effect in IV-VI and III- V narrow gap semimagnetic semiconductors. We studied two different types of materials: Sn 1- xmn x Te and In 1-x Mn x Sb. Samples of the first material were bulks with x=0.07, 0.11, and high carriers (holes) concentration (10 21 cm -3 ). In spite of the fact that Sn 1-x Mn x Te system is well known, behaviour of the anomalous Hall coefficient versus temperature was not studied. The Hall effect and electric conductivity measurements (up to 13 T in the temperature range K) were performed using standard DC technique. Magnetic properties were studied using Lake Shore 7229 Magnetometer/Susceptometer. Magnetization of Sn 1-x Mn x Te samples was measured at magnetic fields up to 9 T for the same temperatures as the ρ dependence. As the result of the studies, both R 0 and R S were evaluated and their dependence on temperature estimated. Fig.1. Hall resistivity vs. B up to 13 T for Sn 1-x Mn x Te 3501 I1 sample. H The second part of our work was devoted to study of transport and magnetooptical (MOKE) properties of new MBE grown ferromagnetic semiconductor In 1-x Mn x Sb. InSb crystals, used for devices working in infrared region, when mixed with Mn and forming ferromagnetic semiconductor, may be useful for new spintronic devices. In the following we present the preliminary results of our investigation. The Hall effect and conductivity of In 1-x Mn x Sb samples in B up to 13 T in superconducting magnet and up to 45 T in pulsed magnetic field using standard DC technique were investigated. The measurements were performed in temperature range K. The magnetooptical Kerr effect, which is proportional to magnetization, was already observed in one of the samples. S 1 Sn 1-x Mn x Te samples were grown in Institute of Physics PAS by B. Witkowska and In 1-x Mn x Sb layers were grown by T. Wojtowicz and G. Cywiński in group of Prof. J. Furdyna in Notre Dame University. 2 Work is conducted with cooperation with W. Dobrowolski IP PAS Warsaw, and O. Portugall and M. Goiran LNCMP Toulouse. 15

16 Properties of heterostructures: ferromagnetic islands placed over DMS quantum wells K.M. Lebecki and J. Kossut Institute of Physics, Polish Academy of Sciences Al. Lotnikow 32/46, Warsaw, Poland Magnetic behavior of small ferromagnetic structures, e.g. iron islands, has been already explored. Such structures are typically in lateral range of few microns till hundred nanometers. Calculations show, that under special conditions, if such islands are magnetized, then their magnetization will be similar to a mono-domain structure. Magnetic fields, generated by such magnetized islands, are called the fringe fields. An additional planar structure can be placed below the sample surface, while the ferromagnetic island is placed on the sample surface. When calculating the fringe field value in such a plane, regions with higher field value (around 0.5 Tesla) can be found. Two major experimental approaches were used so far to observe the fringe field: electrical measurements and optical observations. In the electrical setup, a Hall bar is formed from a twodimensional electron gas below a sample surface. Measured resistance showed, that magnetization of a ferromagnetic island indeed exist and can be observed. On the other hand, an optical setup was described only in few publications. Here, ferromagnetic islands are placed above a quantum well (QW) made of a DMS (diluted magnetic semiconductor). Luminescence from the QW is observed. However, the influence of fringe field on optical phenomena was so far shown only qualitatively. The goal of present work is to continue the investigations of the influence of the fringe field on optical properties in DMS QW and to search for accompanying optical phenomena. Among them there may be effects related to condensation of excitons. Other reasons to explore this subject are possible applications in small magneto-optical devices. Planned activities are focused on one side on measurements using micro-coils and on the other side on observing arrays of many islands. The reason to use micro-coils is the following: there are technical problems in placing strong classical coils in a cryostat used for micro-optical measurements. Small micro-coils (inner diameter is in range of microns) can quite easily produce strong enough field to magnetize an island placed in the center of the coil. Pulse currents are used usually in this case. The question is still open, whether it would be possible to create small enough micro-coil to work in a DC mode. The reason to work with arrays of islands is as follows. In microoptical measurements, the focused spot size has a diameter of ca. 1-3 microns. On the other side, the island must be smaller than a micron (planar size) to maintain a mono-domain structure. So, measuring arrays instead of single islands can improve the signal originated from fringe field. It may be even possible to work with normal optical setup in this case without the micro-optical equipment limitations. 16