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Spring 1999 logo1999 MRS Spring Meeting & Exhibit

April 5-9, 1999 | San Francisco
Meeting Chairs: Katayun Barmak, James S. Speck, Raymond T. Tung, Paul D. Calvert

Symposium K—Hybrid Magnetic, Semiconductor, and Superconductor Structures

S. James Allen 
Dept of Physics 
Univ of California-Santa Barbara 
Santa Barbara, CA 93106 

Paul Crowell
Univ of Minnesota
School of Physics & Astronomy
Minneapolis, MN 55455

Mark Johnson 
Materials Science & Technology Div 
Naval Research Laboratory 
Code 6341 
Washington, DC 20375 

Hideo Ohno
Res Inst Electrical Communication
Tohoku Univ
Sendai, 980-8577 JAPAN

Symposium Support 
*Keithley Instruments, Inc. 
*Office of Naval Research 

1999 Spring Exhibitor

* Invited paper
Chair: S. James Allen 
Monday Morning, April 5, 1999 
Salon 2 (M)
8:30 AM K1.1 
PROXIMITY EFFECTS IN SUPERCONDUCTOR/ FERROMAGNET TRILAYERS. J.M.E. Geers, J. Aarts , Leiden University, Kamerlingh Onnes Laboratory, Leiden, THE NETHERLANDS; A.A. Golubov, Technical University Twente, Enschede, THE NETHERLANDS.

With the increased interest in superconductor / ferromagnet (S/F) structures, it is of use to understand the mechanisms for suppression of superconductivity at the S/F interface, and especially the role of the F-layer magnetic moment . This suppression can be investigated by determining the critical thickness dcr for occurrence of superconductivity in F/S/F trilayers. We compared the behaviour of two different S/F systems, V/(V,Fe) and Nb/(V,Fe), where  of the (V,Fe) alloy can be varied between 2  and 0.25  by choosing different Fe concentrations. In both cases we find that dcr at high  is much lower than expected from proximity effect calculations on the basis of pair breaking by the exchange field in the F layer. This can be described by an interface transparency parameter T which we find to decrease linearly with increasing , probably due to the increasing suppression of Andreev reflection processes by the increasing spin splitting of the F subbands. At low  we find differences between the Nb and the V systems, with a higher transparency for the case of Nb. This is unexpected, since the larger lattice mismatches in the case of Nb/(V,Fe) lead to larger crystalline disorder at the interface. Apparently, the mechanism causing low values for T is quenched by the disorder rather than enhanced, again demonstrating that it is intrinsic to the S/F problem.

8:45 AM K1.2 
SUBMICRON-SCALE PERIODIC HYBRID STRUCTURES. V. Metlushko , U. Welp, G. Crabtree, R.M. Osgood III, S.D. Bader, Materials Science Division, Argonne National Laboratory, Argonne, IL; B. Ilic, K. Chung, P.J. Hesketh, EECS, University of Illinois at Chicago, IL; L.E. DeLong, University of Kentucky, Lexington, KY; Wenjun Fan, S.R.J. Brueck, Center for High Technology Materials, University of New Mexico, Albuquerque, NM.

The interaction of vortex lattice with the periodic arrays of holes, submicron size magnetic dots and antidots (characteristic sizes   100-400 nm, periods of 1-2  ) defined by laser interferometric lithography were studied in superconducting (Nb) /magnetic (Fe) hybrid structures deposited on a Si wafer with a 100-nm thick, thermally grown SiO2 insulating layer and patterned using wet chemical or reactive ion (SF6) etching . A formation of new highly ordered interstitial vortex phases with different symmetry for fields up to the 10th matching field leads to a well defined maxima in the field dependence of the critical current. The possible application for new generation of active magnetic and superconducting devices will be discussed. *This work was supported by the U.S. DOE, BES-Materials Sciences, under contract W-31- 109-ENG-38 (U.W., V.M., G.C.), by DARPA (W.F., S.R.J.B.), by U.S. DOE BES-Materials Sciences, Grant DE-FG02-97ER45653 (L.E.D)

9:00 AM *K1.3 
SPIN INJECTION INTO SUPERCONDUCTORS WITH FERROMAGNETIC CONTACTS. A.M. Goldman , V. Vasíko, P. Kraus, and K. Nikolaev, University of Minnesota, School of Physics and Astronomy, Minneapolis, MN.

Advances in molecular beam epitaxial growth of oxides have facilitated the growth of heterostructures consisting of superconducting oxides (cuprates) and ferromagnetic oxides (manganites). Bilayer structures consisting of dysprosium barium copper oxide and various lanthanum manganites have been investigated. The half-metallic character of the manganites leads to spin polarization of their charge carriers. The injection of carriers from manganite contacts into a cuprate superconductor results in the reduction of the critical current and transition temperature of the superconductor. Control experiments with identical geometries suggest that the reductions are substantial greater than those resulting from current injection from gold contacts, or contacts made from nonmagnetic conducting oxides. Other experiments rule out heating as a source of these observations. Studies of the differential conductance vs. voltage of manganite-cuprate interfaces also reveal unusual behavior. A conductance minimum at zero bias, suggestive of suppression of the Andreev channel for conduction, is found. This presumable occurs because the carriers entering the superconductor from the ferromagnetic contact are at least partially spin polarized. The latter conjecture is supported by measurements of the temperature and magnetic field dependence of a minimum in the conductance of the interface found at zero bias. The procedures used to fabricate these heterostructures along with the latest experimental results will be reported. * Supported in part by the Office of Naval Research under grant N/N00014-98-1-0098.

9:30 AM *K1.4 

I will give an overview of recent developments of the superconducting proximity effect (SPE) in hybrid superconductor/semiconductor and superconductor/metal systems, from both a fundamental as well as applied perspective. One aspect of the SPE is the modification of the dissipative transport in normal conductors coupled to superconductors. This has been studied extensively in so-called Andreev interferometers, where a normal conductor is connected to two superconductors, between which a phase difference can be applied (usually with a magnetic field) As a second manifestation the SPE makes it possible for a dissipationless supercurrent to flow through an S/N/S structure where a normal metal or semiconductor connects two superconducting electrodes. Recently Baselmans et al. [Nature, to be published] studied the critical supercurrent in a SNS device where an additional current could be send between two normal reservoirs connected to the N-region. The dimensions of the structure are smaller than both electron-phonon and electron-electron energy relaxation lengths. In this case a two-step non-thermal electron distribution is generated in the N-region. It was shown that when a voltage is applied between the normal reservoirs which exceeds a threshold value, the direction of the supercurrent can be reversed, resulting in a so-called pi-junction. In SINIS systems, where I represent tunnelbarriers, it is possible to reduce the electron temperature below that of the phonons, and thus effectively cool the electrons. In principle it is also possible to generate distributions which would correspond effectively to negative temperatures. Future prospects and possible applications of the above phenomena will be discussed.

10:30 AM K1.5 
OPTICAL SWITCHING OF JOSEPHSON JUNCTIONS WITH SEMICONDUCTOR BARRIERS. G. Bastian *, E.O. Gobel, Physikalisch-Technische Bundesanstalt, Braunschweig, GERMANY; J. Schmitz, M. Walther, J. Wagner, Fraunhofer-Institut für Angewandte Festkrperphysik, Freiburg, GERMANY; *Present address: NTT Basic Research Laboratories, Morinosato Wakamiya, Atsugi-shi, Kanagawa, JAPAN.

We have fabricated Joesphson junctions with a two-dimensional electron gas based on InAs/AlSb/GaSb as barrier. The advantages of this semiconductor-system for electronic applications due to its high mobilities and low effective masses as well as for optical applications due to its type II structure and the low energy gap have been exploited several times [1]. In particular, the occurrence of both positive and negative persistent photoeffect enables switching the carrier density inside the InAs layer between two states. We have studied the IV-characteristics during and after optical excitation of electron hole pairs inside the heterostructure. During illumination the junction's properties could be influenced by nonequilibrium effects, which reduced the critical current and modified the normal resistance. After the light is switched off, the junction is set to a stable state, which depends on the wavelength of the previous excitation. In junctions with high interface transparencies, the critical current could be altered by about 10%. We discuss, how this optical reversible switching of the junction's properties can be used in optical links to superconducting logics as well as for fundamental research. [1] J. Wagner et al., in Festkörperprobleme / Advances in Solid State Physics, Vol. 36, ed. by R. Helbig (Vieweg, Braunschweig/Wiesbaden), p.59 (1996)

10:45 AM K1.6 
NONEQUILIBRIUM SUPERCONDUCTIVITY IN NB-INAS-NB SNS JUNCTIONS. Konrad W. Lehnert , S. James Allen, Physics Dept and QUEST the NSF Center for Quantized Electronic Structures, Nathan Argaman, Inst for Theoretical Physics and QUEST, Hans-Richard Blank, Ki C. Wong, Evelyn Hu, Herbert Kroemer, Dept of Electrical and Computer Engineering and QUEST, Santa Barbara, CA.

Nonequilibrium dynamics in Nb-InAs-Nb junctions leads to a frequency doubled ac Josephson effect which persists to high temperatures, in the absence of a dc critical current. A nonequilibrium model, based on time-dependent Andreev bound states successfully accounts for this nonequilibrium ac Josephson effect and an enhancement in conductance near zero bias. We distinguish equilibrium and nonequilibrium effects through their very different temperature and bias dependences. Recent measurements also suggest that the phase breaking length in the InAs is about 6 at 4.2 K. This work is supported by the NSF through the NSF Science and Technology Center for Quantized Electronic Structures, Grant No. DMR-91-20007, and the ONR, grant No. N0014-92-J-1452.

11:00 AM *K1.7 

Junctions between a superconducting metal or alloy (SnPb, In, TiN) and various doped semiconductors (GaAs :Si, CdTe :In, doped Silicon) will be considered. These junctions are of Schottky-type, usually with a low transparency at the interface. Nevertheless relatively large sub-gap currents are observed, which are not compatible with a simple superconductor-insulator-semiconductor picture. An important fact is that the semiconducting part of the junction is itself very resistive, providing some effective confinement for carriers near the interface. Quantum interference effects associated to this confinement develop themselves in this semiconducting part, giving rise to sub-gap conductance anomalies. Examples of a zero bias conductance peak as well as a zero bias conductance dip will be presented. These anomalies are very sensitive to a magnetic field and to other depairing effects. An open question is to know if such anomalies are precursors of an enlarged proximity effect inside the bulk semiconductor, which would help for JOsephsonFET realizations. W. Poirier et al, Phys.Rev. Lett. 79, 2105 (1997) W. Poirier et al, to appear in European Physical Review B « mesoscopic physics (1998).

11:30 AM K1.8 
INTERFACE PROPERTIES AND LOW-FREQUENCY NOISE OF YBCO/Au JUNCTIONS. Yizi Xu , J.W. Ekin, National Institute of Standards & Technology, Boulder, CO.

Thin-film c-axis YBCO/Au junctions are characterized by extensive measurement of their conductance-voltage characteristics as well as by low-frequency noise measurement. We show strong evidence that tunneling is the main conduction mechanism at low temperatures for these junctions, and that the junction interface can be adequately modeled by a semiconductor/metal contact with a Schottky barrier . The junctions all exhibit a high level of resistance noise. The normalized resistance fluctuation, , is in  range, much greater than that found for bicrystal grain-boundary Josephson junctions. Moreover, the noise spectra at low temperatures closely resemble a Lorentzian function with a roll-off frequency about 500 Hz, indicating that the noise may be a random-telegraph type. The low-temperature noise spectra also show strong dependence on bias voltage. The source for such noise is believed to be the localized trapping states at the junction interface. Detailed measurements of the temperature and bias voltage dependence of the noise is underway to characterize these interface states. We discuss the noise behavior in light of the Schottky barrier model, and present evidence that the effects of the barrier can be mitigated, and junction noise level reduced, by annealing the contact in oxygen. The mechanism with which annealing improves the contact will also be discussed. 

Chair: Mark Johnson 
Monday Afternoon, April 5, 1999 
Salon 2 (M)
1:30 PM K2.1 
EPITAXIAL GROWTH OF FERROMAGNETIC METALS ON II-VI SEMIMAGNETIC SEMICONDUCTORS. Christophe Bourgognon, Serge Tatarenko and Joel Cibert, Laboratoire de Spectrometrie Physique, Universite Joseph Fourier Grenoble-CNRS, FRANCE; Bruno Gilles, Laboratoire de Thermodynamique et Physico-Chimie Metallurgiques/CNRS, Grenoble, FRANCE; Alain Marty, Yves Samson, Departement de Recherche Fondamentale sur la Matiere Condensee/CEA Grenoble, FRANCE; L. Carbonell, V.H. Etgens, Laboratoire de Mineralogie Cristallographie, Universite P et M Curie, Paris, FRANCE.

The presence of a ferromagnetic transition in single modulation doped quantum well of CdMnTe/CdZnMgTe:N was recently evidenced by photoluminescence magnetospectroscopy. The transition is driven by long range RKKY interaction between Mn spins mediated by the 2D hole gas of typical density 2 E11per square centimeter. The approach to the ordered phase at low temperature (2-5K) is characterized by a giant magnetic susceptibility to a magnetic field applied perpendicular to the QW layer. The growth of ferromagnetic metals exhibiting a perpendicular magnetic anisotropy on such structures will have interesting effects on the properties of the QW. In order to produce thin magnetic layers with a high perpendicular anisotropy, Fe0.5Pd0.5 epitaxial thin films were grown on the semiconductor layer. FePd exhibits a L10 ordered tetragonal structure consisting of alternating Fe and Pd atomic planes so that it has only one fourfold symmetry axis which is expected to be an easy magnetization axis. To accomodate the high lattice mismatch between the CdTe substrate and the FePd layer and to smooth the starting surface, buffer layers of Au and Pd were successively grown. The FePd elaborated by codeposition at 520K was studied by X-ray diffraction. A long range order parameter of 0.6 was determined. Magnetization curves measured with a Vibrating Sample Magnetometer indicate an orientation of the magnetization out of the plane (The uniaxial anisotropy constant Ku=6.6 E6 erg per cubic centimeter). Finally, magnetic domains of the FePd thin film are observed by Magnetic Force Microscopy. Dark and bright areas corresponding to up and down perpendicularly magnetized domains are clearly imaged with a high contrast. Their well defined characteristic width is about 70 nm. A comparison with the properties of the layers grown on GaAs and ZnSe substrates will be given.

1:45 PM K2.2 
ELECTRICAL CONTACT RESISTANCE OF FERROMAGNETIC METALS TO InAs(001)*. B.T. Jonker and B.R. Bennett, Naval Research Laboratory, Washington, DC.

Spin polarized carrier populations may be readily created in semiconductor heterostructures by optical pumping, and surprisingly long spin lifetimes and diffusion lengths have been observed in GaAs [1,2]. It is very desirable to electrically inject spin polarized carriers via a ferromagnetic contact to increase the potential for practical applications. This has been an elusive goal, however, and spin scattering at the interface between the magnetic contact and semiconductor appears to be the limiting factor. Initial efforts to address these concerns have only recently been reported for the case of GaAs [3]. InAs is an attractive material for spin injection / transport devices, since it exhibits high mobility and surface carrier accumulation (rather than depletion), making it relatively easy to form ohmic-like contacts. Planar, non-alloyed contacts with minimal resistance are highly desirable to minimize local heating and scattering. To that end, we have measured the contact resistance of several magnetic metals - permalloy, Fe and Co - on InAs epilayers. The samples consist of 0.1um InAs epilayers grown by MBE on 1.0 um GaSb buffer layers on GaAs(001). The magnetic contacts were deposited ex situ at room temperature using electron beam deposition and standard liftoff techniques. No annealing treatments were performed. The contact resistance was determined from four-point probe measurements on transmission line model contacts. We obtain values of 0.05, 0.5 and 0.1 ohm-millimeter for permalloy, Fe and Co contacts, respectively. These results compare favorably with the value of 0.1 ohm-millimeter reported for a Pd/Pt/Au ohmic contact scheme (with 200 C anneal) developed recently to minimize thermal processing and lateral diffusion [4]. We compare results from our ex situ contacts with those obtained from samples for which the ferromagnetic films are deposited in situ via MBE. 

2:00 PM *K2.3 
HYBRID HALL-EFFECT DEVICES. Brian R. Bennett *, Electronics Science and Technology Division, Naval Research Laboratory, Washington, DC. *In collaboration with M. Johnson, M.J. Yang, P.R. Hammar, M.M. Miller and B.V. Shanabrook

Recent results for a novel magnetoelectronic device with digital applications will be presented. The device is composed of a high-mobility InAs/AlGaSb single quantum well grown by molecular beam epitaxy. A semiconducting Hall cross is formed, and an electrically-isolated, micro-structured ferromagnetic film F is fabricated with one edge directly over the center of the Hall cross. Magnetic fringe fields from the edge of F have a large component Bz perpendicular to the plane of the semiconductor and generate a Hall voltage in the sensor arms of the Hall cross. The sign of the fringe field, as well as the sign of the output Hall voltage, is switched by reversing the in-plane magnetization of the ferromagnet. Structures have been fabricated using patterned films of NiFe and FeCo with dimensions from 300 nm by 950 nm to 2 m by 12 m. Device applications as a nonvolatile memory cell and Boolean logic gate will be discussed. Features include 0.1 V output levels and inverse scalability: output increases as device dimensions shrink.

2:30 PM *K2.4 
PERIODIC MICROMAGNET ARRAYS ON TOP OF TWO-DIMENSIONAL ELECTRON SYSTEMS: MAGNETOTRANSPORT AND MORE. D. Weiss , R. Sattler, J. Raabe, J. Zweck, R. Pulwey, W. Breuer, T. Schweinboeck, Experimentelle und Angewandte Physik, Regensburg, GERMANY; P.D. Ye, NTT-Basic Research Laboratories, Atsugi-shi, JAPAN; V. Umanki, Weizmann Institute, Rehovot, ISRAEL.

A periodic array of micromagnets on top of semiconductor heterojunctions containing a two-dimensional electron gas (2DEG) generates a periodic magnetic field which influences dramatically the electron motion in the 2DEG. Measurements of the resistivity of the electron gas display magnetoresistance oscillations which reflect the commensurability between the relevant lengths scales of the system, the cyclotron radius Rc at the Fermi energy and the period a of the magnetic field modulation. In this presentation a brief overview on the most prominent features observed in experiment will be given. While the observed magnetoresistance can be nicely modelled by semiclassical calculations we lack information about the detailed magnetization reversal of micromagnet arrays or individual micro- and nanomagnets. We present our first attempts to gain more experimental insight on this issue by employing Lorentz- and magnetic force microscopy.

3:30 PM K2.5 
INDUCED MAGNETORESISTANCE IN SEMICONDUCTING DEVICES DUE TO SINGLE SUB-MICRON MAGNETIC BARRIERS AND RANDOM MAGNETIC FIELDS. V. Kubrak, A.W. Rushforth, F. Rahman, B.L. Gallagher , P.C. Main, School of Physics & Astronomy, University of Nottingham, Nottingham, UNITED KINGDOM; J. de Boeck, M. Behest, IMEC vzw, Kapeldreef, Leuven, BELGIUM; C.H. Marrows, M.A. Howson, Department of Physics, University of Leeds, Leeds, UNITED KINGDOM.

We observe strongly hysteretic magnetoresistance (MR) in near-surface two-dimensional electron devices due to the fringing field of ferromagnet elements fabricated on the surface of the devices. We have carried out the first investigation of the effect of single sub-micron magnet barriers due to single sub-micron Co stripes. We are able to use the observed MR to perform nanomagnetometry: obtaining the hysteretic and switch properties of the ferromagnetic element. We also observe a novel MR due to the sub-micron correlation length random fringing fields produced by maze domain patterns in CoPd overlayers. The experimental results are compared with detailed numerical calculations.

3:45 PM K2.6 
SPIN DEPENDENT ELECTRON TRANSPORT THROUGH THE FERROMAGNET/SEMICONDUCTOR INTERFACE. Atsufumi Hirohata, Yongbing Xu, Christian M. Guertler, J.A.C. Bland , Cavendish Laboratory, University of Cambridge, Cambridge, ENGLAND.

Since the possibilities for developing a spin polarized scanning tunneling microscopy (SP-STM) were theoretically raised by Molotkov [1], a great many studies have been recently carried out in the field of spin dependent tunneling through metal/oxide insulator/semiconductor (MOS) junctions [2]. However, due to the presence of the oxide layer, the mechanism of the spin dependent tunneling through the junction is extremely complicated. For the direct ferromagnet (FM)/semiconductor(SC) interface, since a Schottky barrier arises which also gives rise to tunneling under appropriate bias conditions, it is a good candidate for understanding the process of spin polarized electron transport. We report the results of a search for spin dependent electron transport at the ferromagnet/semiconductor interface made by measuring the bias dependence of a photon excited current through the interface. A circularly polarized laser beam was used to excite electrons with a spin polarization perpendicular to the film plane from the FM side. Samples of the form 3 nm Au/5 nm FM (Ni80Fe20, Co and Fe)/GaAs ((110), n+=1024 m-3) were prepared with two Al electrical contacts on the Au layer and one ohmic contact at the bottom of the substrate. The bias dependent I-V characteristic (-1<V<1 V) was obtained using conventional four-terminal measurements both with and without unpolarized laser illumination. The polarization of the beam was then modulated in order to introduce photons with a certain helicity. From the first two types of samples, a significant transport current was detected with a magnitude dependent on the relative orientation of the spin polarization and the magnetization vector under appropriate bias conditions. For the NiFe/GaAs structure, at perpendicular saturation the bias dependence of the photocurrent was observed to change in the range 0.7-0.8 eV when the helicity is reversed. This bias range corresponds to the Schottky barrier height and is consistent with the transport of polarized electrons from the semiconductor to the ferromagnetic layer. [1] S.N. Molotkov, Surf. Sci.287/288 (1993) 1098. [2] M.W. Prins et al.Phys. Rev.B 53 (1996) 8090.

4:00 PM K2.7 
UNUSUAL VOLTAGE DEPENDENCE OF THE MAGNETORESISTANCE IN DOUBLE TUNNEL JUNCTIONS. F. Montaigne , P. Seneor, F. Petroff, F. Nguyen Van Dau, A. Schuhl, A. Vaurës and A. Fert, Unité Mixte de Physique CNRS-THOMSON, Orsay, FRANCE.

Magnetic tunnel junctions show a strong bias voltage dependence of the magnetoresistance ratio. We have reduced this drawback by using a double tunnel junction with the structure Si//Co/Al2O3/Co/Al2O3/FeNi/Au. The layers are deposited by RF sputtering and alumina barriers are formed by oxidation of a 2 nm Al layer in a RF Ar/O2 plasma. Junctions in the micrometer range are then patterned by optical lithography. These junctions exhibit three different resistance states depending on the relative orientation of the three magnetic layers. The maximum magnetoresistive effect reaches 25% at low temperature and 16% at room temperature (identical to the value we obtain with single Co/Al2O3/FeNi junctions). Actually, this value depends on the magnetic behaviour of the intermediate layer which varies strongly as we vary its thickness between 0.4 nm (granular layer) and 5 nm (continuous layer). The voltage dependence of the tunnel conductance presents interesting features like Coulomb blockade at low temperature when the intermediate Co layer is granular. Even more interesting is the bias voltage dependence of the magnetoresistance (theoretically divided by a factor of 2 with two equivalent junctions in series) which is also dramatically influenced by this intermediate layer. The voltage for which the MR is halved ranges from 0.4 to 1 V (to be compared to 0.3V in single junctions) and is maximum for an intermediate Co thickness of 2 nm. This point highlights a richer conduction process than sequential tunneling through the two barriers. It is also a new step towards the applicability of tunnel junctions for magnetic memories as well as sensors.

4:15 PM K2.8 
SPIN-DEPENDENT TRANSMISSION OF ELECTRONS THROUGH THE FERROMAGNETIC METAL BASE OF A HOT-ELECTRON TRANSISTOR-LIKE SYSTEM. A. Filipe, H.J. Drouhin, G. Lampel, Y. Lassailly, J. Peretti, T. Wirth , Laboratoire de Physique de la Matiëre Condensée, Ecole Polytechnique, Palaiseau, FRANCE; J. Nagle, A. Schuhl, Laboratoire Central de Recherche, Thomson-CSF, Orsay, FRANCE.

A quasimonoenergetic spin-polarized electrons beam, emitted in vacuum from a GaAs photocathode, is injected into a thin ferromagnetic metal layer deposited on an n-doped GaAs substrate. The current transmitted through this Schottky barrier is measured. The striking feature of this hot-electron transistorlike system is a current gain spin dependency as high as 20. The measured variations of the current gain and its spin dependency with the injection energy are well explained by a very simple analytical model describing the transport of hot electrons in metallic thin films.