A THOMAS

Publications (43)16.82 Total impact

• Source

  Available from: ArXiv

  Article: Electric breakdown in ultra-thin MgO tunnel barrier junctions for spin-transfer torque switching

  M. Schäfers, V. Drewello, G. Reiss, A. Thomas, K. Thiel, G. Eilers, M. Münzenberg, H. Schuhmann, M. Seibt
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  ABSTRACT: Magnetic tunnel junctions for spin-transfer torque switching were prepared to investigate the dielectric breakdown. The breakdown occurs typically at voltages not much higher than the switching voltages, a bottleneck for the implementation of spin-transfer torque Magnetic Random Access Memory. Intact and broken tunnel junctions are characterized by transport measurements and then prepared for transmission electron microscopy and energy dispersive x-ray spectrometry by focussed ion beam. The comparison to our previous model of the electric breakdown for thicker MgO tunnel barriers reveals significant differences arising from the high current densities.
  07/2009;
• Article: Dielectric breakdown and inelastic electron tunneling spectroscopy of top and bottom pinned Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junctions

  Ayaz Arif Khan, J. Schmalhorst, A. Thomas, V. Drewello, G. Reiss
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  ABSTRACT: The time dependent dielectric breakdown in Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junctions was investigated by voltage ramp experiments. The measurements were done for two types of junctions: one set of junctions had exchange biased (pinned) bottom electrodes and one set had exchange biased (pinned) top electrodes with an additional artificial ferrimagnet. We found a significant polarity dependence in the dielectric breakdown: top as well as bottom pinned tunnel junctions showed higher breakdown voltage when the top electrode was biased positively compared to negative bias. In contrast to this the differential resistance (dV/dI)-V spectra revealed an asymmetry for the top pinned junctions which was reversed in comparison to the bottom pinned system. This indicates that both asymmetries have different origins. Additionally the bottom pinned junctions showed in general slightly lower breakdown voltages and stronger magnon excitation in the inelastic electron tunneling (d²I/dV²)-V spectra than the top pinned junctions. Possible reasons for these correlations are discussed.
  Journal of Applied Physics 05/2009; · 2.17 Impact Factor
• Article: Element-specific study of the temperature dependent magnetization of Co–Mn–Sb thin films

  J. Schmalhorst, D. Ebke, M. Meinert, A. Thomas, G. Reiss, E. Arenholz
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  ABSTRACT: Magnetron sputtered thin Co–Mn–Sb films were investigated with respect to their element-specific magnetic properties. Stochiometric Co ₁ Mn ₁ Sb ₁ crystallized in the C1_b structure has been predicted to be half-metallic and is therefore of interest for spintronic applications. It should show a characteristic antiferromagnetic coupling of the Mn and Co magnetic moments and a transition temperature T_C of about 480 K. Although the observed transition temperature of our 20 nm thick Co _32.4 Mn _33.7 Sb _33.8 , Co _37.7 Mn _34.1 Sb _28.2 , and Co _43.2 Mn _32.6 Sb _24.2 films is in quite good agreement with the expected value, we found a ferromagnetic coupling of the Mn and Co magnetic moments which indicates that the films do not crystallize in the C1_b structure and are probably not fully spin polarized. The ratio of the Co and Mn moments does not change up to the transition temperature and the temperature dependence of the magnetic moments can be well described by the mean-field theory.
  Journal of Applied Physics 04/2009; · 2.17 Impact Factor
• Article: Dielectric breakdown in Co–Fe–B/MgO/Co–Fe–B magnetic tunnel junction

  Ayaz Arif Khan, J. Schmalhorst, A. Thomas, O. Schebaum, G. Reiss
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  ABSTRACT: The time-dependent dielectric breakdown has been investigated in Co–Fe–B/MgO/Co–Fe–B junctions by voltage ramp experiments and focused on its dependence on the barrier thickness, junction area, polarity of the applied voltage, ramp speed, and annealing temperature. The results suggest that the breakdown voltage strongly depends both on the polarity of the applied voltage and the annealing temperature. Magnetic tunnel junctions (MTJs) with positive bias on the top electrode show higher breakdown voltage than MTJs with negative bias. We found that there is a significant decrease in the breakdown voltage when the annealing temperature is increased above 350 ° C .
  Journal of Applied Physics 07/2008; · 2.17 Impact Factor
• Source

  Available from: ArXiv

  Article: Direct imaging of the structural change generated by dielectric breakdown in MgO based magnetic tunnel junctions

  A. Thomas, V. Drewello, M. Schaefers, A. Weddemann, G. Reiss, G. Eilers, M. Muenzenberg, K. Thiel, M. Seibt
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  ABSTRACT: MgO based magnetic tunnel junctions are prepared to investigate the dielectric breakdown of the tunnel barrier. The breakdown is directly visualized by transmission electron microscopy measurements. The broken tunnel junctions are prepared for the microscopy measurements by focussed ion beam out of the junctions characterized by transport investigations. Consequently, a direct comparison of transport behavior and structure of the intact and broken junctions is obtained. Compared to earlier findings in Alumina based junctions, the MgO barrier shows much more microscopic pinholes after breakdown. This can be explained within a simple model assuming a relationship between the current density at the breakdown and the rate of pinhole formation.
  06/2008;
• Article: Chemical and Magnetic Interface Properties of Tunnel Junctions With Co2 MnSi/Co2FeSi Multilayer Electrode Showing Large Tunneling Magnetoresistance

  J. Schmalhorst, D. Ebke, M.D. Sacher, Ning-Ning Liu, A. Thomas, G. Reiss, A. Hutten, E. Arenholz
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  ABSTRACT: Transport, as well as chemical and magnetic interface properties of two kinds of magnetic tunnel junctions (MTJs) with Co₂FeSi electrode, Al-O barrier, and Co-Fe counter electrode, are investigated. For junctions with Co₂FeSi single-layer electrodes, a tunnel magnetoresistance of up to 52% is found after optimal annealing for an optimal Al thickness of 1.5 nm, whereas the room temperature bulk magnetization of the Co₂FeSi film reaches only 75% of the expected value. By using a [Co₂MnSi/Co₂FeSi]_times10 multilayer electrode, the magnetoresistance can be increased to 114%, corresponding to a large spin polarization of 0.74, and the full bulk magnetization is reached. For Al thickness smaller than 1 nm, the TMR of both kinds of MTJs decreases rapidly to zero. On the other hand, for 2- to 3-nm-thick Al, the TMR decreases only slowly. The Al thickness dependence of the TMR is directly correlated to the element-specific magnetic moments of Fe and Co at the Co₂FeSi/Al-O interface for all Al thickness. Especially, for optimal Al thickness and annealing, the interfacial Fe moment of the single-layer electrode is about 20% smaller than for the multilayer electrode, indicating smaller atomic disorder at the barrier interface for the latter MTJ.
  IEEE Transactions on Magnetics 07/2007; · 1.36 Impact Factor
• Source

  Available from: ArXiv

  Article: Influence of chemical and magnetic interface properties of Co-Fe-B / MgO / Co-Fe-B tunnel junctions on the annealing temperature dependence of the magnetoresistance

  J. Schmalhorst, A. Thomas, G. Reiss, X. Kou, E. Arenholz
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  ABSTRACT: The knowledge of chemical and magnetic conditions at the Co40Fe40B20 / MgO interface is important to interpret the strong annealing temperature dependence of tunnel magnetoresistance of Co-Fe-B / MgO / Co-Fe-B magnetic tunnel junctions, which increases with annealing temperature from 20% after annealing at 200C up to a maximum value of 112% after annealing at 350C. While the well defined nearest neighbor ordering indicating crystallinity of the MgO barrier does not change by the annealing, a small amount of interfacial Fe-O at the lower Co-Fe-B / MgO interface is found in the as grown samples, which is completely reduced after annealing at 275C. This is accompanied by a simultaneous increase of the Fe magnetic moment and the tunnel magnetoresistance. However, the TMR of the MgO based junctions increases further for higher annealing temperature which can not be caused by Fe-O reduction. The occurrence of an x-ray absorption near-edge structure above the Fe and Co L-edges after annealing at 350C indicates the recrystallization of the Co-Fe-B electrode. This is prerequisite for coherent tunneling and has been suggested to be responsible for the further increase of the TMR above 275C. Simultaneously, the B concentration in the Co-Fe-B decreases with increasing annealing temperature, at least some of the B diffuses towards or into the MgO barrier and forms a B2O3 oxide.
  01/2007;
• Chapter: Magnetic Tunneling Junctions — Materials, Geometry and Applications

  G. Reiss, H. Koop, D. Meyners, A. Thomas, S. Kämmerer, J. Schmalhorst, M. Brzeska, X. Kou, H. Brückl, A. Hütten
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  ABSTRACT: The discoveries of antiferromagnetic coupling in Fe/Cr multilayers by Grünberg, the Giant MagnetoResistance by Fert and Grünberg and a large tunnelling magnetoresistance at room temperature by Moodera have triggered enormous research on magnetic thin films and magnetoelectronic devices. Large opportunities are especially opened by the spin dependent tunnelling resistance, where a strong dependence of the tunnelling current on an external magnetic field can be found. Within a short time, the quality of these junctions increased dramatically. We will briefly address important basic properties of these junctions depending on the material stacking sequence of the underlying standard thin film system with special regard to complex interdiffusion properties. New materials with potentially 100% spin polarization will be discussed using the example of the full Heusler compound Co2MnSi, where we obtain up to 100% TMR at low temperature. Next, we discuss scaling issues, i.e. the influence of the geometry of small tunnelling junctions especially on the magnetic switching behaviour down to junction sizes below 0.01 µm2. The last part will give a short overview on field programmable logic circuits made from magnetic tunnelling cells, where we demonstrate the clocked operation of a programmed AND gate.
  12/2006: pages 147-165;
• Article: Temperature dependence of the resistance of magnetic tunnel junctions with MgO barrier

  X. Kou, J. Schmalhorst, A. Thomas, G. Reiss
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  ABSTRACT: The temperature dependent tunneling resistance of magnetic tunnel junctions with MgO barriers was characterized. In the junctions prepared by magnetron sputtering, the tunnel magnetoresistance decreases with increasing temperature. Various contributions to the tunnel conductance are discussed using different models. Not only the direct elastic tunneling contributes to the temperature dependence of tunnel magnetoresistance, but also the assisted, spin-independent tunneling plays an important role in determining the temperature dependent behavior in our magnetic tunneling junctions. The process is further investigated assuming magnon and phonon assisted tunneling and compared to junctions with alumina tunnel barrier.
  Applied Physics Letters 06/2006; · 3.84 Impact Factor
• Article: The current potential of Co2MnSi Heusler alloy electrodes in magnetic tunnel junctions

  A. Hütten, S. Kämmerer, J. Schmalhorst, A. Thomas, G. Reiss
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  ABSTRACT: We have integrated Co2MnSi as a representative of the full-Heusler compound family as one magnetic electrode into technological relevant magnetic tunnel junctions. The preparation strategy has been chosen so as to sputter Co2MnSi at room temperature onto a V-buffer layer, which assists in (110) texture formation, and to deposit the Al-barrier layer directly thereafter. After plasma oxidizing the Al-barrier layer subsequent annealing leads (1) to the texture formation and (2) to the appropriate atomic ordering within the Co2MnSi and (3) homogenizes the AlOx-barrier. The resulting magnetoresistance of the Co2MnSi containing magnetic tunnel junctions has been determined as a function of temperature. At a bias voltage of 1 mV a magnetoresistance of 94.6% has been realized at 20 K leading to a spin polarization of 65.5%. Furthermore, the microstructural as well as magnetic quality of the Co2MnSi/AlOx-barrier interface has been investigated in detail. Additionally, the problem of antisite formation at the Co2MnSi/AlOx-barrier interface is experimentally addressed by intercalating thin Co layers in between the Heusler layer and the AlOx barrier. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  physica status solidi (a) 11/2004; 201(15):3271 - 3279. · 1.21 Impact Factor
• Article: New materials and applications for magnetic tunnel junctions

  G. Reiss, H. Brückl, A. Hütten, H. Koop, D. Meyners, A. Thomas, S. Kämmerer, J. Schmalhorst, M. Brzeska
  [show abstract] [hide abstract]
  ABSTRACT: Large opportunities in magnetolectronic devices are opened by the spin dependent tunnelling resistance, where a strong dependence of the tunnelling current on an external magnetic field can be found. Within a short time, the quality of the junctions increased dramatically. We will briefly address some important basics depending on the material stacking sequence of the underlying thin film system with special regard to the ferromagnetic electrodes. Scaling issues, i.e. the influence of the geometry of small tunnelling junctions especially on the magnetic switching behaviour are considered down to junction sizes below 0.01 μm2. The last part will give a short overview on applications beyond the use of the tunnelling elements as storage cells in MRAMs. This concerns mainly field programmable logic circuits, where we demonstrate the clocked operation of a programmed AND gate. The second ‘unconventional’ feature is the use as sensing elements in DNA or protein biochips, where molecules marked magnetically with commercial beads can be detected via the dipole stray field in a highly sensitive and relatively simple way. (© 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)
  physica status solidi (a) 05/2004; 201(8):1628 - 1634. · 1.21 Impact Factor
• Article: Temperature and bias voltage dependence of Co-Fe-AlOx-Py-AlOx-Co-Fe double-barrier junctions

  A. Thomas, H. Bruckl, J. Schmalhorst, G. Reiss
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  ABSTRACT: Exchange-biased magnetic tunnel junctions with single and double barriers are investigated with respect to the temperature and bias voltage dependence of the tunneling magnetoresistance. The single-barrier junctions show a tunneling magnetoresistance ratio of up to 49% at room temperature (72% at 10 K), the double-barrier systems up to 38% (57%), respectively, with three clearly separated magnetic states. The temperature and bias voltage dependence of the double-barrier junctions is explained as a serial connection of two single junctions. Theoretical studies of the tunneling magnetoresistance exhibit that the magnetoresistance ratio can be enhanced by ballistic electrons in double-barrier junctions, but only if the potential of the middle electrode can be shifted.
  IEEE Transactions on Magnetics 10/2003; · 1.36 Impact Factor
• Article: Spinelectronics and its applications

  G. Reiss, H. Brückl, A. Hütten, J. Schmalhorst, M. Justus, A. Thomas, S. Heitmann
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  ABSTRACT: The discovery of antiferromagnetic coupling in metallic Fe/Cr multilayers by Grünberg has triggered enormous research activities in the area of magnetic thin films. Additionally, the resistance of multilayers in the antiferromagnetic state is higher than in the parallel state at magnetic saturation. This Giant Magneto Resistance is caused by spin-dependent scattering of the conduction electrons in the magnetic layers. For applications, however, moderate saturation fields, tailorable resistance characteristics and good temperature stability are required. Additional opportunities are opened by a similar effect in magnetic tunnel junctions. Here, the tunneling probability depends on the relative orientation of the magnetizations of the electrodes and thus a large dependence of the tunneling current on an external magnetic field can be found. This effect is usually called Tunneling Magneto Resistance and can again be used both for detecting external fields as well as for information storage. Much more possible applications are still ahead, especially after the finding of magnetoelectronic effects in semiconductors. In this contribution, we will sketch basic physics of these effects and give examples for current developments.
  physica status solidi (b) 03/2003; 236(2):289 - 302. · 1.32 Impact Factor
• Article: Low B2 crystallization temperature and high tunnel magnetoresistance in Co2FeAl/MgO/Co–Fe magnetic tunnel junctions

  D Ebke, P Thomas, O Schebaum, M Schäfers, D Nissen, V Drewello, A Hütten, A Thomas
  322(8):996-998.
• Article: Room-temperature preparation and magnetic behavior of Co2MnSi thin films

  S KAMMERER, S HEITMANN, D MEYNERS, D SUDFELD, A THOMAS, A HUTTEN, G REISS
  JOURNAL OF APPLIED PHYSICS. 93(10):7945-7947.
• Article: Preparation of Heusler thin films: The quaternary alloy CO2Fe0.5Mn0.5Si

  D EBKE, A THOMAS, A HUTTEN, B BALKE, C FELSER, J SCHMALHORST, G REISS
  PHYSICA STATUS SOLIDI A-APPLICATIONS AND MATERIALS SCIENCE. 205(10):2298-2301.
• Article: Aluminum oxidation by a remote electron cyclotron resonance plasma in magnetic tunnel junctions

  A THOMAS, H BRUCKL, MD SACHER, J SCHMALHORST, G REISS
  JOURNAL OF VACUUM SCIENCE & TECHNOLOGY B. 21(5):2120-2122.
• Article: Direct measurement of the spin polarization of Co2FeAl in combination with MgO tunnel barriers

  O Schebaum, D Ebke, A Niemeyer, G Reiss, JS Moodera, A Thomas
  Journal of Applied Physics. 107(9).
• Article: Interlayer exchange coupling and current induced magnetization switching in magnetic tunnel junctions with MgO wedge barrier

  W Skowronski, T Stobiecki, J Wrona, K Rott, A Thomas, G Reiss, S Dijken
  Journal of Applied Physics. 107(9).
• Article: Dielectric breakdown and inelastic electron tunneling spectroscopy of top and bottom pinned Co-Fe-B/MgO/Co-Fe-B magnetic tunnel junctions

  AA KHAN, J SCHMALHORST, A THOMAS, V DREWELLO, G REISS
  JOURNAL OF APPLIED PHYSICS. 105(8).