Publications (15)140.51 Total impact
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Article: Magnetic tunnel junctions with MgO-EuO composite tunnel barriers
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ABSTRACT: The chalcogenide compound EuO is best known as a highly efficient spin-filter tunnel barrier material. Using the molecular beam epitaxy method, we combine polycrystalline EuO with epitaxial MgO and construct magnetic tunnel junctions with such hybrid tunnel barriers. Tunnel magnetoresistance of over 40% was achieved in junctions with oxygen-rich EuO. For lower oxygen concentration, magnetoresistance decreases dramatically and eventually vanishes, indicating that spin filtering is weakened when the transport is mainly mediated by excess conduction channels through defect sites.Phys. Rev. B. 04/2012; 85(14). -
Article: Spin polarized tunneling in MgO-based tunnel junctions with superconducting electrodes
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ABSTRACT: We prepared magnetic tunnel junctions with one ferromagnetic and one superconducting Al-Si electrode. Pure cobalt electrodes were compared with a Co-Fe-B alloy and the Heusler compound Co2FeAl. The polarization of the tunneling electrons was determined using the Maki-Fulde-model and is discussed along with the spin-orbit scattering and the total pair-breaking parameters. The junctions were post-annealed at different temperatures to investigate the symmetry filtering mechanism responsible for the giant tunneling magnetoresistance ratios in Co-Fe-B/ MgO/ Co-Fe-B junctions.12/2011; -
Article: Seebeck effect in magnetic tunnel junctions.
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ABSTRACT: Creating temperature gradients in magnetic nanostructures has resulted in a new research direction, that is, the combination of magneto- and thermoelectric effects. Here, we demonstrate the observation of one important effect of this class: the magneto-Seebeck effect. It is observed when a magnetic configuration changes the charge-based Seebeck coefficient. In particular, the Seebeck coefficient changes during the transition from a parallel to an antiparallel magnetic configuration in a tunnel junction. In this respect, it is the analogue to the tunnelling magnetoresistance. The Seebeck coefficients in parallel and antiparallel configurations are of the order of the voltages known from the charge-Seebeck effect. The size and sign of the effect can be controlled by the composition of the electrodes' atomic layers adjacent to the barrier and the temperature. The geometric centre of the electronic density of states relative to the Fermi level determines the size of the Seebeck effect. Experimentally, we realized 8.8% magneto-Seebeck effect, which results from a voltage change of about -8.7 μV K⁻¹ from the antiparallel to the parallel direction close to the predicted value of -12.1 μV K⁻¹. In contrast to the spin-Seebeck effect, it can be measured as a voltage change directly without conversion of a spin current.Nature Material 07/2011; 10(10):742-6. · 32.84 Impact Factor -
Article: Tunneling path toward spintronics
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ABSTRACT: The phenomenon of quantum tunneling, which was discovered almost a century ago, has led to many subsequent discoveries. One such discovery, spin polarized tunneling, was made 40 years ago by Robert Meservey and Paul Tedrow (Tedrow and Meservey 1971 Phys. Rev. Lett. 26 192), and it has resulted in many fundamental observations and opened up an entirely new field of study. Until the mid-1990s, this field developed at a steady, low rate, after which a huge increase in activity suddenly occurred as a result of the unraveling of successful spin tunneling between two ferromagnets. In the past 15 years, several thousands of papers related to spin polarized tunneling and transport have been published, making this topic one of the hottest areas in condensed matter physics from both fundamental science and applications viewpoints. Many review papers and book chapters have been written in the past decade on this subject. This paper is not exhaustive by any means; rather, the emphases are on recent progress, technological developments and informing the reader about the current direction in which this topic is moving.Reports on Progress in Physics 02/2011; 74(3):036501. · 14.72 Impact Factor -
Article: Exchange splitting and bias-dependent transport in EuO spin filter tunnel barriers
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ABSTRACT: We report on transport measurements in EuO-based tunnel junctions, studying the characteristic features of spin filter tunneling in magnetic insulators. Current-voltage (I-V) curves show a unique voltage dependence that reveals the interplay between direct and spin-selective Fowler-Nordheim (FN) tunneling. Successive onsets of FN tunneling give direct evidence for a spin filtering effect, without relying on the use of external magnetic fields for spin detection. The variation of the effective tunnel barrier height below the Curie temperature was extracted from temperature-dependent I-V characteristics, and systematically correlates with the spontaneous magnetization of EuO. The magnitude of the exchange splitting was deduced and allows the evaluation of spin filter efficiency of EuO spin filter tunnel barriers by fully electrical means.EPL (Europhysics Letters) 11/2009; 88(4):47006. · 2.17 Impact Factor -
Article: Observation of the triplet exciton in EuS-coated single-walled nanotubes.
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ABSTRACT: Photon absorption by carbon nanotubes creates bound electron-hole pairs called excitons, which can exist in spin-polarized triplet or spin-unpolarized singlet configurations. Triplet excitons are optically inactive owing to the weak spin-orbit coupling in nanotubes. This prevents the optical injection of electron spin into nanotubes for spintronic applications and limits the efficiency of photocurrent generation. Here, we show that it is possible to optically excite the triplet exciton by using a ferromagnetic semiconductor as a spin filter to mix the singlet and triplet excitons. The triplet contribution to the photocurrent is detected, representing the first direct evidence of the triplet exciton in carbon nanotubes.Nature Nanotechnology 08/2009; 4(7):425-9. · 27.27 Impact Factor -
Article: Controlling magnetic switching properties of EuS for constructing double spin filter magnetic tunnel junctions
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ABSTRACT: EuS is a well established spin filter (SF) material and its magnetic properties are shown to be tunable with deposition temperature. In tunnel junctions consisting of double EuS SF barriers, different coercive fields of the two adjacent SFs are achieved by depositing one EuS layer at room temperature and quench condensing the other at liquid nitrogen temperature. A thin Al2O3 spacer layer is introduced in between to prevent direct magnetic coupling. Magnetoresistance with well defined spin-parallel and spin-antiparallel states is obtained this way, and the resistance change originates completely within the SF/I/SF composite tunnel barrier, with no ferromagnetic electrodes involved.Applied Physics Letters 05/2009; 94(18):182504-182504-3. · 3.84 Impact Factor -
Article: Thickness dependence of ferromagnetic- and metal-insulator transition in thin EuO films
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ABSTRACT: We have studied the thickness dependence of the magnetic and transport properties of thin EuO films in the range of 10–60 Å. The ferromagnetic phase transition shows a systematic dependence of the critical temperature T<sub>c</sub> with decreasing EuO film thickness. This behavior has been attributed to the interface layers which play a major role by reducing the number of average magnetic neighbors; we find the effect of interface intermixing becoming relevant in low thickness regime. In addition, we could identify a clear dependence of the onset of the metal-to-insulator transition on the ferromagnetic ordering of thin EuO films.Journal of Applied Physics 05/2009; · 2.17 Impact Factor -
Article: Magnetoresistance in double spin filter tunnel junctions with nonmagnetic electrodes and its unconventional bias dependence.
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ABSTRACT: Spin filtering happens due to the discriminative tunneling probabilities for spin-up and spin-down electrons through a magnetic barrier and can result in highly spin polarized tunnel currents. Combining two such barriers in a tunnel junction thus leads to large magnetoresistance without the necessity of magnetic electrodes. We demonstrate the realization of such unconventional tunnel junctions using double EuS spin filter barriers with Al electrodes. The novel nonmonotonic and asymmetric bias behavior in magnetoresistance can be qualitatively modeled in the framework of WKB approximations.Physical Review Letters 03/2009; 102(7):076601. · 7.37 Impact Factor -
Article: Spin polarization in half-metals probed by femtosecond spin excitation.
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ABSTRACT: Knowledge of the spin polarization is of fundamental importance for the use of a material in spintronics applications. Here, we used femtosecond optical excitation of half-metals to distinguish between half-metallic and metallic properties. Because the direct energy transfer by Elliot-Yafet scattering is blocked in a half-metal, the demagnetization time is a measure for the degree of half-metallicity. We propose that this characteristic enables us vice versa to establish a novel and fast characterization tool for this highly important material class used in spin-electronic devices. The technique has been applied to a variety of materials where the spin polarization at the Fermi level ranges from 45 to 98%: Ni, Co(2)MnSi, Fe(3)O(4), La(0.66)Sr(0.33)MnO(3) and CrO(2).Nature Material 01/2009; 8(1):56-61. · 32.84 Impact Factor -
Article: Infinite magnetoresistance from the spin dependent proximity effect in symmetry driven bcc-Fe/V/Fe heteroepitaxial superconducting spin valves.
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ABSTRACT: Superconductivity in fully epitaxial bcc-Fe/V/Fe hybrid spin valve structures is influenced by the spin currents and supercurrents as well as band symmetry. The transition temperature is spin dependent in the presence of the proximity effect. A unique feature in this system is the band symmetry filtering taking place at the Fe/V interface. The absence of Delta2 Bloch states at the Fermi level in the Fe spin majority channel leads to spin selectivity and reduced transparency at the interface. Infinite magnetoresistance with clear remanence states is obtained, and implies the potential for spintronic applications.Physical Review Letters 10/2008; 101(13):137001. · 7.37 Impact Factor -
Article: Influence of spin-polarized current on superconductivity and the realization of large magnetoresistance.
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ABSTRACT: The superconducting state can be influenced by injecting spin-polarized current in a controlled manner by properly tailoring the interfacial transmittivity between a ferromagnet (F) and a superconductor (S), resulting in a large magnetoresistance of over 1100% for a F/I/S/I/F multilayer system (I insulator). Because of the competition between ferromagnetism and superconductivity, the superconducting transition temperature (T(C)) in the spin-parallel configuration is shifted below that in the spin antiparallel configuration. The T(C) shift is attributed to ferromagnet-induced nonequilibrium spin carriers in the superconductors.Physical Review Letters 07/2007; 98(26):267001. · 7.37 Impact Factor -
Article: The phenomena of spin-filter tunnelling
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ABSTRACT: The spin filtering phenomenon allows one to obtain highly spin-polarized charge carriers generated from nonmagnetic electrodes using magnetic tunnel barriers. The exponential dependence of tunnel current on the tunnel barrier height is operative here. The magnetic, semiconducting europium chalcogenide compounds have strikingly demonstrated this effect. The possibility of employing ferrites and other methods opens the potential for display of this phenomenon at room temperature, which can be expected to lead to huge progress in spin injection and detection in semiconductors. But first, extremely challenging material-related issues have to be addressed. This review covers the field.Journal of Physics Condensed Matter 04/2007; 19(16):165202. · 2.55 Impact Factor -
Article: Photoexcitation of the triplet exciton in single wall carbon nanotubes
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ABSTRACT: The carbon nanotube photoexcitation spectrum is dominated by excitonic transitions, rather than interband transitions between continuum states. There are eight distinct excitonic transitions (four singlet and four triplet), each with two-fold degeneracy. Because the triplet excitons are spin polarized with electron and hole spins both pointing in the same direction, they are optically inactive, and optical spectroscopy has revealed no evidence for their existence. Here, we show that by the interaction with a spin filter ferromagnetic semiconductor, photoexcitation of the carbon nanotube triplet exciton is possible, and its contribution to the photocurrent can be detected. The perturbation provided by the spin filter allows for inter-system mixing between the singlet and triplet excitonic states, and relaxes the spin selection rules. This supplies the first evidence for the existence of the triplet exciton, and provides an avenue for the optical excitation of spin polarized carriers in carbon nanotubes. United States Department of Energy (DE-FG02-07ER46375) National Science Foundation (NSF DMR 0504158) United States Office of Naval Research (ONR N00014-06-1-0228 and ONR N00014-06-1-0235)SPIE. -
Article: Seebeck effect in magnetic tunnel junctions
Nature Materials. 10(10):742-746.
Top co-authors
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Institutions
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2007–2012
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Massachusetts Institute of Technology
- Francis Bitter Magnet Laboratory
Cambridge, MA, USA
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