Facet-independent electric-field-induced volume metallization of tungsten trioxide films
Altendorf, S. G., J. Jeong, D. Passarello, N. B. Aetukuri, M. G. Samant, S. S. P. Parkin
Advanced Materials 28 (26), pp 5284-5292 (2016)
abstractReversible metallization of band and Mott insulators by ionic-liquid gating is accompanied by significant structural changes. A change in conductivity of seven orders of magnitude at room temperature is found in epitaxial films of WO3 with an associated monoclinic-to-cubic structural reorganization. The migration of oxygen ions along open volume channels is the underlying mechanism.
Recombination at Lomer dislocations in multicrystalline silicon for solar cells
Bauer, J., A. Hähnel, P. Werner, N. Zakharov, H. Blumtritt, A. Zuschlag, O. Breitenstein
IEEE Journal of Photovoltaics 6 (1), pp 100-110 (2016)
abstractLomer dislocations at small-angle grain boundaries in multicrystalline silicon solar cells have been identified as responsible for the dominating inherent dark current losses. Resulting efficiency losses have been quantified by dark lock-in thermography to be locally up to several percent absolute, reducing the maximum power of the cells. By electron beam induced current measurements and scanning transmission electron microscopy investigations, it is revealed that the strengths of the dark current losses depend on the density of Lomer dislocations at the small-angle grain boundaries.
THz-driven ultrafast spin-lattice scattering in amorphous metallic ferromagnets
Bonetti, S., M. C. Hoffmann, M. J. Sher, Z. Chen, S. H. Yang, M. G. Samant, S. S. P. Parkin, H. A. Dürr
Physical Review Letters 117 (8), pp 082705/1-10 (2016)
abstractWe use single-cycle THz fields and the femtosecond magneto-optical Kerr effect to, respectively, excite and probe the magnetization dynamics in two thin-film ferromagnets with different lattice structures: crystalline Fe and amorphous CoFeB. We observe Landau-Lifshitz-torque magnetization dynamics of comparable magnitude in both systems, but only the amorphous sample shows ultrafast demagnetization caused by the spin-lattice depolarization of the THz-induced ultrafast spin current. Quantitative modeling shows that such spin-lattice scattering events occur on similar time scales than the conventional spin conserving electronic scattering (similar to 30 fs). This is significantly faster than optical laser-induced demagnetization. THz conductivity measurements point towards the influence of lattice disorder in amorphous CoFeB as the driving force for enhanced spin-lattice scattering.
Growth and properties of self-catalyzed (In,Mn)As nanowires
Bouravleuv, A., G. Cirlin, R. Reznik, A. Khrebtov, Y. Samsonenko, P. Werner, I. Soshnikov, A. Savin, H. Lipsanen
Physica Status Solidi RRL 10 (7), pp 554-557 (2016)
Local solar cell efficiency analysis performed by injection-dependent PL imaging (ELBA) and voltage-dependent lock-in thermography (local I-V)
Breitenstein, O., F. Frühauf, J. Bauer, F. Schindler, B. Michl
Energy Procedia 92, pp 10-15 (2016)
Effective diffusion length and bulk saturation current density imaging in solar cells by spectrally filtered luminescence imaging
Breitenstein, O., F. Frühauf, D. Hinken, K. Bothe
IEEE Journal of Photovoltaics 6 (5), pp 1243-1254 (2016)
An improved method to measure the point spread function of cameras used for electro- and photoluminescence imaging of silicon solar cells
Breitenstein, O., F. Frühauf, A. Teal
IEEE Journal of Photovoltaics 6 (2), pp 522-527 (2016)
abstractWhen silicon solar cells are investigated by electroor photoluminescence (PL) imaging using a silicon-based camera, photon scattering in the detector chip leads to a certain degree of blurring of the images, which can be removed by image deconvolution. The necessary point spread function (PSF) was originally measured directly by evaluating fine light spots, but this procedure needed the evaluation of several images. It was shown recently that evaluating an image with a sharp contrast edge in the middle may lead to a PSF spreading over longer distances by evaluating only one image. Here, we show that the previous backwards substitution method for obtaining this PSF does still lead to residual errors. An alternative method to derive the PSF from a measured contrast edge image is introduced here. It uses an iterative method, which leads to most precise results. In addition to the local deconvolution, nonlocal (homogeneous) light scattering is corrected. The correct reconstruction of the dark part of the image used for obtaining the PSF is a proof for the accuracy of this PSF.
Improved empirical method for calculation short circuit current density images of silicon solar cells from saturation current density images and vice versa
Breitenstein, O., F. Frühauf, M. Turek
Solar Energy Materials and Solar Cells 154, pp 99-103 (2016)
abstractAn empirical dependence of the short circuit current density Jsc as a function of the dark saturation current density J01 is proposed, which describes this dependence down to a bulk lifetime of 1 ns. This method avoids artifacts, which appear when applying the previously proposed quadratic dependence. The parameters of the new dependence are fitted to PC1D simulations and to experimental LBIC results for various wavelengths and AM 1.5 for a typical industrial BSF-type solar cell and a PERC cell. This dependence can also be used to calculate J01 images from LBIC-based Jsc images. It turns out that this method is more reliable in BSF than in PERC cells.
Parametric harmonic generation as a probe of unconstrained spin magnetization precession in the shallow barrier limit
Capua, A., C. Rettner, S. S. P. Parkin
Physical Review Letters 116 (4), pp 047204/1-5 (2016)
abstractWe study the parametric excitation of high orders of magnetization precession in ultrathin films having perpendicular magnetic anisotropy. We observe that for a given driving field amplitude the harmonic generation can be increased by lowering the barrier with the application of an in-plane magnetic field in the manner of the Smit-Beljers effect. In this effect, the magnetic stiffness is reduced not by lowering the magnitude of the magnetic field upon which the spins precess, but rather by effectively releasing the field's änchoring" point. This results in a shallow energy barrier where the electrons' spin is locally unconstrained. While the observation is unveiled in the form of nonlinear high harmonic generation, we believe that the physics whereby the barrier is suppressed by an external magnetic field may apply to other phenomena associated with ultrathin films. In these cases, such unconstrained motion may serve as a sensitive probe of the torques associated with proximate spin currents. Moreover, our approach may be used as a model system for the study of phase transitions in the field of nonlinear dynamics.
Metal seed loss throughout the nanowire growth: Bulk trapping and surface mass transport
Chagnon, D., E. Pippel, S. Senz, O. Moutanabbir
Journal of Physical Chemistry C 120 (5), pp 2932-2940 (2016)
abstractThe physical and chemical properties of metal-catalyzed semiconductor nanowires are very sensitive to their composition and morphology, which are very sensitive to the behavior of the catalyst nanodroplet during growth. Herein, we identify and investigate the main atomic pathways and processes governing the metal mass transport and the associated variation in the nanodroplet size throughout the growth of metal-catalyzed silicon nanowires. This includes surface diffusion and catalyst trapping in addition to the shift in phase boundaries of the eutectic nanodroplet to count for surface effects, capillarity, and related nanoscale stresses. On the basis of thermodynamic and kinetic considerations, a theoretical framework is presented to elucidate these catalyst nanodroplet instabilities. Moreover, we also address the influence of these phenomena on the shape and impurity concentration in silicon nanowires. Modeling results along with experimental data demonstrate that the combined effects of the kinetically driven catalyst trapping and surface diffusion play the key role in tailoring the nanowire morphology and composition. The proposed model can be extended straightforwardly to describe the evolution of the morphology during the growth of any other system of metal-catalyzed semiconductor nanowires.
Enhanced spin-orbit torques by oxygen incorporation in tungsten films
Demasius, K. U., T. Phung, W. F. Zhang, B. P. Hughes, S. H. Yang, A. Kellock, W. Han, A. Pushp, S. S. P. Parkin
Nature Communications 7, pp 10644/1-7 (2016)
abstractThe origin of spin-orbit torques, which are generated by the conversion of charge-to-spin currents in non-magnetic materials, is of considerable debate. One of the most interesting materials is tungsten, for which large spin-orbit torques have been found in thin films that are stabilized in the A15 (beta-phase) structure. Here we report large spin Hall angles of up to approximately -0.5 by incorporating oxygen into tungsten. While the incorporation of oxygen into the tungsten films leads to significant changes in their microstructure and electrical resistivity, the large spin Hall angles measured are found to be remarkably insensitive to the oxygen-doping level (12-44%). The invariance of the spin Hall angle for higher oxygen concentrations with the bulk properties of the films suggests that the spin-orbit torques in this system may originate dominantly from the interface rather than from the interior of the films.
Strength of the symmetry spin-filtering effect in magnetic tunnel junctions
Faleev, S. V., O. N. Mryasov, S. S. P. Parkin
Physical Review B 94 (17), pp 174408/1-7 (2016)
abstractWe developed a general theory that allows us to predict the power factor n in the asymptotics of the tunneling magnetoresistance (TMR), TMR alpha N-n, in the limit of large number of the tunnel barrier layers, N, for a magnetic tunnel junction (MTJ) system that has the so-called symmetry spin-filtering properties. Within this theory the only information required to determine n is the knowledge of the symmetries of the wave functions of the bulk electrode and barrier materials at the Gamma point in the in-plane surface Brillouin zone. In particular, we show that for a MTJ that has the in-plane square symmetry only three values for the power factor are allowed: n = 0,1, and 2 for the asymptotics of the TMR enhanced due to the symmetry spin-filtering mechanism. To verify our theory we performed the density functional theory calculations of transmission functions and TMR for a Fe/MgO/Fe MTJ which confirm predicted values of the power factor n = 0,1, or 2 in specific ranges of energies (in particular, n = 1 at the Fermi energy).
Basics and prospectives of magnetic Heusler compounds
Felser, C. and L. Wollmann, S. Chadov, G. H. Fecher, S. S. P. Parkin
In: Heusler Alloys: Properties, Growth, Applications, Springer Series in Materials Science 222, pp 37-48 (Eds.) Felser, C. and Hirohata, A.,Springer Verlag, Berlin, Germany 2016
abstractManganese-rich Heusler compounds attract much interest in the context of spin transfer torque and rare-earth free hard magnets. Here we give a comprehensive overview of the magnetic properties of non-centrosymmetric cubic Mn-2-based Heusler materials, which are characterized by an antiparallel coupling of magnetic moments on Mn atoms. Such a ferrimagnetic order leads to the emergence of new properties that are absent in ferromagnetic centrosymmetric Heusler structures. In terms of the band structure calculations, we explain the formation of this magnetic order and the Curie temperatures. This overview is intended to establish guidelines for a basic understanding of magnetism in Mn-2-based Heusler compounds.
Probing the spinor nature of electronic states in nanosize non-collinear magnets
Fischer, J. A., L. M. Sandratskii, S. H. Phark, S. Ouazi, A. A. Pasa, D. Sander, S. S. P. Parkin
Nature Communications 7, pp 13000/1-8 (2016)
abstractNon-collinear magnetization textures provide a route to novel device concepts in spintronics. These applications require laterally confined non-collinear magnets (NCM). A crucial aspect for potential applications is how the spatial proximity between the NCM and vacuum or another material impacts the magnetization texture on the nanoscale. We focus on a prototypical exchange-driven NCM given by the helical spin order of bilayer Fe on Cu(111). Spinpolarized scanning tunnelling spectroscopy and density functional theory reveal a nanosizeand proximity-driven modification of the electronic and magnetic structure of the NCM in interfacial contact with a ferromagnet or with vacuum. An intriguing non-collinearity between the local magnetization in the sample and the electronic magnetization probed above its surface results. It is a direct consequence of the spinor nature of electronic states in NCM. Our findings provide a possible route for advanced control of nanoscale spin textures by confinement.
Laplacian photoluminescence image evaluation employing image deconvolution
Frühauf, F., O. Breitenstein
Solar Energy Materials and Solar Cells 146, pp 87-90 (2016)
abstractThe conventional evaluation of photoluminescence (PL) images of inhomogeneous solar cells, which is based on the model of independent diodes, leads to systematic errors in the estimation of the local saturation current density J(01). The Laplacian-based image evaluation, which was proposed already in 2009, does not rely on this model and has the potential to image Jot correctly. However, first applications of this method to PL images also have failed. In this work it is shown that this failing was due to the blurring effect occurring in the luminescence detector. If PL images are deconvoluted with the correct point spread function, the resulting images lead to the correct J(01) distribution if evaluated by the Laplacian method.
Laplacian PL image evaluation implying correction of photon scattering in the luminescence detector
Frühauf, F., O. Breitenstein
Energy Procedia 92, pp 24-28 (2016)
Description of the local series resistance of real solar cells by separate horizontal and vertical components
Frühauf, F., Y. Sayad, O. Breitenstein
Solar Energy Materials and Solar Cells 154, pp 23-34 (2016)
abstractAll previous concepts for describing the effective local series resistance of really existing solar cells, as it can be measured e.g. by luminescence imaging, try to describe it by a single local number. In solar cells showing an inhomogeneous saturation current density, this results in different series resistance images for the dark and illuminated case. The reason is the distributed character of the series resistance and the different diode current profiles under these different conditions. In this work the well-known finite element concept is used for describing a solar cell, which contains separate resistors carrying horizontal and vertical currents. A strategy is proposed how to fit these resistors to results of electroluminescence and lock-in thermography images of a real solar cell, leading to separate images of the local horizontal grid resistance, which may also show broken gridlines, and the local vertical lumped emitter contact resistance. The latter lumps all resistive inhomogeneities of the cell, caused by a possibly inhomogeneous contact-, emitter-, grid-, bulk-, and back contact resistance. It will be shown that this description of the local series resistance reasonably describes both the dark and illuminated case, even in inhomogeneous multicrystalline silicon solar cells.
Isothermal anisotropic magnetoresistance in antiferromagnetic metallic IrMn
Galceran, R., I. Fina, J. Cisneros-Fernandez, B. Bozzo, C. Frontera, L. Lopez-Mir, H. Deniz, K.-W. Park, B.-G. Park, L. Balcells, X. Marti, T. Jungwirth, B. Martinez
Scientific Reports 6, pp 35471/1-6 (2016)
Yttrium iron garnet thin films with very low damping obtained by recrystallization of amorphous material
Hauser, C., T. Richter, N. Homonnay, C. Eisenschmidt, M. Qaid, H. Deniz, D. Hesse, M. Sawicki, S. G. Ebbinhaus, G. Schmidt
Scientific Reports 6, pp 20827/1-8 (2016)
abstractWe have investigated recrystallization of amorphous Yttrium Iron Garnet (YIG) by annealing in oxygen atmosphere. Our findings show that well below the melting temperature the material transforms into a fully epitaxial layer with exceptional quality, both structural and magnetic. In ferromagnetic resonance (FMR) ultra low damping and extremely narrow linewidth can be observed. For a 56 nm thick layer a damping constant of alpha = (6.15 +/- 1.50) . 10(-5) is found and the linewidth at 9.6 GHz is as small as 1.30 +/- 0.05 Oe which are the lowest values for PLD grown thin films reported so far. Even for a 20 nm thick layer a damping constant of a = (7.35 +/- 1.40) . 10(-5) is found which is the lowest value for ultrathin films published so far. The FMR linewidth in this case is 3.49 +/- 0.10 Oe at 9.6 GHz. Our results not only present a method of depositing thin film YIG of unprecedented quality but also open up new options for the fabrication of thin film complex oxides or even other crystalline materials.
Formation of silver nanoparticles in silicate glass using excimer laser radiation: Structural characterization by HRTEM, XRD, EXAFS and optical absorption spectra
Heinz, M., V. V. Srabionyan, A. L. Bugaev, V. V. Pryadchenko, E. V. Ishenko, L. A. Avakyan, Y. V. Zubavichus, J. Ihlemann, J. Meinertz, E. Pippel, M. Dubiel, Bugaev. L. A.
Journal of Alloys and Compounds 681, pp 307-315 (2016)
abstractlasmonic silver nanostructures in surfaces of soda-lime silicate glasses were generated using Ag+ <-> Na+ ion exchange and UV laser irradiation (ArF laser, 193 nm) with different number of ns laser pulses (from 2 to 5000). To identify the correlations between the optical properties (surface plasmon resonance (SPR) parameters) and atomic structure of silver nanoparticles and their agglomerations, characterization of the samples was performed by HRTEM, XRD, optical absorption in visible range and Ag K-edge EXAFS spectra. Analysis of the optical spectra was performed using a Mie theory approach, accounting for the most plausible defect centers in silicate glass like hole trap centers and non-bridging oxygen hole centers. Processing of Ag K-edge EXAFS yielded values of Ag-Ag and Ag-O interactions averaged over ionic and neutral states of silver. The consistent treatment of HRTEM and XRD data, the behavior of features in optical spectra and the obtained dependence of Ag-Ag and Ag-O structural parameters upon the number of laser pulses enabled to suggest a mechanism of plasmonic Ag nanoparticles formation in silicate glass under UV laser irradiation.
Generation mechanism of terahertz coherent acoustic phonons in Fe
Henighan, T., M. Trigo, S. Bonetti, P. Granitzka, D. Higley, Z. Chen, M. P. Jiang, R. Kukreja, A. Gray, A. H. Reid, E. Jal, C. Hoffmann, M. Kozina, S. Song, M. Chollet, D. Zhu, P. F. Xu, J. Jeong, K. Carva, P. Maldonado, P. M. Oppeneer, M. G. Samant, S. S. P. Parkin, D. A. Reis, H. A. Dürr
Physical Review B 93 (22), pp 220301/1-5 (2016)
abstractWe use femtosecond time-resolved hard x-ray scattering to detect coherent acoustic phonons generated during ultrafast laser excitation of ferromagnetic bcc Fe films grown on MgO(001). We observe the coherent longitudinal-acoustic phonons as a function of wave vector through analysis of the temporal oscillations in the x-ray scattering signal. The width of the extracted strain wave front associated with this coherent motion is similar to 100 fs. An effective electronic Gruneisen parameter is extracted within a two-temperature model. However, ab initio calculations show that the phonons are nonthermal on the time scale of the experiment, which calls into question the validity of extracting physical constants by fitting such a two-temperature model.
Heusler alloy films for spintronic devices
Hirohata, A., J. Sagar, L. R. Fleet and S. S. P. Parkin
In: Heusler Alloys: Properties, Growth, Applications, Springer Series in Materials Science 222, pp 219-248 (Eds.) Felser, C., Felser, C., Hirohata, A. and Hirohata, A.,Springer Verlag, Berlin, Germany 2016
abstractThis chapter reviews the requirements for the Heusler-alloy films to be used in spintronic devices. Four key requirements are identified to be large giant magnetoresistance (GMR), large tunnelling magnetoresistance (TMR), large spin-transfer torque and fast spin resonance. These requirements can be achieved by utilising the fundamental properties of the Heusler alloys, such as atomic substitution, generalised Slater-Pauling behaviour, crystalline ordering, half-metallicity, low damping constant, high Curie temperature, good lattice matching and large activation volume. To date the main obstacles for the Heusler-alloy films to be used in spintronic devices are their (i) high crystallisation temperature, (ii) interfacial atomic disordering and (iii) small activation volume. Here, we have investigated these properties for both epitaxial and polycrystalline films and have found a favourable crystallisation orientation to lower the ordering temperature by inducing a two-dimensional growth. We have demonstrated the effect of interfacial dusting to maintain the crystalline ordering from atomic diffusion by annealing. We have also established that the above requirements can be controlled by the competition between the structural and magnetic volume, the latter of which can be defined as activation volume. In all cases, the polycrystalline films have found to be advantageous over the epitaxial ones due to their strain-free growth with controlled grain size. We anticipate that the optimised polycrystalline films can be used in the next generation hard disk read heads and magnetic random access memory cells.
Termination layer compensated tunneling magnetoresistance in ferrimagnetic Heusler compounds with high perpendicular magnetic anisotropy
Jeong, J., Y. Ferrante, S. V. Faleev, M. G. Samant, C. Felser, S. S. P. Parkin
Nature Communications 7, pp 10276/1-8 (2016)
abstractAlthough high-tunnelling spin polarization has been observed in soft, ferromagnetic, and predicted for hard, ferrimagnetic Heusler materials, there has been no experimental observation to date of high-tunnelling magnetoresistance in the latter. Here we report the preparation of highly textured, polycrystalline Mn3Ge films on amorphous substrates, with very high magnetic anisotropy fields exceeding 7 T, making them technologically relevant. However, the small and negative tunnelling magnetoresistance that we find is attributed to predominant tunnelling from the lower moment Mn-Ge termination layers that are oppositely magnetized to the higher moment Mn-Mn layers. The net spin polarization of the current reflects the different proportions of the two distinct termination layers and their associated tunnelling matrix elements that result from inevitable atomic scale roughness. We show that by engineering the spin polarization of the two termination layers to be of the same sign, even though these layers are oppositely magnetized, high-tunnelling magnetoresistance is possible.
Ambient temperature growth of mono- and polycrystalline NbN nanofilms and their surface and composition analysis
Krause, S., V. Afanas'ev, V. Desmaris, D. Meledin, A. Pavolotsky, V. Belitsky, A. Lubenschenko, A. Batrakov, M. Rudzinksi, E. Pippel
IEEE Transactions on Applied Superconductivity 26 (3), pp 7500205/1-5 (2016)
abstractThis paper presents the studies of high-quality 5-nm-thin NbN films deposited by means of reactive dc magnetron sputtering at room temperature. The deposition without substrate heating offers major advantages from a processing point of view and motivates the extensive composition and surface characterization and comparison of the present films with high-quality films grown at elevated temperatures. Monocrystalline NbN films have been epitaxially grown onto hexagonal GaN buffer layers (0002) and show a distinct low defect interface as confirmed by high-resolution TEM. The critical temperature T-c of the films on the GaN buffer layer reached 10.4 K. Furthermore, a polycrystalline structure was observed on films grown onto Si (100) substrates, exhibiting a T-c of 8.1 K, albeit a narrow transition from the normal to the superconducting state. X-ray photoelectron spectroscopy and reflected electron energy loss spectroscopy verified that the composition of NbN was identical irrespective of applied substrate heating. Moreover, the native oxide layer at the surface of NbN has been identified as NbO2 and, thus, is in contrast to the Nb2O5, usually claimed to be formed at the surface of Nb when exposed to air. These findings are of significance since it was proven the possibility of growing epitaxial NbN onto GaN buffer layer in the absence of high temperatures, hence paving the way to employ NbN in more advanced fabrication processes involving a higher degree of complexity. The eased integration and employment of liftoff techniques could, in particular, lead to improved performance of cryogenic ultrasensitive terahertz electronics.
Continuous magnetoelectric control in multiferroic DyMnO3 films with twin-like domains
Lu, C., H. Deniz, X. Li, J.-M. Liu, S.-W. Cheong
Scientific Reports 6, pp 20175/1-8 (2016)
Large anomalous Hall effect driven by a nonvanishing Berry curvature in the noncolinear antiferromagnet Mn3Ge
Nayak, A. K., J. E. Fischer, Y. Sun, B. H. Yan, J. Karel, A. C. Komarek, C. Shekhar, N. Kumar, W. Schnelle, J. Kübler, C. Felser, S. S. P. Parkin
Science Advances 2 (4), pp e1501870/1-5 (2016)
abstractIt is well established that the anomalous Hall effect displayed by a ferromagnet scales with its magnetization. Therefore, an antiferromagnet that has no net magnetization should exhibit no anomalous Hall effect. We show that the noncolinear triangular antiferromagnet Mn3Ge exhibits a large anomalous Hall effect comparable to that of ferromagnetic metals; the magnitude of the anomalous conductivity is similar to ≈ 500 (ohm.cm)−1 at 2 K and similar to ≈ 50 (ohm.cm)−1 at room temperature. The angular dependence of the anomalous Hall effect measurements confirms that the small residual in-plane magnetic moment has no role in the observed effect except to control the chirality of the spin triangular structure. Our theoretical calculations demonstrate that the large anomalous Hall effect in Mn3Ge originates from a nonvanishing Berry curvature that arises from the chiral spin structure, and that also results in a large spin Hall effect of 1100 (h/e) (ohm.cm)−1, comparable to that of platinum. The present results pave the way toward the realization of room temperature antiferromagnetic spintronics and spin Hall effect-based data storage devices.
Photocatalytic activity of CaTaO2N nanocrystals obtained from a hydrothermally synthesized oxide precursor
Oehler, F., R. Naumann, R. Köferstein, D. Hesse, S. G. Ebbinghaus
Materials Research Bulletin 73, pp 276-283 (2016)
Superconductivity in Weyl semimetal candidate MoTe2
Qi, Y. P., P. G. Naumov, M. N. Ali, C. R. Rajamathi, W. Schnelle, O. Barkalov, M. Hanfland, S. C. Wu, C. Shekhar, Y. Sun, V. Suss, M. Schmidt, U. Schwarz, E. Pippel, P. Werner, R. Hillebrand, T. Forster, E. Kampert, S. S. P. Parkin, R. J. Cava, C. Felser, B. H. Yan, S. A. Medvedev
Nature Communications 7, pp 11038/1-7 (2016)
abstractTransition metal dichalcogenides have attracted research interest over the last few decades due to their interesting structural chemistry, unusual electronic properties, rich intercalation chemistry and wide spectrum of potential applications. Despite the fact that the majority of related research focuses on semiconducting transition-metal dichalcogenides (for example, MoS2), recently discovered unexpected properties of WTe2 are provoking strong interest in semimetallic transition metal dichalcogenides featuring large magnetoresistance, pressure-driven superconductivity and Weyl semimetal states. We investigate the sister compound of WTe2, MoTe2, predicted to be a Weyl semimetal and a quantum spin Hall insulator in bulk and monolayer form, respectively. We find that bulk MoTe2 exhibits superconductivity with a transition temperature of 0.10 K. Application of external pressure dramatically enhances the transition temperature up to maximum value of 8.2 K at 11.7 GPa. The observed dome-shaped superconductivity phase diagram provides insights into the interplay between superconductivity and topological physics.
Observation of topological Hall effect in Mn2RhSn films
Rana, K. G., O. Meshcheriakova, J. Kübler, B. Ernst, J. Karel, R. Hillebrand, E. Pippel, P. Werner, A. K. Nayak, C. Felser, S. S. P. Parkin
New Journal of Physics 18 (8), pp 085007/1-7 (2016)
Valence band offset at the Si/SiSn interface by applying deep level transient spectroscopy
Rangel-Kuoppa, V.-T., A. Tonkikh, N. Zakharov, C. Eisenschmidt, P. Werner
Nanotechnology 27 (7), pp 075705/1-7 (2016)
Electronic properties of dislocations
Reiche, M., M. Kittler, H. Uebensee, E. Pippel, A. Haehnel, S. Birner
Applied Physics A, Materials Science & Processing 122 (4), pp 389/1-7 (2016)
abstractDislocations exhibit a number of exceptional electronic properties resulting in a significant increase in the drain current of MOSFETs if defined numbers of these defects are placed in the channel. Measurements on individual dislocations in Si refer to a supermetallic conductivity. A model of the electronic structure of dislocations is proposed based on experimental measurements and tight-binding simulations. It is shown that the high strain level on the dislocation core-exceeding 10 % or more-causes locally dramatic changes in the band structure and results in the formation of a quantum well along the dislocation line. This explains experimental findings (two-dimensional electron gas, single-electron transitions). The energy quantization within the quantum well is most important for supermetallic conductivity.
Current-induced domain-wall motion for electron flow in ferromagnetic Pt/Co/Ni/Co/Pt wires
Ryu, K. S., S. H. Yang, L. Thomas, S. S. P. Parkin
IEEE Transactions on Magnetics 52 (7), pp 1400404/1-4 (2016)
abstractWe report the current-induced domain-wall (CIDW) motion along the electron flow in the perpendicularly magnetized Co/Ni/Co structure sandwiched between the top and bottom Pt layers, observed by means of the Kerr microscope. As the lower Co thickness increases, the DW velocity for the electron flow is significantly increased, and also its motion becomes more stable irrespective of various current pulses. From the longitudinal field dependence of the DW velocity, it is found that the increase of the lower Co thickness increases a Dyzaloshinskii-Moriya interaction in the bottom Co/Pt interface, which results in the increase of the chiral spin torque, responsible for the increase of the DW velocity along the electron flow.
Compensated ferrimagnetic tetragonal Heusler thin films for antiferromagnetic spintronics
Sahoo, R., L. Wollmann, S. Selle, T. Höche, B. Ernst, A. Kalache, C. Shekhar, N. Kumar, S. Chadov, C. Felser, S. S. P. Parkin, A. K. Nayak
Advanced Materials 28 (38), pp 8499-8504 (2016)
abstractFully compensated ferrimagnets with tetragonal crystal structure have the potential for large spin-polarization and strong out-of-plane magnetic anisotropy; hence, they are ideal candidates for high-density-memory applications. Tetragonal Heusler thin films with compensated magnetic state are realized by substitution of Pt in Mn3-xPtxGa. Furthermore, the bilayer formed from compensated/uncompensated Mn-Pt-Ga layers is utilized to accomplish exchange bias up to room temperature.
Producing ZnFe2O4 thin films from ZnO/FeO multilayers
Salcedo Rodriguez, K. L., M. Hoffmann, F. Golmar, G. Pasquevich, P. Werner, W. Hergert, C. E. Rodriguez Torres
Applied Surface Science 393, pp 256-261 (2016)
Toward versatile Sr2FeMoO6-based spintronics by exploiting nanoscale defects
Saloaro, M., M. Hoffmann, W. A. Adeagbo, S. Granroth, H. Deniz, H. Palonen, H. Huhtinen, S. Majumdar, P. Laukkanen, W. Hergert, A. Ernst, P. Paturi
ACS Applied Materials and Interfaces 8 (31), pp 20440-20447 (2016)
abstractTo actualize the high spintronic application potential of complex magnetic oxides, it is essential to fabricate these materials as thin films with the best possible magnetic and electrical properties. Sr2FeMoO6 is an outstanding candidate for such applications, but presently no thin film synthesis route, which would preserve the magnetic properties of bulk Sr2FeMoO6, is currently known. In order to address this problem, we present a comprehensive experimental and theoretical study where we link the magnetic and half metallic properties of Sr2FeMoO6 thin films to lattice strain, Fe-Mo antisite disorder and oxygen vacancies. We find the intrinsic effect of strain on the magnetic properties to be very small, but also that an increased strain will significantly stabilize the Sr2FeMoO6 lattice against the formation of antisite disorder and oxygen vacancies. These defects, on the other hand, are recognized to drastically influence the magnetism of Sr2FeMoO6 in a nonlinear manner. On the basis of the findings, we propose strain manipulation and reductive annealing as optimization pathways for improving the spintronic functionality of Sr2FeMoO6.
Dirac cone protected by non-symmorphic symmetry and three-dimensional Dirac line node in ZrSiS
Schoop, L. M., M. N. Ali, C. Strasser, A. Topp, A. Varykhalov, D. Marchenko, V. Duppel, S. S. P. Parkin, B. V. Lotsch, C. R. Ast
Nature Communications 7, pp 11696/1-7 (2016)
abstractMaterials harbouring exotic quasiparticles, such as massless Dirac and Weyl fermions, have garnered much attention from physics and material science communities due to their exceptional physical properties such as ultra-high mobility and extremely large magnetoresistances. Here, we show that the highly stable, non-toxic and earth-abundant material, ZrSiS, has an electronic band structure that hosts several Dirac cones that form a Fermi surface with a diamond-shaped line of Dirac nodes. We also show that the square Si lattice in ZrSiS is an excellent template for realizing new types of two-dimensional Dirac cones recently predicted by Young and Kane. Finally, we find that the energy range of the linearly dispersed bands is as high as 2 eV above and below the Fermi level; much larger than of other known Dirac materials. This makes ZrSiS a very promising candidate to study Dirac electrons, as well as the properties of lines of Dirac nodes.
Quantifying the structural integrity of nanorod arrays
Thöle, F., L. Xue, C. Heß, R. Hillebrand, S. N. Gorb, M. Steinhart
Journal of Microscopy 265 (2), pp 222-231 (2016)
Structural and electronics properties of epitaxial multilayer h-BN on Ni(111) for spintronics applications
Tonkikh, A. A., E. N. Voloshina, P. Werner, H. Blumtritt, B. Senkovskiy, G. Güntherodt, S. S. P. Parkin, Y. S. Dedkov
Scientific Reports 6, pp 23547/1-8 (2016)
abstractHexagonal boron nitride (h-BN) is a promising material for implementation in spintronics due to a large band gap, low spin-orbit coupling, and a small lattice mismatch to graphene and to close-packed surfaces of fcc-Ni(111) and hcp-Co(0001). Epitaxia I deposition of h-BN on ferromagnetic metals is aimed at small interface scattering of charge and spin carriers. We report on the controlled growth of h-BN/Ni(111) by means of molecular beam epitaxy (MBE). Structural and electronic properties of this system are investigated using cross-section transmission electron microscopy (TEM) and electron spectroscopies which confirm good agreement with the properties of bulk h-BN. The latter are also corroborated by density functional theory (DFT) calculations, revealing that the first h-BN layer at the interface to Ni is metallic. Our investigations demonstrate that MBE is a promising, versatile alternative to both the exfoliation approach and chemical vapour deposition of h-BN.
The effect of interface roughness on exchange bias in La0.7Sr0.3MnO3-BiFeO3 heterostructures
Vafaee, M., S. Finizio, H. Deniz, D. Hesse, H. Zabel, G. Jakob, M. Kläui
Applied Physics Letters 108 (7), pp 072401/1-5 (2016)
abstractWe characterized the interfaces of heterostructures with different stack sequences of La0.7Sr0.3MnO3-BiFeO3 (LSMO/BFO) and BFO/LSMO using TEM revealing sharp and rough interfaces, respectively. Magnetometry and magnetoresistance measurements do not show a detectable exchange bias coupling for the multistack with sharp interface. Instead, the heterostructures with rough and chemically intermixed interfaces exhibit a sizable exchange bias coupling. Furthermore, we find a temperature-dependent irreversible magnetization behavior and an exponential decay of coercive and exchange bias fields with temperature suggesting a possible spin-glass-like state at the interface of both stacks.
Transparent conducting oxide induced by liquid electrolyte gating
Viol Barbosa, C., J. Karel, J. Kiss, O. D. Gordan, S. G. Altendorf, Y. Utsumi, M. G. Samant, Y. H. Wu, K. D. Tsuei, C. Felser, S. S. P. Parkin
Proceedings of the National Academy of Sciences 113 (40), pp 11148-11151 (2016)
abstractOptically transparent conducting materials are essential in modern technology. These materials are used as electrodes in displays, photovoltaic cells, and touchscreens; they are also used in energy-conserving windows to reflect the infrared spectrum. The most ubiquitous transparent conducting material is tin-doped indium oxide (ITO), a wide-gap oxide whose conductivity is ascribed to n-type chemical doping. Recently, it has been shown that ionic liquid gating can induce a reversible, nonvolatile metallic phase in initially insulating films of WO3. Here, we use hard X-ray photoelectron spectroscopy and spectroscopic ellipsometry to show that the metallic phase produced by the electrolyte gating does not result from a significant change in the bandgap but rather originates from new in-gap states. These states produce strong absorption below similar to 1 eV, outside the visible spectrum, consistent with the formation of a narrow electronic conduction band. Thus WO3 is metallic but remains colorless, unlike other methods to realize tunable electrical conductivity in this material. Core-level photoemission spectra show that the gating reversibly modifies the atomic coordination of Wand O atoms without a substantial change of the stoichiometry; we propose a simple model relating these structural changes to the modifications in the electronic structure. Thus we show that ionic liquid gating can tune the conductivity over orders of magnitude while maintaining transparency in the visible range, suggesting the use of ionic liquid gating for many applications.
Preface to special topic: 2D spintronics
Wang, X., S. S. P. Parkin, Q. K. Xue
APL Materials 4 (3), pp 032201/1 (2016)
Investigation of the unidirectional spin heat conveyer effect in a 200 nm thin Yttrium Iron Garnet film
Wid, O., J. Bauer, A. Müller, O. Breitenstein, S. S. P. Parkin, G. Schmidt
Scientific Reports 6, pp 28233/1-7 (2016)
Experimental investigation of temperature-dependent Gilbert damping in permalloy thin films
Zhao, Y. L., Q. Song, S. H. Yang, T. Su, W. Yuan, S. S. P. Parkin, J. Shi, W. Han
Scientific Reports 6, pp 22890/1-8 (2016)
abstractThe Gilbert damping of ferromagnetic materials is arguably the most important but least understood phenomenological parameter that dictates real-time magnetization dynamics. Understanding the physical origin of the Gilbert damping is highly relevant to developing future fast switching spintronics devices such as magnetic sensors and magnetic random access memory. Here, we report an experimental study of temperature-dependent Gilbert damping in permalloy (Py) thin films of varying thicknesses by ferromagnetic resonance. From the thickness dependence, two independent contributions to the Gilbert damping are identified, namely bulk damping and surface damping. Of particular interest, bulk damping decreases monotonically as the temperature decreases, while surface damping shows an enhancement peak at the temperature of similar to 50 K. These results provide an important insight to the physical origin of the Gilbert damping in ultrathin magnetic films.
Mesoscopic structural phase progression in photo-excited VO2 revealed by time-resolved x-ray diffraction microscopy
Zhu, Y., Z. H. Cai, P. C. Chen, Q. T. Zhang, M. J. Highland, I. W. Jung, D. A. Walko, E. M. Dufresne, J. Jeong, M. G. Samant, S. S. P. Parkin, J. W. Freeland, P. G. Evans, H. D. Wen
Scientific Reports 6, pp 21999/1-7 (2016)
abstractDynamical phase separation during a solid-solid phase transition poses a challenge for understanding the fundamental processes in correlated materials. Critical information underlying a phase transition, such as localized phase competition, is difficult to reveal by measurements that are spatially averaged over many phase separated regions. The ability to simultaneously track the spatial and temporal evolution of such systems is essential to understanding mesoscopic processes during a phase transition. Using state-of-the-art time-resolved hard x-ray diffraction microscopy, we directly visualize the structural phase progression in a VO2 film upon photoexcitation. Following a homogenous in-plane optical excitation, the phase transformation is initiated at discrete sites and completed by the growth of one lattice structure into the other, instead of a simultaneous isotropic lattice symmetry change. The time-dependent x-ray diffraction spatial maps show that the in-plane phase progression in laser-superheated VO2 is via a displacive lattice transformation as a result of relaxation from an excited monoclinic phase into a rutile phase. The speed of the phase front progression is quantitatively measured, and is faster than the process driven by in-plane thermal diffusion but slower than the sound speed in VO2. The direct visualization of localized structural changes in the time domain opens a new avenue to study mesoscopic processes in driven systems.

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