Publications by authors named "Gerhard Jakob"

15 Publications

  • Page 1 of 1

Terahertz Spin-to-Charge Conversion by Interfacial Skew Scattering in Metallic Bilayers.

Adv Mater 2021 Mar 27;33(9):e2006281. Epub 2021 Jan 27.

Department of Physics, Freie Universität Berlin, Arnimallee 14, 14195, Berlin, Germany.

The efficient conversion of spin to charge transport and vice versa is of major relevance for the detection and generation of spin currents in spin-based electronics. Interfaces of heterostructures are known to have a marked impact on this process. Here, terahertz (THz) emission spectroscopy is used to study ultrafast spin-to-charge-current conversion (S2C) in about 50 prototypical F|N bilayers consisting of a ferromagnetic layer F (e.g., Ni Fe , Co, or Fe) and a nonmagnetic layer N with strong (Pt) or weak (Cu and Al) spin-orbit coupling. Varying the structure of the F/N interface leads to a drastic change in the amplitude and even inversion of the polarity of the THz charge current. Remarkably, when N is a material with small spin Hall angle, a dominant interface contribution to the ultrafast charge current is found. Its magnitude amounts to as much as about 20% of that found in the F|Pt reference sample. Symmetry arguments and first-principles calculations strongly suggest that the interfacial S2C arises from skew scattering of spin-polarized electrons at interface imperfections. The results highlight the potential of skew scattering for interfacial S2C and propose a promising route to enhanced S2C by tailored interfaces at all frequencies from DC to terahertz.
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http://dx.doi.org/10.1002/adma.202006281DOI Listing
March 2021

Harnessing Orbital-to-Spin Conversion of Interfacial Orbital Currents for Efficient Spin-Orbit Torques.

Phys Rev Lett 2020 Oct;125(17):177201

Institute of Physics, Johannes Gutenberg University Mainz, Staudingerweg 7, 55128 Mainz, Germany.

Current-induced spin-orbit torques (SOTs) allow for the efficient electrical manipulation of magnetism in spintronic devices. Engineering the SOT efficiency is a key goal that is pursued by maximizing the active interfacial spin accumulation or modulating the nonequilibrium spin density that builds up through the spin Hall and inverse spin galvanic effects. Regardless of the origin, the fundamental requirement for the generation of the current-induced torques is a net spin accumulation. We report on the large enhancement of the SOT efficiency in thulium iron garnet (TmIG)/Pt by capping with a CuO_{x} layer. Considering the weak spin-orbit coupling (SOC) of CuO_{x}, these surprising findings likely result from an orbital current generated at the interface between CuO_{x} and Pt, which is injected into the Pt layer and converted into a spin current by strong SOC. The converted spin current decays across the Pt layer and exerts a "nonlocal" torque on TmIG. This additional torque leads to a maximum colossal enhancement of the SOT efficiency of a factor 16 for 1.5 nm of Pt at room temperature, thus opening a path to increase torques while at the same time offering insights into the underlying physics of orbital transport, which has so far been elusive.
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http://dx.doi.org/10.1103/PhysRevLett.125.177201DOI Listing
October 2020

Phonon Bridge Effect in Superlattices of Thermoelectric TiNiSn/HfNiSn With Controlled Interface Intermixing.

Nanomaterials (Basel) 2020 Jun 25;10(6). Epub 2020 Jun 25.

Institute of Physics, Johannes Gutenberg University, Staudingerweg 7, 55128 Mainz, Germany.

The implementation of thermal barriers in thermoelectric materials improves their power conversion rates effectively. For this purpose, material boundaries are utilized and manipulated to affect phonon transmissivity. Specifically, interface intermixing and topography represents a useful but complex parameter for thermal transport modification. This study investigates epitaxial thin film multilayers, so called superlattices (SL), of TiNiSn/HfNiSn, both with pristine and purposefully deteriorated interfaces. High-resolution transmission electron microscopy and X-ray diffractometry are used to characterize their structural properties in detail. A differential 3 ω -method probes their thermal resistivity. The thermal resistivity reaches a maximum for an intermediate interface quality and decreases again for higher boundary layer intermixing. For boundaries with the lowest interface quality, the interface thermal resistance is reduced by 23% compared to a pristine SL. While an uptake of diffuse scattering likely explains the initial deterioration of thermal transport, we propose a phonon bridge interpretation for the lowered thermal resistivity of the interfaces beyond a critical intermixing. In this picture, the locally reduced acoustic contrast of the less defined boundary acts as a mediator that promotes phonon transition.
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http://dx.doi.org/10.3390/nano10061239DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC7353130PMC
June 2020

Electric-Field Control of Spin-Orbit Torques in Perpendicularly Magnetized W/CoFeB/MgO Films.

Phys Rev Lett 2020 May;124(21):217701

Institute of Physics, Johannes Gutenberg University, 55099 Mainz, Germany.

Controlling magnetism by electric fields offers a highly attractive perspective for designing future generations of energy-efficient information technologies. Here, we demonstrate that the magnitude of current-induced spin-orbit torques in thin perpendicularly magnetized CoFeB films can be tuned and even increased by electric-field generated piezoelectric strain. Using theoretical calculations, we uncover that the subtle interplay of spin-orbit coupling, crystal symmetry, and orbital polarization is at the core of the observed strain dependence of spin-orbit torques. Our results open a path to integrating two energy efficient spin manipulation approaches, the electric-field-induced strain and the current-induced magnetization switching, thereby enabling novel device concepts.
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http://dx.doi.org/10.1103/PhysRevLett.124.217701DOI Listing
May 2020

Propagation Length of Antiferromagnetic Magnons Governed by Domain Configurations.

Nano Lett 2020 Jan 17;20(1):306-313. Epub 2019 Dec 17.

Institut für Physik , Johannes Gutenberg Universität-Mainz , 55099 , Mainz , Germany.

The compensated magnetic order and characteristic terahertz frequencies of antiferromagnetic materials make them promising candidates to develop a new class of robust, ultrafast spintronic devices. The manipulation of antiferromagnetic spin-waves in thin films is anticipated to lead to new exotic phenomena such as spin-superfluidity, requiring an efficient propagation of spin-waves in thin films. However, the reported decay length in thin films has so far been limited to a few nanometers. In this work, we achieve efficient spin-wave propagation over micrometer distances in thin films of the insulating antiferromagnet hematite with large magnetic domains while evidencing much shorter attenuation lengths in multidomain thin films. Through transport and magnetic imaging, we determine the role of the magnetic domain structure and spin-wave scattering at domain walls to govern the transport. We manipulate the spin transport by tailoring the domain configuration through field cycle training. For the appropriate crystalline orientation, zero-field spin transport is achieved across micrometers, as required for device integration.
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http://dx.doi.org/10.1021/acs.nanolett.9b03837DOI Listing
January 2020

Thermal skyrmion diffusion used in a reshuffler device.

Nat Nanotechnol 2019 Jul 22;14(7):658-661. Epub 2019 Apr 22.

Institut für Physik, Johannes Gutenberg-Universität Mainz, Mainz, Germany.

Magnetic skyrmions in thin films can be efficiently displaced with high speed by using spin-transfer torques and spin-orbit torques at low current densities. Although this favourable combination of properties has raised expectations for using skyrmions in devices, only a few publications have studied the thermal effects on the skyrmion dynamics. However, thermally induced skyrmion dynamics can be used for applications such as unconventional computing approaches, as they have been predicted to be useful for probabilistic computing devices. In our work, we uncover thermal diffusive skyrmion dynamics by a combined experimental and numerical study. We probed the dynamics of magnetic skyrmions in a specially tailored low-pinning multilayer material. The observed thermally excited skyrmion motion dominates the dynamics. Analysing the diffusion as a function of temperature, we found an exponential dependence, which we confirmed by means of numerical simulations. The diffusion of skyrmions was further used in a signal reshuffling device as part of a skyrmion-based probabilistic computing architecture. Owing to its inherent two-dimensional texture, the observation of a diffusive motion of skyrmions in thin-film systems may also yield insights in soft-matter-like characteristics (for example, studies of fluctuation theorems, thermally induced roughening and so on), which thus makes it highly desirable to realize and study thermal effects in experimentally accessible skyrmion systems.
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http://dx.doi.org/10.1038/s41565-019-0436-8DOI Listing
July 2019

Microstructure Design for Fast Lifetime Measurements of Magnetic Tunneling Junctions.

Sensors (Basel) 2019 Jan 30;19(3). Epub 2019 Jan 30.

Fachbereich Physik and Forschungszentrum OPTIMAS, Technische Universität Kaiserslautern, 67663 Kaiserslautern, Germany.

The estimation of the reliability of magnetic field sensors against failure is a critical point concerning their application for industrial purposes. Due to the physical stochastic nature of the failure events, this can only be done by means of a statistical approach which is extremely time consuming and prevents a continuous observation of the production. Here, we present a novel microstructure design for a parallel measurement of the lifetime characteristics of a sensor population. By making use of two alternative designs and the Weibull statistical distribution function, we are able to measure the lifetime characteristics of a CoFeB/MgO/CoFeB tunneling junction population. The main parameters governing the time evolution of the failure rate are estimated and discussed and the suitability of the microstructure for highly reliable sensor application is proven.
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http://dx.doi.org/10.3390/s19030583DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6387249PMC
January 2019

Femtosecond formation dynamics of the spin Seebeck effect revealed by terahertz spectroscopy.

Nat Commun 2018 07 24;9(1):2899. Epub 2018 Jul 24.

Department of Physical Chemistry, Fritz Haber Institute of the Max Planck Society, 14195, Berlin, Germany.

Understanding the transfer of spin angular momentum is essential in modern magnetism research. A model case is the generation of magnons in magnetic insulators by heating an adjacent metal film. Here, we reveal the initial steps of this spin Seebeck effect with <27 fs time resolution using terahertz spectroscopy on bilayers of ferrimagnetic yttrium iron garnet and platinum. Upon exciting the metal with an infrared laser pulse, a spin Seebeck current j arises on the same ~100 fs time scale on which the metal electrons thermalize. This observation highlights that efficient spin transfer critically relies on carrier multiplication and is driven by conduction electrons scattering off the metal-insulator interface. Analytical modeling shows that the electrons' dynamics are almost instantaneously imprinted onto j because their spins have a correlation time of only ~4 fs and deflect the ferrimagnetic moments without inertia. Applications in material characterization, interface probing, spin-noise spectroscopy and terahertz spin pumping emerge.
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http://dx.doi.org/10.1038/s41467-018-05135-2DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6057952PMC
July 2018

Magnetic Exchange Interaction in Nitronyl Nitroxide Radical-Based Single Crystals of 3d Metal Complexes: A Combined Experimental and Theoretical Study.

ACS Omega 2018 Mar 9;3(3):2918-2933. Epub 2018 Mar 9.

Max Planck Institute for Polymer Research, Ackermannweg 10, D-55128 Mainz, Germany.

Two stable nitronyl nitroxide free radicals { = 4'-methoxy-phenyl-4,4,5,5,-tetramethylimidazoline-1-oxyl-3-oxide (NNPhOMe) and = 2-(2'-thienyl)-4,4,5,5-tetramethylimidazoline 3-oxide 1-oxyl (NNT)} are successfully synthesized using Ullmann condensation. The reactions of these two radicals with 3d transition metal ions, in the form of M(hfac) (where M = Co or Mn, hfac: hexafluoroacetylacetone), result in four metal-organic complexes Co(hfac)(NNPhOMe), ; Co(hfac)(NNT)·(HO), ; Mn(hfac)(NNPhOMe)·(CH), ; and Mn(hfac)(NNT), . The crystal structure and magnetic properties of these complexes are investigated by single-crystal X-ray diffraction, dc magnetization, infrared, and electron paramagnetic resonance spectroscopies. The compounds and crystallize in the triclinic, 1̅, space group, whereas complex crystallizes in the monoclinic structure with the 2/ space group and forms chain-like structure along the direction. The complex crystallizes in the monoclinic symmetry with the 2/ space group in which the N-O unit of the radical coordinates with the Co ion through hydrogen bonding of a water molecule. All compounds exhibit antiferromagnetic interactions between the transition metal ions and nitronyl nitroxide radicals. The magnetic exchange interactions (/ ) are derived using isotropic spin Hamiltonian = -2∑( ) for the model fitting to the magnetic susceptibility data for , , , and . The exchange interaction strengths are found to be -328, -1.25, -248, and -256 K, for the , , , and metal-organic complexes, respectively. Quantum chemical density functional theory (DFT) computations are carried out on several models of the metal-radical complexes to elucidate the magnetic interactions at the molecular level. The calculations show that a small part of the inorganic spins are delocalized over the oxygens from hfac {∼0.03 for Co(II) and ∼0.015 for Mn(II)}, whereas a more significant fraction {∼0.24 for Mn(II) and ∼0.13 for Co(II)} of delocalized spins from the metal ion is transferred to the coordinated oxygen atom(s) of nitronyl nitroxide.
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http://dx.doi.org/10.1021/acsomega.7b01669DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC6641457PMC
March 2018

Complex Terahertz and Direct Current Inverse Spin Hall Effect in YIG/CuIr Bilayers Across a Wide Concentration Range.

Nano Lett 2018 02 2;18(2):1064-1069. Epub 2018 Feb 2.

Institute of Physics, Johannes Gutenberg-University Mainz , 55099 Mainz, Germany.

We measure the inverse spin Hall effect of CuIr thin films on yttrium iron garnet over a wide range of Ir concentrations (0.05 ⩽ x ⩽ 0.7). Spin currents are triggered through the spin Seebeck effect, either by a continuous (dc) temperature gradient or by ultrafast optical heating of the metal layer. The spin Hall current is detected by electrical contacts or measurement of the emitted terahertz radiation. With both approaches, we reveal the same Ir concentration dependence that follows a novel complex, nonmonotonous behavior as compared to previous studies. For small Ir concentrations a signal minimum is observed, whereas a pronounced maximum appears near the equiatomic composition. We identify this behavior as originating from the interplay of different spin Hall mechanisms as well as a concentration-dependent variation of the integrated spin current density in CuIr. The coinciding results obtained for dc and ultrafast stimuli provide further support that the spin Seebeck effect extends to terahertz frequencies, thus enabling a transfer of established spintronic measurement schemes into the terahertz regime. Our findings also show that the studied material allows for efficient spin-to-charge conversion even on ultrafast time scales.
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http://dx.doi.org/10.1021/acs.nanolett.7b04538DOI Listing
February 2018

: calculate X-ray diffraction of epitaxial multilayers.

J Appl Crystallogr 2017 Feb 1;50(Pt 1):288-292. Epub 2017 Feb 1.

Institute of Physics, University of Mainz, Staudinger Weg 7, 55128 Mainz, Germany; Graduate School Materials Science in Mainz, Staudinger Weg 9, 55128 Mainz, Germany.

Epitaxial multilayers and superlattice (SL) structures are gaining increasing importance as they offer the opportunity to create artificial crystals with new functionalities. These crystals deviate from the parent bulk compounds not only in terms of the lattice constants but also in the symmetry classification, which renders calculation of their X-ray diffraction (XRD) patterns tedious. Nevertheless, XRD is essential to get information on the multilayer/SL structure such as, for example, out-of-plane lattice constants, strain relaxation and period length of the crystalline SL. This article presents a powerful yet simple program, based on the general one-dimensional kinematic X-ray diffraction theory, which calculates the XRD patterns of tailor-made multilayers and thus enables quantitative comparison of measured and calculated XRD data. As the multilayers are constructed layer by layer, the final material stack can be entirely arbitrary. Moreover, is very flexible and can be straightforwardly adapted to any material system. The source code of is available as supporting material for this article.
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http://dx.doi.org/10.1107/S1600576716018379DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC5294394PMC
February 2017

Origin of the spin Seebeck effect in compensated ferrimagnets.

Nat Commun 2016 Feb 4;7:10452. Epub 2016 Feb 4.

Institute of Physics, Johannes Gutenberg-University Mainz, 55099 Mainz, Germany.

Magnons are the elementary excitations of a magnetically ordered system. In ferromagnets, only a single band of low-energy magnons needs to be considered, but in ferrimagnets the situation is more complex owing to different magnetic sublattices involved. In this case, low lying optical modes exist that can affect the dynamical response. Here we show that the spin Seebeck effect (SSE) is sensitive to the complexities of the magnon spectrum. The SSE is caused by thermally excited spin dynamics that are converted to a voltage by the inverse spin Hall effect at the interface to a heavy metal contact. By investigating the temperature dependence of the SSE in the ferrimagnet gadolinium iron garnet, with a magnetic compensation point near room temperature, we demonstrate that higher-energy exchange magnons play a key role in the SSE.
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http://dx.doi.org/10.1038/ncomms10452DOI Listing
http://www.ncbi.nlm.nih.gov/pmc/articles/PMC4742853PMC
February 2016

Length Scale of the Spin Seebeck Effect.

Phys Rev Lett 2015 Aug 28;115(9):096602. Epub 2015 Aug 28.

Institute of Physics, Johannes Gutenberg-University Mainz, 55099 Mainz, Germany.

We investigate the origin of the spin Seebeck effect in yttrium iron garnet (YIG) samples for film thicknesses from 20 nm to 50  μm at room temperature and 50 K. Our results reveal a characteristic increase of the longitudinal spin Seebeck effect amplitude with the thickness of the insulating ferrimagnetic YIG, which levels off at a critical thickness that increases with decreasing temperature. The observed behavior cannot be explained as an interface effect or by variations of the material parameters. Comparison to numerical simulations of thermal magnonic spin currents yields qualitative agreement for the thickness dependence resulting from the finite magnon propagation length. This allows us to trace the origin of the observed signals to genuine bulk magnonic spin currents due to the spin Seebeck effect ruling out an interface origin and allowing us to gauge the reach of thermally excited magnons in this system for different temperatures. At low temperature, even quantitative agreement with the simulations is found.
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http://dx.doi.org/10.1103/PhysRevLett.115.096602DOI Listing
August 2015

Ellipsoid-shaped superparamagnetic nanoclusters through emulsion electrospinning.

Chem Commun (Camb) 2015 Mar;51(18):3758-61

Empa, Swiss Federal Laboratories for Materials Science and Technology, Laboratory for Protection and Physiology, Lerchenfeldstrasse 5, CH-9014 St. Gallen, Switzerland.

Ellipsoid-shaped nanoclusters composed of single superparamagnetic nanoparticles can be generated by emulsion electrospinning. Stretching and subsequent solvent evaporation of iron oxide loaded emulsion droplets during the emulsion electrospinning process enables the creation of such structures embedded in polymer nanofibers. Dissolution of the polymer fibers yields an aqueous dispersion of the inorganic clusters which are the first example of ellipsoid-shaped superparamagnetic nanoclusters with a high saturation magnetization (∼47 emu g(-1)).
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http://dx.doi.org/10.1039/c4cc10076jDOI Listing
March 2015

Tailor-made nanocontainers for combined magnetic-field-induced release and MRI.

Macromol Biosci 2014 Sep 9;14(9):1205-14. Epub 2014 May 9.

Max Planck Institute for Polymer Research, Ackermannweg 10, 55128, Mainz, Germany; Graduate School Materials Science in Mainz, Staudinger Weg 9, 55128, Mainz, Germany.

The synthesis of a novel nanocapsule-based carrier system is described, possessing a triggered release in remote-controlled fashion upon application of an external magnetic field in combination with the possibility to use the capsules as contrast agents for magnetic resonance imaging (MRI). Therefore, polymeric nanocontainers containing a high amount of superparamagnetic MnFe2 O4 nanoparticles and a thermo-degradable shell are fabricated via a miniemulsion route. The process allows the facile encapsulation of hydrophilic compounds, as demonstrated for a model dye. Release of the encapsulated dye is achieved upon application of an external alternating magnetic field. While the magnetic nanoparticles here act as heat generators to stimulate the decomposition of the shell and subsequently a release of the payload, they additionally enable the use of the nanocapsules as imaging agents for MRI. Due to the encapsulated magnetic nanoparticles, the nanocapsules possess high r2 relaxivity values of 96-120 Hz mmol(-1) , which makes them suitable for MRI. In toxicity experiments, the nanocapsules show no cell toxicity up to fairly high concentrations (600 µg mL(-1) ). Due to their dual-functionality, the nanocapsules possess high potential as nanocarriers with combined magnetic-field-induced release capability and as contrast agents for MRI.
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http://dx.doi.org/10.1002/mabi.201400122DOI Listing
September 2014
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