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Projekt Druckansicht

Boltzmann Zugang zum gekoppelten Spin- und Wärmetransport in Ferromagneten

Antragsteller Professor Dr. Felix von Oppen, seit 10/2016
Fachliche Zuordnung Theoretische Physik der kondensierten Materie
Förderung Förderung von 2011 bis 2018
Projektkennung Deutsche Forschungsgemeinschaft (DFG) - Projektnummer 198246422
 
Erstellungsjahr 2018

Zusammenfassung der Projektergebnisse

5. Summary of activities The scientific reports of the projects will be published in a special issue on Spincaloritronics in the Journal of Physics D (http://iopscience.iop.org/journal/0022-3727/page/Special-issue-on- Spincaloritronics). This special issue is expected to be completed in the fall of 2018. Below, we briefly summarize the highlights of each project. Ansermet (Lausanne): The SPINCAT project has been very important because it is during this time that my group was able to establish the theory of heat-driven spin torques in insulators and verify this theory experimentally. We introduced the notion of a magnetic Seebeck effect and showed that the propagation of a wave packet was less attenuated when going from cold to hot than conversely. We derived a quantitive model based on a modified Landau-Lishitz equation and could account for all the spectroscopic data we accumulated. Back/Strunk (Regensburg): We have clarified inconsistent evidence from different experiments in the literature, by showing that the so-called transversal spin-Seebeck effect in metals is unobservably small, when compared to the competing (established) magnetothermoelectric effects. In the course of these efforts, we have established an experimental platform for the simultaneous measurement of the electric, thermoelectric and thermal transport coefficients of metal films. The platform was applied to the alloy system FeCo, which allows to systematically tune the Fermi energy through the band structure, and investigated the evolution of the transport properties with composition. Most prominent result is a pronounced magnon drag contribution to the thermopower that changes sign at xCo~0.6. Bauer (Tohoku/Delft): We carried out theoretical research on spin caloritronic phenomena with emphasis on the magnetic insulator yttrium iron garnet using a combination of analytic and numerical techniques. In collaboration with experimental groups in the Priority Program we helped discover new effects such as the spin Hall magnetoresistance. Belzig (Konstanz): We have predicted effcient cooling of spin-split superconducting devices using spin-polarized currents in multi-terminal geometries and in the non-linear regime. We have developed the theory of strongly spin-dependent boundary conditions for quasiclassical Greens functions to describe electric, thermal and spin transport far from equilibrium. We have predicted spin-polarized Shiba bands at interfaces between a 10 SPP 1538 „Spin Caloric Transport“ strongly spin-polarized magnetic insulator and a superconductor, which can be used for long-range spin transport of heat and charge. Deac/Lindner (Dresden): The main achievement of the project was to develop and fabricate a microresonator-based setup to investigate thermal spin-torque effects in layered thin film structures under the presence of a thermal gradient. The latter is generated by laser heating, while the influence of the thermal gradient on the spin-torque properties is monitored by microwave absorption. The setup enables one to investigate single mesoscopic samples without need of any electrical contacts. Demokritov (Münster): We have studied the interaction of thermal gradients introduced by spin waves with electrons in graphene. We have demonstrated a dc electric field/voltage in graphene as a response to the dynamic magnetic excitations in an adjacent out-ofplane magnetized YIG film. We show that the induced voltage changes its sign when the orientation of the static magnetization is reversed, clearly indicating the broken mirror reflection symmetry about the planes normal to the graphene/YIG interface. In addition, we show that, due to the refraction of spin waves in the thermal gradients, the heated region acts as a defocusing lens for Damon-Eshbach spin waves and as a focusing for backward volume modes. Finaly, we have shown that spin waves under influence of spin current obey the thermodynamic Bose-Einstein statistics with a non-zero chemical potential. Fabian (Regensburg): The standard theory of spin, charge, and heat transport was formulated and tunneling spin caloric effects in ferromagnetic junctions were predicted. The suitability black phosphorous and phosphorene for spin transport application were established. Using DFT methods spin relaxation time in phosphorene was calculated and its large anisotropy was predicted. Goennenwein/Gross (Dresden/Garching): Within SPP 1538, we successfully established the pulsed laser epitaxy of high quality yttrium iron garnet (Y3Fe5O12) as well as gadolinium iron garnet (Gd3Fe5O12) thin films and heterostructures (see, e.g., Althammer et al., Phys. Rev. B 87, 224401 (2013)). Furthermore, in collaboration with colleagues in Japan, we discovered the so-called spin Hall magnetoresistance effect (Nakayama et al, Phys. Rev. Letters 110, 206601 (2013)), which arises from spin transfer across a magnetic insulator/metal interface. Finally, we experimentally observed the spin Nernst effect in Pt (Meyer et al, Nat. Mat. 16, 977 (2017)). Grundler (Lausanne): In our project we prepared ferromagnetic nanotubes on semiconductor nanowires and investigated their domain configurations depending on different materials. Using a scanning laser focus we generated thermal gradients and performed spatially resolved investigations on the anomalous Nernst effect. Exploiting the grating coupler effect and ferroelectric/ferromagnetic hybrid structures we explored the generation of exchange-dominated spin waves with wavelengths smaller than 100 nm. 11 SPP 1538 „Spin Caloric Transport“ Kampfrath (Berlin): We successfully transferred central spintronic effects to highest (i.e. Terahertz) frequencies, including the spin-dependent/spin Seebeck effect to generate spin currents and the inverse spin Hall effect to convert these spin currents into charge currents. We took advantage of these effects to build a spintronic emitter of Terahertz electromagnetic pulses that cover the full range from 1 to 30 THz with an efficiency comparable or even better than standard Terahertz sources. Moreover, we revealed the elementary steps leading to the formation of the spin-Seebeck effect in archetypal YIG|Pt bilayers whose ultrafast rise is determined by the thermalization dynamics of the optically excited electrons in Pt. Kläui (Mainz): We demonstrated in a joint experimental and theoretical work that the spin Seebeck effect can generate spin currents in the bulk of an insulating ferrimagnet. By varying materials and interfaces, we identified different magnon modes that contribute to the spin transport generated by a heat current experimentally. Theoretical calculations analyzed the effect of thermal spin currents on the displacement of domain walls. Kratzer/Popescu (Duisburg): The anistropic magnetothermopower in ferromagnetic/nonmagnetic heterostructures is much larger than the corresponding anisotropic magnetoresistance, a feature that could be related to quantum well states occurring in the minority spin channel of the nonmagnetic partner. Heterostructures formed between Heusler alloys and nonmagnetic leads (Al, Pd, Pt) are stable upon formation, may provide a measurable spin accumulation, but the transport properties of the Heusler systems may vary significantly as a result of unintentional, intrinsic doping. Krause (Hamburg): Using a spin-polarized scanning tunneling microscope at low temperature, we realized model-type magnetic tunnel junction experiments with well-defined interfaces and vacuum serving as tunnel barrier between a magnetic probe tip and an ultrathin film sample exhibiting a inhomogeneous spin spiral. A temperature drop at the junction is generated and controlled by heating only the scanning probe tip with a laser beam. Recording the thermovoltage at the junction while scanning the tip above the magnetic surface we precisely measured angel-resolved coefficients for magneto- Seebeck tunneling across a vacuum barrier. Kuschel (Bielefeld): As main achievements of this project we investigated alternative spin Seebeck effect (SSE) detection methods using magnetooptic techniques instead of the classical electrical detection based on the inverse spin Hall effect [Kimling et al., Phys. Rev. Lett. 118, 057201 (2017); Kehlberger et al., Phys. Rev. Appl. 4, 014008 (2015)] avoiding parasitic thermoelectric effects. However, we further found a way to separate the thermoelectric effects from the SSE for the electrical detection [Bougiatioti et al., Phys. Rev. Lett. 119, 227205 (2017)], thus observing the SSE in metals. For this achievement two new techniques had to be used, which we have introduced to the spin caloric community, this is synchrotron-based x-ray resonant magnetic reflectivity [Kuschel et al., Phys. Rev. Lett. 115, 097401 (2015); Klewe et al., Phys. Rev. B 93, 214440 (2016)] for the detection of the magnetic proximity effect in Pt and the heat flux detection method [Sola et al., Sci. 12 SPP 1538 „Spin Caloric Transport“ Rep. 7, 46752 (2017)] to reliably compare SSE signals from different samples and between different setups. We further studied the non-local SSE [Shan et al., Phys. Rev. B 94, 174437 (2016), Shan et al., Appl. Phys. Lett. 110, 132406 (2017), Liu et al., Phys. Rev. B 95, 140402(R) (2017), Cornelissen et al., Phys. Rev. B 96, 104441 (2017)] during a 18-month research stay of the PI at the University of Groningen, Netherlands. Kuschel/Reiss/Schmalhorst (Bielefeld): We could refute the experimental observation of the transverse spin Seebeck effect in metals [Schmid et al., Phys. Rev. Lett. 111, 187201 (2013); Meier et al., Phys. Rev. B 88, 184425 (2013); Shestakov et al., Phys. Rev. B 92, 224425 (2015)] and insulators [Meier et al., Nat. Commun. 6, 8211 (2015)], while we observed the longitudinal spin Seebeck effect in nickel ferrite for the first time [Meier et al., Phys. Rev. B 87, 054421 (2013)]. Beside these two geometries with fixed orientation of the thermal gradient, we developed a new tool to generate a thermal gradient in various in-plane directions [Reimer et al., Sci. Rep. 7, 40586 (2017)] for anisotropy studies of thermoelectric and spin caloric effects. Furthermore, we observed the tunnel magneto- Seebeck effect in magnetic tunnel junctions [Walter et al., Nat. Mater. 10, 742 (2011); Liebing et al., Phys. Rev. Lett. 107, 177201 (2011)], studied parasitic effects [Boehnke et al., Rev. Sci. Instrum. 84, 063905 (2013); Huebner et al., Phys. Rev. B 93, 224433 (2016)] and increased the effect sizes by choosing proper material properties for the tunnel barrier [Huebner et al., Phys. Rev. B 96, 214435 (2017)] and the electrodes [Boehnke et al., Nat. Commun. 8, 1626 (2017)] or by applying a bias voltage [Boehnke et al., Sci. Rep. 5, 8945 (2015)]. Mertig (Halle): Topology has conquered the field of condensed matter physics with the discovery of the quantum Hall effect. Since then the zoo of topological materials is steadily increasing. In this project, we discovered how to realize different topological phases with magnons: the magnon pendants to topological insulators as well as Weyl and nodal-line semimetals are presented. Similar to the electronic case, nonzero Berry curvature causes transverse transport, that is, magnon Hall effects. We developed a method to show how these effects can be quantified by classical spin dynamics simulations. Mokrousov (Jülich): Within our project, we applied various ab-initio-based techniques to study thermal transport and response characteristics of complex magnetic materials, with a particular focus on the origin of these phenomena in the underlying topology of the electronic structure in the reciprocal space. On the side of transport properties, the main achievements of the project in the course of the SpinCat were the predictions concerning the topological part of the anomalous Nernst and spin Nernst effects in elemental ferromagnets and paramagnets. In addition, we have pioneered the effects related to the manipulation of the magnetization in ferromagnets by applied thermal gradients, i.e. the effect of thermal spin-orbit torque, as well as the effect of generation of thermal currents by magnetization dynamics via the effect of the inverse spin-orbit torque. Our road to these milestones lead to exiting developments in the area of chiral magnetism and current-driven magnetization dynamics. 13 SPP 1538 „Spin Caloric Transport“ Molenkamp/Gould (Würzburg): We mapped out the diffusive thermopower and Nearnst contributions in the ferromagnetic semiconductor (Ga,Mn)As. These show very large anisotropy contributions, which relate to the direction of the magnetization in the crystal, which yield additional insight on the details of the band structure in this material. Münzenberg: We have published a public outreach article in in the Greman journal “Physik unserer Zeit”: Vom Seebeck-Effekt zur Spinkaloritronik - Heiße Elektronik, A. Thomas, M. Münzenberg, Physik in unserer Zeit 6, (2012). In the first period we developed the idea of extreme temperature gradient by femtosecond lasers and thermal spin torques (J. C. Leutenantsmeyer, et al, SPIN 3, 1350002 (2013).) which was a close theory and experimental work with the Heiliger group (joint publictaions), but also Hans Werner Schumachers project at the PTB. During that period we also discovered the emission of THz radiation from femtosecond laser driven double layers, a breakthrough opening a novel field of THz spintronics, which can be seen as a kind of ultrafast spin-dependent Seebeck effect with inverse spin Hall effect (T. Kampfrath, et al, Nat. Nano. 8, 256 (2013). This led to a new project for the second period, led by Tobias Kampfrath, FHI Berlin. In the last period, we focused on new materials (Heusler electrodes), novel oxide barriers and in especial and increase of the lateral resolution of temperature gradients to extend a magnetic memory to three dimensions by using thermocurrent-maps with spatial resolution. This allows the controlled application of also of lateral heat gradients and we can generate different voltage signals, giving separation x-y-z direction independently at around a single tunnel junction. We identify the Anomalous Nernst effect (ANE) for the first time inside a single micron sized magnetic tunnel junction (MTJs). Latest experiments show that by detecting the ANE much more sensitively and on smaller length scales by the tunnel junction, opens new possibilities for three-dimensional applications based on this effect. Nowak (Konstanz): We demonstrated in a joint experimental and theoretical work that the spin Seebeck effect can generate spin currents in the bulk of insulating ferri- and antiferromagnets and we explored the possible application of this effect in a magnonic spin valvue device. By varying materials and interfaces, we identified different magnon modes that contribute to the spin transport generated by a heat current experimentally. Theoretical calculations analyzed the effect of thermal spin currents on the displacement of domain walls. Schumacher (Braunschweig): Among the highlights of the project of Santiago Serrano-Guisan and Hans Werner Schumacher were the measurement of the tunnel magneto thermo power and tunnel magneto thermo current of nanopatterned magnetic tunnel junctions. Furthermore, the magneto Seebeck contribution of a single domain wall in a magnetic nanowire was experimentally accessed for the first time. As an extension of this work, the anomalous Nernst effect was established as a tool to measure domain wall propagation in magnetic nanowires with nano scale resolution. 14 SPP 1538 „Spin Caloric Transport“ Vasyuchka (Kaiserslautern): In this project, linear and nonlinear phenomena induced by coherently excited magnon currents in the presence of a thermal gradient were investigated and a high level of understanding was achieved. A new approach for creation of fully tunable, two-dimensionally structured magnetic materials was proposed and realized: it was shown that thermal landscape created by a laser heating in a magnetic insulator results in modulations of the saturation magnetization and in the control of spinwave characteristics. As well, key characteristics of parametric processes in a magnetic insulator – normal metal structures subject to a thermal gradient were revealed for both dipolar and exchange magnons. It is found, e.g., that the parametric instability threshold strongly depends on the temperature of the spin-wave excitation area, whereas temperature gradients do not influence the threshold. Vasyuchka/Serga/Hillebrands (Kaiserslautern): The project was focused on the investigation of magnon currents generated by a thermal gradient in low-damping magnetic structures and on understanding the dynamic and static magnon Seebeck effect in hetero-structures comprising non-magnetic metals and low-damping magnets. It was found that the temporal dynamics of the longitudinal spin Seebeck effect in the ferromagneticinsulator/normal-metal system depends on the diffusion of bulk thermal magnons in the thermal gradient and can be described by a characteristic magnon diffusion length. From the transient voltage response of yttrium iron garnet/platinum bilayers subject to periodic heating the characteristic response times were identified, which depend on the thickness of the magnetic layer and can drop down to the nanosecond timescale. Zierold/Nielsch (Hamburg): Magnetic-field dependent electrical and thermal transport properties have been successfully studied on metallic multilayer thin films and nanowires. Novel approaches for the characterization of the thermovoltage/Seebeck coefficient as well as the measurement of the thermal conductivity on nanostructures have been developed and assigned to research fields such as investigation of Topological Insulators and Wey semimetals. Finally, the influence of temperature gradients on the magnetization reversal of magnetic nanowires have been explored revealing that a stressinduced magneto-elastic anisotropy can counteract a thermally assisted magnetization reversal process.

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