Technical Physics

Walther Meißner Institute, Technical University of Munich

Walther-Meißner-Institute for Low Temperature Research

Walther-Meißner Strasse 8

85748 Garching

Tel. +49 89 289 14249


Group Webpage

Quantum interference and coherence effects in solid-state systems are inspiring me since decades. Even better that we can use them now in many applications such as quantum computing, sensing and communication.


Research focus: solid-state quantum technology, superconducting quantum circuits, quantum microwave communication, quantum sensing, quantum materials

The understanding of solid-state quantum systems as well as engineering, controlling and exploiting them for applications in quantum computing, sensing and communication is one of the key challenges in today’s solid-state physics.

Solid-State Quantum Technology

We study the fundamental physics of solid-state based quantum systems and advance their fabrication technology to lay the basis for applications in quantum computing, quantum communication, and quantum sensing. Key directions are superconducting quantum circuits and hybrid quantum systems combining e.g. superconducting, spin and mechanical systems.


Quantum Microwave Communication

We study the physical foundations of quantum microwave communication and sensing. We also develop quantum microwave technologies for the realization of quantum local area networks and advanced sensing methods such as quantum radar. Our research focuses on the continuous-variable regime, which offers advantages in terms of robustness and technological implementation.

Quantum Materials

Solid-state research at an international top level is not possible without excellent materials in form of single crystals and thin film heterostructures, state-of-the-art characterization methods and advanced techniques for fabricating nanostructures. We synthesize and characterize novel quantum materials and devices as the basis for modern solid-state quantum technologies.

Major achievements

  • realization of ultra-strong coupling in superconducting circuit-QED
  • development of dry dilution refrigerators
  • realization of a novel dual-path state reconstruction scheme for propagating quantum microwaves
  • demonstration of strong magnon-photon coupling
  • demonstration of path entanglement of continuous-variable quantum microwaves and implementation of relevant protocols such as remote state preparation and teleportation.

Roles within MCQST


Hybrid magnetization dynamics in Cu2OSeO3/NiFe heterostructures

Lüthi, C., Flacke, L., Aqeel, A., Kamra, A., Gross, R., Back, C. & Weiler, M.

Appl. Phys. Lett. 122(1), 012401 (2023).

Show Abstract

We investigate the coupled magnetization dynamics in heterostructures of a single crystal of the chiral magnet Cu2OSeO3 (CSO) and a polycrystalline ferromagnet NiFe (Py) thin film using broadband ferromagnetic resonance (FMR) at cryogenic temperatures. We observe the excitation of a hybrid mode (HM) below the helimagnetic transition temperature of CSO. This HM is attributed to the spin dynamics at the CSO/Py interface. We study the HM by measuring its resonance frequencies for in plane rotations of the external magnetic field. We find that the HM exhibits dominantly fourfold anisotropy in contrast to the FMR of CSO and Py.

DOI: 10.1063/5.0128733

Magnon transport in Y3Fe5O12/Pt nanostructures with reduced effective magnetization

J. Gückelhorn, T. Wimmer, M. Müller, S. Geprägs, H. Huebl, R. Gross, M. Althammer

Physical Review B 104, L180410 (2021).

Show Abstract

For applications making use of magnonic spin currents damping effects, which decrease the spin conductivity, have to be minimized. We here investigate the magnon transport in a yttrium iron garnet thin film with strongly reduced effective magnetization. We show that in a three-terminal device the effective magnon conductivity can be increased by a factor of up to six by a current applied to a modulator electrode, which generates damping compensation above a threshold current. Moreover, we find a linear dependence of this threshold current on the applied magnetic field. We can explain this behavior by the reduced effective magnetization and the associated nearly circular magnetization precession.

DOI: 10.1103/PhysRevB.104.L180410

Beyond the standard quantum limit for parametric amplification of broadband signals

M. Renger, S. Pogorzalek, Q. Chen, Y. Nojiri, K. Inomata, Y. Nakamura, M. Partanen, A. Marx, R. Gross, F. Deppe, K.G. Fedorov

Npj Quantum Information 7, 160 (2021).

Show Abstract

The low-noise amplification of weak microwave signals is crucial for countless protocols in quantum information processing. Quantum mechanics sets an ultimate lower limit of half a photon to the added input noise for phase-preserving amplification of narrowband signals, also known as the standard quantum limit (SQL). This limit, which is equivalent to a maximum quantum efficiency of 0.5, can be overcome by employing nondegenerate parametric amplification of broadband signals. We show that, in principle, a maximum quantum efficiency of unity can be reached. Experimentally, we find a quantum efficiency of 0.69 +/- 0.02, well beyond the SQL, by employing a flux-driven Josephson parametric amplifier and broadband thermal signals. We expect that our results allow for fundamental improvements in the detection of ultraweak microwave signals.

DOI: 10.1038/s41534-021-00495-y

Low-temperature suppression of the spin Nernst angle in Pt

T. Wimmer, J. Gückelhorn, S. Wimmer, S. Mankovsky, H. Ebert, M. Opel, S. Geprägs, R. Gross, H. Huebl, M. Althammer

Physical Review B 104, L140404 (2021).

Show Abstract

The coupling between electrical, thermal, and spin transport results in a plethora of novel transport phenomena. However, disentangling different effects is experimentally very challenging. We demonstrate that bilayers consisting of the antiferromagnetic insulator hematite (alpha-Fe2O3) and Pt allow one to precisely measure the transverse spin Nernst magnetothermopower (TSNM) and observe the low-temperature suppression of the platinum (Pt) spin Nernst angle. We show that the observed signal stems from the interplay between the interfacial spin accumulation in Pt originating from the spin Nernst effect and the orientation of the Neel vector of alpha-Fe2O3, rather than its net magnetization. Since the latter is negligible in an antiferromagnet, our device is superior to ferromagnetic structures, allowing one to unambiguously distinguish the TSNM from thermally excited magnon transport, which usually dominates in ferri/ferromagnets due to their nonzero magnetization. Evaluating the temperature dependence of the effect, we observe a vanishing TSNM below similar to 100 K. We compare these results with theoretical calculations of the temperature-dependent spin Nernst conductivity and find excellent agreement. This provides evidence for a vanishing spin Nernst angle of Pt at low temperatures and the dominance of extrinsic contributions to the spin Nernst effect.

DOI: 10.1103/PhysRevB.104.L140404

Robust formation of nanoscale magnetic skyrmions in easy-plane anisotropy thin film multilayers with low damping

L. Flacke, V. Ahrens, S. Mendisch, L. Körber, T. Böttcher, E. Meidinger, M. Yaqoob, M. Müller, L. Liensberger, A. Kákay, M. Becherer, P. Pirro, M. Althammer, S. Geprägs, H. Huebl, R. Gross, M. Weiler

Physical Review B 104, L100417 (2021).

Show Abstract

We experimentally demonstrate the formation of room-temperature skyrmions with radii of about 25 nm in easy-plane anisotropy multilayers with an interfacial Dzyaloshinskii-Moriya interaction (DMI). We detect the formation of individual magnetic skyrmions by magnetic force microscopy and find that the skyrmions are stable in out-of-plane fields up to about 200 mT. We determine the interlayer exchange coupling as well as the strength of the interfacial DMI. Additionally, we investigate the dynamic microwave spin excitations by broadband magnetic resonance spectroscopy. From the uniform Kittel mode we determine the magnetic anisotropy and low damping alpha(G) < 0.04. We also find clear magnetic resonance signatures in the nonuniform (skyrmion) state. Our findings demonstrate that skyrmions in easy-plane multilayers are promising for spin-dynamical applications.

DOI: 10.1103/PhysRevB.104.L100417

Tunable cooperativity in coupled spin-cavity systems

L. Liensberger, F.X. Haslbeck, A. Bauer, H. Berger, R. Gross, H. Huebl, C. Pfleiderer, M. Weiler

Physical Review B 104, L100415 (2021).

Show Abstract

We experimentally study the tunability of the cooperativity in coupled spin-cavity systems by changing the magnetic state of the spin system via an external control parameter. As a model system, we use the skyrmion host material Cu2OSeO3 coupled to a microwave cavity resonator. We measure a dispersive coupling between the resonator and magnon modes in different magnetic phases of the material and model our results by using the input-output formalism. Our results show a strong tunability of the normalized coupling rate by magnetic field, allowing us to change the magnon-photon cooperativity from 1 to 60 at the phase boundaries of the skyrmion lattice state.

DOI: 10.1103/PhysRevB.104.L100415

RF Antenna Design for 3D Quantum Memories

F. Deppe, E. Xie, K.G. Fedorov, G. Andersson, J. Muller, A. Marx, R. Gross

International Symposium of the Applied-Computational-Electromagnetics-Society (ACES) (2021).

Show Abstract

A quantum memory has to meet the conflicting requirements of strong coupling for fast readout and weak coupling for long storage. Multimode rectangular superconducting 3D cavities are known to satisfy both properties. Here, we systematically study the external coupling to the two lowest-frequency modes of an aluminum cavity. First, we introduce a general analytical scheme to describe the capacitive coupling of the antenna pin and validate this model experimentally. On this basis, we engineer an antenna which is overcoupled to the first mode, but undercoupled to the second mode.

DOI: 10.1109/aces53325.2021.00104

Growth of aluminum nitride on a silicon nitride substrate for hybrid photonic circuits

G. Terrasanta, M. Müller, T. Sommer, S. Geprägs, R. Gross, M. Althammer, M. Poot

Materials for Quantum Technology 1, 21002 (2021).

Show Abstract

Aluminum nitride (AlN) is an emerging material for integrated quantum photonics with its excellent linear and nonlinear optical properties. In particular, its second-order nonlinear susceptibility χ(2) allows single-photon generation. We have grown AlN thin films on silicon nitride (Si3N4) via reactive DC magnetron sputtering. The thin films have been characterized using x-ray diffraction (XRD), optical reflectometry, atomic force microscopy (AFM), and scanning electron microscopy. The crystalline properties of the thin films have been improved by optimizing the nitrogen to argon ratio and the magnetron DC power of the deposition process. XRD measurements confirm the fabrication of high-quality c-axis oriented AlN films with a full width at half maximum of the rocking curves of 3.9° for 300 nm-thick films. AFM measurements reveal a root mean square surface roughness below 1 nm. The AlN deposition on SiN allows us to fabricate hybrid photonic circuits with a new approach that avoids the challenging patterning of AlN.

DOI: 10.1088/2633-4356/ac08ed

Quantifying the spin mixing conductance of EuO/W heterostructures by spin Hall magnetoresistance experiments

P. Rosenberger, M. Opel, S. Geprägs, H. Huebl, R. Gross, M. Müller, M. Althammer

Applied Physics Letters 118, 192401 (2021).

Show Abstract

The spin Hall magnetoresistance (SMR) allows to investigate the magnetic textures of magnetically ordered insulators in heterostructures with normal metals by magnetotransport experiments. We here report the observation of the SMR in in situ prepared ferromagnetic EuO/W thin film bilayers with magnetically and chemically well-defined interfaces. We characterize the magnetoresistance effects utilizing angle-dependent and field-dependent magnetotransport measurements as a function of temperature. Applying the established SMR model, we derive and quantify the real and imaginary parts of the complex spin mixing interface conductance. We find that the imaginary part is by one order of magnitude larger than the real part. Both decrease with increasing temperature. This reduction is in agreement with thermal fluctuations in the ferromagnet.

DOI: 10.1063/5.0049235

All-Electrical Magnon Transport Experiments in Magnetically Ordered Insulators

M. Althammer

Phys. Status Solidi-Rapid Res. Lett. 15 (8), 2100130 (2021).

Show Abstract

Angular momentum transport is one of the cornerstones of spintronics. Spin angular momentum is not only transported by mobile charge carriers but also by the quantized excitations of the magnetic lattice in magnetically ordered systems. In this regard, magnetically ordered insulators (MOIs) provide a platform for magnon spin transport experiments without additional contributions from spin currents carried by mobile electrons. In combination with charge-to-spin current conversion processes in conductors with finite spin-orbit coupling, it is possible to realize all-electrical magnon transport schemes in thin-film heterostructures. Herein, an insight into such experiments and recent breakthroughs achieved is provided. Special attention is given to charge-current-based manipulation via an adjacent normal metal of magnon transport in MOIs in terms of spin-transfer torque. Moreover, the influence of two magnon modes with opposite spin in antiferromagnetic insulators on all-electrical magnon transport experiments is discussed.

DOI: 10.1002/pssr.202100130

In-situ tunable nonlinearity and competing signal paths in coupled superconducting resonators

M. Fischer, Q.-M. Chen, C. Besson, P. Eder, J. Goetz, S. Pogorzalek, M. Renger, E. Xie, M.J. Hartmann, K.G. Fedorov, A. Marx, F. Deppe, R. Gross

Physical Review B 103, 94515 (2021).

Show Abstract

We have fabricated and studied a system of two tunable and coupled nonlinear superconducting resonators. The nonlinearity is introduced by galvanically coupled dc superconducting quantum interference devices. We simulate the system response by means of a circuit model, which includes an additional signal path introduced by the electromagnetic environment. Furthermore, we present two methods allowing us to experimentally determine the nonlinearity. First, we fit the measured frequency and flux dependence of the transmission data to simulations based on the equivalent circuit model. Second, we fit the power dependence of the transmission data to a model that is predicted by the nonlinear equation of motion describing the system. Our results show that we are able to tune the nonlinearity of the resonators by almost two orders of magnitude via an external coil and two on-chip antennas. The studied system represents a basic building block for larger systems, allowing for quantum simulations of bosonic many-body systems with a larger number of lattice sites.

DOI: 10.1103/PhysRevB.103.094515

Temperature-Dependent Spin Transport and Current-Induced Torques in Superconductor-Ferromagnet Heterostructures

M. Mueller, L. Liensberger, L. Flacke, H. Huebl, A. Kamra, W. Belzig, R. Gross, M. Weiler, M. Althammer

Physical Review Letters 126 (8), 087201 (2021).

Show Abstract

We investigate the injection of quasiparticle spin currents into a superconductor via spin pumping from an adjacent ferromagnetic metal layer. To this end, we use NbN-Ni80Fe20(Py) heterostructures with a Pt spin sink layer and excite ferromagnetic resonance in the Permalloy layer by placing the samples onto a coplanar waveguide. A phase sensitive detection of the microwave transmission signal is used to quantitatively extract the inductive coupling strength between the sample and the coplanar waveguide, interpreted in terms of inverse current-induced torques, in our heterostructures as a function of temperature. Below the superconducting transition temperature T-c, we observe a suppression of the dampinglike torque generated in the Pt layer by the inverse spin Hall effect, which can be understood by the changes in spin current transport in the superconducting NbN layer. Moreover, below T-c we find a large fieldlike current-induced torque.

DOI: 10.1103/PhysRevLett.126.087201

Experimental evidence for Zeeman spin-orbit coupling in layered antiferromagnetic conductors

R. Ramazashvili, P.D. Grigoriev, T. Helm, F. Kollmannsberger, M. Kunz, W. Biberacher, E. Kampert, H. Fujiwara, A. Erb, J. Wosnitza, R. Gross, M.V. Kartsovnik

NPJ Quantum Materials 6 (1), 11 (2021).

Show Abstract

Most of solid-state spin physics arising from spin-orbit coupling, from fundamental phenomena to industrial applications, relies on symmetry-protected degeneracies. So does the Zeeman spin-orbit coupling, expected to manifest itself in a wide range of antiferromagnetic conductors. Yet, experimental proof of this phenomenon has been lacking. Here we demonstrate that the Neel state of the layered organic superconductor kappa-(BETS)(2)FeBr4 shows no spin modulation of the Shubnikov-de Haas oscillations, contrary to its paramagnetic state. This is unambiguous evidence for the spin degeneracy of Landau levels, a direct manifestation of the Zeeman spin-orbit coupling. Likewise, we show that spin modulation is absent in electron-doped Nd1.85Ce0.15CuO4, which evidences the presence of Neel order in this cuprate superconductor even at optimal doping. Obtained on two very different materials, our results demonstrate the generic character of the Zeeman spin-orbit coupling.

DOI: 10.1038/s41535-021-00309-6

Static magnetic proximity effects and spin Hall magnetoresistance in Pt/Y3Fe5O12 and inverted Y3Fe5O12/Pt bilayers

S. Gepraegs, C. Klewe, S. Meyer, D. Graulich, F. Schade, M. Schneider, S. Francoual, S.P. Collins, K. Ollefs, F. Wilhelm, A. Rogalev, Y. Joly, S.T.B. Goennenwein, M. Opel, T. Kuschel, R. Gross

Physical Review B 102 (21), 214438 (2020).

Show Abstract

The magnetic state of heavy metal Pt thin films in proximity to the ferrimagnetic insulator Y3Fe5O12 has been investigated systematically by means of x-ray magnetic circular dichroism and x-ray resonant magnetic reflectivity measurements combined with angle-dependent magnetotransport studies. To reveal intermixing effects as the possible cause for induced magnetic moments in Pt, we compare thin film heterostructures with different orders of the layer stacking and different interface properties. For standard Pt layers on Y3Fe5O12 thin films, we do not detect any static magnetic polarization in Pt. These samples show an angle-dependent magnetoresistance behavior, which is consistent with the established spin Hall magnetoresistance. In contrast, for the inverted layer sequence, Y3Fe5O12 thin films grown on Pt layers, Pt displays a finite induced magnetic moment comparable to that of all-metallic Pt/Fe bilayers. This magnetic moment is found to originate from finite intermixing at the Y3Fe5O12/Pt interface. As a consequence, we found a complex angle-dependent magnetoresistance indicating a superposition of the spin Hall and the anisotropic magnetoresistance in these types of samples. Both effects can be disentangled from each other due to their different angle dependence and their characteristic temperature evolution.

DOI: 10.1103/PhysRevB.102.214438

Sideband-resolved resonator electromechanics based on a nonlinear Josephson inductance probed on the single-photon level

P. Schmidt, M.T. Amawi, S. Pogorzalek, F. Deppe, A. Marx, R. Gross, H. Huebl

Communication Physics 3 (1), 233 (2020).

Show Abstract

Light-matter interaction in optomechanical systems is the foundation for ultra-sensitive detection schemes as well as the generation of phononic and photonic quantum states. Electromechanical systems realize this optomechanical interaction in the microwave regime. In this context, capacitive coupling arrangements demonstrated interaction rates of up to 280Hz. Complementary, early proposals and experiments suggest that inductive coupling schemes are tunable and have the potential to reach the single-photon strong-coupling regime. Here, we follow the latter approach by integrating a partly suspended superconducting quantum interference device (SQUID) into a microwave resonator. The mechanical displacement translates into a time varying flux in the SQUID loop, thereby providing an inductive electromechanical coupling. We demonstrate a sideband-resolved electromechanical system with a tunable vacuum coupling rate of up to 1.62kHz, realizing sub-aNHz(-1/2) force sensitivities. The presented inductive coupling scheme shows the high potential of SQUID-based electromechanics for targeting the full wealth of the intrinsically nonlinear optomechanics Hamiltonian. Recently, inductively-coupled optomechanical systems have been realized. They represent an important step forward towards achieving strong light-matter interaction, offer extreme sensitivity to mechanical displacement, and allow to study quantum phenomena on a single quantum level. In this work, a superconducting device is inductively coupled to a microwave resonator forming an electromechanical system operating at the single-photon level.

DOI: 10.1038/s42005-020-00501-3

Observation of Antiferromagnetic Magnon Pseudospin Dynamics and the Hanle Effect

T. Wimmer, A. Kamra, J. Gueckelhorn, M. Opel, S. Gepraegs, R. Gross, H. Huebl, M. Althammer

Physical Review Letters 125 (24), 247204 (2020).

Show Abstract

We report on experiments demonstrating coherent control of magnon spin transport and pseudospin dynamics in a thin film of the antiferromagnetic insulator hematite utilizing two Pt strips for all-electrical magnon injection and detection. The measured magnon spin signal at the detector reveals an oscillation of its polarity as a function of the externally applied magnetic field. We quantitatively explain our experiments in terms of diffusive magnon transport and a coherent precession of the magnon pseudospin caused by the easy-plane anisotropy and the Dzyaloshinskii-Moriya interaction. This experimental observation can be viewed as the magnonic analog of the electronic Hanle effect and the Datta-Das transistor, unlocking the high potential of antiferromagnetic magnonics toward the realization of rich electronics-inspired phenomena.

DOI: 10.1103/PhysRevLett.125.247204

Precise control of J(eff)=1/2 magnetic properties in Sr2IrO4 epitaxial thin films by variation of strain and thin film thickness

S. Geprags, B.E. Skovdal , M. Scheufele, M. Opel, D. Wermeille, P. Thompson, A. Bombardi, V. Simonet, S. Grenier, P. Lejay, G.A. Chahine, D.L. Quintero-Castro, R. Gross, D. Mannix

Physical Review B 102 (21), 214402 (2020).

Show Abstract

We report on a comprehensive investigation of the effects of strain and film thickness on the structural and magnetic properties of epitaxial thin films of the prototypal J(eff) = 1/2 compound Sr2IrO4 by advanced x-ray scattering. We find that the Sr2IrO4 thin films can be grown fully strained up to a thickness of 108 nm. By using x-ray resonant scattering, we show that the out-of-plane magnetic correlation length is strongly dependent on the thin film thickness, but independent of the strain state of the thin films. This can be used as a finely tuned dial to adjust the out-of-plane magnetic correlation length and transform the magnetic anisotropy from two-dimensional to three-dimensional behavior by incrementing film thickness. These results provide a clearer picture for the systematic control of the magnetic degrees of freedom in epitaxial thin films of Sr2IrO4 and bring to light the potential for a rich playground to explore the physics of 5d transition-metal compounds.

DOI: 10.1103/PhysRevB.102.214402

Quantitative comparison of magnon transport experiments in three-terminal YIG/Pt nanostructures acquired via dc and ac detection techniques

J. Gueckelhorn, T. Wimmer, S. Gepraegs, H. Huebl, R. Gross, M. Althammer

Applied Physics Letters 117 (18), 182401 (2020).

Show Abstract

All-electrical generation and detection of pure spin currents are promising ways toward controlling the diffusive magnon transport in magnetically ordered insulators. We quantitatively compare two measurement schemes, which allow us to measure the magnon spin transport in a three-terminal device based on a yttrium iron garnet thin film. We demonstrate that the dc charge current method based on the current reversal technique and the ac charge current method utilizing first and second harmonic lock-in detection can both efficiently distinguish between electrically and thermally injected magnons. In addition, both measurement schemes allow us to investigate the modulation of magnon transport induced by an additional dc charge current applied to the center modulator strip. However, while at a low modulator charge current both schemes yield identical results, we find clear differences above a certain threshold current. This difference originates from nonlinear effects of the modulator current on the magnon conductance.

DOI: 10.1063/5.0023307

Echo Trains in Pulsed Electron Spin Resonance of a Strongly Coupled Spin Ensemble

S. Weichselbaumer, M. Zens, C.W. Zollitsch, M.S. Brandt, S. Rotter, R. Gross, H. Huebl.

Physical Review Letters 125, 137701 (2020).

Show Abstract

We report on a novel dynamical phenomenon in electron spin resonance experiments of phosphorus donors. When strongly coupling the paramagnetic ensemble to a superconducting lumped element resonator, the coherent exchange between these two subsystems leads to a train of periodic, self-stimulated echoes after a conventional Hahn echo pulse sequence. The presence of these multiecho signatures is explained using a simple model based on spins rotating on the Bloch sphere, backed up by numerical calculations using the inhomogeneous Tavis-Cummings Hamiltonian.


Resonant nanodiffraction x-ray imaging reveals role of magnetic domains in complex oxide spin caloritronics

P.G. Evans, S.D. Marks, S. Gepraegs, M. Dietlein, Y. Joly, M.Y. Dai, J.M. Hu, L. Bouchenoire, P.B.J. Thompson, T.U. Schulli, M.I. Richard, R. Gross, D. Carbone, D. Mannix

Science Advances 6 (40), eaba9351 (2020).

Show Abstract

Spin electronic devices based on crystalline oxide layers with nanoscale thicknesses involve complex structural and magnetic phenomena, including magnetic domains and the coupling of the magnetism to elastic and plastic crystallographic distortion. The magnetism of buried nanoscale layers has a substantial impact on spincaloritronic devices incorporating garnets and other oxides exhibiting the spin Seebeck effect (SSE). Synchrotron hard x-ray nanobeam diffraction techniques combine structural, elemental, and magnetic sensitivity and allow the magnetic domain configuration and structural distortion to be probed in buried layers simultaneously. Resonant scattering at the Gd L-2 edge of Gd3Fe5O12 layers yields magnetic contrast with both linear and circular incident x-ray polarization. Domain patterns facet to form low-energy domain wall orientations but also are coupled to elastic features linked to epitaxial growth. Nanobeam magnetic diffraction images reveal diverse magnetic microstructure within emerging SSE materials and a strong coupling of the magnetism to crystallographic distortion.

DOI: 10.1126/sciadv.aba9351

Effect of interfacial oxidation layer in spin pumping experiments on Ni80Fe20/SrIrO3 heterostructures

T.S. Suraj, M. Mueller, S. Gelder, S. Gepraegs, M. Opel, M. Weiler, K. Sethupathi, H. Huebl, R. Gross, M.S.R. Rao, M. ALthammer

Journal of Applied Physics 128 (8), 083903 (2020).

Show Abstract

SrIrO3 with its large spin-orbit coupling and low charge conductivity has emerged as a potential candidate for efficient spin-orbit torque magnetization control in spintronic devices. Here we report on the influence of an interfacial oxide layer on spin pumping experiments in Ni80Fe20 (NiFe)/SrIrO3 bilayer heterostructures. To investigate this scenario, we have carried out broadband ferromagnetic resonance (BBFMR) measurements, which indicate the presence of an interfacial antiferromagnetic oxide layer. We performed in-plane BBFMR experiments at cryogenic temperatures, which allowed us to simultaneously study dynamic spin pumping properties (Gilbert damping) and static magnetic properties (such as the effective magnetization and magnetic anisotropy). The results for NiFe/SrIrO3 bilayer thin films were analyzed and compared to those from a NiFe/NbN/SrIrO3 trilayer reference sample, where a spin-transparent, ultra-thin NbN layer was inserted to prevent the oxidation of NiFe. At low temperatures, we observe substantial differences in the magnetization dynamics parameters of these samples. In particular, the Gilbert damping in the NiFe/SrIrO3 bilayer sample drastically increases below 50 K, which can be well explained by enhanced spin fluctuations at the antiferromagnetic ordering temperature of the interfacial oxide layer. Our results emphasize that this interfacial oxide layer plays an important role for the spin current transport across the NiFe/SrIrO3 interface.

DOI: 10.1063/5.0021741

Spin Hall magnetoresistance in antiferromagnetic insulators

S. Gepraegs, M. Opel, J. Fischer, O. Gomonay, P. Schwenke, M. Althammer, H. Huebl, R. Gross

Journal of Applied Physics 127 (24), (2020).

Show Abstract

Antiferromagnetic materials promise improved performance for spintronic applications as they are robust against external magnetic field perturbations and allow for faster magnetization dynamics compared to ferromagnets. The direct observation of the antiferromagnetic state, however, is challenging due to the absence of a macroscopic magnetization. Here, we show that the spin Hall magnetoresistance (SMR) is a versatile tool to probe the antiferromagnetic spin structure via simple electrical transport experiments by investigating the easy-plane antiferromagnetic insulators

alpha -

Fe 2

O 3 (hematite) and NiO in bilayer heterostructures with a Pt heavy-metal top electrode. While rotating an external magnetic field in three orthogonal planes, we record the longitudinal and the transverse resistivities of Pt and observe characteristic resistivity modulations consistent with the SMR effect. We analyze both their amplitude and phase and compare the data to the results from a prototypical collinear ferrimagnetic

Y 3

Fe 5

O 12/Pt bilayer. The observed magnetic field dependence is explained in a comprehensive model, based on two magnetic sublattices and taking into account magnetic field-induced modifications of the domain structure. Our results show that the SMR allows us to understand the spin configuration and to investigate magnetoelastic effects in antiferromagnetic multi-domain materials. Furthermore, in


- Fe 2

O 3/Pt bilayers, we find an unexpectedly large SMR amplitude of

2.5 x


- 3, twice as high as for prototype

Y 3

Fe 5

O 12/Pt bilayers, making the system particularly interesting for room-temperature antiferromagnetic spintronic applications.

DOI: 10.1063/5.0009529

Large Spin Hall Magnetoresistance in Antiferromagnetic alpha-Fe2O3/Pt Heterostructures

J. Fischer, M. Althammer, N. Vlietstra, H. Huebl, S.T.B. Goennenwein, R. Gross, S. Gepraegs, M. Opel

Physical Review Applied 13 (1), 014019 (2020).

Show Abstract

We investigate the spin Hall magnetoresistance (SMR) at room temperature in thin-film heterostructures of antiferromagnetic insulating (0001)-oriented alpha-Fe2O3 (hematite) and Pt. We measure their longitudinal and transverse resistivities while rotating an applied magnetic field of up to 17 T in three orthogonal planes. For out-of-plane magnetotransport measurements, we find indications for a multidomain antiferromagnetic configuration whenever the field is aligned along the film normal. For in-plane field rotations, we clearly observe a sinusoidal resistivity oscillation characteristic for the SMR due to a coherent rotation of the Neel vector. The maximum SMR amplitude of 0.25% is, surprisingly, twice as high as for prototypical ferrimagnetic Y3Fe5O12/Pt heterostructures. The SMR effect saturates at much smaller magnetic fields than in comparable antiferromagnets, making the alpha-Fe2O3/Pt system particularly interesting for roomtemperature antiferromagnetic spintronic applications.

DOI: 10.1103/PhysRevApplied.13.014019

Spin Transport in a Magnetic Insulator with Zero Effective Damping

T. Wimmer, M. Althammer, L. Liensberger, N. Vlietstra, S. Geprägs, M. Weiler, R. Gross, H. Huebl

Physical Review Letters 123 (25), 257201 (2019).

Show Abstract

Applications based on spin currents strongly rely on the control and reduction of their effective damping and their transport properties. We here experimentally observe magnon mediated transport of spin (angular) momentum through a 13.4-nm thin yttrium iron garnet film with full control of the magnetic damping via spin-orbit torque. Above a critical spin-orbit torque, the fully compensated damping manifests itself as an increase of magnon conductivity by almost 2 orders of magnitude. We compare our results to theoretical expectations based on recently predicted current induced magnon condensates and discuss other possible origins of the observed critical behavior.

DOI: 10.1103/PhysRevLett.123.257201

High spin-wave propagation length consistent with low damping in a metallic ferromagnet

L. Flacke, L. Liensberger, M. Althammer, H. Huebl, S. Geprags, K. Schultheiss, A. Buzdakov, T. Hula, H. Schultheiss, E.R.J. Edwards, H.T. Nembach, J.M. Shaw, R. Gross, M. Weiler

Applied Physics Letters 115 (12), 122402 (2019).

Show Abstract

We report ultralow intrinsic magnetic damping in Co25Fe75 heterostructures, reaching the low 10(-4) regime at room temperature. By using a broadband ferromagnetic resonance technique in out-of-plane geometry, we extracted the dynamic magnetic properties of several Co25Fe75-based heterostructures with varying ferromagnetic layer thicknesses. By measuring radiative damping and spin pumping effects, we found the intrinsic damping of a 26 nm thick sample to be alpha 0 less than or similar to 3.18x10-4. Furthermore, using Brillouin light scattering microscopy, we measured spin-wave propagation lengths of up to (21 +/- 1) mu m in a 26 nm thick Co25Fe75 heterostructure at room temperature, which is in excellent agreement with the measured damping.

DOI: 10.1063/1.5102132

Magnetoelasticity of Co25Fe75 thin films

D. Schwienbacher, M. Pernpeintner, L. Liensberger, E.R.J. Edwards, H.T. Nembach, J.M. Shaw, M. Weiler, R. Gross, H. Huebl

Journal of Applied Physics 126 (10), 103902 (2019).

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We investigate the magnetoelastic properties of Co25Fe75 and Co10Fe90 thin films by measuring the mechanical properties of a doubly clamped string resonator covered with multilayer stacks containing these films. For the magnetostrictive constants, we find lambda Co25Fe75=(-20.68 +/- 0.25)x10-6 and lambda Co10Fe90=(-9.80 +/- 0.12)x10-6 at room temperature, in contrast to the positive magnetostriction previously found in bulk CoFe crystals. Co25Fe75 thin films unite low damping and sizable magnetostriction and are thus a prime candidate for micromechanical magnonic applications, such as sensors and hybrid phonon-magnon systems.

DOI: 10.1063/1.5116314

Exchange-Enhanced Ultrastrong Magnon-Magnon Coupling in a Compensated Ferrimagnet

L. Liensberger, A. Kamra, H. Maier-Flaig, S. Geprags, A. Erb, S.T.B. Goennenwein, R. Gross, W. Belzig, H. Huebl, M. Weiler

Physical Review Letters 123 (11), 117204 (2019).

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We experimentally study the spin dynamics in a gadolinium iron garnet single crystal using broadband ferromagnetic resonance. Close to the ferrimagnetic compensation temperature, we observe ultrastrong coupling of clockwise and counterclockwise magnon modes. The magnon-magnon coupling strength reaches almost 40% of the mode frequency and can be tuned by varying the direction of the external magnetic field. We theoretically explain the observed mode coupling as arising from the broken rotational symmetry due to a weak magnetocrystalline anisotropy. The effect of this anisotropy is exchange enhanced around the ferrimagnetic compensation point.

DOI: 10.1103/PhysRevLett.123.117204

Anomalous spin Hall angle of a metallic ferromagnet determined by a multiterminal spin injection/detection device

T. Wimmer, B. Coester, S. Geprags, R. Gross, S.T.B. Goennenwein, H. Huebl, M. Althammer

Applied Physics Letters 115 (9), 092404 (2019).

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We report on the determination of the anomalous spin Hall angle in the ferromagnetic metal alloy cobalt-iron (Co25Fe75, CoFe). This is accomplished by measuring the spin injection/detection efficiency in a multiterminal device with nanowires of platinum (Pt) and CoFe deposited onto the magnetic insulator yttrium iron garnet (Y3Fe5O12, YIG). Applying a spin-resistor model to our multiterminal spin transport data, we determine the magnon conductivity in YIG, the spin conductance at the YIG/CoFe interface, and finally the anomalous spin Hall angle of CoFe as a function of its spin diffusion length in a single device. Our experiments clearly reveal a negative anomalous spin Hall angle of the ferromagnetic metal CoFe, but a vanishing ordinary spin Hall angle. This work, therefore, adds new observations to the results reported in Tian et al. [Phys. Rev. B 94, 020403 (2016)] and Das et al. [Phys. Rev. B 96, 220408(R) (2017)] , where the authors found finite contributions of the ordinary spin Hall angle in the ferromagnetic metals Co and Permalloy. Published under license by AIP Publishing.

DOI: 10.1063/1.5101032

Secure quantum remote state preparation of squeezed microwave states

S. Pogorzalek, K. G. Fedorov, M. Xu, A. Parra-Rodriguez, M. Sanz, M. Fischer, E. Xie, K. Inomata, Y. Nakamura, E. Solano, A. Marx, F. Deppe, R. Gross.

Nature Communications 10, 2604 (2019).

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Quantum communication protocols based on nonclassical correlations can be more efficient than known classical methods and offer intrinsic security over direct state transfer. In particular, remote state preparation aims at the creation of a desired and known quantum state at a remote location using classical communication and quantum entanglement. We present an experimental realization of deterministic continuous-variable remote state preparation in the microwave regime over a distance of 35 cm. By employing propagating two-mode squeezed microwave states and feedforward, we achieve the remote preparation of squeezed states with up to 1.6 dB of squeezing below the vacuum level. Finally, security of remote state preparation is investigated by using the concept of the one-time pad and measuring the von Neumann entropies. We find nearly identical values for the entropy of the remotely prepared state and the respective conditional entropy given the classically communicated information and, thus, demonstrate close-to-perfect security.


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