##### Electrically Induced Angular Momentum Flow between Separated Ferromagnets

R. Schlitz, M. Grammer, T. Wimmer, J. Gückelhorn, L. Flacke, S. T. B. Goennenwein, R. Gross, H. Hübl, A. Kamra, M. Althammer

Physical Review Letters 132 (25), 256701 (2024).

Converting angular momentum between different degrees of freedom within a magnetic material results from a dynamic interplay between electrons, magnons, and phonons. This interplay is pivotal to implementing spintronic device concepts that rely on spin angular momentum transport. We establish a new concept for long-range angular momentum transport that further allows us to address and isolate the magnonic contribution to angular momentum transport in a nanostructured metallic ferromagnet. To this end, we electrically excite and detect spin transport between two parallel and electrically insulated ferromagnetic metal strips on top of a diamagnetic substrate. Charge-to-spin current conversion within the ferromagnetic strip generates electronic spin angular momentum that is transferred to magnons via electron-magnon coupling. We observe a finite angular momentum flow to the second ferromagnetic strip across a diamagnetic substrate over micron distances, which is electrically detected in the second strip by the inverse charge-to-spin current conversion process. We discuss phononic and dipolar interactions as the likely cause to transfer angular momentum between the two strips. Moreover, our Letter provides the experimental basis to separate the electronic and magnonic spin transport and thereby paves the way towards magnonic device concepts that do not rely on magnetic insulators.

##### Temperature dependence of the magnon-phonon interaction in hybrids of high-overtone bulk acoustic resonators with ferromagnetic thin films

M. Müller, J. Weber, S. T. B. Goennenwein, S. V. Kusminskiy, R. Gross, M. Althammer, H. Hübl

Physical Review Applied 21 (3), 34032 (2024).

Tailored magnon-phonon hybrid systems, in which high-overtone bulk acoustic resonators couple resonantly to the Kittel mode of a ferromagnetic thin film, are considered optimal for the creation of acoustic phonons with a defined circular polarization. This class of devices is therefore ideal for the investigation of phonon-propagation properties and assessing their capacity to transport angular momentum in the classical, and potentially even in the quantum, regime. Here, we study the coupling between the magnons in a ferromagnetic Co25Fe75 thin film and the transverse acoustic phonons in bulk acoustic wave resonators formed by the sapphire substrate onto which the film is deposited. Using broadband ferromagnetic resonance experiments as a function of temperature, we investigate the strength of the coherent magnon-phonon interaction and the individual damping rates of the magnons and phonons participating in the process. This demonstrates that this coupled magnon-phonon system can reach a cooperativity C approximate to 1 at cryogenic temperatures. Our experiments also showcase the potential of strongly coupled magnon-phonon systems for strain-sensing applications.

##### Chiral phonons and phononic birefringence in ferromagnetic metal-bulk acoustic resonator hybrids

M. Müller, J. Weber, F. Engelhardt, V. Bittencourt, T. Luschmann, M. Cherkasskii, M. Opel, S. T. B. Goennenwein, S. V. Kusminskiy, S. Geprägs, R. Gross, M. Althammer, H. Hübl

Physical Review B 109 (2), 24430 (2024).

Magnomechanical devices, in which magnetic excitations couple to mechanical vibrations, have been discussed as efficient and broadband microwave signal transducers in the classical and quantum limit. We experimentally investigate the resonant magnetoelastic coupling between the ferromagnetic resonance modes in metallic Co25Fe75 thin films, featuring ultralow magnetic damping as well as sizable magnetostriction, and standing transverse elastic phonon modes in sapphire, silicon, and gadolinium gallium garnet at cryogenic temperatures. For all substrates, we observe a coherent interaction between the acoustic and magnetic modes. We identify the phonon modes as transverse shear waves propagating with slightly different velocities (Av/v similar or equal to 10(-5)),. i.e., all investigated substrates show potential for phononic birefringence as well as phonon-mediated angular momentum transport. Our magnon-phonon hybrid systems operate in a coupling regime analogous to the Purcell enhanced damping in cavity magnonics.

##### Imperfect photon detection in quantum illumination

F. Kronowetter, M. Wuerth, W. Utschick, R. Gross, K. G. Fedorov

Physical Review Applied 21 (1), 14007 (2024).

In quantum illumination, various detection schemes have been proposed for harnessing the remaining quantum correlations of the entanglement-based resource state. To date, the only successful implementation in the microwave domain [R. Assouly, R. Dassonneville, T. Peronnin, A. Bienfait, and B. Huard, Nat. Phys. 19, 1418 (2023)] has relied on a specific mixing operation of the respective return and idler modes, followed by single-photon counting in one of the two mixer outputs. We investigate the performance of this scheme for realistic detection parameters in terms of the detection efficiency, dark-count probability, and photon-number resolution. Furthermore, we take the second mixer output into account and investigate the advantage of correlated photon counting (CPC) for a varying thermal background and optimum postprocessing weighting in CPC. We find that the requirements for photon-number resolution in the two mixer outputs are highly asymmetric due to different associated photon-number expectation values.

##### Site-Selective Enhancement of Superconducting Nanowire Single-Photon Detectors via Local Helium Ion Irradiation

S. Strohauer, F. Wietschorke, L. Zugliani, R. Flaschmann, C. Schmid, S. Grotowski, M. Müller, B. Jonas, M. Althammer, R. Gross, K. Müller, J. J. Finley

Advanced Quantum Technologies 6 (12), 12 (2023).

Achieving homogeneous performance metrics between nominally identical pixels is challenging for the operation of arrays of superconducting nanowire single-photon detectors (SNSPDs). Here, local helium ion irradiation is utilized to post-process and tune single-photon detection efficiency, switching current, and critical temperature of individual devices on the same chip. For 12 nm thick highly absorptive SNSPDs, which are barely sensitive to single photons with a wavelength of 780 nm prior to He ion irradiation, an increase of the system detection efficiency from <0.05% to (55.3 +/- 1.1)% is observed following irradiation. Moreover, the internal detection efficiency saturates at a temperature of 4.5 K after irradiation with 1800 ions nm(-2). Compared to 8 nm SNSPDs of similar detection efficiency, a doubling of the switching current (to 20 <mu>A) is observed for irradiated 10 nm thick detectors, increasing the amplitude of detection voltage pulses. Investigations of the scaling of superconducting thin film properties with irradiation up to a fluence of 2600 ions nm(-2) revealed an increase of sheet resistance and a decrease of critical temperature towards high fluences. A physical model accounting for defect generation and sputtering during helium ion irradiation is presented and shows good qualitative agreement with experiments.

##### Impact of growth conditions on magnetic anisotropy and magnon Hanle effect in *a*-Fe_{2}O_{3}

M. Scheufele, J. Gückelhorn, M. Opel, A. Kamra, H. Hübl, R. Gross, S. Geprägs, M. Althammer

Apl Materials 11 (9), 91115 (2023).

The antiferromagnetic insulator a-Fe2O3 (hematite), widely used in spintronics and magnonics, features a spin-reorientation transition (Morin transition) at 263 K. Thin films, however, often lack this Morin transition, limiting their potential applications. Here, we investigate the impact of different growth conditions on the magnetic anisotropy in a-Fe2O3 films to tune the Morin transition temperature. To this end, we compare the structural, magnetic, and magnon-based spin transport properties of a-Fe2O3 films with different thicknesses grown by pulsed laser deposition in molecular and atomic oxygen atmospheres. We observe a finite Morin transition for those grown by atomic-oxygen-assisted deposition, interestingly even down to 19 nm thickness, where we find a Morin transition at 125 K. In easy-plane antiferromagnets, the nature and time-evolution of the elementary excitations of the spin system are captured by the orientation and precession of the magnon pseudospin around its equilibrium pseudofield, manifesting itself in the magnon Hanle effect. We characterize this effect in these a-Fe2O3 films via all-electrical magnon transport measurements. The films grown with atomic oxygen show a markedly different magnon spin signal from those grown in molecular oxygen atmospheres. Most importantly, the maximum magnon Hanle signal is significantly enhanced, and the Hanle peak is shifted to lower magnetic field values for films grown with atomic oxygen, suggesting changes in the magnetic anisotropy due to an increased oxygen content in these films. Our findings provide new insights into the possibility to fine-tune the magnetic anisotropy in a-Fe2O3 and thereby to engineer the magnon Hanle effect.

##### Optimizing the growth conditions of Al mirrors for superconducting nanowire single-photon detectors

R. Flaschmann, C. Schmid, L. Zugliani, S. Strohauer, F. Wietschorke, S. Grotowski, B. Jonas, M. Mueller, M. Althammer, R. Gross, J. J. Finley, K. Müller

Materials for Quantum Technology 3 (3), 35002 (2023).

We investigate the growth conditions for thin ( <= 200 nm) sputtered aluminum films. These coatings are needed for various applications, e.g. for advanced manufacturing processes in the aerospace industry or for nanostructures for quantum devices. Obtaining high-quality films, with low roughness, requires precise optimization of the deposition process. To this end, we tune various sputtering parameters such as the deposition rate, temperature and power, which enables 50 nm thin films with a root mean square roughness of less than 1 nm and high reflectivity. Finally, we confirm the high-quality of the deposited films by realizing superconducting single-photon detectors integrated into multi-layer heterostructures consisting of an aluminum mirror and a silicon dioxide dielectric spacer. We achieve an improvement in detection efficiency at 780 nm from 40% to 70% by this integration approach.

##### High-*Q* Magnetic Levitation and Control of Superconducting Microspheres at Millikelvin Temperatures

J. Hofer, R. Gross, G. Higgins, H. Hübl, O. F. Kieler, R. Kleiner, D. Koelle, P. Schmidt, J. A. Slater, M. Trupke, K. Uhl, T. Weimann, W. Wieczorek, M. Aspelmeyer

Physical Review Letters 131 (4), 43603 (2023).

We report the levitation of a superconducting lead-tin sphere with 100 mu m diameter (corresponding to a mass of 5.6 mu g) in a static magnetic trap formed by two coils in an anti-Helmholtz configuration, with adjustable resonance frequencies up to 240 Hz. The center-of-mass motion of the sphere is monitored magnetically using a dc superconducting quantum interference device as well as optically and exhibits quality factors of up to 2.6 x 10(7). We also demonstrate 3D magnetic feedback control of the motion of the sphere. The setup is housed in a dilution refrigerator operating at 15 mK. By implementing a cryogenic vibration isolation system, we can attenuate environmental vibrations at 200 Hz by approximately 7 orders of magnitude. The combination of low temperature, large mass, and high quality factor provides a promising platform for testing quantum physics in previously unexplored regimes with high mass and long coherence times.

##### Surface acoustic wave resonators on thin film piezoelectric substrates in the quantum regime

T. Luschmann, A. Jung, S. Geprägs, F. X. Haslbeck, A. Marx, S. Filipp, S. Gröblacher, R. Gross, H. Hübl

Materials for Quantum Technology 3 (2), 21001 (2023).

Lithium niobate (LNO) is a well established material for surface acoustic wave (SAW) devices including resonators, delay lines and filters. Recently, multi-layer substrates based on LNO thin films have become commercially available. Here, we present a systematic low-temperature study of the performance of SAW devices fabricated on LNO-on-insulator and LNO-on-Silicon substrates and compare them to bulk LNO devices. Our study aims at assessing the performance of these substrates for quantum acoustics, i.e. the integration with superconducting circuits operating in the quantum regime. To this end, we design SAW resonators with a target frequency of 5 GHz and perform experiments at millikelvin temperatures and microwave power levels corresponding to single photons or phonons. The devices are investigated regarding their internal quality factors as a function of the excitation power and temperature, which allows us to characterize and quantify losses and identify the dominating loss mechanism. For the measured devices, fitting the experimental data shows that the quality factors are limited by the coupling of the resonator to a bath of two-level-systems. Our results suggest that SAW devices on thin film LNO on silicon have comparable performance to devices on bulk LNO and are viable for use in SAW-based quantum acoustic devices.

##### Quantum behavior of the Duffing oscillator at the dissipative phase transition

Q. M. Chen, M. Fischer, Y. Nojiri, M. Renger, E. D. Xie, M. Partanen, S. Pogorzalek, K. G. Fedorov, A. Marx, F. Deppe, R. Gross

Nature Communications 14 (1), 2896 (2023).

The non-deterministic behavior of the Duffing oscillator is classically attributed to the coexistence of two steady states in a double-well potential. However, this interpretation fails in the quantum-mechanical perspective which predicts a single unique steady state. Here, we measure the non-equilibrium dynamics of a superconducting Duffing oscillator and experimentally reconcile the classical and quantum descriptions as indicated by the Liouvillian spectral theory. We demonstrate that the two classically regarded steady states are in fact quantum metastable states. They have a remarkably long lifetime but must eventually relax into the single unique steady state allowed by quantum mechanics. By engineering their lifetime, we observe a first-order dissipative phase transition and reveal the two distinct phases by quantum state tomography. Our results reveal a smooth quantum state evolution behind a sudden dissipative phase transition and form an essential step towards understanding the intriguing phenomena in driven-dissipative systems. Classical mechanics predicts a bistability in the dynamics of the Duffing oscillator, a key model of nonlinear dynamics. By performing quantum simulations of the model, Chen et al. explain the bistability by quantum metastable states with long lifetimes and reveal a first-order dissipative phase transition.

##### Observation of the Nonreciprocal Magnon Hanle Effect

J. Gnckelhorn, S. de-la-Peña, M. Scheufele, M. Grammer, M. Opel, S. Geprägs, J. C. Cuevas, R. Gross, H. Hübl, A. Kamra, M. Althammer

Physical Review Letters 130 (21), 216703 (2023).

The precession of magnon pseudospin about the equilibrium pseudofield, the latter capturing the nature of magnonic eigenexcitations in an antiferromagnet, gives rise to the magnon Hanle effect. Its realization via electrically injected and detected spin transport in an antiferromagnetic insulator demonstrates its high potential for devices and as a convenient probe for magnon eigenmodes and the underlying spin interactions in the antiferromagnet. Here, we observe a nonreciprocity in the Hanle signal measured in hematite using two spatially separated platinum electrodes as spin injector or detector. Interchanging their roles was found to alter the detected magnon spin signal. The recorded difference depends on the applied magnetic field and reverses sign when the signal passes its nominal maximum at the so-called compensation field. We explain these observations in terms of a spin transport direction-dependent pseudofield. The latter leads to a nonreciprocity, which is found to be controllable via the applied magnetic field. The observed nonreciprocal response in the readily available hematite films opens interesting opportunities for realizing exotic physics predicted so far only for antiferromagnets with special crystal structures.

##### Coherent heavy charge carriers in an organic conductor near the bandwidth-controlled Mott transition

S. Oberbauer, S. Erkenov, W. Biberacher, N. D. Kushch, R. Gross, M. V. Kartsovnik

Physical Review B 107 (7), 75139 (2023).

The physics of the Mott metal-insulator transition (MIT) has attracted huge interest in the last decades. However, despite broad efforts, some key theoretical predictions are still lacking experimental confirmation. In particular, it is not clear whether the large coherent Fermi surface survives in immediate proximity to the bandwidth-controlled first-order MIT. A quantitative experimental verification of the predicted behavior of the quasiparticle effective mass, renormalized by many-body interactions, is also missing. Here we address these issues by employing organic K-type salts as exemplary quasi-two-dimensional bandwidth-controlled Mott insulators and gaining direct access to their charge-carrier properties via magnetic quantum oscillations. We trace the evolution of the effective cyclotron mass as the conduction bandwidth is tuned very close to the MIT by means of precisely controlled external pressure. We find that the sensitivity of the mass renormalization to tiny changes of the bandwidth is significantly stronger than theoretically predicted and is even further enhanced upon entering the transition region where the metallic and insulating phases coexist. On the other hand, even on the very edge of its existence, the metallic ground state preserves a large coherent Fermi surface with no significant enhancement of scattering.

##### Hybrid magnetization dynamics in Cu_{2}OSeO_{3}/NiFe heterostructures

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

Applied Physics Letters 122 (1), 12401 (2023).

We investigate the coupled magnetization dynamics in heterostructures of a single crystal of the chiral magnet Cu 2 OSeO 3 (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.

##### Reduced effective magnetization and damping by slowly relaxing impurities in strained ?-Fe2O3 thin films

M. Muller, M. Scheufele, J. Guckelhorn, L. Flacke, M. Weiler, H. Hübl, S. Gepraegs, R. Gross, M. Althammer

Journal of Applied Physics 132 (23), 233905 (2022).

Magnetically ordered insulators are of key interest for spintronics applications, but most of them have not yet been explored in depth regarding their magnetic properties, in particular with respect to their dynamic response. We study the static and dynamic magnetic properties of epitaxially strained gamma-Fe2O3 (maghemite) thin films grown via pulsed-laser deposition on MgO substrates by SQUID magnetometry and cryogenic broadband ferromagnetic resonance experiments. SQUID magnetometry measurements reveal hysteretic magnetization curves for magnetic fields applied both in- and out of the sample plane. From the magnetization dynamics of our thin films, we find a small negative effective magnetization in agreement with a strain induced perpendicular magnetic anisotropy. Moreover, we observe a non-linear evolution of the ferromagnetic resonance-linewidth as a function of the microwave frequency and explain this finding with the so-called slow relaxor model. We investigate the magnetization dynamics and non-linear damping mechanisms present in our samples as a function of frequency and temperature and in particular, observe a sign change in the effective magnetization from the transition of the magnetic anisotropy from a perpendicular easy axis to an easy in-plane anisotropy for reduced temperatures. Its nonlinear damping properties and strain-induced perpendicular anisotropy render gamma-Fe2O3 an interesting material platform for spintronics devices. Published under an exclusive license by AIP Publishing.

##### Scattering coefficients of superconducting microwave resonators. I. Transfer matrix approach

Q. M. Chen, M. Pfeiffer, M. Partanen, F. Fesquet, K. E. Honasoge, F. Kronowetter, Y. Nojiri, M. Renger, K. G. Fedorov, A. Marx, F. Deppe, R. Gross

Physical Review B 106 (21), 214505 (2022).

We describe a unified classical approach for analyzing the scattering coefficients of superconducting microwave resonators with a variety of geometries. To fill the gap between experiment and theory, we also consider the influences of small circuit asymmetry and the finite length of the feedlines, and describe a procedure to correct their influences in typical experiments. We show that, similar to the transmission coefficient of a hanger-type resonator, the reflection coefficient of a necklace- or cross-type resonator also contains a so-called reference point that can be used to characterize the internal quality factor of the resonator. Our results provide a comprehensive understanding of superconducting microwave resonators from the design concepts to the characterization details.

##### Scattering coefficients of superconducting microwave resonators. II. System-bath approach

Q. M. Chen, M. Partanen, F. Fesquet, K. E. Honasoge, F. Kronowetter, Y. Nojiri, M. Renger, K. G. Fedorov, A. Marx, F. Deppe, R. Gross

Physical Review B 106 (21), 214506 (2022).

We describe a unified quantum approach for analyzing the scattering coefficients of superconducting mi-crowave resonators with a variety of geometries, and demonstrate its consistency with the classical approach [Q.-M. Chen et al., Phys. Rev. B 106, 214505 (2022)]. We also generalize the result to a chain of resonators with time delays, and reveal several transport properties similar to a photonic crystal and can be used to design high-quality resonators. These results form a firm theoretical ground for analyzing the scattering coefficients of an arbitrary resonator network. They set a step forward to designing and characterizing superconducting microwave resonators in a complex superconducting quantum circuit.

##### Flow of quantum correlations in noisy two-mode squeezed microwave states

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

Physical Review A 106 (5), 52415 (2022).

We study nonclassical correlations in propagating two-mode squeezed microwave states in the presence of noise. We focus on two different types of correlations, namely, quantum entanglement and quantum discord. Quantum discord has various intriguing fundamental properties which require experimental verification, such as the asymptotic robustness to environmental noise. Here, we experimentally investigate quantum discord in propagating two-mode squeezed microwave states generated via superconducting Josephson parametric ampli-fiers. By exploiting an asymmetric noise injection into these entangled states, we demonstrate the robustness of quantum discord against thermal noise while verifying the sudden death of entanglement. Furthermore, we investigate the difference between quantum discord and entanglement of formation, which can be directly related to the flow of locally inaccessible information between the environment and the bipartite subsystem. We observe a crossover behavior between quantum discord and entanglement for low noise photon numbers, which is a result of the tripartite nature of noise injection. We demonstrate that the difference between entanglement and quantum discord can be related to the security of certain quantum key distribution protocols.

##### Low-temperature nanoscale heat transport in a gadolinium iron garnet heterostructure probed by ultrafast x-ray diffraction

D. S. Gyan, D. Mannix, D. Carbone, J. L. Sumpter, S. Geprags, M. Dietlein, R. Gross, A. Jurgilaitis, V. T. Pham, H. Coudert-Alteirac, J. Larsson, D. Haskel, J. Strempfer, P. G. Evans

Structural Dynamics-Us 9 (4), 45101 (2022).

Time-resolved x-ray diffraction has been used to measure the low-temperature thermal transport properties of a Pt/Gd3Fe5O12//Gd3Ga5O12 metal/oxide heterostructure relevant to applications in spin caloritronics. A pulsed femtosecond optical signal produces a rapid temperature rise in the Pt layer, followed by heat transport into the Gd3Fe5O12 (GdIG) thin film and the Gd3Ga5O12 (GGG) substrate. The time dependence of x-ray diffraction from the GdIG layer was tracked using an accelerator-based femtosecond x-ray source. The ultrafast diffraction measurements probed the intensity of the GdIG (1 -1 2) x-ray reflection in a grazing-incidence x-ray diffraction geometry. The comparison of the variation of the diffracted x-ray intensity with a model including heat transport and the temperature dependence of the GdIG lattice parameter allows the thermal conductance of the Pt/GdIG and GdIG//GGG interfaces to be determined. Complementary synchrotron x-ray diffraction studies of the low-temperature thermal expansion properties of the GdIG layer provide a precise calibration of the temperature dependence of the GdIG lattice parameter. The interfacial thermal conductance of the Pt/GdIG and GdIG//GGG interfaces determined from the time-resolved diffraction study is of the same order of magnitude as previous reports for metal/oxide and epitaxial dielectric interfaces. The thermal parameters of the Pt/GdIG//GGG heterostructure will aid in the design and implementation of thermal transport devices and nanostructures. (C) 2022 Author(s).

##### Influence of low-energy magnons on magnon Hanle experiments in easy-plane antiferromagnets

J. Guckelhorn, A. Kamra, T. Wimmer, M. Opel, S. Geprags, R. Gross, H. Hübl, M. Althammer

Physical Review B 105 (9), 94440 (2022).

Antiferromagnetic materials host pairs of spin-up and spin-down magnons which can be described in terms of a magnonic pseudospin. The close analogy between this magnonic pseudospin system and that of electronic charge carriers led to the prediction of fascinating phenomena in antiferromagnets. Recently, the associated dynamics of antiferromagnetic pseudospin has been experimentally demonstrated and, in particular, an observation of the magnon Hanle effect has been reported. We here expand the magnonic spin transport description by explicitly taking into account contributions of finite-spin low-energy magnons. In our experiments we realize the spin injection and detection process by two platinum strips and investigate the influence of the Pt strips on the generation and diffusive transport of magnons in films of the antiferromagnetic insulator hematite. For both a 15 and a 100 nm thick film, we find a distinct signal caused by the magnon Hanle effect. However, the magnonic spin signal exhibits clear differences in both films. In contrast to the thin film, for the thicker one, we observe an oscillating behavior in the high magnetic field range as well as an additional offset signal in the low magnetic field regime. We attribute this offset signal to the presence of finite-spin low-energy magnons.

##### Aluminum nitride integration on silicon nitride photonic circuits: a hybrid approach towards on-chip nonlinear optics

G. Terrasanta, T. Sommer, M. Muller, M. Althammer, R. Gross, M. Poot

Optics Express 30 (6), 8537-8549 (2022).

Aluminum nitride (AlN) is an emerging material for integrated quantum photonics due to its large chi((2)) nonlinearity. Here we demonstrate the hybrid integration of AlN on silicon nitride (SiN) photonic chips. Composite microrings are fabricated by reactive DC sputtering of caxis oriented AlN on top of pre-patterned SiN. This new approach does not require any patterning of AlN and depends only on reliable SiN nanofabrication. This simplifies the nanofabrication process drastically. Optical characteristics, such as the quality factor, propagation losses and group index, are obtained. Our hybrid resonators can have a one order of magnitude increase in quality factor after the AlN integration, with propagation losses down to 0.7 dB/cm. Using finite-clement simulations, phase matching in these waveguides is explored. (C) 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement

##### Mechanical frequency control in inductively coupled electromechanical systems

T. Luschmann, P. Schmidt, F. Deppe, A. Marx, A. Sanchez, R. Gross, H. Hübl

Scientific Reports 12 (1), 1608 (2022).

Nano-electromechanical systems implement the opto-mechanical interaction combining electromagnetic circuits and mechanical elements. We investigate an inductively coupled nano-electromechanical system, where a superconducting quantum interference device (SQUID) realizes the coupling. We show that the resonance frequency of the mechanically compliant string embedded into the SQUID loop can be controlled in two different ways: (1) the bias magnetic flux applied perpendicular to the SQUID loop, (2) the magnitude of the in-plane bias magnetic field contributing to the nano-electromechanical coupling. These findings are quantitatively explained by the inductive interaction contributing to the effective spring constant of the mechanical resonator. In addition, we observe a residual field dependent shift of the mechanical resonance frequency, which we attribute to the finite flux pinning of vortices trapped in the magnetic field biased nanostring.

##### Tuning and amplifying the interactions in superconducting quantum circuits with subradiant qubits

Q. M. Chen, F. Kronowetter, F. Fesquet, K. E. Honasoge, Y. Nojiri, M. Renger, K. G. Fedorov, A. Marx, F. Deppe, R. Gross

Physical Review A 105 (1), 12405 (2022).

We propose a tunable coupler consisting of N fixed-frequency qubits, which can tune and even amplify the effective interaction between two superconducting quantum circuits. The tuning range of the interaction is proportional to N, with a minimum value of zero and a maximum that can exceed the physical coupling rates between the coupler and the circuits. The effective coupling rate is determined by the collective magnetic quantum number of the qubit ensemble, which takes only discrete values and is free from collective decay and decoherence. Using single-photon pi-pulses, the coupling rate can be switched between arbitrary choices of the initial and final values within the dynamic range in a single step without going through intermediate values. A cascade of the couplers for amplifying small interactions or weak signals is also discussed. These results should not only stimulate interest in exploring the collective effects in quantum information processing, but also enable development of applications in tuning and amplifying the interactions in a general cavity-QED system.

##### Experimental quantum teleportation of propagating microwaves

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

Science Advances 7 (52), eabk0891 (2021).

The field of quantum communication promises to provide efficient and unconditionally secure ways to exchange information, particularly, in the form of quantum states. Meanwhile, recent breakthroughs in quantum computation with superconducting circuits trigger a demand for quantum communication channels between spatially separated superconducting processors operating at microwave frequencies. In pursuit of this goal, we demonstrate the unconditional quantum teleportation of propagating coherent microwave states by exploiting two-mode squeezing and analog feedforward over a macroscopic distance of d = 0.42 m. We achieve a teleportation fidelity of F = 0.689 +/- 0.004, exceeding the asymptotic no-cloning threshold. Thus, the quantum nature of the teleported states is preserved, opening the avenue toward unconditional security in microwave quantum communication.

##### Magnon transport in Y3Fe5O12/Pt nanostructures with reduced effective magnetization

J. Guckelhorn, T. Wimmer, M. Muller, S. Geprags, H. Hübl, R. Gross, M. Althammer

Physical Review B 104 (18), L180410 (2021).

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.

##### 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 (1), 160 (2021).

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.

##### Low-temperature suppression of the spin Nernst angle in Pt

T. Wimmer, J. Guckelhorn, S. Wimmer, S. Mankovsky, H. Ebert, M. Opel, S. Geprags, R. Gross, H. Hübl, M. Althammer

Physical Review B 104 (14), L140404 (2021).

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.

##### Tunable cooperativity in coupled spin-cavity systems

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

Physical Review B 104 (10), L100415 (2021).

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.

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

L. Flacke, V. Ahrens, S. Mendisch, L. Korber, T. Bottcher, E. Meidinger, M. Yaqoob, M. Muller, L. Liensberger, A. Kakay, M. Becherer, P. Pirro, M. Althammer, S. Geprags, H. Hübl, R. Gross, M. Weiler

Physical Review B 104 (10), L100417 (2021).

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.

##### RF Antenna Design for 3D Quantum Memories

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

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

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.

##### All-Electrical Magnon Transport Experiments in Magnetically Ordered Insulators

M. Althammer

Physica Status Solidi-Rapid Research Letters 15 (8), 2100130 (2021).

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.

##### 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).

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.

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

P. Rosenberger, M. Opel, S. Geprags, H. Hübl, R. Gross, M. Muller, M. Althammer

Applied Physics Letters 118 (19), 192401 (2021).

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.

##### 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 (9), 94515 (2021).

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.

##### 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).

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.

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

M. Muller, L. Liensberger, L. Flacke, H. Hübl, A. Kamra, W. Belzig, R. Gross, M. Weiler, M. Althammer

Physical Review Letters 126 (8), 87201 (2021).

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.

##### 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. Hübl

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

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.

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

S. Geprags, 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).

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.

##### 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).

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.

##### Observation of Antiferromagnetic Magnon Pseudospin Dynamics and the Hanle Effect

T. Wimmer, A. Kamra, J. Guckelhorn, M. Opel, S. Geprags, R. Gross, H. Hübl, M. Althammer

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

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.

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

J. Guckelhorn, T. Wimmer, S. Geprags, H. Hübl, R. Gross, M. Althammer

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

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.

##### 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. Hübl

Physical Review Letters 125 (13), 137701 (2020).

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. Geprags, 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).

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.

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

T. S. Suraj, M. Muller, S. Gelder, S. Geprags, M. Opel, M. Weiler, K. Sethupathi, H. Hübl, R. Gross, M. S. R. Rao, M. Althammer

Journal of Applied Physics 128 (8), 83903 (2020).

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.

##### Spin Hall magnetoresistance in antiferromagnetic insulators

S. Geprags, M. Opel, J. Fischer, O. Gomonay, P. Schwenke, M. Althammer, H. Hübl, R. Gross

Journal of Applied Physics 127 (24), 10 (2020).

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 alpha - Fe 2 O 3/Pt bilayers, we find an unexpectedly large SMR amplitude of 2.5 x 10 - 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.

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

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

Physical Review Applied 13 (1), 14019 (2020).

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.

##### Spin Transport in a Magnetic Insulator with Zero Effective Damping

T. Wimmer, M. Althammer, L. Liensberger, N. Vlietstra, S. Geprags, M. Weiler, R. Gross, H. Hübl

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

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.

##### Spin-Wave Propagation in Metallic Co25Fe75 Films Determined by Microfocused Frequency-Resolved Magneto-Optic Kerr Effect

L. Liensberger, L. Flacke, D. Rogerson, M. Althammer, R. Gross, M. Weiler

Ieee Magnetics Letters 10, 5503905 (2019).

We investigated the magnetization dynamics of a patterned Co2Fe75-based heterostructure with a novel optical measurement technique that we call microfocused frequency-resolved magneto-optic Kerr effect. We measured the magnetic field dependence of the dynamical spin-wave susceptibility and recorded a spatial map of the spin waves excited by a microwave antenna. We compare these results to those obtained on the same sample with the established microfocused Brillouin light scattering technique. With both techniques, we find a spin-wave propagation length of 5.6 mu m at 10 GHz. We also measured the dispersion of the wave vector and the spin-wave propagation length as a function of the external magnetic field. These results are in good agreement with the existing literature and with the Kalinikos-Slavin model.

##### 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. Hübl, M. Weiler

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

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.

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

L. Flacke, L. Liensberger, M. Althammer, H. Hübl, 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).

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.

##### 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. Hübl

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

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.

##### 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. Hübl, M. Althammer

Applied Physics Letters 115 (9), 92404 (2019).

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.

##### 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).

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.