arXiv subjects

Rudolf Gross

Publications and source records attributed to Rudolf Gross.

At least 55 records · Page 3Linked to original sources

arXiv2021

Mechanical frequency control in inductively coupled electromechanical systems

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: (i) the bias magnetic flux applied perpendicular to the SQUID loop, (ii) 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.

quant-ph↗

arXiv2021

Low Temperature Suppression of the Spin Nernst Angle in Pt

We demonstrate the low temperature suppression of the platinum (Pt) spin Nernst angle in bilayers consisting of the antiferromagnetic insulator hematite ($α$-Fe$_2$O$_3$) and Pt upon measuring the transverse spin Nernst magnetothermopower (TSNM). 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 Néel vector of $α$-Fe$_2$O$_3$, rather than its net magnetization. Since the latter is negligible in an antiferromagnet, our device is superior to ferromagnetic structures, allowing to unambiguously distinguish the TSNM from thermally excited magnon transport (TMT), which usually dominates in ferri/ferromagnets due to their non-zero magnetization. Evaluating the temperature dependence of the effect, we observe a vanishing TSNM below ~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.

cond-mat.mtrl-sci↗

arXiv2021

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

We have fabricated and studied a system of two tunable and coupled nonlinear superconducting resonators. The nonlinearity is introduced by galvanically coupled dc-SQUIDs. 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 the basic building block for larger systems, allowing for quantum simulations of bosonic many-body systems with a larger number of lattice sites.

cond-mat.supr-con↗

arXiv2021

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

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.

cond-mat.mes-hall↗

arXiv2021

Tunable Cooperativity in Coupled Spin--Cavity Systems

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 model system, we use the skyrmion host material Cu$_2$OSeO$_3$ coupled to a microwave cavity resonator. In the different magnetic phases we measure a dispersive coupling between the resonator and the magnon modes 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.

cond-mat.mes-hall↗

arXiv2021

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

We experimentally demonstrate the formation of room-temperature skyrmions with radii of about 25\,nm in easy-plane anisotropy multilayers with 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 $α_{\mathrm{G}} < 0.04$. We also find clear magnetic resonance signatures in the non-uniform (skyrmion) state. Our findings demonstrate that skyrmions in easy-plane multilayers are promising for spin-dynamical applications.

cond-mat.mtrl-sci↗

arXiv2021

Growth optimization of TaN for superconducting spintronics

We have optimized the growth of superconducting TaN thin films on \ch{SiO2} substrates via dc magnetron sputtering and extract a maximum superconducting transition temperature of $T_{\mathrm{c}}=5$ K as well as a maximum critical field $μ_0H_{\mathrm{c2}}=(13.8\pm0.1)$ T. To investigate the impact of spin-orbit interaction in superconductor/ferromagnet heterostructures, we then analyze the magnetization dynamics of both normal state and superconducting TaN/\ch{Ni80Fe20}(Permalloy, Py)-bilayers as a function of temperature using broadband ferromagnetic resonance (bbFMR) spectroscopy. The phase sensitive detection of the microwave transmission signal is used to quantitatively extract the inverse current-induced torques of the bilayers. The results are compared to our previous study on NbN/Py-bilayers. In the normal state of TaN, we detect a positive damping-like current-induced torque $σ_{\mathrm{d}}$ from the inverse spin Hall effect (iSHE) and a small field-like torque $σ_{\mathrm{f}}$ attributed to the inverse Rashba-Edelstein effect (iREE) at the TaN/Py-interface. In the superconducting state of TaN, we detect a negative $σ_{\mathrm{d}}$ attributed to the quasiparticle mediated inverse spin Hall effect (QMiSHE) and the unexpected manifestation of a large positive field-like $σ_{\mathrm{f}}$ of unknown origin matching our previous results for NbN/Py-bilayers.

cond-mat.mes-hall↗

arXiv2021

Superconducting Wireless Power Transfer Beyond 5 kW at High Power Density for Industrial Applications and Fast Battery Charging

State of the Art Wireless Power Transfer (WPT) systems, based on conventional copper coils, are known to exhibit efficiencies well above 90% when operated in the resonantly coupled mid-range regime. Besides full system efficiency, the area- and weight-related power densities of the transmission coils are key figures of merit for high power applications. This paper reports on a fully functional WPT system, consisting of single pancake high-temperature superconducting (HTS) coils on the transmitter and the receiver side, which exceeds the power density of most conventional systems. Despite a compact coil size, a DC-to-DC efficiency above 97% is achieved at 6 kW output power. Next to the fundamental coil design, analytical and numerical simulations of the AC loss in the HTS coils are shown, taking into account both hysteresis and eddy current contributions. The results are validated by experimental AC loss measurements of single coils, obtained by a standard lock-in technique up to frequencies of 4 kHz. Finally, experimental results of the full system performance at different frequencies and load conditions are presented.

physics.app-ph↗

arXiv2020

Observation of Antiferromagnetic Magnon Pseudospin Dynamics and the Hanle effect

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 analogue of the electronic Hanle effect and the Datta-Das transistor, unlocking the high potential of antiferromagnetic magnonics towards the realization of rich electronics-inspired phenomena.

cond-mat.mtrl-sci↗

arXiv2020

$\mathrm{La}_{\mathrm{1-}x} \mathrm{Mn}_{\mathrm{1-}y} \mathrm{O}_{\mathrm{1\pm}δ}$ buffer layers on inclined substrate deposited MgO templates for coated conductors

Most commercial high-temperature superconducting coated conductors based on ion beam assisted MgO deposited templates use $\mathrm{LaMnO_3}$ (LMO) films as the terminating buffer layer. In contrast, coated conductors based on inclined substrate deposition (ISD)-MgO technology are still produced with homoepitaxial (homoepi)-MgO as the cap layer. In this work we report on the deposition of LMO buffer layers on ISD-MgO/homoepi-MgO by electron beam physical vapor deposition. The growth parameters of textured LMO films were studied systematically and their properties were optimized regarding the critical current densitiy ($J_\mathrm{c}$) of the subsequently deposited $\mathrm{DyBa_2Cu_3O}_{\mathrm{7-}δ}$ (DyBCO) superconducting films. LMO films without outgrowths at the surface were obtained at growth rates of up to $4\,\mathrm{Å/s}$. Despite the formation of non-stoichiometric LMO films containing $59\,\%$ La, single-phase films were obtained at substrate temperatures below $775^{\circ}\mathrm{C}$ and at oxygen partial pressures of up to $4\times10^{-4}\,\mathrm{mbar}$ due to a large homogeneity region towards La. The $J_\mathrm{c}$ values of DyBCO films deposited on LMO were found to be independent of the LMO thickness in a range from $50\,\mathrm{nm}$ to $450\,\mathrm{nm}$. DyBCO films on LMO reach $J_\mathrm{c}=0.83\,\mathrm{MA/cm^2}$ at $77\,\mathrm{K}$ in zero applied field. This value is up to $30\,\%$ higher than those of DyBCO films grown directly on homoepi-MgO. The wide range of LMO growth parameters and higher $J_\mathrm{c}$ values of DyBCO on LMO compared to DyBCO on homoepi-MgO make this material attractive for its use in manufacturing coated conductors based on ISD-MgO technology.

cond-mat.supr-con↗

arXiv2020

Dia- and adiabatic dynamics in a phononic network

Mechanical resonator networks are currently discussed in the context of model systems giving insight into problems of condensed matter physics including effects in topological phases. Here, we discuss networks based on three high-quality factor nanomechanical string resonators (nanostrings) made from highly tensile-stressed Si$_{3}$N$_{4}$. The strings are strongly coupled via a shared support and thus can form a fully mechanical, classical multi-level system. Moreover, the individual strings are tunable in frequency, which allows one to explore their coupling behaviour using continuous wave spectroscopy and time domain techniques. In particular, such systems allow for the experimental exploration of quantum phenomena such as Landau-Zener transition dynamics by studying their classical analogues. Here, we extend the previous work performed on two coupled strings to three coupled resonators and discuss the additional features of the inter-string dynamics, such as the classical analog of Landau-Zener transition dynamics in a three-mode system. classical analog of Landau-Zener transition dynamics in a three-mode system.

cond-mat.mes-hall↗

arXiv2020

Static magnetic proximity effects and spin Hall magnetoresistance in Pt/Y$_{3}$Fe$_{5}$O$_{12}$ and inverted Y$_{3}$Fe$_{5}$O$_{12}$/Pt bilayers

The magnetic state of heavy metal Pt thin films in proximity to the ferrimagnetic insulator Y$_{3}$Fe$_{5}$O$_{12}$ 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 order of the layer stacking and different interface properties. For standard Pt layers on Y$_{3}$Fe$_{5}$O$_{12}$ 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, Y$_{3}$Fe$_{5}$O$_{12}$ 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 Y$_{3}$Fe$_{5}$O$_{12}$/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 type of samples. Both effects can be disentangled from each other due to their different angle dependence and their characteristic temperature evolution.

cond-mat.mtrl-sci↗

arXiv2020

Precise control of $J_\mathrm{eff}=1/2$ magnetic properties in Sr$_2$IrO$_4$ epitaxial thin films by variation of strain and thin film thickness

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 $J_\mathrm{eff}=1/2$ compound Sr$_2$IrO$_4$ by advanced X-ray scattering. We find that the Sr$_2$IrO$_4$ 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 (2D) to three-dimensional (3D) 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 Sr$_2$IrO$_4$ and bring to light the potential for a rich playground to explore the physics of $5d$-transition metal compounds.

cond-mat.str-el↗

arXiv2020

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

All-electrical generation and detection of pure spin currents is a promising way towards controlling the diffusive magnon transport in magnetically ordered insulators. We quantitatively compare two measurement schemes, which allow 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 to investigate the modulation of magnon transport induced by an additional dc charge current applied to the center modulator strip. However, while at 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.

cond-mat.mes-hall↗

arXiv2020

Temperature-dependent spin-transport and current-induced torques in superconductor/ferromagnet heterostructures

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

cond-mat.mes-hall↗

arXiv2020

Echo trains in pulsed electron spin resonance of a strongly coupled spin ensemble

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 echos after a conventional Hahn echo pulse sequence. The presence of these multi-echo 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.

cond-mat.mes-hall↗

arXiv2020

Spin Hall magnetoresistance in antiferromagnetic insulators

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 $α$-Fe2O3 (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 Y3Fe5O12/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 $α$-Fe2O3/Pt bilayers, we find an unexpectedly large SMR amplitude of $2.5 \times 10^{-3}$, twice as high as for prototype Y3Fe5O12/Pt bilayers, making the system particularly interesting for room-temperature antiferromagnetic spintronic applications.

cond-mat.mtrl-sci↗

arXiv2020

Effect of interfacial oxidation layer in spin pumping experiments on Ni$_{80}$Fe$_{20}$/SrIrO$_3$ heterostructures

SrIrO$_3$ 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. We here report on the influence of an interfacial oxide layer on spin pumping experiments in Ni$_{80}$Fe$_{20}$ (NiFe)/SrIrO$_3$ 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/SrIrO$_3$ bilayer thin films were analyzed and compared to those from a NiFe/NbN/SrIrO$_3$ trilayer reference sample, where a spin-transparent, ultra-thin NbN layer was inserted to prevent oxidation of NiFe. At low temperatures, we observe substantial differences in the magnetization dynamics parameters of these samples, which can be explained by an antiferromagnetic interfacial layer in the NiFe/SrIrO$_3$ bilayers.

cond-mat.mes-hall↗