arXiv subjects

Matthias Althammer

Publications and source records attributed to Matthias Althammer.

At least 19 recordsLinked to original sources

arXiv2026

Nonreciprocal Magnon Hanle Effect in Antiferromagnetic $\alpha$-Fe$_2$O$_3$

The precession of the 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 non-reciprocity in the Hanle signal measured in alpha-Fe2O3 using two spatially separated Pt electrodes as spin injector and detector. Interchanging their roles alters 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. Our findings unlock the high potential of antiferromagnetic magnonics towards the realization of electronics-inspired phenomena.

cond-mat.mes-hall↗

arXiv2026

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. 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-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 effect allows to readout the spin configuration and to investigate magnetoelastic effects in antiferromagnetic multi-domain materials. We demonstrate that the SMR amplitude scales with the sum of the absolute sublattice magnetizations in ferrimagnetic and antiferromagnetic materials. In alpha-Fe2O3/Pt bilayers, we find an unexpectedly large SMR amplitude of 2.5E-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↗

arXiv2026

Niobium Titanium Nitride as a High Tensile Stress Material for Nanomechanics

Over the past decades, high-coherence mechanical resonators have been continuously pushed to new limits, using techniques such as dissipation dilution and clamp-tapering to enhance their quality factor beyond intrinsic material limitations. Today, these mechanical resonators are often fabricated from silicon-nitride, silicon-carbide or aluminum. Recently, however, interest in novel material platforms has grown, especially those allowing for the integration within superconducting circuits. Among these, superconducting nitrides stand out as particularly promising due to their high transition temperatures compared to elementary superconductors. Here, we introduce them as nanomechanical resonators and report on the fabrication and characterization of highly stressed, doubly clamped niobium titanium nitride (NbTiN) nanostring resonators. Using optical interferometry, we determine the elastic properties and mechanical quality factor from room temperature to 13 K. We observe high tensile stress up to 0.81 GPa along with a Young's modulus of 181 GPa and an intrinsic mechanical quality factor up to 850. With these favorable mechanical properties, NbTiN constitutes a promising material platform for future applications in cavity electro- and nanomechanics.

quant-ph↗

arXiv2026

Second-Harmonic Imaging of Magnetic Domains in Thin Film Hematite

Hematite is an antiferromagnetic oxide and candidate altermagnetic insulator whose N\'eel order reorients from an easy-axis to an easy-plane phase at the Morin transition. Interpreting altermagnetic transport and symmetry sensitive optical responses requires knowledge of the N\'eel vector orientation relative to the crystal axes and of the domain structure within the probed device. Here, we show that polarization-resolved second-harmonic generation (SHG) microscopy resolves magnetic symmetry and domains in epitaxial (0001)-oriented hematite films. Across the Morin temperature, the SHG polarization anisotropy evolves from an approximately sixfold pattern consistent with the easy-axis orientation to pronounced twofold patterns consistent with the easy-plane orientation. A symmetry analysis based on magnetic-dipole and electric-quadrupole contributions reproduces this evolution. Importantly, the interference between SHG amplitudes that are odd and even under reversal of the magnetic order renders opposite N\'eel-vector orientations optically distinguishable. Consistently, opposite directions of an applied in-plane magnetic field produce distinct SHG responses in our experiments. Using this magnetic contrast, we image micrometer-scale domains, their reorganization across the Morin transition, and their reconfiguration under magnetic and thermal cycling, including a remanent change after cycling through the spin-flop transition. These results establish SHG microscopy as a local probe of magnetic symmetry, N\'eel-vector orientation, and domain evolution in hematite films.

cond-mat.mtrl-sci↗

arXiv2026

GHz non-reciprocal optical conductivity in hematite ($\alpha$-$\text{Fe}_2\text{O}_3$)

We study the non-reciprocal properties of the iron oxide $\alpha$-Fe$_2$O$_3$ (hematite) in the canted easy-plane antiferromagnetic phase, specifically in the GHz to THz frequency range. First, using the the microscopic spin Hamiltonian, we obtain the correct classical ground state where the canting is induced by the Dzyaloshinskii-Moriya interactions (DMI). The magnon spectrum is simulated using linear spin wave theory. We then compute the polarizability and the sub-gap optical conductivities using linear response. We find that the conductivity tensor contains frequency peaks at the zero momentum magnon gaps of order $0.1~$meV which can be tuned by the DMI and on-site anisotropic spin interactions. Furthermore, we show that the canting-induced net magnetic moment $\mathbf{m}$ represents a measure for the effective time-reversal-symmetry breaking and non-reciprocity of the system: a finite $\mathbf{m}$ results in a non-zero Hall conductivity. Finally, we discuss the prospective application of hematite in non-reciprocal circulator design, by computing the non-reciprocal circulator transmission amplitude using the conductivities as input.

cond-mat.str-el↗

arXiv2026

Temperature dependence of the magnon-phonon coupling in yttrium iron garnet/gadolinium gallium garnet high overtone bulk acoustic resonators

Weexperimentally study the temperature dependence of the magnon-phonon coupling in a yttrium iron garnet (YIG)/gadolinium gallium garnet (GGG) heterostructure. More specifically, we use broadband ferromagnetic resonance to investigate the magneto-elastic coupling between the Kittel mode of a YIG thin film and the transverse acoustic phonon modes of the YIG/GGG high overtone bulk acoustic wave acoustic resonator for in and out-of-plane field directions in the temperature range between T = 5K and 300K. We find that for a magnetic field applied normal to the film surface, magneto-elastic coupling decreases with decreasing temperature, whereas it increases for the in-plane magnetic field configuration. The observed temperature dependence differs from earlier observations on bulk YIG samples, which might be due to the temperature dependent stress imposed by the GGG substrate.

cond-mat.mtrl-sci↗

arXiv2026

Impact of Stoichiometry of MoSi Thin Films for Enhanced Sensitivity of Superconducting Nanowire Single-Photon Detectors

We report on the impact of the stoichiometry of superconducting MoSi thin films on the performance of superconducting nanowire single-photon detectors (SNSPDs). Specifically, we investigate the relation between the film parameters critical temperature Tc , sheet resistance Rs and superconductor thickness d and observe a universal scaling behavior. To benchmark the performance of SNSPDs fabricated from films having different stoichiometry, we measure the bias dependent count rate curves, while the detector is illuminated with wavelengths between 780 nm and 1550 nm. The detector performance as a function photon energy for different nanowire widths reveals a linear relation between the detection current and the photon energy. Furthermore, we determine the interfacial thermal boundary conductance $\beta$ between the superconducting thin film and the substrate, by measuring the return current of the SNSPD and find an increase of $\beta$ with increasing Mo concentration. The highest sensitivity amongst all compared devices is achieved for Mo$_{0.53}$Si$_{0.47}$, with low Tc (4.1 K) and high Rs (397$\Omega$/sq) at a film thickness of 5.4 nm.

cond-mat.supr-con↗

arXiv2026

On-chip Dicke-type magnon polaritons in the ultrastrong coupling regime via spatially separated nanomagnets

Quantum electrodynamics lies at the heart of hybrid quantum systems essential for future technologies. The thermodynamic limit of the Dicke model, a fundamental model describing these systems, predicts exotic quantum phenomena, such as equilibrium superradiant phase transitions and ground-state two-mode squeezing. However, the experimental realization of genuine Dicke systems has remained elusive due to the inevitable existence of gauge-invariant self-interaction terms that hinder such phenomena. Here, we report on the on-chip realization of a Dicke-type system utilizing ultrastrong magnetic-dipole interactions between collective excitations in a spatially separated ferromagnetic array and a superconducting resonator, resulting in creation of magnon polaritons. Crucially, this spatially separated architecture allows the cooperative enhancement of the coupling strength without increasing the self-interaction energy. We experimentally confirm the Bloch-Siegert shift, originating from the counter-rotating terms, alongside the suppression of self-excitation terms required to observe critical Dicke physics. Our results establish a versatile platform, which provides the playground to explore quantum collective coupling physics and open pathways towards integrated quantum devices harnessing Dicke physics.

quant-ph↗

arXiv2025

Spin Hall magnetoresistance at the altermagnetic insulator/Pt interface

The resistance of a heavy metal can be modulated by an adjacent magnetic material through the combined effects of the spin Hall effect, inverse spin Hall effect, and dissipation of the spin accumulation at the interface. This phenomenon is known as the spin Hall magnetoresistance. The dissipation of the spin accumulation can occur via various mechanisms, with spin-transfer torque being the most extensively studied. In this work, we report the observation of spin Hall magnetoresistance at the interface between platinum and an insulating altermagnetic candidate, Ba$_2$CoGe$_2$O$_7$. Our findings reveal that this heterostructure exhibits a relatively large spin Hall magnetoresistance signal, which is anisotropic with respect to the crystal orientation of the current channel. We explore and rule out several potential explanations for this anisotropy and propose that our results may be understood in the context of the anisotropies of the spin current channels across the Pt/altermagnetic Ba$_2$CoGe$_2$O$_7$ interface.

cond-mat.mes-hall↗

arXiv2025

Influence of Platinum Thin Films on the Photophysical and Quantum Properties of Near-Surface NV Centers

Nitrogen-vacancy (NV) centers in diamond are optically addressable spin defects with great potential for nanoscale quantum sensing. A key application of NV centers is the detection of external spins at the diamond surface. Among metals, platinum thin films - widely used in spintronics, catalysis and electrochemistry - provide a particularly interesting system for such studies. However, the interaction between NV centers and metals is known to affect their quantum sensing capabilities. In this work, we study five platinum-covered diamond samples containing shallow NVs created via nitrogen implantation with different energies (2.5-60 keV) and investigate the optical and quantum properties of NV ensembles beneath the metal films. We find a substantial reduction of the photoluminescence lifetime and a pronounced decrease of the NV$^{-}$ population for NV ensembles located near the platinum layer. As a result, optically detected magnetic resonance experiments could only be efficiently performed on diamonds implanted with at least 20 keV, where we observed a strong increase in the T$_{2}$ coherence time beneath the platinum thin films. Our study describes the various processes affecting NV centers near platinum films and provides guidance for the integration of thin metal films with near-surface NV centers.

cond-mat.mes-hall↗

arXiv2025

Theory of polarization-dependent phonon pumping in ferromagnetic/non-magnetic bilayers

We develop a theoretical model for polarization-selective phonon pumping induced by magnon-phonon coupling in a ferromagnetic/non-magnetic acoustic bilayer structure, focusing on the effects arising from a misalignment between the magnetic and crystallographic symmetry axes. Our model considers the coupled equations of motion describing uniform magnetization dynamics (the Kittel mode) and elastic waves in both layers, incorporating phonon pumping and boundary conditions at the interface. We show that even small misalignments lift the degeneracy of transverse shear elastic modes, resulting in phononic birefringence characterized by distinct propagation velocities for linearly polarized modes. Furthermore, our analysis reveals that magnon-phonon hybridization gives magnetic-field-dependent properties to otherwise non-magnetic phonons. We show that the polarization transfer between linearly polarized phonons and the circularly polarized Kittel mode can be tuned with an external magnetic field. Our theoretical results quantitatively reproduce recent experimental findings [1].

cond-mat.mes-hall↗

arXiv2024

Planar scanning probe microscopy enables vector magnetic field imaging at the nanoscale

Planar scanning probe microscopy is a recently emerging alternative approach to tip-based scanning probe imaging. It can scan an extended planar sensor, such as a polished bulk diamond doped with magnetic-field-sensitive nitrogen-vacancy (NV) centers, in nanometer-scale proximity of a planar sample. So far, this technique has been limited to optical near-field microscopy, and has required nanofabrication of the sample of interest. Here we extend this technique to magnetometry using NV centers, and present a modification that removes the need for sample-side nanofabrication. We harness this new ability to perform a hitherto infeasible measurement - direct imaging of the three-dimensional vector magnetic field of magnetic vortices in a thin film magnetic heterostructure, based on repeated scanning with NV centers with different orientations within the same scanning probe. Our result opens the door to quantum sensing using multiple qubits within the same scanning probe, a prerequisite for the use of entanglement-enhanced and massively parallel schemes.

cond-mat.mes-hall↗

arXiv2023

Temperature dependence of the magnon-phonon interaction in high overtone bulk acoustic resonator-ferromagnetic thin film hybrids

Tailored magnon-phonon hybrid systems, where high overtone bulk acoustic resonators couple resonantly to the magnonic 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 \ch{Co25Fe75} thin film and the transverse acoustic phonons in a 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 $C\approx 1$ at cryogenic temperatures. Our experiments also showcase the potential of strongly coupled magnon-phonon systems for strain sensing applications.

cond-mat.mes-hall↗

arXiv2023

Electrically induced angular momentum flow between separated ferromagnets

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 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 work 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.

cond-mat.mes-hall↗

arXiv2023

Impact of magnetic anisotropy on the magnon Hanle effect in $α$-Fe$_2$O$_3$

In easy-plane antiferromagnets, the nature of the elementary excitations of the spin system is captured by the precession of the magnon pseudospin around its equilibrium pseudofield, manifesting itself in the magnon Hanle effect. Here, we investigate the impact of growth-induced changes in the magnetic anisotropy on this effect in the antiferromagnetic insulator $α$-Fe$_2$O$_3$ (hematite). To this end, we compare the structural, magnetic, and magnon-based spin transport properties of $α$-Fe$_2$O$_3$ films with different thicknesses grown by pulsed laser deposition in molecular and atomic oxygen atmospheres. While in films grown with molecular oxygen a spin-reorientation transition (Morin transition) is absent down to $10\,$K, we observe a Morin transition for those grown by atomic-oxygen-assisted deposition, indicating a change in magnetic anisotropy. Interestingly, even for a $19\,$nm thin $α$-Fe$_2$O$_3$ film grown with atomic oxygen we still detect a Morin transition at $125\,$K. We characterize the magnon Hanle effect in these $α$-Fe$_2$O$_3$ 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. These observations suggest a change of magnetic anisotropy for $α$-Fe$_2$O$_3$ films fabricated by atomic-oxygen-assisted deposition resulting in an increased oxygen content in these films. Our findings provide new insights into the possibility to fine-tune the magnetic anisotropy in $α$-Fe$_2$O$_3$ and thereby to engineer the magnon Hanle effect.

cond-mat.mes-hall↗

arXiv2023

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

Achieving homogeneous performance metrics between nominally identical pixels is challenging for the operation of arrays of superconducting nanowire single-photon detectors (SNSPDs). Here, we utilize local helium ion irradiation to post-process and tune single-photon detection efficiency, switching current, and critical temperature of individual devices on the same chip. For 12nm thick highly absorptive SNSPDs, which are barely single-photon sensitive prior to irradiation, we observe an increase of the system detection efficiency from $< 0.05\,\%$ to $(55.3 \pm 1.1)\,\%$ following irradiation. Moreover, the internal detection efficiency saturates at a temperature of 4.5 K after irradiation with $1800\, \mathrm{ions}\, \mathrm{nm}^{-2}$. For irradiated 10 nm thick detectors we observe a doubling of the switching current (to $20\, μ\mathrm{A}$) compared to 8 nm SNSPDs of similar detection efficiency, increasing the amplitude of detection voltage pulses. Investigations of the scaling of superconducting thin film properties with irradiation up to a fluence of $2600\, \mathrm{ions}\, \mathrm{nm}^{-2}$ 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.

quant-ph↗

arXiv2023

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

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 magnetoelastic coupling between the ferromagnetic resonance (FMR) modes in a metallic Co$_{25}$Fe$_{75}$ thin film, featuring ultra-low magnetic damping as well as sizable magnetostriction, and standing transverse elastic phonon modes in sapphire, silicon and gadolinium gallium garnet by performing broadband FMR spectroscopy at cryogenic temperatures. For all these substrate materials, we observe an interaction between the resonant acoustic and magnetic modes, which can be tailored by the propagation direction of the acoustic mode with respect to the crystallographic axes. We identify these phonon modes as transverse shear waves propagating with slightly different velocities with relative magnitudes of $Δv/v\simeq10^{-5}$, i.e., all substrates show phononic birefringence. Upon appropriately choosing the phononic mode, the hybrid magnomechanical system enters the Purcell enhanced coupling regime.

cond-mat.mes-hall↗

arXiv2022

Observation of nonreciprocal magnon Hanle effect

The precession of magnon pseudospin about the equilibrium pseudofield, the latter capturing the nature of magnonic eigen-excitations 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/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.

cond-mat.mes-hall↗