arXiv subjects

Romain Lebrun

Publications and source records attributed to Romain Lebrun.

At least 19 recordsLinked to original sources

arXiv2026

Apparent Zero-Momentum Signals from Magnon--Magnon Interference in Near-Field Spin-Wave Imaging

Spatially resolved measurements of coherent waves are commonly read as direct maps of the underlying eigenmode spectrum. We show that this interpretation fails in near-field imaging of multimode spin-wave transport. Scanning nitrogen-vacancy magnetometry of yttrium iron garnet reveals pronounced apparent low-wave-vector signals at frequencies for which no propagating spin-wave mode exists. By tuning the probe-sample distance and modeling the anisotropic near-field response, we identify their origin as coherent mixing between directly excited Damon-Eshbach waves and defect- or transducer-scattered spin waves: the measured signal acquires spatial Fourier components at the difference wave vector $\mathbf{k}_{\mathrm{exc}}-\mathbf{k}_{\mathrm{scat}}$, which can vanish even though both constituent waves carry finite momentum. These results establish that near-field spectra of coherent excitations are interferometric field spectra rather than eigenmode maps, providing both a caution and a framework for identifying scattering processes in nanoscale magnonic devices.

cond-mat.mes-hall↗

arXiv2026

Reprogrammable magnonic logic in a multiferroic heterostructure via magnetoelectric coupling

The realization of fully reconfigurable, voltage-controlled, and programmable on-chip magnonic devices is essential to fully harness the potential of spin waves for signal processing, logic and neuromorphic computing. Yet, existing demonstrations of electrical tuning of magnonic responses are either volatile, current-driven and thus energy-inefficient, or rely on local strain modification limiting their scalability for wafer-scale integration. Here, we address this challenge using a BiFeO3/La0.67Sr0.33MnO3 multiferroic thin film heterostructure. We show that ferroelectric domain engineering in BiFeO3 enables deterministic tuning of the magnon dispersion of La0.67Sr0.33MnO3, producing frequency shifts up to $\sim 150 MHz$ and allowing reconfigurable waveguiding. Micro-focused Brillouin light scattering directly images these effects, revealing electrically defined magnonic waveguides and spatially programmable dispersion. Compared to conventional approaches, this method provides non-volatile and reversible control. Furthermore, using an inverse-design simulation code, we demonstrate the capability of our platform to perform advanced magnonic functions such as frequency demultiplexing. Our results open a new avenue for using magnetoelectric heterostructures for magnonic logic, with further applicability to reservoir and neuromorphic computing and AI driven magnonic devices.

cond-mat.mes-hall↗

arXiv2025

Phonon-tunable THz magnonic emission in multiferroic heterostructures

Collective excitations such as magnons and polar phonons provide natural access to the terahertz (THz) regime, but efficient generation and tunability remain elusive. Multiferroic BiFeO3 combines both orders at room temperature, offering a unique platform for narrowband THz emission. Here, we achieve efficient sub-bandgap optical rectification of coupled phonon-polaritons near 2 THz in bare epitaxial thin films. In Pt/BiFeO3 bilayers, we demonstrate that coupling the electromagnon branch with ultrafast strain waves, optically generated in Pt layers with various thicknesses, can produce tunable and narrowband emission between 0.4-0.8 THz. These results uncover the intertwined role of phonons, magnons, and magneto-acoustic dynamics in antiferromagnetic multiferroics, and establish these hybrid platforms as versatile engineered narrowband THz sources.

cond-mat.mes-hall↗

arXiv2025

Controllable Non-reciprocity in Multi-sphere Loaded Chiral Resonator

Cavity magnonics explores the hybridization of photons and magnons within microwave resonators. One of the hallmarks of these systems is their ability to exhibit non-reciprocity, which is a key feature for radio frequency (RF) applications. One way to control non-reciprocal behaviors in cavity magnonics is the design of chiral cavities that allow selective coupling between photons and magnons depending on their polarization. However, a built-in chiral platform to harness and control non-reciprocity remains to be achieved. Here, we experimentally demonstrate controllable non-reciprocity (with an absolute isolation ratio reaching 46 dB) in a chiral resonator loaded with multiple yttrium iron garnet spheres. We develop a theoretical model of the S-parameters based on input-output formalism which highlights the links between the phases occurring in the system and its non-reciprocal behavior. Controllable non-reciprocity in cavity magnonics could enable the development of programmable isolators, circulators, and RF switches with improved performance. Such developments could pave the way toward more versatile and scalable information processing systems.

cond-mat.mtrl-sci↗

arXiv2025

Terahertz time-domain signatures of the inverse Edelstein effect in topological-insulator|ferromagnet heterostructures

Three-dimensional topological insulators possess topologically protected surface states with spin-momentum locking, which enable spin-charge-current interconversion (SCI) by the inverse Edelstein effect (IEE). However, it remains experimentally challenging to separate the surface-related IEE from the bulk-type inverse spin Hall effect (ISHE). Here, we search for distinct time-domain signatures of the two SCI phenomena in a $\mathcal{F}$|TI model stack of a ferromagnetic-metal layer $\mathcal{F}$ (Co and Fe) and a topological-insulator layer TI (Bi$_2$Te$_3$, SnBi$_2$Te$_4$ and Bi$_{1-x}$Sb$_x$ with $x$ = 0.15 and 0.3), where the focus is on Bi$_2$Te$_3$. A femtosecond laser pulse serves to induce a transient spin voltage $μ_s^{\mathcal{F}}$ in $\mathcal{F}$ and, thus, drive an ultrafast spin current out of $\mathcal{F}$. SCI results in a transverse charge current with a sheet density $I_c$ that is detected by sampling the emitted terahertz electric field. Analysis of the dynamics of $I_c(t)$ vs time $t$ relative to $μ_s^{\mathcal{F}}(t)$ reveals two components with distinct time scales: (i) a quasi-instantaneous response and (ii) a longer-lived response with a relaxation time of 270 fs, which is independent of the chosen $\mathcal{F}$ material. Component (i) is consistently ascribed to the ISHE. In contrast, we interpret component (ii) as a signature of interfacial spin accumulation and the IEE at the $\mathcal{F}$/Bi$_2$Te$_3$ interface, with a fraction of $< 10^{-2}$ of the incident spins participating. This assignment is fully consistent with respect to its dynamics and magnitude. We rate other possible signal contributions, such as spin trapping in intermediate states, as less likely. Our results show that the femtosecond dynamics of photocurrents provide important insights into the mechanisms of spin transport and SCI in $\mathcal{F}$|TI stacks.

cond-mat.mes-hall↗

arXiv2024

Level attraction from interference in two-tone driving

Coherent and dissipative couplings, respectively characterised by energy level repulsion and attraction, each have different applications for quantum information processing. Thus, a system in which both coherent and dissipative couplings are tunable on-demand and in-situ is tantalising. A first step towards this goal is the two-tone driving of two bosonic modes, whose experimental signature was shown to exhibit controllable level repulsion and attraction by changing the phase and amplitude of one drive. However, whether the underlying physics is that of coherent and dissipative couplings has not been clarified, and cannot be concluded solely from the measured resonances (or anti-resonances) of the system. Here, we show how the physics at play can be analysed theoretically. Combining this theory with realistic finite-element simulations, we deduce that the observation of level attraction originates from interferences due to the measurement setup, and not dissipative coupling. Beyond the clarification of a novel origin for level attraction attributed to interference, our work demonstrate how effective Hamiltonians can be derived to appropriately describe the physics.

quant-ph↗

arXiv2024

True amplification of spin waves in magnonic nano-waveguides

Magnonic nano-devices exploit magnons -- quanta of spin waves -- to transmit and process information within a single integrated platform that has the potential to outperform traditional semiconductor-based electronics for low power applications. The main missing cornerstone of this information nanotechnology is an efficient scheme for the direct amplification of propagating spin waves. The recent discovery of spin-orbit torque provided an elegant mechanism for propagation losses compensation. While partial compensation of the spin-wave damping has allowed for spin-wave signal modulation, true amplification - the exponential increase in the spin-wave intensity during propagation - has so far remained elusive. Here we evidence the operating conditions to achieve unambiguous amplification using clocked nanoseconds-long spin-orbit torque pulses in sub-micrometer wide magnonic waveguides, where the effective magnetization has been engineered to be close to zero to suppress the detrimental magnon-magnon scattering. As a result, we achieve an exponential increase in the intensity of propagating spin waves up to 500 % at a propagation distance of several micrometers. These results pave the way towards the implementation of energy efficient, cascadable magnonic architectures for wave-based information processing and complex on-chip computation.

cond-mat.mes-hall↗

arXiv2023

Non-hermiticity in spintronics: oscillation death in coupled spintronic nano-oscillators through emerging exceptional points

The emergence of exceptional points (EPs) in the parameter space of a non-hermitian (2D) eigenvalue problem is studied in a general sense in mathematical physics, and has in the last decade successively reached the scope of experiments. In coupled systems, it gives rise to unique physical phenomena, which enable novel approaches for the development of seminal types of highly sensitive sensors. Here, we demonstrate at room temperature the emergence of EPs in coupled spintronic nanoscale oscillators and hence exploit the system's non-hermiticity. We describe the observation of amplitude death of self-oscillations and other complex dynamics, and develop a linearized non-hermitian model of the coupled spintronic system, which properly describes the main experimental features. Interestingly, these spintronic nanoscale oscillators are deployment-ready in different applicational technologies, such as field, current or rotation sensors, radiofrequeny and wireless devices and, more recently, novel neuromorphic hardware solutions. Their unique and versatile properties, notably their large nonlinear behavior, open up unprecedented perspectives in experiments as well as in theory on the physics of exceptional points. Furthermore, the exploitation of EPs in spintronics devises a new paradigm for ultrasensitive nanoscale sensors and the implementation of complex dynamics in the framework of non-conventional computing.

cond-mat.mes-hall↗

arXiv2023

Temperature-independent ferromagnetic resonance shift in Bi-doped YIG garnets through magnetic anisotropy tuning

Thin garnet films are becoming central for magnon-spintronics and spin-orbitronics devices as they show versatile magnetic properties together with low magnetic losses. These fields would benefit from materials in which heat does not affect the magnetization dynamics, an effect known as the non-linear thermal frequency shift. In this study, low damping Bi substituted Iron garnet (Bi:YIG) ultra-thin films have been grown using Pulsed Laser Deposition. Through a fine tuning of the growth parameters, the precise control of the perpendicular magnetic anisotropy allows to achieve a full compensation of the dipolar magnetic anisotropy. Strikingly, once the growth conditions are optimized, varying the growth temperature from 405 °C to 475 °C as the only tuning parameter induces the easy-axis to go from out-of-plane to in-plane. For films that are close to the dipolar compensation, Ferromagnetic Resonance measurements yield an effective magnetization $μ_{0}M_{eff} (T)$ that has almost no temperature dependence over a large temperature range (260 K to 400 K) resulting in an anisotropy temperature exponent of 2. These findings put Bi:YIG system among the very few materials in which the temperature dependence of the magnetic anisotropy varies at the same rate than the saturation magnetization. This interesting behavior is ascribed phenomenologically to the sizable orbital moment of $Bi^{3+}$.

cond-mat.mtrl-sci↗

arXiv2021

Electrical detection of the spin-flop and room-temperature magnetic ordering in van der Waals CrPS$_{4}$/(Pt, Pd) heterostructures

We study magneto-transport in heterostructures composed of the van der Waals antiferromagnet CrPS$_{4}$ and the heavy metals Pt and Pd. The transverse resistance (R$_{xy}$) signal reveals the spin-flop transition of CrPS$_{4}$ and a strongly enhanced magnetic ordering temperature (>300 K), which might originate from a strong spin-orbit coupling at the interface. While CrPS$_{4}$/Pt devices allow for easy detection of the spin-flop transition, CrPS$_{4}$/Pd devices show a more substantial enhancement in magnetic ordering temperature and exhibit a topological Hall effect signal, possibly related to chiral spin structures at the interface. The longitudinal magnetoresistance (R$_{xx}$) results from a combination of spin-Hall magnetoresistance and the negative magnetoresistance that can be explained by a field-induced change of the electronic band structure of CrPS$_{4}$.

physics.app-ph↗

arXiv2021

Stabilization of phase noise in spin torque nano oscillators by a phase locked loop

The main limitation in order to exploit spin torque nano-oscillators (STNOs) in various potential applications is their large phase noise. In this work, we demonstrate its efficient reduction by a highly reconfigurable, compact, specifically on-chip designed PLL based on custom integrated circuits. First, we thoroughly study the parameter space of the PLL+STNO system experimentally. Second, we present a theory which describes the locking of a STNO to an external signal in a general sense. In our developed theory, we do not restrict ourselves to the case of a perfect phase locking but also consider phase slips and the corresponding low offset frequency $1/f^2$ noise, so far the main drawback in such systems. Combining experiment and theory allows us to reveal complex parameter dependences of the system's phase noise. The results provide an important step for the optimization of noise properties and thus leverage the exploitation of STNOs in prospective real applications.

physics.app-ph↗

arXiv2021

Voltage-Controlled Reconfigurable Magnonic Crystal at the Submicron Scale

Multiferroics offer an elegant means to implement voltage-control and on the fly reconfigurability in microscopic, nanoscaled systems based on ferromagnetic materials. These properties are particularly interesting for the field of magnonics, where spin waves are used to perform advanced logical or analogue functions. Recently, the emergence of nano-magnonics {\color{black} is expected to} eventually lead to the large-scale integration of magnonic devices. However, a compact voltage-controlled, on demand reconfigurable magnonic system has yet to be shown. Here, we introduce the combination of multiferroics with ferromagnets in a fully epitaxial heterostructure to achieve such voltage-controlled and reconfigurable magnonic systems. Imprinting a remnant electrical polarization in thin multiferroic $\mathrm{BiFeO_3}$ with a periodicity of $500\,\mathrm{nm}$ yields a modulation of the effective magnetic field in the micron-scale, ferromagnetic $\mathrm{La_{2/3}Sr_{1/3}MnO_3}$ magnonic waveguide. We evidence the magneto-electrical coupling by characterizing the spin wave propagation spectrum in this artificial, voltage induced, magnonic crystal and demonstrate the occurrence of a robust magnonic bandgap with $>20 \,\mathrm{dB}$ rejection.

cond-mat.mes-hall↗

arXiv2021

Long-distance spin-transport across the Morin phase transition up to room temperature in ultra-low damping single crystals of the antiferromagnet α-Fe2O3

Antiferromagnetic materials can host spin-waves with polarizations ranging from circular to linear depending on their magnetic anisotropies. Until now, only easy-axis anisotropy antiferromagnets with circularly polarized spin-waves were reported to carry spin-information over long distances of micrometers. In this article, we report long-distance spin-transport in the easy-plane canted antiferromagnetic phase of hematite and at room temperature, where the linearly polarized magnons are not intuitively expected to carry spin. We demonstrate that the spin-transport signal decreases continuously through the easy-axis to easy-plane Morin transition, and persists in the easy-plane phase through current induced pairs of linearly polarized magnons with dephasing lengths in the micrometer range. We explain the long transport distance as a result of the low magnetic damping, which we measure to be below 0.0001 as in the best ferromagnets. All of this together demonstrates that long-distance transport can be achieved across a range of anisotropies and temperatures, up to room temperature, highlighting the promising potential of this insulating antiferromagnet for magnon-based devices.

cond-mat.mes-hall↗

arXiv2021

Flicker and random telegraph noise between gyrotropic and dynamic C-state of a vortex based spin torque nano oscillator

Vortex based spin torque nano oscillators (STVOs) can present more complex dynamics than the spin torque induced gyrotropic (G) motion of the vortex core. The respective dynamic modes and the transition between them can be controlled by experimental parameters such as the applied dc current. An interesting behavior is the stochastic transition from the G- to a dynamic C-state occurring for large current densities. Moreover, the C-state oscillations exhibit a constant active magnetic volume. We present noise measurements in the different dynamic states that allow accessing specific properties of the stochastic transition, such as the characteristic state transition frequency. Furthermore,we confirm, as theoretically predicted, an increase of flicker noise with $I_{dc}^2$ when the oscillation volume remains constant with the current. These results bring insight into the potential optimization of noise properties sought for many potential rf applications with spin torque oscillators. Furthermore, the investigated stochastic characteristics open up new potentialities, for instance in the emerging field of neuromorphic computing schemes.

cond-mat.mes-hall↗

arXiv2021

Efficient spin torques in antiferromagnetic CoO/Pt quantified by comparing field- and current- induced switching

We achieve current-induced switching in collinear insulating antiferromagnetic CoO/Pt, with fourfold in-plane magnetic anisotropy. This is measured electrically by spin Hall magnetoresistance and confirmed by the magnetic field-induced spin-flop transition of the CoO layer. By applying current pulses and magnetic fields, we quantify the efficiency of the acting current-induced torques and estimate a current-field equivalence ratio of $4x10^{-11} T A^{-1} m^2$. The Néel vector final state ($n \perp j$) is in line with a thermomagnetoelastic switching mechanism for a negative magnetoelastic constant of the CoO.

cond-mat.mtrl-sci↗

arXiv2021

Propagation length of antiferromagnetic magnons governed by domain configurations

The compensated magnetic order and characteristic, terahertz frequencies of antiferromagnetic materials makes them promising candidates to develop a new class of robust, ultra-fast spintronic devices. The manipulation of antiferromagnetic spin-waves in thin films is anticipated to lead to new exotic phenomena such as spin-superfluidity, requiring an efficient propagation of spin-waves in thin films. However, the reported decay length in thin films has so far been limited to a few nanometers. In this work, we achieve efficient spin-wave propagation, over micrometer distances, in thin films of the insulating antiferromagnet hematite with large magnetic domains whilst evidencing much shorter attenuation lengths in multidomain thin films. Through transport and magnetic imaging, we conclude on the role of the magnetic domain structure and spin-wave scattering at domain walls to govern the transport. We manipulate the spin transport by tailoring the domain configuration through field cycle training. For the appropriate crystalline orientation, zero-field spin-transport is achieved across micrometers, as required for device integration.

cond-mat.mtrl-sci↗

arXiv2020

Spin-torque Dynamics for Noise Reduction in Vortex-based Sensors

Performance of magnetoresistive sensors is today mainly limited by their 1/f low-frequency noise. Here, we study this noise component in vortex-based TMR sensors. We compare the noise level in different magnetization configurations of the device, i.e vortex state or uniform parallel or antiparallel states. We find that the vortex state is at least an order of magnitude noisier than the uniform states. Nevertheless, by activating the spin-transfer induced dynamics of the vortex configuration, we observe a reduction of the 1/f noise, close to the values measured in the AP state, as the vortex core has a lower probability of pinning into defect sites. Additionally, by driving the dynamics of the vortex core by a non-resonant rf field or current we demonstrate that the 1/f noise can be further decreased. The ability to reduce the 1/f low-frequency noise in vortex-based devices by leveraging their spin-transfer dynamics thus enhances their applicability in the magnetic sensors' landscape.

cond-mat.mes-hall↗

arXiv2020

An insulating doped antiferromagnet with low magnetic symmetry as a room temperature spin conduit

We report room temperature long-distance spin transport of magnons in antiferromagnetic thin film hematite doped with Zn. The additional dopants significantly alter the magnetic anisotropies, resulting in a complex equilibrium spin structure that is capable of efficiently transporting spin angular momentum at room temperature without the need for a well-defined, pure easy-axis or easy-plane anisotropy. We find intrinsic magnon spin-diffusion lengths of up to 1.5 μm, and magnetic domain governed decay lengths of 175 nm for the low frequency magnons, through electrical transport measurements demonstrating that the introduction of non-magnetic dopants does not strongly reduce the transport length scale showing that the magnetic damping of hematite is not significantly increased. We observe a complex field dependence of the non-local signal independent of the magnetic state visible in the local magnetoresistance and direct magnetic imaging of the antiferromagnetic domain structure. We explain our results in terms of a varying and applied-field-dependent ellipticity of the magnon modes reaching the detector electrode allowing us to tune the spin transport.

cond-mat.mes-hall↗