arXiv subjects

P. Pirro

Publications and source records attributed to P. Pirro.

At least 19 recordsLinked to original sources

arXiv2026

Near-zero effective magnetization enabling ultra-low threshold currents in spin Hall micro-oscillators

Reducing the electrical current required to excite magnetization dynamics is a central challenge, e.g. for energy-efficient magnonic devices or oscillator-based computing. Spin Hall oscillators typically rely on large current densities to compensate intrinsic magnetic damping, so these systems are usually studied on the nanoscale (Spin Hall Nano Oscillators, SHNOs) to work with moderate currents and a favorable heat dissipation geometry. Here, we demonstrate that engineering a near-zero effective magnetization ($M_\mathrm{eff}$) enables a drastic reduction of the magnetization oscillation threshold current density for Spin Hall oscillators. This makes it possible to excite even comparably large systems with micrometer lateral sizes, so-called "Spin Hall Micro-Oscillators" (SHMOs). Using micro-focused Brillouin light scattering spectroscopy, we quantify the threshold current density in SHMOs based on W/CoFeB/MgO/Ta with near-zero $M_\mathrm{eff}$. We observe threshold current densities as low as $J_{\mathrm{th}}$ = (0.292 $\pm$ 0.025) $\times 10^{10}$ A/m$^{2}$, representing a reduction of more than two orders of magnitude compared with most recent reported SHNOs. Using systematic micromagnetic simulations, we investigate the breaking down of the macrospin approximation and underline the high influence of $M_\mathrm{eff}$ on magnetization dynamics under applied spin currents. Our results establish $M_\mathrm{eff}$ engineering as a powerful strategy for realizing ultra-low-power spin Hall oscillators and energy-efficient magnetization control.

cond-mat.mes-hall↗

arXiv2025

Hybrid magnonic spintronic system for tunable broadband signal filtering and microwave generation

Non-conventional beyond-the-state-of-the-art signal processing schemes require parallelism, scalability, robustness and energy efficiency to meet the demands of complex data-driven applications. With further research, magnonic and spintronic circuits can potentially help to fulfill these requirements. We present an experimental proof-of-concept of a hybrid device that can employ broad deteriorated microwave signals to excite and detect low energy propagating spin waves (SWs). For this, we use the output signal of a spin-transfer torque nano-oscillator (STNO) and connect it to a RF filter based on a magnonic delay-line. The STNO serves as a tunable nano-scaled signal generator with a broad output linewidth. Its RF output is fed as input into the magnonic delay-line circuit. Tuning the magnetic field solely at the magnonic circuit, we demonstrate the capability to selectively filter a broad RF input, obtaining a spin-wave output signal with a much narrower linewidth. This allows to tune the frequency of the RF signal at the output simply by tuning the magnetic field. Our findings are a first step towards a versatile, energy-efficient and compact wave-based filter with high sensitivity. Such a device can use even low-power, degraded signals and convert them into tunable SW outputs, effectively reducing the need for charge-based signal processing.

physics.app-ph↗

arXiv2025

Nonreciprocal spin waves in out-of-plane magnetized waveguides reconfigured by domain wall displacements

Wave-based platforms for novel unconventional computing approaches like neuromorphic computing require a well-defined, but adjustable flow of wave information combined with non-volatile data storage elements to implement weights which allow for training and learning. Due to their inherent nonreciprocal properties and their direct physical interaction with magnetic data storage, spin waves are ideal candidates to realize such platforms. In the present study, we show how spin-wave nonreciprocity induced by dipolar interactions of nanowaveguides with antiparallel, out-of-plane magnetization orientations can be used to create a spin-wave circulator allowing for unidirectional information transport and complex signal routing. In addition, the device can be reconfigured by a magnetic domain wall with adjustable position, which allows for a non-volatile tuning of the nonreciprocity and signal propagation. These properties are demonstrated for a spin-wave directional coupler through a combination of micromagnetic simulations and analytical modeling also showing that it functions as a waveguide crossing element, tunable power splitter, isolator, and frequency multiplexer. As magnetic material, out-of-plane magnetized Bismuth-doped Yttrium Iron Garnet has been considered. For this material, the motion of domain walls by magnonic spin transfer torque has been recently experimentally demonstrated which enables to store results from spin-wave computation. In combination with the presented concept of domain wall based reconfiguration and nonlinear spin-wave dynamics, this enables for the creation of a nano-scaled nonlinear wave computing platform with the capability for self-learning.

cond-mat.mes-hall↗

arXiv2025

Diverse dynamics in interacting vortices systems through tunable conservative and non-conservative coupling strengths

Magnetic vortices are highly tunable, nonlinear systems with ideal properties for being applied in spin wave emission, data storage, and neuromorphic computing. However, their technological application is impaired by a limited understanding of non conservative forces, that results in the open challenge of attaining precise control over vortex dynamics in coupled vortex systems. Here, we present an analytical model for the gyrotropic dynamics of coupled magnetic vortices within nano pillar structures, revealing how conservative and non conservative forces dictate their complex behavior. Validated by micromagnetic simulations, our model accurately predicts dynamic states, controllable through external current and magnetic field adjustments. The experimental verification in a fabricated nano pillar device aligns with our predictions, and it showcases the system's adaptability in dynamical coupling. The unique dynamical states, combined with the system's tunability and inherent memory, make it an exemplary foundation for reservoir computing. This positions our discovery at the forefront of utilizing magnetic vortex dynamics for innovative computing solutions, marking a leap towards efficient data processing technologies.

cond-mat.mes-hall↗

arXiv2024

Injection locking in DC-driven spintronic vortex oscillators via surface acoustic wave modulation

Control of the microwave signal generated by spin-transfer torque oscillators (STOs) is crucial for their applications in spin wave generation and neuromorphic computing. This study investigates injection locking of a DC-driven vortex STO using surface acoustic waves (SAWs) to enhance the STO's signal and allow for its synchronization with external inputs. We employ a simplified model based on Thiele's formalism and highlight the role of vortex deformations in achieving injection locking. Micromagnetic simulations are conducted to validate our theoretical predictions, revealing how the locking bandwidth depends on SAW amplitude, as well as on the amplitude and direction of an applied external field. Our findings are pivotal for advancing experimental research and developing efficient low-power synchronization methods for large-scale STO networks.

cond-mat.mes-hall↗

arXiv2024

Influence of buffer/protective layers on the structural and magnetic properties of SmCo films on Silicon

Integration of Samarium Cobalt hard magnets on silicon requires buffer/protective layers that can enhance the magnetic properties of the magnet while preserving its structure and chemical composition after post-annealing treatments needed for the formation of the magnetically hard phase. In this work, a comparison of Samarium-Cobalt films for five different buffer/protective layers, namely Ti, W, TiW, Ta, Cr and two different annealing temperatures, 650{\deg}C and 750{\deg}C, is presented. Depending on materials and annealing temperatures, magnetic properties such as saturation and coercivity of the SmCo film can be finely tuned. We show that coercivity up to 3.65 T or saturation magnetization up to 0.95 T can be reached by proper choice of the relevant process parameters: deposition temperature, material for the buffer/protective layer and annealing temperature. Such value of coercivity is among the highest found in literature for thin films of SmCo.

cond-mat.mtrl-sci↗

arXiv2023

Resonant excitation of vortex gyrotropic mode via surface acoustic waves

Finding new energy-efficient methods for exciting magnetization dynamics is one of the key challenges in magnonics. In this work, we present an approach to excite the gyrotropic dynamics of magnetic vortices through the phenomenon of inverse magnetostriction, also known as the Villari effect. We develop an analytical model based on the Thiele formalism that describes the gyrotropic motion of the vortex core including the energy contributions due to inverse magnetostriction. Based on this model, we predict excitations of the vortex core resonances by surface acoustic waves whose frequency is resonant with the frequency of the vortex core. We verify the model's prediction using micromagnetic simulations, and show the dependence of the vortex core's oscillation radius on the surface acoustic wave amplitude and the static bias field. Our study contributes to the advancement of energy-efficient magnetic excitations by relying on voltage-induced driven dynamics, which is an alternative to conventional current-induced excitations.

physics.app-ph↗

arXiv2023

Reversal of coupled vortices in advanced spintronics: A mechanistic study

This study conducts a comprehensive investigation into the reversal mechanism of magnetic vortex cores in a nanopillar system composed of two coupled ferromagnetic dots under zero magnetic field conditions. The research employs a combination of experimental and simulation methods to gain a deeper understanding of the dynamics of magnetic vortex cores. The findings reveal that by applying a constant direct current, the orientation of the vortex cores can be manipulated, resulting in a switch in one of the dots at a specific current value. The micromagnetic simulations provide evidence that this switch is a consequence of a deformation in the vortex profile caused by the increasing velocity of the vortex cores resulting from the constant amplitude of the trajectory as frequency increases. These findings offer valuable new insights into the coupled dynamics of magnetic vortex cores and demonstrate the feasibility of manipulating their orientation using direct currents under zero magnetic field conditions. The results of this study have potential implications for the development of vortex-based non-volatile memory technologies. \end{abstract}

cond-mat.mtrl-sci↗

arXiv2023

Amplification and frequency conversion of spin waves using acoustic waves

We numerically study the acoustic parametric amplification of spin waves using surface acoustic waves (SAW) in a magnetic thin film. First, we illustrate how the process of parametric spin-wave generation using short-waved SAWs with a fixed frequency allows to tune frequencies of the generated spin waves by the applied magnetic field. We further present the amplification of microwave driven spin waves using this method. The decay length and the amplitude of the driven spin waves can be amplified up to approximately 2.5 and 10 times compared to the reference signal, respectively. More importantly, the proposed design can be used as a frequency converter, in which a low (high) frequency spin-wave mode stimulates the excitation of a high (low) frequency mode. Our results pave the way in designing highly flexible and efficient hybrid magnonic device architectures for microwave data transport and processing.

physics.app-ph↗

arXiv2022

Hybrid Magnonic-Oscillator System

We propose a hybrid magnonic-oscillator system based on the combination of a spin transfer auto-oscillator and a magnonic waveguide to open new perspectives for spin-wave based circuits. The system is composed of a spin transfer oscillator based on a vortex state which is dipolarly coupled to a nanoscale spin-wave waveguide with longitudinal magnetization. In its auto-oscillating regime, the oscillator emits coherent spin waves with tunable and controllable frequencies, directions and amplitudes into the waveguide. We demonstrate the principle of this method using micromagnetic simulations and show that reconfiguration of the system is possible by changing the chirality and polarity of the magnetic vortex. Spin waves are emitted into the waveguide with high non-reciprocity and the preferred direction depends on the core polarity of the vortex. In contrast, different vortex chiralities lead to different amplitudes of the emitted waves. Our findings open up a novel way to design an agile spintronic device for the coherent and tunable generation of propagating spin waves.

cond-mat.other↗

arXiv2022

The role of damping rate amplitude in the synchronization of two coupled oscillators

We investigate the synchronization phenomenon between two Spin-transfer Torque Nano-Oscillators (STNOs) of different frequencies in two pillar systems under vortex configuration detunings or driving frequencies. The oscillators' mutual synchronization occurs through magnetic dipolar interaction. Our micromagnetic simulations show that an amplitude fluctuation referred to as $\Gamma_p$ has a significant impact on determining the synchronization frequency. The evolution of frequency and amplitude fluctuation rate in two different oscillator sizes versus external perpendicular field are compared and discussed. Our results reveal that the oscillator with lower $\Gamma_p$, referred to as the "Leader" oscillator, leads the synchronization process. As such, the "follower" oscillator adjusts its frequency as to that of the "Leader", thus achieving synchronization. We believe that taking $\Gamma_p$ into consideration can help in controlling synchronization frequencies in future building blocks of any network multi-array spintronics' devices.

cond-mat.other↗

arXiv2022

Simultaneous multitone microwave emission by DC-driven spintronic nano-element

Current-induced self-sustained magnetization oscillations in spin-torque nano-oscillators (STNOs) are promising candidates for ultra-agile microwave sources or detectors. While usually STNOs behave as a monochrome source, we report here clear bimodal simultaneous emission of incommensurate microwave oscillations, where the two tones correspond to two parametrically coupled eigenmodes with tunable splitting. The emission range is crucially sensitive to the change in hybridization of the eigenmodes of free and fixed layers, for instance, through a slight tilt of the applied magnetic field from the normal of the nano-pillar. Our experimental findings are supported both analytically and by micromagnetic simulations, which ascribe the process to four-magnon scattering between a pair of radially symmetric magnon modes and a pair of magnon modes with opposite azimuthal index. Our findings open up new possibilities for cognitive telecommunications and neuromorphic systems that use frequency multiplexing to improve communication performance.

cond-mat.mes-hall↗

arXiv2021

Fast long-wavelength exchange spin waves in partially-compensated Ga:YIG

Spin waves in yttrium iron garnet (YIG) nano-structures attract increasing attention from the perspective of novel magnon-based data processing applications. For short wavelengths needed in small-scale devices, the group velocity is directly proportional to the spin-wave exchange stiffness constant $\lambda_\mathrm{ex}$. Using wave vector resolved Brillouin Light Scattering (BLS) spectroscopy, we directly measure $\lambda_\mathrm{ex}$ in Ga-substituted YIG thin films and show that it is about three times larger than for pure YIG. Consequently, the spin-wave group velocity overcomes the one in pure YIG for wavenumbers $k > 4$ rad/$\mu$m, and the ratio between the velocities reaches a constant value of around 3.4 for all $k > 20$ rad/$\mu$m. As revealed by vibrating-sample magnetometry (VSM) and ferromagnetic resonance (FMR) spectroscopy, Ga:YIG films with thicknesses down to 59 nm have a low Gilbert damping ($\alpha < 10^{-3}$), a decreased saturation magnetization $\mu_0 M_\mathrm{S}~\approx~20~$mT and a pronounced out-of-plane uniaxial anisotropy of about $\mu_0 H_{\textrm{u1}} \approx 95 $ mT which leads to an out-of-plane easy axis. Thus, Ga:YIG opens access to fast and isotropic spin-wave transport for all wavelengths in nano-scale systems independently of dipolar effects.

cond-mat.mtrl-sci↗

arXiv2021

Roadmap on Spin-Wave Computing

Magnonics is a field of science that addresses the physical properties of spin waves and utilizes them for data processing. Scalability down to atomic dimensions, operations in the GHz-to-THz frequency range, utilization of nonlinear and nonreciprocal phenomena, and compatibility with CMOS are just a few of many advantages offered by magnons. Although magnonics is still primarily positioned in the academic domain, the scientific and technological challenges of the field are being extensively investigated, and many proof-of-concept prototypes have already been realized in laboratories. This roadmap is a product of the collective work of many authors that covers versatile spin-wave computing approaches, conceptual building blocks, and underlying physical phenomena. In particular, the roadmap discusses the computation operations with Boolean digital data, unconventional approaches like neuromorphic computing, and the progress towards magnon-based quantum computing. The article is organized as a collection of sub-sections grouped into seven large thematic sections. Each sub-section is prepared by one or a group of authors and concludes with a brief description of the current challenges and the outlook of the further development of the research directions.

physics.app-ph↗

arXiv2020

Controlling the nonlinear relaxation of quantized propagating magnons in nanodevices

Relaxation of linear magnetization dynamics is well described by the viscous Gilbert damping processes. However, for strong excitations, nonlinear damping processes such as the decay via magnon-magnon interactions emerge and trigger additional relaxation channels. Here, we use space- and time-resolved microfocused Brillouin light scattering spectroscopy and micromagnetic simulations to investigate the nonlinear relaxation of strongly driven propagating spin waves in yttrium iron garnet nanoconduits. We show that the nonlinear magnon relaxation in this highly quantized system possesses intermodal features, i.e., magnons scatter to higher-order quantized modes through a cascade of scattering events. We further show how to control such intermodal dissipation processes by quantization of the magnon band in single-mode devices, where this phenomenon approaches its fundamental limit. Our study extends the knowledge about nonlinear propagating spin waves in nanostructures which is essential for the construction of advanced spin-wave elements as well as the realization of Bose-Einstein condensates in scaled systems.

cond-mat.mes-hall↗

arXiv2019

Parametric generation of propagating spin-waves in ultra thin yttrium iron garnet waveguides

We present the experimental demonstration of the parallel parametric generation of spin-waves in a microscaled yttrium iron garnet waveguide with nanoscale thickness. Using Brillouin light scattering microscopy, we observe the excitation of the first and second waveguide modes generated by a stripline microwave pumping source. Micromagnetic simulations reveal the wave vector of the parametrically generated spin-waves. Based on analytical calculations, which are in excellent agreement with our experiments and simulations, we prove that the spin-wave radiation losses are the determinative term of the parametric instability threshold in this miniaturized system. The used method enables the direct excitation and amplification of nanometer spin-waves dominated by exchange interactions. Our results pave the way for integrated magnonics based on insulating nano-magnets.

cond-mat.mes-hall↗

arXiv2019

Spin pinning and spin-wave dispersion in nanoscopic ferromagnetic waveguides

Spin waves are investigated in Yttrium Iron Garnet (YIG) waveguides with a thickness of 39 nm and widths ranging down to 50 nm, i.e., with aspect ratios thickness over width approaching unity, using Brillouin Light Scattering spectroscopy. The experimental results are verified by a semi-analytical theory and micromagnetic simulations. A critical width is found, below which the exchange interaction suppresses the dipolar pinning phenomenon. This changes the quantization criterion for the spin-wave eigenmodes and results in a pronounced modification of the spin-wave characteristics. The presented semi-analytical theory allows for the calculation of spin-wave mode profiles and dispersion relations in nano-structures.

cond-mat.mes-hall↗

arXiv2019

Chiral excitations of magnetic droplet solitons driven by their own inertia

The inertial effects of magnetic solitons play a crucial role in their dynamics and stability. Yet governing their inertial effects is a challenge for their use in real devices. Here, we show how to control the inertial effects of magnetic droplet solitons. Magnetic droplets are strongly nonlinear and localized autosolitons than can form in current-driven nanocontacts. Droplets can be considered as dynamical particles with an effective mass. We show that the dynamical droplet bears a second excitation under its own inertia. These excitations comprise a chiral profile, and appear when the droplet resists the force induced by the Oersted field of the current injected into the nanocontact. We reveal the role of the spin torque on the excitation of these chiral modes and we show how to control these modes using the current and the field.

cond-mat.mes-hall↗