arXiv subjects

G. Finocchio

Publications and source records attributed to G. Finocchio.

At least 19 recordsLinked to original sources

arXiv2026

Remotely programming the weights of a spintronic neural network by a radiofrequency broadcast signal

Selectively programming large number of non-volatile synaptic weights without compromising scalability is a key challenge for in-memory computing. Here, we demonstrate remote programming of synaptic weights in series-connected chains of 11 vortex-based magnetic tunnel junctions using broadcast radiofrequency signals applied through a shared strip line. The programming relies on frequency-selective reversal of the vortex-core polarity and therefore does not require individual access lines or selector devices. By reconfiguring the binary states of these chains, we reshape the weighted sums they perform on frequency-multiplexed RF inputs. Using a 22-synapse network composed of two such chains, we remotely reconfigure the same hardware to perform two distinct tasks: handwritten-digit classification and drone RF-signature identification. The digit-optimized configuration reaches 94.91 +/- 0.26% accuracy on handwritten digits but only 13.17 +/- 0.47% on drone RF signatures, whereas the drone-optimized configuration reaches 97.33 +/- 0.62% on drones but only 47.59 +/- 1.5% on digits. Broadcast RF programming thus provides a compact and scalable route to rapidly reconfigurable spintronic neuromorphic hardware.

cs.ET↗

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

Modeling the spatial resolution of magnetic solitons in Magnetic Force Microscopy and the effect on their sizes

In this work, we explored theoretically the spatial resolution of magnetic solitons and the variations of their sizes when subjected to a Magnetic Force Microscopy (MFM) measurement. Next to tip-sample separation, we considered reversal in the magnetization direction of the tip, showing that the magnetic soliton size measurement can be strongly affected by the magnetization direction of the tip. In addition to previous studies that only consider thermal fluctuations, we developed a theoretical method to obtain the minimum observable length of a magnetic soliton and its length variation due to the influence of the MFM tip by minimizing the soliton's magnetic energy. Our model uses analytical and numerical calculations and prevents overestimating the characteristic length scales from MFM images. We compared our method with available data from MFM measurements of domain wall widths, and we performed micromagnetic simulations of a skyrmion-tip system, finding a good agreement for both attractive and repulsive domain wall profile signals and for the skyrmion diameter in the presence of the magnetic tip. Our results provide significant insights for a better interpretation of MFM measurements of different magnetic solitons and will be helpful in the design of potential reading devices based on magnetic solitons as information carriers.

cond-mat.mes-hall↗

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

Field-driven collapsing dynamics of skyrmions in magnetic multilayers

Magnetic skyrmions are fascinating topological particle-like textures promoted by a trade-off among interfacial properties (perpendicular anisotropy and Dzyaloshinskii-Moriya interaction (DMI)) and dipolar interactions. Depending on the dominant interaction, complex spin textures, including pure N\'eel and hybrid skyrmions have been observed in multilayer heterostructures. A quantification of these different spin textures typically requires a depth-reoslved magnetic imaging or scattering techniques. In the present work, we will show qualitatively different collapsing dynamics for pure N\'eel and hybrid skyrmions induced by a perpendicular magnetic field in two representative systems, [Pt/Co/Ir]15 and [Ta/CoFeB/MgO]15 multilayers. Skyrmions in the former stack undergo two morphological transitions, upon reversing the perpendicular field direction. Skyrmions in [Ta/CoFeB/MgO]15 multilayers exhibit a continuous transition, which is mainly linked to a reversible change of the skyrmion size. A full micromagnetic phase diagram is presented to identify these two collapsing mechanisms as a function of material parameters. Since the two distinct collapsing dynamics rely on the detailed layer-dependent spin structures of skyrmions, they could be used as potential fingerprints for identifying the skyrmion type in magnetic multilayers. Our work suggests the employment of pure and hybrid skyrmions for specific applications, due to the strong correlation between the skyrmion dynamics and 3-dimentional spin profiles.

cond-mat.mes-hall↗

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↗

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

Robust-to-outliers square-root LASSO, simultaneous inference with a MOM approach

We consider the least-squares regression problem with unknown noise variance, where the observed data points are allowed to be corrupted by outliers. Building on the median-of-means (MOM) method introduced by Lecue and Lerasle Ann.Statist.48(2):906-931(April 2020) in the case of known noise variance, we propose a general MOM approach for simultaneous inference of both the regression function and the noise variance, requiring only an upper bound on the noise level. Interestingly, this generalization requires care due to regularity issues that are intrinsic to the underlying convex-concave optimization problem. In the general case where the regression function belongs to a convex class, we show that our simultaneous estimator achieves with high probability the same convergence rates and a similar risk bound as if the noise level was unknown, as well as convergence rates for the estimated noise standard deviation. In the high-dimensional sparse linear setting, our estimator yields a robust analog of the square-root LASSO. Under weak moment conditions, it jointly achieves with high probability the minimax rates of estimation $s^{1/p} \sqrt{(1/n) \log(p/s)}$ for the $\ell_p$-norm of the coefficient vector, and the rate $\sqrt{(s/n) \log(p/s)}$ for the estimation of the noise standard deviation. Here $n$ denotes the sample size, $p$ the dimension and $s$ the sparsity level. We finally propose an extension to the case of unknown sparsity level $s$, providing a jointly adaptive estimator $(\widetilde β, \widetilde σ, \widetilde s)$. It simultaneously estimates the coefficient vector, the noise level and the sparsity level, with proven bounds on each of these three components that hold with high probability.

math.ST↗

arXiv2020

Electrically tunable detector of THz-frequency signals based on an antiferromagnet

A concept of an electrically tunable resonance detector of THz-frequency signals based on antiferromagnetic/heavy metal (AFM/HM) hetero-structure is proposed. The conversion of a THz-frequency input signal into DC voltage is done using the inverse spin Hall effect in an (AFM/HM) bilayer. An additional bias DC current in the HM layer can be used to vary the effective anisotropy of the AFM, and, therefore, to tune the AFMR frequency. The proposed AFM/HM hetero-structure works as a resonance-type quadratic detector which can be tuned by the bias current in the range of at least 10 percent of the AFMR frequency and our estimations show that the sensitivity of this detector could be comparable to that of modern detectors based on the Schottky, Gunn or graphene-based diodes.

cond-mat.str-el↗

arXiv2019

Spin-orbit torque based physical unclonable function

This paper introduces the concept of spin-orbit-torque-MRAM (SOT-MRAM) based physical unclonable function (PUF). The secret of the PUF is stored into a random state of a matrix of perpendicular SOT-MRAMs. Here, we show experimentally and with micromagnetic simulations that this random state is driven by the intrinsic nonlinear dynamics of the free layer of the memory excited by the SOT. In detail, a large enough current drives the magnetization along an in-plane direction. Once the current is removed, the in-plane magnetic state becomes unstable evolving towards one of the two perpendicular stable configurations randomly. In addition, an hybrid CMOS/spintronics model is used to evaluate the electrical characteristics of a PUF realized with an array of 16x16 SOT-MRAM cells. Beyond robustness against voltage and temperature variations, hardware authentication based on this PUF scheme has additional advantages over other PUF technologies such as non-volatility (no power consumption in standby mode), reconfigurability (the secret can be rewritten), and scalability. We believe that this work is a step forward the design of spintronic devices for application in security.

physics.app-ph↗

arXiv2019

Configurational entropy of magnetic skyrmions as an ideal gas

The study of thermodynamics of topological defects is an important challenge to understand their underlying physics. Among them, magnetic skyrmions have a leading role for their physical properties and potential applications in storage and neuromorphic computing. In this paper, the thermodynamic statistics of magnetic skyrmions is derived. It is shown that the skyrmion free energy can be modelled via a parabolic function and the diameters statistics obeys the Maxwell-Boltzmann distribution. This allows for making an analogy between the behavior of the distribution of skyrmion diameters statistics and the diluted gas Maxwell-Boltzmann molecules distribution at thermodynamical equilibrium. The calculation of the skyrmion configurational entropy, due to thermally-induced changes of size and shape of the skyrmion, is essential for the determination of thermal fluctuations of the skyrmion energy around its average value. These results can be employed to advance the field of skyrmionics.

cond-mat.mes-hall↗

arXiv2019

Stabilizing zero-field skyrmions in Ir/Fe/Co/Pt thin film multilayers by magnetic history control

We present a study of the stability of room-temperature skyrmions in [Ir/Fe/Co/Pt] thin film multilayers, using the First Order Reversal Curve (FORC) technique and magnetic force microscopy (MFM). FORC diagrams reveal irreversible changes in magnetization upon field reversals, which can be correlated with the evolution of local magnetic textures probed by MFM. Using this approach, we have identified two different mechanisms - (1) skyrmion merger and (2) skyrmion nucleation followed by stripe propagation - which facilitate magnetization reversal in a changing magnetic field. Analysing the signatures of these mechanisms in the FORC diagram allows us to identify magnetic "histories" - i.e. precursor field sweep protocols - capable of enhancing the final zero-field skyrmion density. Our results indicate that FORC measurements can play a useful role in characterizing spin topology in thin film multilayers, and are particularly suitable for identifying samples in which skyrmion populations can be stabilized at zero field.

cond-mat.mes-hall↗

arXiv2018

Nonlinear dispersion relation in anharmonic periodic mass-spring and mass-in-mass systems

The study of wave propagation in chains of anharmonic periodic systems is of fundamental importance to understand the response of dynamical absorbers of vibrations and acoustic metamaterials working in nonlinear regime. Here, we derive an analytical nonlinear dispersion relation for periodic chains of anharmonic mass-spring and mass-in-mass systems resulting from considering the hypothesis of weak anharmonic energy and a periodic distribution function as ansatz of a general solution of the nonlinear equations of motion. Numerical simulations show that this expression is valid for anharmonic potential energy up to 50% of the harmonic one. This work provides a simple tool to design and study nonlinear dynamics for a class of seismic metamaterials.

physics.geo-ph↗

arXiv2018

Theory of nonreciprocal spin waves excitation in spin-Hall oscillators with Dzyaloshinkii-Moriya interaction

A two-dimensional analytical model for the description of the excitation of nonreciprocal spin waves by spin current in spin-Hall oscillators in the presence of the interfacial Dzyaloshinskii-Moriya interaction (i-DMI) is developed. The theory allows one to calculate the threshold current for the excitation of spin waves, as well as the frequencies and spatial profiles of the excited spin wave modes. It is found, that the frequency of the excited spin waves exhibits a quadratic red shift with the i-DMI strength. At the same time, in the range of small and moderate values of the i-DMI constant, the averaged wave number of the excited spin waves is almost independent of the i-DMI, which results in a rather weak dependence on the i-DMI of the threshold current of the spin wave excitation. The obtained analytical results are confirmed by the results of micromagnetic simulations.

cond-mat.mes-hall↗

arXiv2017

Origin of temperature and field dependence of magnetic skyrmion size in ultrathin nanodots

Understanding the physical properties of magnetic skyrmions is important for fundamental research with the aim to develop new spintronic device paradigms where both logic and memory can be integrated at the same level. Here, we show a universal model based on the micromagnetic formalism that can be used to study skyrmion stability as a function of magnetic field and temperature. We consider ultrathin, circular ferromagnetic magnetic dots. Our results show that magnetic skyrmions with a small radius compared to the dot radius are always metastable, while large radius skyrmions form a stable ground state. The change of energy profile determines the weak (strong) size dependence of the metastable (stable) skyrmion as a function of temperature and/or field. These results can open a path toward the design of optimal materials for skyrmion based devices.

cond-mat.mes-hall↗

arXiv2016

Performance of synthetic antiferromagnetic racetrack memory: domain wall vs skyrmion

A storage scheme based on racetrack memory, where the information can be coded in a domain or a skyrmion, seems to be an alternative to conventional hard disk drive for high density storage. Here, we perform a full micromagnetic study of the performance of synthetic antiferromagnetic (SAF) racetrack memory in terms of velocity and sensitivity to defects by using experimental parameters. We find that to stabilize a SAF skyrmion, the Dzyaloshinskii-Moriya Interaction in the top and the bottom ferromagnet should have an opposite sign. The velocity of SAF skyrmion and SAF Neel domain wall are of the same order and can reach values larger than 1200 m/s if a spin-orbit torque from the spin-Hall effect with opposite sign is applied to both ferromagnets.

cond-mat.mtrl-sci↗

arXiv2016

On the R2* Relaxometry in Complex Multi-Peak Multi-Echo Chemical Shift-Based Water-Fat Quantification: Applications to the Neuromuscular Diseases

Purpose: Investigation of the feasibility of the R2* mapping techniques by using latest theoretical models corrected for confounding factors and optimized for signal to noise ratio. Theory and Methods: The improvement of the performance of state of the art MRI relaxometry algorithms is challenging because of a non-negligible bias and still unresolved numerical instabilities. Here, R2* mapping reconstructions, including complex-fitting with multi-spectral fat-correction by using single-decay (1D) and double-decay (2D) formulation, are studied in order to identify optimal configuration parameters and minimize numerical artifacts. The effects of echo number, echo spacing, and fat/water relaxation model are evaluated through simulated and in-vivo data. We also explore the stability of such models by analyzing the impact of high percentage of fat infiltrations and local transverse relaxation differences among biological species. Results: The main limits of the MRI relaxometry are the presence of bias and the occurrence of artifacts which affect its accuracy. Chemical-shift complex reconstructions R2*-corrected with 1D formulation exhibit a large bias in presence of a significant difference in the relaxation rates of fat and water and with fat concentration larger than 30%. We find that for fat-dominated tissues or in patients affected by iron overload, MRI reconstructions accounting for multi-exponential relaxation time provide accurate R2* measurements and are less prone to numerical artifacts. Conclusions: Complex fitting 2D formulation outperforms the conventional 1D approximation in various diagnostic scenarios. Although it still lacks of numerical stability which requires model enhancement and support from spectroscopy, it offers promising perspectives for the development of relaxometry as a reliable tool to improve tissue characterization and monitoring of neuromuscular disorders.

physics.med-ph↗