arXiv subjects

P. Bortolotti

Publications and source records attributed to P. Bortolotti.

At least 19 recordsLinked to original sources

arXiv2024

Controlling encirclement of an exceptional point using coupled spintronic nano-oscillators

Exceptional points (EPs), branch singularities parameter space of non-Hermitian eigenvalue manifolds, display unique topological phenomena linked to eigenvalue and eigenvector switching: the parameter space states are highly sensitive to the system's parameter changes. Therefore, we suggest investigating the parameter space in the presence of an EP by experimentally accessing and exploiting the topological nature of the coupled system around an EP. We demonstrate control over exceptional points in coupled vortex spin-transfer torque oscillators by adjusting the system's damping through the spin-transfer torque effect and their relative phase. This approach allows for precise manipulation of the coupling behavior in the vicinity of an exceptional point. We report the presence of both level attraction/repulsion by adjusting the system's parameters. Moreover, we evidence the topological nature of the EP by dynamically encircling it in the phase-current parameter space, leading to a switch of the eigenstates. Our study introduces a new method for exploring non-Hermitian physics in spintronic systems at room temperature.

cond-mat.mes-hall↗

arXiv2024

Probing nonlinear spin dynamics in canted easy-plane antiferromagnets using spin-rectification effects

We investigate spin-rectification phenomena in canted antiferromagnets, closely connected to the family of altermagnetic materials. Our results show that excitation efficiency is significantly enhanced by the Dzyaloshinskii-Moriya interaction. Antiferromagnetic dynamics can be detected through spin-Hall magnetoresistance and bolometric effects, with an efficiency reaching up to mV/W. The rectified voltage shape is influenced by both the symmetry of the exciting torques, detection mechanisms (continuous spin-pumping and spin-Hall magnetoresistance), and the antiferromagnetic crystalline axis. Under high pumping power, we observe a saturation effect related to Suhl-like spin-wave instabilities and a nonlinear redshift of the antiferromagnetic resonance. These findings open new avenues for studying nonlinear dynamics in antiferromagnetic and altermagnetic spintronic devices.

cond-mat.mes-hall↗

arXiv2023

Antiferromagnetic magnon spintronic based on non-reciprocal and non-degenerated ultra-fast spin-waves in the canted antiferromagnet α-Fe2O3

Spin-waves in antiferromagnets hold the prospects for the development of faster, less power-hungry electronics, as well as promising physics based on spin-superfluids and coherent magnon-condensates. For both these perspectives, addressing electrically coherent antiferromagnetic spin-waves is of importance, a prerequisite that has so far been elusive, because unlike ferromagnets,antiferromagnets couple weakly to radiofrequency fields. Here, we demonstrate the detection of ultra-fast non-reciprocal spin-waves in the dipolar-exchange regime of a canted antiferromagnet using both inductive and spintronic transducers. Using time-of-flight spin-wave spectroscopy on hematite (α-Fe2O3), we find that the magnon wave packets can propagate as fast as 20 km/s for reciprocal bulk spin-wave modes and up to 6 km/s for surface-spin waves propagating parallel to the antiferromagnetic Neel vector. We finally achieve efficient electrical detection of non-reciprocal spin-wave transport using non-local inverse spin-Hall effects. The electrical detection of coherent non-reciprocal antiferromagnetic spin waves paves the way for the development of antiferromagnetic and altermagnet-based magnonic devices.

cond-mat.mes-hall↗

arXiv2021

Evidence for spin current driven Bose-Einstein condensation of magnons

The quanta of magnetic excitations - magnons - are known for their unique ability to undergo Bose-Einstein condensation at room temperature. This fascinating phenomenon reveals itself as a spontaneous formation of a macroscopic coherent state under the influence of incoherent stimuli. Spin currents have been predicted to offer electronic control of magnon Bose-Einstein condensates, but this phenomenon has not been experimentally evidenced up to now. Here we experimentally show that current-driven Bose-Einstein condensation can be achieved in nanometer-thick films of magnetic insulators with tailored dynamic magnetic nonlinearities and minimized magnon-magnon interactions. We demonstrate that, above a certain threshold, magnons injected by the spin current overpopulate the lowest-energy level forming a highly coherent spatially extended state. By accessing magnons with essentially different energies, we quantify the chemical potential of the driven magnon gas and show that, at the critical current, it reaches the energy of the lowest magnon level. Our results pave the way for implementation of integrated microscopic quantum magnonic and spintronic devices.

cond-mat.mes-hall↗

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↗

arXiv2021

Dispersionless propagation of ultra-short spin-wave pulses in ultrathin yttrium iron garnet waveguides

We study experimentally the propagation of nanosecond spin-wave pulses in microscopic waveguides made of nanometer-thick yttrium iron garnet films. For these studies, we use micro-focus Brillouin light scattering spectroscopy, which provides the possibility to observe propagation of the pulses with high spatial and temporal resolution. We show that, for most spin-wave frequencies, dispersion leads to broadening of the pulse by several times at propagation distances of 10 micrometers. However, for certain frequency interval, the dispersion broadening is suppressed almost completely resulting in a dispersionless pulse propagation. We show that the formation of the dispersion-free region is caused by the competing effects of the dipolar and the exchange interaction, which can be controlled by the variation of the waveguide geometry. These conclusions are supported by micromagnetic simulations and analytical calculations. Our findings provide a simple solution for the implementation of high-speed magnonic systems that require undisturbed propagation of short information-carrying spin-wave pulses.

cond-mat.mes-hall↗

arXiv2020

Ultrafast spin-currents and charge conversion at 3d-5d interfaces probed by time-domain terahertz spectroscopy

Spintronic structures are extensively investigated for their spin orbit torque properties, required for magnetic commutation functionalities. Current progress in these materials is dependent on the interface engineering for the optimization of spin transmission. Here, we advance the analysis of ultrafast spin-charge conversion phenomena at ferromagnetic-transition metal interfaces due to their inverse spin-Hall effect properties. In particular the intrinsic inverse spin Hall effect of Pt-based systems and extrinsic inverse spin-Hall effect of Au:W and Au:Ta in NiFe/Au:(W,Ta) bilayers are investigated. The spin-charge conversion is probed by complementary techniques -- ultrafast THz time domain spectroscopy in the dynamic regime for THz pulse emission and ferromagnetic resonance spin-pumping measurements in the GHz regime in the steady state -- to determine the role played by the material properties, resistivities, spin transmission at metallic interfaces and spin-flip rates. These measurements show the correspondence between the THz time domain spectroscopy and ferromagnetic spin-pumping for the different set of samples in term of the spin mixing conductance. The latter quantity is a critical parameter, determining the strength of the THz emission from spintronic interfaces. This is further supported by ab-initio calculations, simulations and analysis of the spin-diffusion and spin relaxation of carriers within the multilayers in the time domain, permitting to determine the main trends and the role of spin transmission at interfaces. This work illustrates that time domain spectroscopy for spin-based THz emission is a powerful technique to probe spin-dynamics at active spintronic interfaces and to extract key material properties for spin-charge conversion.

physics.app-ph↗

arXiv2019

Temporal pattern recognition with delayed feedback spin-torque nano-oscillators

The recent demonstration of neuromorphic computing with spin-torque nano-oscillators has opened a path to energy efficient data processing. The success of this demonstration hinged on the intrinsic short-term memory of the oscillators. In this study, we extend the memory of the spin-torque nano-oscillators through time-delayed feedback. We leverage this extrinsic memory to increase the efficiency of solving pattern recognition tasks that require memory to discriminate different inputs. The large tunability of these non-linear oscillators allows us to control and optimize the delayed feedback memory using different operating conditions of applied current and magnetic field.

physics.app-ph↗

arXiv2019

Analog and digital phase modulation of spin torque nano-oscillators

Spin torque nano-oscillators (STNO) are nanoscale devices with wide band frequency tunability. Their multifunctional RF properties are well suited to define novel schemes for wireless communications that use basic protocols for data transmission such as amplitude, frequency and phase shift keying (ASK, FSK, PSK). In contrast to ASK and FSK, implementation of PSK is more challenging for STNOs because of their relatively high phase noise. Here we introduce a special PSK technique by combining their modulation and injection locking functionality. The concept is validated using magnetic tunnel junction based vortex STNOs for injection locking at 2f and f/2 showing phase shifts up to 2.1rad and data transmission rates up to 4Mbit/s. Quadrature phase shift keying and analog phase modulation are also implemented, where the latter is employed for voice transmission over a distance of 10 meters. This demonstrates that STNO phase noise and output power meet the requested performances for operation in existing communication schemes.

physics.app-ph↗

arXiv2019

Enhancing the injection locking range of spin torque oscillators through mutual coupling

We investigate how the ability of the vortex oscillation mode of a spin-torque nano-oscillator to lock to an external microwave signal is modified when it is coupled to another oscillator. We show experimentally that mutual electrical coupling can lead to locking range enhancements of a factor 1.64. Furthermore, we analyze the evolution of the locking range as a function of the coupling strength through experiments and numerical simulations. By uncovering the mechanisms at stake in the locking range enhancement, our results will be useful for designing spin-torque nano-oscillators arrays with high sensitivities to external microwave stimuli.

cond-mat.mes-hall↗

arXiv2019

Neuromorphic Computing through Time-Multiplexing with a Spin-Torque Nano-Oscillator

Fabricating powerful neuromorphic chips the size of a thumb requires miniaturizing their basic units: synapses and neurons. The challenge for neurons is to scale them down to submicrometer diameters while maintaining the properties that allow for reliable information processing: high signal to noise ratio, endurance, stability, reproducibility. In this work, we show that compact spin-torque nano-oscillators can naturally implement such neurons, and quantify their ability to realize an actual cognitive task. In particular, we show that they can naturally implement reservoir computing with high performance and detail the recipes for this capability.

cs.ET↗

arXiv2019

Microwave neural processing and broadcasting with spintronic nano-oscillators

Can we build small neuromorphic chips capable of training deep networks with billions of parameters? This challenge requires hardware neurons and synapses with nanometric dimensions, which can be individually tuned, and densely connected. While nanosynaptic devices have been pursued actively in recent years, much less has been done on nanoscale artificial neurons. In this paper, we show that spintronic nano-oscillators are promising to implement analog hardware neurons that can be densely interconnected through electromagnetic signals. We show how spintronic oscillators maps the requirements of artificial neurons. We then show experimentally how an ensemble of four coupled oscillators can learn to classify all twelve American vowels, realizing the most complicated tasks performed by nanoscale neurons.

physics.app-ph↗

arXiv2018

Emission of coherent propagating magnons by insulator-based spin-orbit torque oscillators

We experimentally demonstrate generation of coherent propagating magnons in ultra-thin magnetic-insulator films by spin-orbit torque induced by dc electric current. We show that this challenging task can be accomplished by utilizing magnetic-insulator films with large perpendicular magnetic anisotropy. We demonstrate simple and flexible spin-orbit torque devices, which can be used as highly efficient nanoscale sources of coherent propagating magnons for insulator-based spintronic applications.

cond-mat.mes-hall↗

arXiv2017

Nano-patterned magnonic crystals based on ultrathin YIG films

We demonstrate a microscopic magnonic-crystal waveguide produced by nano-patterning of a 20 nm thick film of Yttrium Iron Garnet. By using the phase-resolved micro-focus Brillouin light scattering spectroscopy, we map the intensity and the phase of spin waves propagating in such a periodic magnetic structure. Based on these maps, we obtain the dispersion and the attenuation characteristics of spin waves providing detailed information about the physics of spin-wave propagation in the magnonic crystal. We show that, in contrast to the simplified physical picture, the maximum attenuation of spin waves is achieved close to the edge of the magnonic band gap, which is associated with non-trivial reflection characteristics of spin waves in non-uniform field potentials.

cond-mat.mes-hall↗

arXiv2016

Driven energy transfer between coupled modes in spin-torque oscillators

The mutual interaction between the different eigenmodes of a spin-torque oscillator can lead to a large variety of physical mechanisms from mode hopping to multi-mode generation, that usually reduce their performances as radio-frequency devices. To tackle this issue for the future applications, we investigate the properties of a model spin-torque oscillator that is composed of two coupled vortices with one vortex in each of the two magnetic layers of the oscillator. In such double-vortex system, the remarkable properties of energy transfer between the coupled modes, one being excited by spin transfer torque while the second one being damped, result into an alteration of the damping parameters. As a consequence, the oscillator nonlinear behavior is concomitantly drastically impacted. This efficient coupling mechanism, driven mainly by the dynamic dipolar field generated by the spin transfer torque induced motion of the vortices, gives rise to an unexpected dynamical regime of self-resonance excitation. These results show that mode coupling can be leveraged for controlling the synchronization process as well as the frequency tunability of spin-torque oscillators.

cond-mat.mes-hall↗

arXiv2016

Spin-wave propagation in ultra-thin YIG based waveguides

Spin-wave propagation in an assembly of microfabricated 20 nm thick, 2.5 μm wide Yttrium Iron Garnet (YIG) waveguides is studied using propagating spin-wave spectroscopy (PSWS) and phase resolved micro-focused Brillouin Light Scattering (μ-BLS) spectroscopy. We show that spin-wave propagation in 50 parallel waveguides is robust against microfabrication induced imperfections. Spin-wave propagation parameters are studied in a wide range of excitation frequencies for the Damon-Eshbach (DE) configuration. As expected from its low damping, YIG allows the propagation of spin waves over long distances (the attenuation lengths is 25 μm at μ$_{0}$H = 45 mT). Direct mapping of spin waves by μ-BLS allows us to reconstruct the spin-wave dispersion relation and to confirm the multi-mode propagation in the waveguides, glimpsed by propagating spin-wave spectroscopy.

cond-mat.mes-hall↗

arXiv2016

Direct observation of dynamic modes excited in a magnetic insulator by pure spin current

Excitation of magnetization dynamics by pure spin currents has been recently recognized as an enabling mechanism for spintronics and magnonics, which allows implementation of spin-torque devices based on low-damping insulating magnetic materials. Here we report the first spatially-resolved study of the dynamic modes excited by pure spin current in nanometer-thick microscopic insulating Yttrium Iron Garnet disks. We show that these modes exhibit nonlinear self-broadening preventing the formation of the self-localized magnetic bullet, which plays a crucial role in the stabilization of the single-mode magnetization oscillations in all-metallic systems. This peculiarity associated with the efficient nonlinear mode coupling in low-damping materials can be among the main factors governing the interaction of pure spin currents with the dynamic magnetization in high-quality magnetic insulators.

cond-mat.mes-hall↗

arXiv2016

Spin wave amplification using the spin Hall effect in permalloy/platinum bilayers

We investigate the effect of an electrical current on the attenuation length of a 900 nm wavelength spin-wave in a permalloy/Pt bilayer using propagating spin-wave spectroscopy. The modification of the spin-wave relaxation rate is linear in current density, reaching up to 14% for a current density of 2.3$\times10^{11} $A/m$^2$ in Pt. This change is attributed to the spin transfer torque induced by the spin Hall effect and corresponds to an effective spin Hall angle of 0.13, which is among the highest values reported so far. The spin Hall effect thus appears as an efficient way of amplifying/attenuating propagating spin waves.

cond-mat.mtrl-sci↗