arXiv subjects

Yanis Sassi

Publications and source records attributed to Yanis Sassi.

8 recordsLinked to original sources

arXiv2023

Neuromorphic weighted sums with magnetic skyrmions

Integrating magnetic skyrmions into neuromorphic computing could help improve hardware efficiency and computational power. However, developing a scalable implementation of the weighted sum of neuron signals - a core operation in neural networks - has remained a challenge. Here, we show that weighted sum operations can be performed in a compact, biologically-inspired manner by using the non-volatile and particle-like characteristics of magnetic skyrmions that make them easily countable and summable. The skyrmions are electrically generated in numbers proportional to an input with an efficiency given by a non-volatile weight. The chiral particles are then directed using localized current injections to a location where their presence is quantified through non-perturbative electrical measurements. Our experimental demonstration, which currently has two inputs, can be scaled to accommodate multiple inputs and outputs using a crossbar array design, potentially nearing the energy efficiency observed in biological systems.

cs.ET↗

arXiv2023

Control of the magnetic anisotropy in multi-repeat Pt/Co/Al heterostructures using magneto-ionic gating

Controlling magnetic properties through the application of an electric field is a significant challenge in modern nanomagnetism. In this study, we investigate the magneto-ionic control of magnetic anisotropy in the topmost Co layer in Ta/Pt/[Co/Al/Pt]$_n$/Co/Al/AlO$_\text{x}$ multilayer stacks comprising $n +1$ Co layers and its impact on the magnetic properties of the multilayers. We demonstrate that the perpendicular magnetic anisotropy can be reversibly quenched through gate-driven oxidation of the intermediary Al layer between Co and AlO$_\text{x}$, enabling dynamic control of the magnetic layers contributing to the out-of-plane remanence - varying between $n$ and $n +1$. For multilayer configurations with $n = 2$ and $n = 4$, we observe reversible and non-volatile additions of 1/3 and 1/5, respectively, to the anomalous Hall effect amplitude based on the applied gate voltage. Magnetic imaging reveals that the gate-induced spin-reorientation transition occurs through the propagation of a single 90$^{\circ}$ magnetic domain wall separating the perpendicular and in-plane anisotropy states. In the 5-repetition multilayer, the modification leads to a doubling of the period of the magnetic domains at remanence. These results demonstrate that the magneto-ionic control of the anisotropy of a single magnetic layer can be used to control the magnetic properties of coupled multilayer systems, extending beyond the gating effects on a single magnetic layer.

cond-mat.mtrl-sci↗

arXiv2023

Driving skyrmions in flow regime in synthetic ferrimagnets

Despite significant advances in the last decade regarding the room temperature stabilization of skyrmions or their current induced dynamics, the impact of local material inhomogeneities still remains an important issue that impedes to reach the regime of steady state motion of these spin textures. Here, we study the spin-torque driven motion of skyrmions in synthetic ferrimagnetic multilayers with the aim of achieving high mobility and reduced skyrmion Hall effect. We consider Pt|Co|Tb multilayers of various thicknesses with antiferromagnetic coupling between the Co and Tb magnetization. The increase of Tb thickness in the multilayers allows to reduce the total magnetic moment and increases the spin-orbit torques allowing to reach velocities up to 400 m.s-1 for skyrmions with diameters of about 160 nm. We demonstrate that due to reduced skyrmion Hall effect, combined with the edge repulsion of the magnetic track making the skyrmions moving along the track without any transverse deflection. Further, by comparing the field-induced domain wall motion and current-induced skyrmion motion, we demonstrate that the skyrmions at the largest current densities present all the characteristics of a dynamical flow regime.

cond-mat.mtrl-sci↗

arXiv2022

Densely packed skyrmions stabilized at zero magnetic field by indirect exchange coupling in multilayers

Room-temperature stabilization of skyrmions in magnetic multilayered systems is the result of a fine balance between different magnetic interactions namely symmetric and antisymmetric exchange, dipolar, perpendicular magnetic anisotropy as well as, in most cases, Zeeman through an applied external field. Such field-driven stabilization approach is not compatible with most of the anticipated skyrmion based applications, e.g. skyrmion memories, logic or neuromorphic computing which motivates a reduction or a cancellation of field requirements. Here we present a method to stabilize at room-temperature and zero-field, a densely packed skyrmion phase in ferromagnetic multilayers with moderate number of repetitions. To this aim, we finely tune the multilayer parameters to stabilize a dense skyrmion phase. Then, relying on the interlayer electronic coupling to an adjacent bias magnetic layer with strong perpendicular magnetic anisotropy and uniform magnetization, we demonstrate the stabilization of sub-60 nm diameter skyrmion at zero-field with adjustable skyrmion density.

cond-mat.mtrl-sci↗

arXiv2021

Resonant dynamics of skyrmion lattices in thin film multilayers: Localised modes and spin wave emission

The spectral signatures of magnetic skyrmions under microwave field excitation are of fundamental interest and can be an asset for high frequency applications. These topological solitons can be tailored in multilayered thin films, but the experimental observation of their spin wave dynamics remains elusive, in particular due to large damping. Here, we study Pt/FeCoB/AlO$_x$ multilayers hosting dense and robust skyrmion lattices at room temperature with Gilbert damping of $\sim 0.02$. We use magnetic force microscopy to characterise their static magnetic phases and broadband ferromagnetic resonance to probe their high frequency response. Micromagnetic simulations reproduce the experiments with accuracy and allow us to identify distinct resonant modes detected in the skyrmion lattice phase. Low ($<$ 2 GHz) and intermediate frequency ($2-8$ GHz) modes involve excitations localised to skyrmion edges in conjunction with precession of the uniform background magnetisation, while a high frequency ($>$ 12 GHz) mode corresponds to in-phase skyrmion core precession emitting spin waves into uniform background with wavelengths in the 50--80 nm range commensurate with the lattice structure. These findings could be instrumental in the investigation of room temperature wave scattering and the implementation of novel microwave processing schemes in reconfigurable arrays of solitons.

cond-mat.mes-hall↗

arXiv2021

Element-selective modulation of interfacial Dzyaloshinskii-Moriya interaction in Pt|Co|Metal based multilayers

Despite a decade of research, the precise mechanisms occurring at interfaces underlying the Dzyaloshinskii-Moriya interaction (DMI), and thus the possibility of fine-tuning it, are not yet fully identified. In this study, we investigate the origin of the interfacial DMI, aiming at disentangling how independent are the interfaces around the ferromagnetic layer, and what are their relative contributions to the effective DMI amplitude. For this purpose, we have grown and investigated a large variety of systems with a common structure Pt$|$Co$|M$ with $M =$ Ni, Pd, Ru, Al, Al$|$Ta and MoSi. We explore the correlation between the effective interfacial DMI, and different intrinsic properties of metals, namely atomic number, electronegativity and work function difference at the Co$|M$ interfaces. We find a linear relationship between interfacial DMI and the work function difference between the two elements, hence relating the nature of this behavior to the interfacial potential gradient at the metallic interfaces. The understanding of the DMI mechanism is of utmost importance since it opens up the possibility of precisely engineering the magnetic and hence the spintronic properties for future devices.

cond-mat.mtrl-sci↗

arXiv2021

Chiral spin spiral in synthetic antiferromagnets probed by circular dichroism in x-ray resonant magnetic scattering

Noncollinear chiral spin textures in ferromagnetic multilayers are at the forefront of recent research in nano-magnetism with the promise for fast and energy-efficient devices. The recently demonstrated possibilities to stabilize such chiral structures in synthetic antiferromagnets (SAF) has raised interests as they are immune to dipolar field, hence favoring the stabilization of ultra small textures, improve mobility and avoid the transverse deflections of moving skyrmions limiting the efficiency in some foreseen applications. However, such systems with zero net magnetization are hence difficult to characterize by most of the standard techniques. Here, we report that the relevant parameters of a magnetic SAF texture, those being its period, its type (N\'eel or Bloch) and its chirality (clockwise or counterclockwise), can be directly determined using the circular dichroism in x-ray resonant scattering (CD-XRMS) at half integer multilayer Bragg peaks in reciprocal space. The analysis of the dependence in temperature down to 40K allows us moreover to address the question of the temperature stability of a spin spiral in a SAF sample and of the temperature scaling of the symmetric and antisymmetric exchange interactions.

cond-mat.mtrl-sci↗

arXiv2020

Ultrafast time-evolution of chiral N\'eel magnetic domain walls probed by circular dichroism in x-ray resonant magnetic scattering

Non-collinear spin textures in ferromagnetic ultrathin films are attracting a renewed interest fueled by possible fine engineering of several magnetic interactions, notably the interfacial Dzyaloshinskii-Moriya interaction. This allows the stabilization of complex chiral spin textures such as chiral magnetic domain walls (DWs), spin spirals, and magnetic skyrmions. We report here on the ultrafast behavior of chiral DWs after optical pumping in perpendicularly magnetized asymmetric multilayers, probed using time-resolved circular dichroism in x-ray resonant magnetic scattering (CD-XRMS). We observe a picosecond transient reduction of the CD-XRMS, which is attributed to the spin current-induced coherent and incoherent torques within the continuously dependent spin texture of the DWs. We argue that a specific demagnetization of the inner structure of the DW induces a flow of hot spins from the interior of the neighboring magnetic domains. We identify this time-varying change of the DW textures shortly after the laser pulse as a distortion of the homochiral N'eel shape toward a transient mixed Bloch-N\'eel-Bloch textures along a direction transverse to the DW. Our study highlights how time-resolved CD-XRMS can be a unique tool for studying the time evolution in other systems showing a non-collinear electric/magnetic ordering such as skyrmion lattices, conical/helical phases, as well as the recently observed antiskyrmion lattices, in metallic or insulating materials.

cond-mat.mtrl-sci↗