arXiv subjects

Ricardo C. Sousa

Publications and source records attributed to Ricardo C. Sousa.

6 recordsLinked to original sources

arXiv2026

Double-pulse control of all optical magnetization reversal in Tb/Co multilayers

Recent experiments have shown that femtosecond laser pulse with a Gaussian intensity profile can induce magnetization reversal in Tb/Co multilayers with a ring-shaped switching pattern within the laser-irradiated area. Here, we investigate the ultrafast magnetization dynamics leading to such a ring-shaped switching by using double-pulse laser excitation. The laser pulses cause heat-induced quenching and subsequent recovery of the magnetic anisotropy in the multilayers and drive the precessional magnetization switching in the magnetic multilayers. By adjusting the delay between the two pump pulses, we demonstrate that the recovery process can be manipulated and show, experimentally and numerically, that this allows control over the final magnetic domain pattern.

cond-mat.mtrl-sci↗

arXiv2026

Orbital and Spin-Orbit Torque Interplay in Ta/W-based Magnetic Tunnel Junctions with Vertical Non-local Switching

Spin-orbit torque (SOT) enables ultra-fast, energy-efficient magnetization switching, making it a promising mechanism for introducing MRAMs for cache memory applications. However, current SOT-MRAM devices face write efficiency limitations, with charge-to-spin conversion ($ξ_{DL}$) reaching $\sim$ 45\%, far below the projected $\sim$ 80\% needed to comply with the current delivery of advanced transistor nodes. Recent advances in orbital current physics, evidenced in a wide class of materials, offer a path to enhance $ξ_{DL}$. Here, we study the Ta(3-30 nm)\slash W(1-4 nm) system, revealing a large additional spin-orbit torque contribution arising from Ta, a four-fold increase compared to the spin Hall effect in Ta alone, attributed to the orbital Hall contribution. This system exhibits larger $ξ_{DL}$ than W-based SOT systems with more robust perpendicular magnetic anisotropy and compatibility with 400$^\circ$C annealing. Leveraging these advantages, we integrate the Ta/W system into 3-terminal SOT-MTJ devices, showing a level of performance similar to that of W-based systems. Our results show that orbital physics can be easily integrated into SOT-MTJ systems, offering a viable strategy to enhance SOT-MRAM efficiency. In addition, we propose and demonstrate a proof-of-concept for vertical non-local switching of SOT-MTJ using orbital torques, simplifying bottom-pinned SOT-MRAM fabrication.

cond-mat.mes-hall↗

arXiv2025

Low-Noise Nanoscale Vortex Sensor for Out-of-Plane Magnetic Field Detection

This study investigates a vortex sensor based on a nanoscale (sub-100 nm) magnetic tunnel junction (MTJ) with a strong shape anisotropy, designed for sensitivity to the out-of-plane magnetic field component ($H_z$). The sensor comprises a free layer with a vortex configuration and a perpendicularly magnetized reference layer, which provides a reproducible and linear response when excited by a perpendicular magnetic field. Experimental measurements and micromagnetic simulations were combined to systematically assess the influence of structural parameters, specifically aspect ratio and defect landscape, on key sensor performance metrics, including dynamic range, sensitivity, and detectivity. The out-of-plane vortex sensor demonstrates a significantly improved dynamic range exceeding 200 mT, compared to the 40-80 mT typical of conventional in-plane vortex sensors. Frequency-dependent noise measurements reveal that the sensor exhibits low intrinsic noise, along with improved detectivity and resolution. This performance is ascribed to the field-dependent expansion and contraction of the vortex core, which reduces Barkhausen-type noise caused by defect-induced pinning potentials. Moreover, the sub-100\,nm lateral dimensions of the sensor enable scalable array integration, providing further enhancements in noise and detectivity through collective averaging. These results underscore the potential of this sensor architecture for advanced magnetic field sensing applications requiring a wide dynamic range and high measurement accuracy at the same time.

cond-mat.mes-hall↗

arXiv2022

Direct observation of the perpendicular shape anisotropy and thermal stability of p-STT-MRAM nano-pillars

Perpendicular shape anisotropy (PSA) offers a practical solution to downscale spin-transfer torque Magnetic Random-Access Memory (STT-MRAM) beyond the sub-20 nm technology node whilst retaining thermal stability of the storage layer magnetization. However, our understanding of the thermomagnetic behavior of PSA-STT-MRAM is often indirect, relying on magnetoresistance measurements and micromagnetic modelling. Here, the magnetism of a FeCoB / NiFe PSA-STT-MRAM nano-pillar is investigated using off-axis electron holography, providing spatially resolved magnetic information as a function of temperature, which has been previously inaccessible. Magnetic induction maps reveal the micromagnetic configuration of the NiFe storage layer (60 nm high, 20 nm diameter), confirming the PSA induced by its 3:1 aspect ratio. In-situ heating demonstrates that the PSA of the FeCoB / NiFe composite storage layer is maintained up to at least 250 degrees centigrade, and direct quantitative measurements reveal the very moderate decrease of magnetic induction with temperature. Hence, this study shows explicitly that PSA provides significant stability in STT-MRAM applications that require reliable performance over a range of operating temperatures.

cond-mat.mtrl-sci↗

arXiv2021

All-optical spin switching probability in [Tb/Co] multilayers

Since the first experimental observation of all-optical switching phenomena, intensive research has been focused on finding suitable magnetic systems that can be integrated as storage elements within spintronic devices and whose magnetization can be controlled through ultra-short single laser pulses. We report here atomistic spin simulations of all-optical switching in multilayered structures alternating n monolayers of Tb and m monolayers of Co. By using a two temperature model, we numerically calculate the thermal variation of the magnetization of each sublattice as well as the magnetization dynamics of [Tbn/Com] multilayers upon incidence of a single laser pulse. In particular, the condition to observe thermally-induced magnetization switching is investigated upon varying systematically both the composition of the sample (n,m) and the laser fluence. The samples with one monolayer of Tb as [Tb1/Co2] and [Tb1/Co3] are showing thermally induced magnetization switching above a fluence threshold. The reversal mechanism is mediated by the residual magnetization of the Tb lattice while the Co is fully demagnetized in agreement with the models developed for ferrimagnetic alloys. The switching is however not fully deterministic but the error rate can be tuned by the damping parameter. Increasing the number of monolayers the switching becomes completely stochastic. The intermixing at the Tb/Co interfaces appears to be a promising way to reduce the stochasticity. These results predict for the first time the possibility of TIMS in [Tb/Co] multilayers and suggest the occurrence of sub-picosecond magnetization reversal using single laser pulses.

physics.app-ph↗

arXiv2020

Realizing an Isotropically Coercive Magnetic Layer for Memristive Applications by Analogy to Dry Friction

We investigate the possibility of realizing a spintronic memristive device based on the dependence of the tunnel conductance on the relative angle between the magnetization of the two magnetic electrodes in in-plane magnetized tunnel junctions. For this, it is necessary to design a free layer whose magnetization can be stabilized along several or even any in-plane direction between the parallel and the antiparallel magnetic configurations. We experimentally show that this can be achieved by exploiting antiferromagnet-ferromagnet exchange interactions in a regime where the antiferromagnet is thin enough to induce enhanced coercivity and no exchange bias. The frustration of exchange interactions at the interfaces due to competing ferro- and antiferromagnetic interactions is at the origin of an isotropic dissipation mechanism yielding isotropic coercivity. From a modeling point of view, it is shown that this isotropic dissipation can be described by a dry friction term in the Landau-Lifshitz-Gilbert equation. The influence of this dry friction term on the magnetization dynamics of an in-plane magnetized layer submitted to a rotating in-plane field is investigated both analytically and numerically. The possibility to control the free layer magnetization orientation in an in-plane magnetized magnetic tunnel junction by using the spin transfer torque from an additional perpendicular polarizer is also investigated through macrospin simulation. It is shown that the memristor function can be achieved by the injection of current pulses through the stack in the presence of an in-plane static field transverse to the reference layer magnetization, the aim of which is to limit the magnetization rotation between 0° and 180°.

physics.app-ph↗