arXiv subjects

R. C. Sousa

Publications and source records attributed to R. C. Sousa.

10 recordsLinked to original sources

arXiv2026

In-plane magnetic field control of anomalous Hall response enabled by magnetic anisotropy engineering

Engineering magnetic anisotropy provides a powerful route to control magnetization orientation and unlock emerging functionalities in opto-spintronic and current-driven devices. Beyond its role in magnetization reversal, the effective anisotropy can strongly influence the magnetotransport response, offering an additional degree of freedom to tune new device functionalities. In this work, we report a magnetotransport study of a ferrimagnetic [Tb/Co]$_{\times 5}$ multilayer grown with a Tb thickness gradient, whose wedge-shaped tilts the uniaxial anisotropy axis slightly away from the film normal. Anomalous Hall resistivity measurements from 80 K to 300 K reveal a spin reorientation transition, while the angular dependence of the magnetotransport responses exposes the crucial role of the tilted anisotropy. A simplified macrospin model reproduces the full angular response across the transition and shows that the observed anomalous Hall effect when the in-plane magnetic field is applied originates from the tilt of the uniaxial anisotropy axis, which supplies a built-in symmetry-breaking mechanism, enabling in-plane field control over the out-of-plane anomalous Hall response, sign included. These findings establish tilted magnetic anisotropy as a promising route toward Hall effect-based sensor applications and highlight Tb/Co multilayers as a versatile platform for anisotropy-engineered spintronic devices.

cond-mat.mes-hall↗

arXiv2026

He-CF4-CH4 ternary mixtures as target gas for the CYGNO directional dark matter experiment

The CYGNO collaboration is advancing a high-resolution optical Time Projection Chamber (TPC) for directional dark matter searches and solar neutrino spectroscopy at LNGS. The detector uses a He-40%CF4 gas mixture at atmospheric pressure and a triple-GEM cascade for ionization signal amplification. Scintillation light from GEM electron avalanches is read out using sCMOS cameras, enabling high sensitivity to interactions in the few keV range, alongside precise ionization event tracking and particle identification. This study investigates the effects of adding 3-10% methane to the He-40%CF4 mixture. Methane improves the electrical stability of the TPC, allowing for higher GEM voltages before discharge onset, which compensates for its scintillation quenching and leads to enhanced overall scintillation yield. Compared to prior studies performed with isobutane, methane demonstrates a lower quenching effect on visible and UV photons while maintaining good energy resolution. Importantly, the inclusion of methane lowers the dark matter detection threshold by providing a lighter target and extending track lengths of light nuclear recoils, thus enhancing directional discrimination. These results establish methane as a promising additive for optimizing CYGNO's performance in detecting low-mass dark matter candidates.

physics.ins-det↗

arXiv2026

Perpendicularly magnetized Tb/Co multilayers featuring tilted uniaxial anisotropy: Experiments and modeling

Rare earth/transition metal (RE/TM) multilayers with perpendicular magnetic anisotropy are key ingredients for the development of spintronic applications. Their compensation temperature depends on the ratio of the thicknesses of rare earth and transition metal, allowing their magnetic properties to be tuned with temperature while maintaining their anisotropy even in nanometer-scale devices. In this work, we performed a thorough structural characterization and systematically investigate the magnetic properties of a whole family of ferrimagnetic [Tb/Co]$_{\times 5}$ multilayers varying the Tb thickness in the range of 0.4 nm - 1.25 nm. A linear dependence of the compensation temperature on the Tb layer thickness was observed. Moreover, a uniaxial anisotropy constant of 330$\pm$30 kJ/m$^3$, which is close to the values reported by other authors, was estimated. Additionally, we proposed a model to gain a better understanding of the angular dependence of the magnetization loops and the linear dependence of the compensation temperature. We present strong evidence demonstrating that the perpendicular anisotropy must be tilted away from the perpendicular axis in order to explain the observed features, particularly the hysteresis in the in-plane loops. Our work advances the understanding of DC magnetic properties in thin RE/TM ferrimagnetic films, which has the potential to impact different fields where these materials are involved.

cond-mat.mtrl-sci↗

arXiv2023

Field-free all-optical switching and electrical read-out of Tb/Co-based magnetic tunnel junctions

Switching of magnetic tunnel junction using femto-second laser enables a possible path for THz frequency memory operation, which means writing speeds 2 orders of magnitude faster than alternative electrical approaches based on spin transfer or spin orbit torque. In this work we demonstrate successful field-free 50fs single laser pulse driven magnetization reversal of [Tb/Co] based storage layer in a perpendicular magnetic tunnel junction. The nanofabricated magnetic tunnel junction devices have an optimized bottom reference electrode and show Tunnel Magnetoresistance Ratio values (TMR) up to 74\% after patterning down to sub-100nm lateral dimensions. Experiments on continuous films reveal peculiar reversal patterns of concentric rings with opposite magnetic directions, above certain threshold fluence. These rings have been correlated to patterned device switching probability as a function of the applied laser fluence. Moreover, the magnetization reversal is independent on the duration of the laser pulse. According to our macrospin model, the underlying magnetization reversal mechanism can be attributed to an in-plane reorientation of the magnetization due to a fast reduction of the out-of-plane uniaxial anisotropy. These aspects are of great interest both for the physical understanding of the switching phenomenon and their consequences for all-optical-switching memory devices, since they allow for a large fluence operation window with high resilience to pulse length variability.

physics.app-ph↗

arXiv2022

In plane reorientation induced single laser pulse magnetization reversal in rare-earth based multilayer

Single Pulse All Optical Helicity Independent Switching (AO-HIS) represents the ability to reverse the magnetic moment of a nanostructure using a femtosecond single laser pulse. It is an ultrafast method to manipulate magnetization without the use of any applied field. Since the first switching experiments carried on GdFeCo ferrimagnetic systems, single pulse AO-HIS has been restricted for a while to Gd-based alloys or Gd/FM bilayers where FM is a ferromagnetic layer. Only recently has AO-HIS been extended to a few other materials: MnRuGa ferrimagnetic Heusler alloys and Tb/Co multilayers with a very specific range of thickness and composition. Here, we demonstrate that single pulse AO-HIS observed in Tb/Co results from a different mechanism than the one for Gd based samples and that it can be obtained for a large range of rare earth-transition metal (RE-TM) multilayers, making this phenomenon much more general. Surprisingly, in this large family of (RE-TM) multilayer systems, the threshold fluence for switching is observed to be independent of the pulse duration, up to at least 12 ps. Moreover, at high laser intensities, concentric ring domain structures are induced, unveiling multiple fluence thresholds. These striking switching features, which are in contrast to those of AO-HIS in GdFeCo alloys, concomitant with the demonstration of an in-plane reorientation of the magnetization, point towards an intrinsic precessional reversal mechanism. Our results allow expanding the variety of materials with tunable magnetic properties that can be integrated in complex heterostructures and provide a pathway to engineer materials for future applications based on all-optical control of magnetic order.

cond-mat.mtrl-sci↗

arXiv2021

Magnetization reversal driven by spin-transfer-torque in perpendicular shape anisotropy magnetic tunnel junctions

The concept of perpendicular shape anisotropy spin-transfer torque magnetic random-access memory (PSA-STT-MRAM) consists in increasing the storage layer thickness to values comparable to the cell diameter, to induce a perpendicular shape anisotropy in the magnetic storage layer. Making use of that contribution, the downsize scalability of the STT-MRAM may be extended towards sub-20 nm technological nodes, thanks to a reinforcement of the thermal stability factor $Δ$. Although the larger storage layer thickness improves $Δ$, it is expected to negatively impact the writing current and switching time. Hence, optimization of the cell dimensions (diameter, thickness) is of utmost importance for attaining a sufficiently high $Δ$ while keeping a moderate writing current. Micromagnetic simulations were carried out for different pillar thicknesses of fixed lateral size 20 nm. The switching time and the reversal mechanism were analysed as a function of the applied voltage and aspect-ratio (AR) of the storage layer. For AR $<$ 1, the magnetization reversal resembles a macrospin-like mechanism, while for AR $>$ 1 a non-coherent reversal is observed, characterized by the nucleation of a transverse domain wall at the ferromagnet/insulator interface which then propagates along the vertical axis of the pillar. It was further observed that the inverse of the switching time is linearly dependent on the applied voltage. This study was extended to sub-20 nm width with a value of $Δ$ around 80. It was observed that the voltage necessary to reverse the magnetic layer increases as the lateral size is reduced, accompanied with a transition from macrospin-reversal to a buckling-like reversal at high aspect-ratios.

cond-mat.mes-hall↗

arXiv2018

Highly thermally stable sub-20nm magnetic random-access memory based on perpendicular shape anisotropy

A new approach to increase the downsize scalability of perpendicular STT-MRAM is presented. It consists in significantly increasing the thickness of the storage layer in out-of-plane magnetized tunnel junctions (pMTJ) as compared to conventional pMTJ in order to induce a perpendicular shape anisotropy (PSA) in this layer. This PSA is obtained by depositing a thick ferromagnetic (FM) layer on top of an MgO/FeCoB based magnetic tunnel junction (MTJ) so that the thickness of the storage layer becomes of the order or larger than the diameter of the MTJ pillar. In contrast to conventional spin transfer torque magnetic random access memory (STT-MRAM) wherein the demagnetizing energy opposes the interfacial perpendicular magnetic anisotropy (iPMA), in these novel memory cells, both PSA and iPMA contributions favor out-of-plane orientation of the storage layer magnetization. Using thicker storage layers in these PSA-STT-MRAM has several advantages. Thanks to the PSA, very high and easily tunable thermal stability factors can be achieved, even down to sub-10 nm diameters. Moreover, low damping material can be used for the thick FM material thus leading to a reduction of the write current. The paper describes this new PSA-STT-MRAM concept, practical realization of such memory arrays, magnetic characterization demonstrating thermal stability factor above 200 for MTJs as small as 8nm in diameter and possibility to maintain thermal stability factor above 60 down to 4nm diameter.

cond-mat.mtrl-sci↗

arXiv2014

Modulating spin transfer torque switching dynamics with two orthogonal spin-polarizers by varying the cell aspect ratio

We study in-plane magnetic tunnel junctions with additional perpendicular polarizer for subnanosecond-current-induced switching memories. The spin-transfer-torque switching dynamics was studied as a function of the cell aspect ratio both experimentally and by numerical simulations using the macrospin model. We show that the anisotropy field plays a significant role in the dynamics, along with the relative amplitude of the two spin-torque contributions. This was confirmed by micromagnetic simulations. Real-time measurements of the reversal were performed with samples of low and high aspect ratio. For low aspect ratios, a precessional motion of the magnetization was observed and the effect of temperature on the precession coherence was studied. For high aspect ratios, we observed magnetization reversals in less than 1 ns for high enough current densities, the final state being controlled by the current direction in the magnetic tunnel junction cell.

cond-mat.mes-hall↗

arXiv2002

Quasi ballistic magnetization reversal

We demonstrate a quasi ballistic switching of the magnetization in a microscopic mag-neto resistive memory cell. By means of time resolved magneto transport we follow the large angle precession of the free layer magnetization of a spin valve cell upon applica-tion of transverse magnetic field pulses. Stopping the field pulse after a 180 degree precession rotation leads to magnetization reversal with reversal times as short as 165 ps. This switching mode represents the fundamental ultra fast limit of field induced magnetization reversal.

cond-mat.mes-hall↗

arXiv2002

Phase Coherent Precessional Magnetization Reversal in Micro-scopic Spin Valve Elements

We study the precessional switching of the magnetization in microscopic spin valve cells induced by ultra short in-plane hard axis magnetic field pulses. Stable and highly efficient switching is monitored following pulses as short as 140 ps with energies down to 15 pJ. Multiple application of identical pulses reversibly toggles the cell's magnetization be-tween the two easy directions. Variations of pulse duration and amplitude reveal alter-nating regimes of switching and non-switching corresponding to transitions from in-phase to out-of-phase excitations of the magnetic precession by the field pulse. In the low field limit damping becomes predominant and a relaxational reversal is found allowing switching by hard axis fields below the in-plane anisotropy field threshold.

cond-mat↗