arXiv subjects

Fèlix Casanova

Publications and source records attributed to Fèlix Casanova.

At least 55 records · Page 3Linked to original sources

arXiv2024

Odd non-linear conductivity under spatial inversion in chiral Tellurium

Electrical transport in non-centrosymmetric materials departs from the well-established phenomenological Ohm's law. Instead of a linear relation between current and electric field, a non-linear conductivity emerges along specific crystallographic directions. This non-linear transport is fundamentally related to the lack of spatial inversion symmetry. However, the experimental implications of an inversion symmetry operation on the non-linear conductivity remain to be explored. Here, we report on a large, non-linear conductivity in chiral Tellurium. By measuring samples with opposite handedness, we demonstrate that the non-linear transport is odd under spatial inversion. Furthermore, by applying an electrostatic gate, we modulate the non-linear output by a factor of 300, reaching the highest reported value excluding engineered heterostructures. Our results establish chiral Te as an ideal compound not just to study the fundamental interplay between crystal structure, symmetry operations and non-linear transport, but also to develop wireless rectifiers and energy-harvesting chiral devices.

cond-mat.mes-hall↗

arXiv2024

Gate-tunable spin Hall effect in an all-light-element heterostructure: graphene with copper oxide

Graphene is a light material for long-distance spin transport due to its low spin-orbit coupling, which at the same time is the main drawback to exhibit a sizeable spin Hall effect. Decoration by light atoms has been predicted to enhance the spin Hall angle in graphene while retaining a long spin diffusion length. Here, we combine a light metal oxide (oxidized Cu) with graphene to induce the spin Hall effect. Its efficiency, given by the product of the spin Hall angle and the spin diffusion length, can be tuned with the Fermi level position, exhibiting a maximum (1.8 $\pm$ 0.6 nm at 100 K) around the charge neutrality point. This all-light-element heterostructure shows a larger efficiency than conventional spin Hall materials. The gate-tunable spin Hall effect is observed up to room temperature. Our experimental demonstration provides an efficient spin-to-charge conversion system free from heavy metals and compatible with large-scale fabrication.

cond-mat.mes-hall↗

arXiv2023

Twist-angle tunable spin texture in WSe$_2$/graphene van der Waals heterostructures

Angle-twisting engineering has emerged as a powerful tool for modulating electronic properties in van der Waals heterostructures. Recent theoretical works have predicted the modulation of spin texture in graphene-based heterostructures by twist angle, although an experimental verification is missing. Here, we demonstrate the tunability of the spin texture and associated spin-charge interconversion with twist angle in WSe$_2$/graphene heterostructures by using spin precession experiments. For specific twist angles, we experimentally detect a spin component radial with the electron's momentum, in addition to the standard orthogonal component. Our results show that the helicity of the spin texture can be reversed by angle twisting, highlighting its critical role on the spin-orbit properties of WSe$_2$/graphene heterostructures and paving the way for the development of novel spin-twistronic devices.

cond-mat.mes-hall↗

arXiv2023

Electrical control of magnetism by electric field and current-induced torques

While early magnetic memory designs relied on magnetization switching by locally generated magnetic fields, key insights in condensed matter physics later suggested the possibility to do it electrically. In the 1990s, Slonczewzki and Berger formulated the concept of current-induced spin torques in magnetic multilayers through which a spin-polarized current may switch the magnetization of a ferromagnet. This discovery drove the development of spin-transfer-torque magnetic random-access memories (STT-MRAMs). More recent research unveiled spin-orbit-torques (SOTs) and will lead to a new generation of devices including SOT-MRAMs. Parallel to these advances, multiferroics and their magnetoelectric coupling experienced a renaissance, leading to novel device concepts for information and communication technology such as the MESO transistor. The story of the electrical control of magnetization is that of a dance between fundamental research (in spintronics, condensed matter physics, and materials science) and technology (MRAMs, MESO, microwave emitters, spin-diodes, skyrmion-based devices, components for neuromorphics, etc). This pas de deux led to major breakthroughs over the last decades (pure spin currents, magnetic skyrmions, spin-charge interconversion, etc). As a result, this field has propelled MRAMs into consumer electronics products but also fueled discoveries in adjacent research areas such as ferroelectrics or magnonics. Here, we cover recent advances in the control of magnetism by electric fields and by current-induced torques. We first review fundamental concepts in these two directions, then discuss their combination, and finally present various families of devices harnessing the electrical control of magnetic properties for various application fields. We conclude by giving perspectives in terms of both emerging fundamental physics concepts and new directions in materials science.

physics.app-ph↗

arXiv2023

Magnon currents excited by the spin Seebeck effect in ferromagnetic EuS thin films

A magnetic insulator is an ideal platform to propagate spin information by exploiting magnon currents. However, until now, most studies have focused on Y$_3$Fe$_5$O$_{12}$ (YIG) and a few other ferri- and antiferromagnetic insulators, but not on pure ferromagnets. In this study, we demonstrate for the first time that magnon currents can propagate in ferromagnetic insulating thin films of EuS. By performing both local and non-local transport measurements in 18-nm-thick films of EuS using Pt electrodes, we detect magnon currents arising from thermal generation by the spin Seebeck effect. By comparing the dependence of the local and non-local signals with the temperature (< 30 K) and magnetic field (< 9 T), we confirm the magnon transport origin of the non-local signal. Finally, we extract the magnon diffusion length in the EuS film (~140 nm), a short value in good correspondence with the large Gilbert damping measured in the same film.

cond-mat.mes-hall↗

arXiv2023

Quantification of spin-charge interconversion in highly resistive sputtered Bi$_x$Se$_{1-x}$ with non-local spin valves

The development of spin-orbitronic devices, such as magneto-electric spin-orbit logic devices, calls for materials with a high resistivity and a high spin-charge interconversion efficiency. One of the most promising candidates in this regard is sputtered Bi$_x$Se$_{1-x}$. Although there are several techniques to quantify spin-charge interconversion, to date reported values for sputtered Bi$_x$Se$_{1-x}$ have often been overestimated due to spurious effects related to local currents combined with a lack of understanding of the effect of the interfaces and the use of approximations for unknown parameters, such as the spin diffusion length. In the present study, non-local spin valves are used to inject pure spin currents into Bi$_x$Se$_{1-x}$, allowing us to directly obtain its spin diffusion length as well as its spin Hall angle, from 10 K up to 300 K. These values, which are more accurate than those previously reported in sputtered Bi$_x$Se$_{1-x}$, evidence that the efficiency of this material is not exceptional. Indeed, the figure of merit for spin-charge interconversion, given by the product of these two parameters, is slightly under 1 nm. Our work demonstrates the importance of considering all material parameters and interfaces when quantifying the spin transport properties of materials with strong spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2023

Out-of-plane spin-to-charge conversion at low temperatures in graphene/MoTe$_2$ heterostructures

Multi-directional spin-to-charge conversion - in which spin polarizations with different orientations can be converted into a charge current in the same direction - has been demonstrated in low-symmetry materials and interfaces. This is possible because, in these systems, spin to charge conversion can occur in unconventional configurations in which spin polarization and charge current where charge current, spin current and polarization do not need to be mutually orthogonal. Here, we explore, in the low temperature regime, the spin-to-charge conversion in heterostructures of graphene with the low-symmetry 1T' phase of MoTe$_2$. First, we observe the emergence of charge conversion for out-of-plane spins at temperatures below 100 K. This unconventional component is allowed by the symmetries of both MoTe$_2$ and graphene and likely arises from spin Hall effect in the spin-orbit proximitized graphene. Moreover, we examine the low-temperature evolution of non-local voltage signals arising from the charge conversion of the two in-plane spin polarizations, which have been previously observed at higher temperature. As a result, we report omni-directional spin-to-charge conversion - for all spin polarization orientations - in graphene/MoTe${_2}$ heterostructures at low temperatures.

cond-mat.mes-hall↗

arXiv2023

Spin-to-charge conversion by spin pumping in sputtered polycrystalline Bi$_x$Se$_{1-x}$

Topological materials are of high interest due to the promise to obtain low power and fast memory devices based on efficient spin-orbit torque switching or spin-orbit magnetic state read-out. In particular, sputtered polycrystalline Bi$_x$Se$_{1-x}$ is one of the materials with more potential for this purpose since it is relatively easy to fabricate and has been reported to have a very high spin Hall angle. We study the spin-to-charge conversion in Bi$_x$Se$_{1-x}$ using the spin pumping technique coming from the ferromagnetic resonance in a contiguous permalloy thin film. We put a special emphasis on the interfacial properties of the system. Our results show that the spin Hall angle of Bi$_x$Se$_{1-x}$ has an opposite sign to the one of Pt. The charge current arising from the spin-to-charge conversion is, in contrast, lower than Pt by more than one order of magnitude. We ascribe this to the interdiffusion of Bi$_x$Se$_{1-x}$ and permalloy and the changes in chemical composition produced by this effect, which is an intrinsic characteristic of the system and is not considered in many other studies.

cond-mat.mtrl-sci↗

arXiv2023

Exchange bias in molecule/Fe3GeTe2 van der Waals heterostructures via spinterface effects

The exfoliation of layered magnetic materials generates atomically thin flakes characterized by an ultrahigh surface sensitivity, which makes their magnetic properties tunable via external stimuli, such as electrostatic gating and proximity effects. Another powerful approach to tailor magnetic materials is molecular functionalization, which leads to hybrid interface states with peculiar magnetic properties, called spinterfaces. However, spinterface effects have not yet been explored on layered magnetic materials. Here, we demonstrate the emergence of spinterface effects at the interface between flakes of the prototypical layered magnetic metal Fe3GeTe2 and thin films of paramagnetic Co-phthalocyanine. Magnetotransport measurements show that the molecular layer induces a magnetic exchange bias in Fe3GeTe2, indicating that the unpaired spins in Co-phthalocyanine develop antiferromagnetic ordering by proximity and pin the magnetization reversal of Fe3GeTe2. The effect is strongest for a Fe3GeTe2 thickness of 20 nm, for which the exchange bias field reaches -840 Oe and is measurable up to approximately 110 K. This value compares very favorably with previous exchange bias fields reported for Fe3GeTe2 in all-inorganic van der Waals heterostructures, demonstrating the potential of molecular functionalization to tailor the magnetism of van der Waals layered materials.

cond-mat.mtrl-sci↗

arXiv2022

Unconventional charge-to-spin conversions in graphene/MoTe2 van der Waals heterostructures

Spin-charge interconversion (SCI) is a central phenomenon to the development of spintronic devices from materials with strong spin-orbit coupling (SOC). In the case of materials with high crystal symmetry, the only allowed SCI processes are those where the spin current, charge current and spin polarization directions are orthogonal to each other. Consequently, standard SCI experiments are designed to maximize the signals arising from the SCI processes with conventional mutually orthogonal geometry. However, in low-symmetry materials, certain non-orthogonal SCI processes are also allowed. Since the standard SCI experiment is limited to charge current flowing only in one direction in the SOC material, certain allowed SCI configurations remain unexplored. In this work, we performed a thorough SCI study in a graphene-based lateral spin valve combined with low-symmetry MoTe$_2$. Due to a very low contact resistance between the two materials, we could detect SCI signals using both a standard configuration, where the charge current is applied along the MoTe$_2$, and a recently introduced (3D-current) configuration, where the charge current flow can be controlled in three directions within the heterostructure. As a result, we observed three different SCI components, one orthogonal and two non-orthogonal, giving new insight into the SCI processes in low-symmetry materials. The large SCI signals obtained at room temperature, along with the versatility of the 3D-current configuration, provide feasibility and flexibility to the design of the next generation of spin-based devices.

cond-mat.mes-hall↗

arXiv2022

Emergence of large spin-charge interconversion at an oxidized Cu/W interface

Spin-orbitronic devices can integrate memory and logic by exploiting spin-charge interconversion (SCI), which is optimized by design and materials selection. In these devices, such as the magnetoelectric spin-orbit (MESO) logic, interfaces are crucial elements as they can prohibit or promote spin flow in a device as well as possess spin-orbit coupling resulting in interfacial SCI. Here, we study the origin of SCI in a Py/Cu/W lateral spin valve and quantify its efficiency. An exhaustive characterization of the interface between Cu and W electrodes uncovers the presence of an oxidized layer (WO$_x$). We determine that the SCI occurs at the Cu/WO$_x$ interface with a temperature-independent interfacial spin-loss conductance of $G_{||} \approx$ 20 $\times$ 10$^{13} Ω^{-1}m^{-2}$ and an interfacial spin-charge conductivity $σ_{SC}=-$1610 $Ω^{-1}cm^{-1}$ at 10 K ($-$830 $Ω^{-1}cm^{-1}$ at 300 K). This corresponds to an efficiency given by the inverse Edelstein length $λ_{IEE}=-$0.76 nm at 10 K ($-$0.4 nm at 300 K), which is remarkably larger than in metal/metal and metal/oxide interfaces and bulk heavy metals. The large SCI efficiency at such an oxidized interface is a promising candidate for the magnetic readout in MESO logic devices.

cond-mat.mes-hall↗

arXiv2022

All-electrical spin-to-charge conversion in sputtered Bi$_x$Se$_{1-x}$

One of the major obstacles to realizing spintronic devices such as MESO logic devices is the small signal magnitude used for magnetization readout, making it important to find materials with high spin-to-charge conversion efficiency. Although intermixing at the junction of two materials is a widely occurring phenomenon, its influence on material characterization and the estimation of spin-to-charge conversion efficiencies is easily neglected or underestimated. Here, we demonstrate all electrical spin-to-charge conversion in Bi$_x$Se$_{1-x}$ nanodevices and show how the conversion efficiency can be overestimated by tens of times depending on the adjacent metal used as a contact. We attribute this to the intermixing-induced compositional change and the properties of a polycrystal that lead to drastic changes in resistivity and spin Hall angle. Strategies to improve the spin-to-charge conversion signal in similar structures for functional devices are discussed.

cond-mat.mes-hall↗

arXiv2022

Charge-to-spin conversion in twisted graphene/WSe$_2$ heterostructures

We investigate the twist angle dependence of spin-orbit coupling (SOC) proximity effects and charge-to-spin conversion (CSC) in graphene/WSe$_2$ heterostructures from first principles. The CSC is shown to strongly depend on the twist angle, with both the spin Hall and standard Rashba-Edelstein efficiencies optimized at or near 30° twisting. Symmetry breaking due to twisting also gives rise to an unconventional Rashba-Edelstein effect, with electrically generated non-equilibrium spin densities possessing spins collinear to the applied electric field. We further discuss how the carrier doping concentration and band broadening control the crossover between the Fermi-sea and -surface spin response, which reconciles the seemingly disparate experimental observations of different CSC phenomena.

cond-mat.mtrl-sci↗

arXiv2022

Omnidirectional spin-to-charge conversion in graphene/NbSe$_2$ van der Waals heterostructures

The conversion of spin currents polarized in different directions into charge currents is a keystone for novel spintronic devices. Van der Waals heterostructures with tailored symmetry are a very appealing platform for such a goal. Here, by performing nonlocal spin precession experiments, we demonstrate the spin-to-charge conversion (SCC) of spins oriented in all three directions (x, y, and z). By analyzing the magnitude and temperature dependence of the signal in different configurations, we argue that the different SCC components measured are likely due to spin-orbit proximity and broken symmetry at the twisted graphene/NbSe$_2$ interface. Such efficient omnidirectional SCC opens the door to the use of new architectures in spintronic devices, from spin-orbit torques that can switch any magnetization to the magnetic state readout of magnetic elements pointing in any direction.

cond-mat.mes-hall↗

arXiv2022

Spin Hall magnetoresistance effect from a disordered interface

The Spin Hall magnetoresistance (SMR) emerged as a reference tool to investigate the magnetic properties of materials with an all-electrical set-up. Its sensitivity to the magnetization of thin films and surfaces may turn it into a valuable technique to characterize Van der Waals magnetic materials, which support long range magnetic order in atomically thin layers. However, realistic surfaces can be affected by defects and disorder, which may result in unexpected artifacts in the SMR, rather than the sole appearance of electrical noise. Here, we study the SMR response of heterostructures combining a platinum (Pt) thin film with the Van der Waals antiferromagnet MnPSe3 and observe a robust SMR-like signal, which turns out to originate from the presence of strong interfacial disorder in the system. We use transmission electron microscopy (TEM) to characterize the interface between MnPSe3 and Pt, revealing the formation of a few-nanometer-thick platinum-chalcogen amorphous layer. The analysis of the transport and TEM measurements suggests that the signal arises from a disordered magnetic system formed at the Pt/MnPSe3 interface, washing out the interaction between the spins of the Pt electrons and the MnPSe3 magnetic lattice. Our results show that damaged interfaces can yield an important contribution to SMR, questioning a widespread assumption on the role of disorder in such measurements.

cond-mat.mes-hall↗

arXiv2022

Gate-tuneable and chirality-dependent charge-to-spin conversion in Tellurium nanowires

Chiral materials are the ideal playground for exploring the relation between symmetry, relativistic effects, and electronic transport. For instance, chiral organic molecules have been intensively studied to electrically generate spin-polarized currents in the last decade, but their poor electronic conductivity limits their potential for applications. Conversely, chiral inorganic materials such as Tellurium are excellent electrical transport materials, but have not been explored to enable the electrical control of spin polarization in devices. Here, we demonstrate the all-electrical generation, manipulation, and detection of spin polarization in chiral single-crystalline Tellurium nanowires. By recording a large (up to 7%) and chirality-dependent unidirectional magnetoresistance, we show that the orientation of the electrically generated spin polarization is determined by the nanowire handedness and uniquely follows the current direction, while its magnitude can be manipulated by an electrostatic gate. Our results pave the way for the development of magnet-free chirality-based spintronic devices.

cond-mat.mes-hall↗

arXiv2021

Room-temperature operation of a molecular spin photovoltaic device on a transparent substrate

Incorporating multifunctionality along with the spin-related phenomenon in a single device is of great interest for the development of next generation spintronic devices. One of these challenges is to couple the photo-response of the device together with its magneto-response to exploit the multifunctional operation at room temperature. Here, the multifunctional operation of a single layer p-type molecular spin valve is presented, where the device shows a photovoltaic effect at the room temperature on a transparent glass substrate. The generated photovoltage is almost three times larger than the applied bias to the device which facilitates the modulation of the magnetic response of the device both with bias and light. It is observed that the photovoltage modulation with light and magnetic field is linear with the light intensity. The device shows an increase in power conversion efficiency under magnetic field, an ability to invert the current with magnetic field and under certain conditions it can act as a spin-photodetector with zero power consumption in the standby mode. The room temperature exploitation of the interplay among light, bias and magnetic field in the single device with a p-type molecule opens a way towards more complex and efficient operation of a complete spin-photovoltaic cell.

physics.app-ph↗

arXiv2021

Reliability of spin-to-charge conversion measurements in graphene-based lateral spin valves

Understanding spin physics in graphene is crucial for developing future two-dimensional spintronic devices. Recent studies show that efficient spin-to-charge conversions via either the inverse spin Hall effect or the inverse Rashba-Edelstein effect can be achieved in graphene by proximity with an adjacent spin-orbit coupling material. Lateral spin valve devices, made up of a graphene Hall bar and ferromagnets, are best suited for such studies. Here, we report that signals mimicking the inverse Rashba-Edelstein effect can be measured in pristine graphene possessing negligible spin-orbit coupling, confirming that these signals are unrelated to spin-to-charge conversion. We identify either the anomalous Hall effect in the ferromagnet or the ordinary Hall effect in graphene induced by stray fields as the possible sources of this artefact. By quantitatively comparing these options with finite-element-method simulations, we conclude the latter better explains our results. Our study deepens the understanding of spin-to-charge conversion measurement schemes in graphene, which should be taken into account when designing future experiments.

cond-mat.mes-hall↗