arXiv subjects

A. Azevedo

Publications and source records attributed to A. Azevedo.

At least 19 recordsLinked to original sources

arXiv2026

Interfacial Spin-to-Charge Conversion in Sputtered MoTe2 Heterostructures Probed by Spin Pumping and Spin-Torque Ferromagnetic Resonance

Transition metal dichalcogenides (TMDs) and their Weyl semimetal phases, such as MoTe$_2$, have attracted significant attention for spin-orbit torque applications due to their efficient charge-to-spin conversion. However, whether this conversion originates predominantly from the bulk or the interface remains unclear. Here, we investigate spin-charge interconversion in MoTe$_2$ using spin-pumping and spin-torque ferromagnetic resonance (SP-FMR and ST-FMR). Thickness-dependent measurements reveal large spin-to-charge conversion and spin-torque efficiencies that are essentially independent of MoTe$_2$ thickness, indicating that the conversion is predominantly governed by the Rashba-Edelstein effect at the Py/MoTe$_2$ interface rather than by bulk spin transport. This behavior contrasts with the characteristic thickness dependence observed in Pt heterostructures and is further supported by bidirectional SP-FMR and interface-separation measurements. Our results highlight the dominant role of the Py/MoTe$_2$ interface in enabling efficient spin-charge conversion and spin-orbit torques in TMD-based spintronic devices. These findings highlight the potential of sputtered MoTe$_2$/Py heterostructures for low-power spintronic applications, including magnetic memory and logic devices.

cond-mat.mes-hall↗

arXiv2026

Phase-selective orbital-charge conversion in $\mathrm{MoTe_2}$

Two-dimensional transition metal dichalcogenides (TMDs) have emerged as promising materials for spin--orbitronics owing to their strong spin--orbit coupling and rich electronic phases. However, their orbital transport properties remain largely unexplored. Here, we demonstrate that the orbitronic response of $\mathrm{MoTe_2}$ is governed by a thickness-driven structural phase transition. RF-sputtered $\mathrm{MoTe_2}$ thin films exhibit a crossover at a critical thickness of approximately $4.5\,\mathrm{nm}$, stabilizing in the metallic $1T^\prime$ phase below this threshold and in the semiconducting $2H$ phase above it. Raman spectroscopy and scanning tunneling spectroscopy (STS) confirm the structural and electronic transition, revealing gapless behavior in ultrathin films and a finite band gap in thicker samples. Spin-pumping measurements detect an additional transverse charge-conversion signal exclusively in metallic $1T^\prime$-$\mathrm{MoTe_2}$, in agreement with first-principles calculations that identify a dominant orbital Rashba--Edelstein response as the underlying conversion mechanism.

cond-mat.mtrl-sci↗

arXiv2026

Investigating spin and orbital effects via spin-torque ferromagnetic resonance

In this work, we experimentally investigate spin and orbital torque phenomena using the spin-torque ferromagnetic resonance (ST-FMR) technique in a series of bilayer systems composed of different normal metal (NM) materials. Permalloy (Py) and Ni were employed as ferromagnetic (FM) layers to probe the spin and orbital torque responses, respectively. For the SiO$_2$/FM/NM bilayers, we extracted the damping-like and field-like torque components, as well as the damping-like torque efficiency for each sample, and compared our results with previously reported numerical and experimental data in the literature. Additionally, we experimentally demonstrate the presence of an out-of-plane torque component, which we attribute to interfacial mechanisms and associate with a spin-orbital polarized current along the $z$-direction. This interpretation is supported by the azimuthal angular dependence of the applied magnetic field. Our results provide compelling evidence of orbital torque associated with the orbital Hall effect (OHE) in several materials, thereby broadening the prospects for magnetization switching driven by orbital torque.

cond-mat.mes-hall↗

arXiv2025

Probing orbital currents through inverse orbital Hall and Rashba effects

We report a comprehensive experimental investigation of orbital-to-charge conversion in metallic and semiconductor materials, emphasizing the fundamental roles of the inverse orbital Hall effect (IOHE) and the inverse orbital Rashba effect. Using spin pumping driven by ferromagnetic resonance (SP-FMR) and the spin Seebeck effect (SSE), we demonstrate efficient orbital current generation and detection in YIG/Pt/NM structures, where NM is either a metal or a semiconductor. A central finding is the dominance of orbital contributions over spin-related effects, even in systems with weak spin-orbit coupling. In particular, a large enhancement of the SP-FMR and SSE signals is observed in the presence of naturally oxidized Cu in different heterostructures. Furthermore, we identify positive and negative IOHE signals in Ti and Ge, respectively, and extract orbital diffusion lengths in both systems using a diffusive model. Our results confirm the presence of orbital transport and offer valuable insights that may guide the further development of orbitronics.

cond-mat.mes-hall↗

arXiv2025

Anisotropy-Driven Anomalous Inverse Orbital Hall Effect in Fe Films

This study investigates the anomalous orbital effects in iron (Fe) films with strong uniaxial anisotropy, highlighting the interactions between spin and orbital currents. Using heterogeneous YIG/Fe and YIG/Pt/Fe structures, fabricated by oblique deposition in a magnetic field, spin pumping ferromagnetic resonance (SP-FMR) measurements were performed. It was observed that the uniaxial anisotropy enables the emergence of spin-to-charge (AISHE) and orbital-to-charge (AIOHE) conversion signals in out-of-plane configurations, where the spin polarization is parallel to the direction of the spin current. Experimental analysis revealed that orbital dynamics, mediated by orbital Hall conductivity, are more prominent in Fe films due to the low spin-orbit interaction (SOC) and high orbital response. These findings provide fundamental insights for the advancement of orbitronics devices, indicating the potential for controlling orbital and spin currents through magnetic and anisotropic parameters.

cond-mat.mes-hall↗

arXiv2025

Revealing the Dominance of the Orbital Hall Effect over Spin in Transition Metal Heterostructures

We study inverse spin and orbital Hall effects in 19 transition metals using spin-pumping driven by ferromagnetic resonance. Spin-to-charge conversion was measured in YIG/X(5), while orbital-to-charge conversion was probed in YIG/Pt(2)/X(5) heterostructures. Here, X represents the different transition metals. Surprisingly, the orbital contribution overwhelmingly dominates over the spin response, clarifying the challenge of disentangling these effects. Our results largely agree with first-principles predictions for spin and orbital Hall conductivities but reveal discrepancies in select materials. These findings emphasize the fundamental role of the orbital Hall effect, and position orbitronics as a pivotal frontier in condensed matter physics.

cond-mat.mes-hall↗

arXiv2024

Anomalous Spin and Orbital Hall Phenomena in Antiferromagnetic Systems

We investigate anomalous spin and orbital Hall phenomena in antiferromagnetic (AF) materials via orbital pumping experiments. Conducting spin and orbital pumping experiments on YIG/Pt/Ir20Mn80 heterostructures, we unexpectedly observe strong spin and orbital anomalous signals in an out-of-plane configuration. We report a sevenfold increase in the signal of the anomalous inverse orbital Hall effect (AIOHE) compared to conventional effects. Our study suggests expanding the Orbital Hall angle (θ_OH) to a rank 3 tensor, akin to the Spin Hall angle (θ_SH), to explain AIOHE. This work pioneers converting spin-orbital currents into charge current, advancing the spin-orbitronics domain in AF materials.

cond-mat.mes-hall↗

arXiv2024

Probing the Spin-Momentum Locking in Rashba Surfaces via Spin Current

In this work, we explore the intriguing spin-momentum locking phenomenon within the Rashba states of antimony (Sb) films. By combining spin pumping with the flow of an external charge current, we reveal the topological properties of surface states in Sb films. Taking advantage of the well-defined spin polarization of both spin-momentum-locked charge currents and spin-pumped currents, we demonstrate precise manipulation over the direction and magnitude of the resulting charging current, generated through the inverse Rashba-Edelstein effect. This fascinating phenomenon is attributed to the dynamic interaction between the accumulation of out-of-equilibrium pumped spins and the flowing spins, intrinsically locked perpendicular to the direction of the charge current. The results show that Sb as a promising material for basic and applied investigation of spintronics phenomena. We believe that the nanostructures investigated here open the way for the development of low-power logic gates operating in the range of a few tens of microamperes.

cond-mat.mes-hall↗

arXiv2024

Negative orbital Hall effect in Germanium

Our investigation reveals a groundbreaking discovery of a negative inverse orbital Hall effect (IOHE) in Ge thin films. We employed the innovative orbital pumping technique where spin-orbital coupled current is injected into Ge films using YIG/Pt(2)/Ge($t_{Ge}$) and YIG/W(2)/Ge($t_{Ge}$) heterostructures. Through comprehensive analysis, we observe significant reductions in the signals generated by coherent (RF-driven) and incoherent (thermal-driven) spin-orbital pumping techniques. These reductions are attributed to the presence of a remarkable strong negative IOHE in Ge, showing its magnitude comparable to the spin-to-charge signal in Pt. Our findings reveal that although the spin-to-charge conversion in Ge is negligible, the orbital-to-charge conversion exhibits large magnitude. Our results are innovative and pioneering in the investigation of negative IOHE by the injection of spin-orbital currents.

cond-mat.mes-hall↗

arXiv2024

Exploring orbital-charge conversion mediated by interfaces with copper through spin-orbital pumping

We investigated how different materials affect the orbital-charge conversion in heterostructures with the naturally oxidized cooper capping layer. When we added a thin layer of $CuOx(3nm)$ onto yttrium iron garnet $(YIG)/W$ stacks, we observed a significant reduction in the charge current signal measured by means the spin pumping effect technique. This finding contrasts with the results of a prior study conducted on YIG/Pt/CuOx, which reported the opposite effect. On the other hand, when we added the same $CuOx(3nm)$ layer to $YIG/Ti(4nm)$ structures, there was not much change in the spin pumping signal. This occurred because Ti does not generate much orbital current at the $Ti/CuOx$ interface, unlike Pt, due to its weaker spin-orbit coupling. Interestingly, when we added the $CuOx(3nm)$ layer to $SiO_{2}/Py(5nm)/Pt(4nm)$ structures, the spin pumping signal increased. However, in $SiO_{2}/CuOx(3nm)/Pt(4nm)/Py(5nm)$ structures, the signal decreased. Finally, we delve into a theoretical analysis of the spin (orbital) Hall effect in YIG/Heavy-metal systems. These findings have the potential to advance research in the innovative field of orbitronics and contribute to the development of new technologies based on spin-orbital conversion.

cond-mat.mes-hall↗

arXiv2023

Experimental verification of the inverse Anomalous spin Hall effect with perpendicular magnetic anisotropy Materials

In this work, the spin pumping technique was employed to investigate the anomalous inverse spin Hall effect in BIG/NiO/Fe samples where BIG[(Bi,Tm)3(Fe,Ga)5O12] exhibits perpendicular magnetic anisotropy. Our results reveal an intriguing phenomenon: when the magnetizations of both ferromagnetic layers align perpendicularly, a distinct spin-to-charge current conversion mechanism occurs. This conversion is intricately linked to the magnetization of the converting layer, spin polarization, and the spin current orientation.

cond-mat.mes-hall↗

arXiv2023

Surface-state mediated spin-to-charge conversion in Sb films via bilateral spin current injection

The spin-to-charge conversion phenomena is investigated in a trilayer structure consisting of Co(12 nm)/Sb(t)/Py(12 nm), where the thickness t of the antimony layer is varied. Using the spin-pumping technique, a pure spin current is injected from both FM layers into the middle layer, the DC voltage is then measured. We observe a spin-to-charge mechanism in the Sb layer that exhibits striking similarities to the inverse Rashba-Edelstein effect (IREE), driven by surface states.

cond-mat.mtrl-sci↗

arXiv2022

Inverse Orbital Torque via Spin-Orbital Entangled States

While current-induced torque by orbital current has been experimentally found in various structures, evidence for its reciprocity has been missing so far. Here, we report experimental evidence of strong inverse orbital torque in YIG/Pt/CuOx (YIG = Y3Fe5O12) mediated by spin-orbital entangled electronic states in Pt. By injecting spin current from YIG to Pt by the spin pumping via ferromagnetic resonance and by the spin Seebeck effect, we find a pronounced inverse spin Hall effect-like signal. While a part of the signal is explained as due to the inverse spin-orbital Hall effect in Pt, we also find substantial increase of the signal in YIG/Pt/CuOx structures compared to the signal in YIG/Pt. We attribute this to the inverse orbital Rashba-Edelstein effect at Pt/CuOx interface mediated by the spin-orbital entangled states in Pt. Our work paves the way toward understanding of spin-orbital entangled physics in nonequilibrium and provides a way for electrical detection of the orbital current in orbitronic device applications.

cond-mat.mes-hall↗

arXiv2022

Efficient spin-to-charge interconversion in Weyl semimetal TaP at room temperature

In this paper we present spin-to-charge current conversion properties in the Weyl semimetal TaP by means of the inverse Rashba-Edelstein effect (IREE) with the integration of this quantum material with the ferromagnetic metal Permalloy $(Py=Ni_{81}Fe_{19})$. The spin currents are generated in the Py layer by the spin pumping effect (SPE) from microwave-driven ferromagnetic resonance and are detected by a dc voltage along the TaP crystal, at room temperature. We observe a field-symmetric voltage signal without the contamination of asymmetrical lines due to spin rectification effects observed in studies using metallic ferromagnets. The observed voltage is attributed to spin-to-charge current conversion based on the IREE, made possible by the spin-orbit coupling induced intrinsically by the bulk band structure of Weyl semimetals. The measured IREE coefficient $λ_{IREE}=(0.30 \pm{0.01})$ nm is two orders of magnitude larger than in graphene and is comparable to or larger than the values reported for some metallic interfaces and for several topological insulators.

cond-mat.mes-hall↗

arXiv2022

On a class of nonlocal problems with fractional gradient constraint

We consider a Hilbertian and a charges approach to fractional gradient constraint problems of the type $|D^σu|\leq g$, involving the distributional fractional Riesz gradient $D^σ$, $0<σ<1$, extending previous results on the existence of solutions and Lagrange multipliers of these nonlocal problems. We also prove their convergence as $σ\nearrow1$ towards their local counterparts with the gradient constraint $|D u|\leq g$.

math.AP↗

arXiv2020

Magnon-mediated spin currents in Tm3Fe5O12/Pt with perpendicular magnetic anisotropy

The control of pure spin currents carried by magnons in magnetic insulator (MI) garnet films with a robust perpendicular magnetic anisotropy (PMA) is of great interest to spintronic technology as they can be used to carry, transport and process information. Garnet films with PMA present labyrinth domain magnetic structures that enrich the magnetization dynamics, and could be employed in more efficient wave-based logic and memory computing devices. In MI/NM bilayers, where NM being a normal metal providing a strong spin-orbit coupling, the PMA benefits the spin-orbit torque (SOT) driven magnetization's switching by lowering the needed current and rendering the process faster, crucial for developing magnetic random-access memories (SOT-MRAM). In this work, we investigated the magnetic anisotropies in thulium iron garnet (TIG) films with PMA via ferromagnetic resonance measurements, followed by the excitation and detection of magnon-mediated pure spin currents in TIG/Pt driven by microwaves and heat currents. TIG films presented a Gilbert damping constant α~0.01, with resonance fields above 3.5 kOe and half linewidths broader than 60 Oe, at 300 K and 9.5 GHz. The spin-to-charge current conversion through TIG/Pt was observed as a micro-voltage generated at the edges of the Pt film. The obtained spin Seebeck coefficient was 0.54 μV/K, confirming also the high interfacial spin transparency.

physics.app-ph↗

arXiv2020

Unveiling the polarity of the spin-to-charge current conversion in $Bi_2Se_3$

We report an investigation of the spin- to charge-current conversion in sputter-deposited films of topological insulator $Bi_2Se_{3}$ onto single crystalline layers of YIG $(Y_{3}Fe_{5}O_{12})$ and polycrystalline films of Permalloy $(Py = Ni_{81}Fe_{19})$. Pure spin current was injected into the $Bi_{2}Se_{3}$ layer by means of the spin pumping process in which the spin precession is obtained by exciting the ferromagnetic resonance of the ferromagnetic film. The spin-current to charge-current conversion, occurring at the $Bi_{2}Se_{3}/$ferromagnet interface, was attribute to the inverse Rashba-Edelstein effect (IREE). By analyzing the data as a function of the $Bi_{2}Se_{3}$ thickness we calculated the IREE length used to characterize the efficiency of the conversion process and found that 1.2 pm $\leq|λ_{IREE}|\leq$ 2.2 pm. These results support the fact that the surface states of $Bi_{2}Se_{3}$ have a dominant role in the spin-charge conversion process, and the mechanism based on the spin diffusion process plays a secondary role. We also discovered that the spin- to charge-current mechanism in $Bi_{2}Se_{3}$ has the same polarity as the one in Ta, which is the opposite to the one in Pt. The combination of the magnetic properties of YIG and Py, with strong spin-orbit coupling and dissipationless surface states topologically protected of $Bi_{2}Se_{3}$ might lead to spintronic devices with fast and efficient spin-charge conversion.

cond-mat.mes-hall↗

arXiv2018

Direct detection of induced magnetic moment and efficient spin-to-charge conversion in graphene/ferromagnetic structures

This article shows that the spin-to-charge current conversion in single-layer graphene (SLG) by means of the inverse Rashba-Edelstein effect (IREE) is made possible with the integration of this remarkable 2D-material with the unique ferrimagnetic insulator yttrium iron garnet (YIG = $Y_{3}Fe_{5}O_{12}$) as well as with the ferromagnetic metal permalloy (Py = $Ni_{81}Sb_{19}$). By means of X-ray absorption spectroscopy (XAS) and magnetic circular dichroism (XMCD) techniques, we show that the carbon atoms of the SLG acquires an induced magnetic moment due to the proximity effect with the magnetic layer. The spin currents are generated in the magnetic layer by spin pumping from microwave driven ferromagnetic resonance and are detected by a dc voltage along the graphene layer, at room temperature. The spin-to-charge current conversion, occurring at the graphene layer, is explained by the extrinsic spin-orbit interaction (SOI) induced by the proximity effect with the ferromagnetic layer. The results obtained for the SLG/YIG and SLG/Py systems confirm very similar values for the IREE parameter, which are larger than the values reported in previous studies for SLG. We also report systematic investigations of the electronic and magnetic properties of the SLG/YIG by means of scanning tunneling microscopy (STM).

cond-mat.mes-hall↗