arXiv subjects

Amilcar Bedoya-Pinto

Publications and source records attributed to Amilcar Bedoya-Pinto.

At least 19 recordsLinked to original sources

arXiv2026

Atomic-Scale Detection of Néel Vector Switching in the Single-Layer A-type Antiferromagnet Cr2S3-2D

The detection of Néel vector switching in a single-layer A-type antiferromagnet marks an important step toward functional two-dimensional spintronics. Here, Cr$_2$S$_3$-2D, grown on graphene on Ir(110), is established as a first single-layer A-type antiferromagnet. Spin-polarized scanning tunneling microscopy reveals hysteresis loops with a large switching field and a pronounced dependence on island size. X-ray magnetic circular dichroism at the Cr L$_{2,3}$ edges exhibits a tiny signal with a linear magnetic field dependence, consistent with a nearly compensated antiferromagnetic ground state and a Néel temperature of about 160 K. Quantitative analysis of the island-size dependence of the switching field, together with first principles calculations, indicates a slight imbalance between the magnetic moments of the two Cr planes of Cr$_2$S$_3$-2D when supported on a substrate. This imbalance results in a net magnetization for the A-type antiferromagnet, which enables the 180$^\circ$ rotation of the Néel vector. Moreover, Cr$_2$S$_3$-2D retains its magnetic properties after several days of exposure to air.

cond-mat.mes-hall↗

arXiv2026

A molecular-spin photovoltaic device

We fabricated a C60-based molecular spin photovoltaic device that integrated a photovoltaic response with the spin transport across the molecular layer. The photovoltaic response can be modified under the application of a small magnetic field, with a magnetophotovoltage of up to 5% at room temperature. Device functionalities include a magnetic current inverter and the presence of diverging magnetocurrent at certain illumination levels that could be useful for sensing. Completely spin-polarized currents could be created by balancing the external partially spin polarized injection with the photogenerated carriers.

cond-mat.mtrl-sci↗

arXiv2025

Terahertz third-harmonic generation of lightwave driven Weyl fermions far from equilibrium

We report on time-resolved ultrafast terahertz third-harmonic generation spectroscopy of nonequilibrium dynamics of Weyl fermions in a nanometer thin film of the Weyl semimetal TaP. Terahertz third-harmonic generation is observed at room temperature under the drive of a multicycle narrowband terahertz pulse with a peak field strength of down to tens of kV/cm. The observed terahertz third-harmonic generation exhibits a perturbative cubic power-law dependence on the terahertz drive. By varying the polarization of the drive pulse from linear to elliptical, we realize a sensitive tuning of the third harmonic yield. By carrying out theoretical analysis based on the Boltzmann transport theory, we can properly describe the experimental results and ascribe the observed THz nonlinearity to field-driven kinetics of the Weyl fermions.

cond-mat.str-el↗

arXiv2025

Broad-band THz emission by Spin-to-Charge Conversion in Topological Material -- Ferromagnet Heterostructures

Terahertz spintronic devices combine ultrafast operation with low power consumption, making them strong candidates for next-generation memory technologies. In this study, we use time-domain terahertz emission spectroscopy to investigate spin-to-charge conversion (SCC) in bilayer heterostructures comprising topological insulators (TIs) or Weyl semimetals (WSMs) with ferromagnetic metals (FMs). SCC is studied in TI materials \ce{Bi2Se3}, Pb-doped \ce{Bi2Se3}, and (Bi$_{1-x}$Sb$_x$)$_2$Te$_3$, and the WSM NbP. Our results reveal that the dependence of SCC on TI thickness varies with interface quality, indicating that thickness dependence alone is not a reliable criterion for distinguishing between inverse spin Hall effect and the inverse Rashba--Edelstein effect mechanisms. We find efficient SCC in TIs depends on both \textit{in-situ} growth to prevent surface oxidation and proper composition. In NbP$\vert$FM bilayers, we observe THz emission with efficiency and bandwidth comparable to that of TIs, highlighting the broader potential of topological materials. Finally, broadband spectral measurements demonstrate that both TIs and WSMs can generate THz pulses with frequencies extending up to 8\,THz. These findings underscore the promise of topological materials as efficient platforms for ultrafast, broadband spintronic applications.

cond-mat.mtrl-sci↗

arXiv2024

Population inversion and ultrafast terahertz nonlinearity of transient Dirac fermions in Cd$_3$As$_2$

Harmonic generation provides an efficient tool for the study of ultrafast nonlinear dynamics. We report on time-resolved optical-pump terahertz-harmonic-generation spectroscopic investigation of ultrafast nonlinearity in a prototypical three-dimensional Dirac semimetal Cd$_3$As$_2$. A transient population inversion characterized by excessive nonthermal Dirac electrons and holes is found to be very sensitive and responsive to a periodic terahertz drive, leading to very efficient terahertz third-harmonic generation. Based on the Boltzmann transport theory, we analyze the terahertz field-driven kinetics of the transient Dirac fermions that is responsible for the observed strong terahertz nonlinearity.

cond-mat.mtrl-sci↗

arXiv2022

Terahertz spin-to-charge current conversion in stacks of ferromagnets and the transition-metal dichalcogenide NbSe$_2$

Transition-metal dichalcogenides (TMDCs) are an aspiring class of materials with unique electronic and optical properties and potential applications in spin-based electronics. Here, we use terahertz emission spectroscopy to study spin-to-charge current conversion (S2C) in the TMDC NbSe$_2$ in ultra-high-vacuum-grown F|NbSe$_2$ thin-film stacks, where F is a layer of ferromagnetic Fe or Ni. Ultrafast laser excitation triggers an ultrafast spin current that is converted into an in-plane charge current and, thus, a measurable THz electromagnetic pulse. The THz signal amplitude as a function of the NbSe$_2$ thickness shows that the measured signals are fully consistent with an ultrafast optically driven injection of an in-plane-polarized spin current into NbSe$_2$. Modeling of the spin-current dynamics reveals that a sizable fraction of the total S2C originates from the bulk of NbSe$_2$ with the same, negative, sign as the spin Hall angle of pure Nb. By quantitative comparison of the emitted THz radiation from F|NbSe$_2$ to F|Pt reference samples and the results of ab-initio calculations, we estimate that the spin Hall angle of NbSe$_2$ for an in-plane polarized spin current lies between -0.2% and -1.1%, while the THz spin-current relaxation length is of the order of a few nanometers.

cond-mat.mes-hall↗

arXiv2021

Intrinsic 2D-XY ferromagnetism in a van der Waals monolayer

Long before the recent fascination with two-dimensional materials, the critical behaviour and universality scaling of phase transitions in low-dimensional systems has been a topic of great interest. Particularly intriguing is the case of long-range magnetic order in two dimensions, once considered to be excluded in systems with continuous symmetry by the Hohenberg-Mermin-Wagner theorem. While an out-of-plane anisotropy has been shown to stabilize 2D magnetic order, this proof has remained elusive for a 2D magnet with in-plane rotational symmetry. Here, we construct a nearly ideal easy-plane system, a CrCl3 monolayer grown on Graphene/6H-SiC (0001), and unambiguously demonstrate robust in-plane ferromagnetic ordering with a critical scaling behaviour characteristic of a 2D-XY system. These observations suggest the first realization of a finite-size Berezinskii-Kosterlitz-Thouless (BKT) phase transition in a large-area, quasi-freestanding, van der Waals monolayer magnet with a XY universality class; and further constitute an ideal platform to study exotic phenomena like superfluid spin transport or 2D topological in-plane spin textures -- such as merons.

cond-mat.mes-hall↗

arXiv2020

Doping-induced spin Hall ratio enhancement in A15-phase, Ta-doped Beta-W thin films

As spintronic devices become more and more prevalent, the desire to find Pt free materials with large spin Hall effects is increasing. Previously it was shown that Beta W, the metastable A15 structured variant of pure W, has charge-spin conversion efficiencies on par with Pt, and it was predicted that Beta W(Ta) alloys should be even more efficient. Here we demonstrate the enhancement of the spin Hall ratio (SHR) in A15-phase Beta W films doped with Ta (W(4-x)Tax where x is between 0.28 and 0.4, deposited at room temperature using DC magnetron co-sputtering. In close agreement with theoretical predictions, we find that the SHR of the doped films was approx. 9 percent larger than pure Beta W films. We also found that the SHRs in devices with Co2Fe6B2 were nearly twice as large as the SHRs in devices with Co4Fe4B2. This work shows that by optimizing deposition parameters and substrates, the fabrication of the optimum W3Ta alloy should be feasible, opening the door to commercially viable, Pt free, spintronic devices.

cond-mat.mtrl-sci↗

arXiv2019

Towards Weyltronics: Realization of epitaxial NbP and TaP Weyl Semimetal thin films

Weyl Semimetals (WSMs), a recently discovered topological state of matter, exhibit an electronic structure governed by linear band dispersions and degeneracy (Weyl) points leading to rich physical phenomena, which are yet to be exploited in thin film devices. While WSMs were established in the monopnictide compound family several years ago, the growth of thin films has remained a challenge. Here, we report the growth of epitaxial thin films of NbP and TaP by means of molecular beam epitaxy. Single crystalline films are grown on MgO (001) substrates using thin Nb (Ta) buffer layers, and are found to be tensile strained (1%) and with slightly P-rich stoichiometry with respect to the bulk crystals. The resulting electronic structure exhibits topological surface states characteristic of a P-terminated surface and linear dispersion bands in agreement with the calculated band structure, and a Fermi-level shift of -0.2 eV with respect to the Weyl points. Consequently, the electronic transport is dominated by both holes and electrons with carrier mobilities close to 10^3 cm2/Vs at room-temperature. The growth of epitaxial thin films opens up the use of strain and controlled doping to access and tune the electronic structure of Weyl Semimetals on demand, paving the way for the rational design and fabrication of electronic devices ruled by topology.

cond-mat.mes-hall↗

arXiv2018

11,11,12,12-tetracyanonaphtho-2,6-quinodimethane in Contact with Ferromagnetic Electrodes for Organic Spintronics

Spinterface engineering has shown quite important roles in organic spintronics as it can improve spin injection or extraction. In this study, 11,11,12,12-tetracyanonaptho-2,6-quinodimethane (TNAP) is introduced as an interfacial layer for a prototype interface of Fe/TNAP. We report an element-specific investigation of the electronic and magnetic structures of Fe/TNAP system by use of near edge X-Ray absorption fine structure (NEXAFS) and X-ray magnetic circular dichroism (XMCD). Strong hybridization between TNAP and Fe and induced magnetization of N atoms in TNAP molecule are observed. XMCD sum rule analysis demonstrates that the adsorption of TNAP reduces the spin moment of Fe by 12%. In addition, induced magnetization in N K-edge of TNAP has also been found with other commonly used ferromagnets in organic spintronics, such as La0.7Sr0.3MnO3 and permalloy, which makes TNAP a very promising molecule for spinterface engineering in organic spintronics.

cond-mat.mtrl-sci↗

arXiv2016

Competing effects at Pt/YIG interfaces: spin Hall magnetoresistance, magnon excitations and magnetic frustration

We study the spin Hall magnetoresistance (SMR) and the magnon spin transport (MST) in Pt/Y3Fe5O12(YIG)-based devices with intentionally modified interfaces. Our measurements show that the surface treatment of the YIG film results in a slight enhancement of the spin-mixing conductance and an extraordinary increase in the efficiency of the spin-to-magnon excitations at room temperature. The surface of the YIG film develops a surface magnetic frustration at low temperatures, causing a sign change of the SMR and a dramatic suppression of the MST. Our results evidence that SMR and MST could be used to explore magnetic properties of surfaces, including those with complex magnetic textures, and stress the critical importance of the non-magnetic/ferromagnetic interface properties in the performance of the resulting spintronic devices.

cond-mat.mes-hall↗

arXiv2016

Spin Hall magnetoresistance as a probe for surface magnetization in Pt/CoFe$_2$O$_4$ bilayers

We study the spin Hall magnetoresistance (SMR) in Pt grown $\textit{in situ}$ on CoFe$_2$O$_4$ (CFO) ferrimagnetic insulating (FMI) films. A careful analysis of the angle-dependent and field-dependent longitudinal magnetoresistance indicates that the SMR contains a contribution that does not follow the bulk magnetization of CFO but it is a fingerprint of the complex magnetism at the surface of the CFO layer, thus signaling SMR as a tool for mapping surface magnetization. A systematic study of the SMR for different temperatures and CFO thicknesses gives us information impossible to obtain with any standard magnetometry technique. On one hand, surface magnetization behaves independently of the CFO thickness and does not saturate up to high fields, evidencing that the surface has its own anisotropy. On the other hand, characteristic zero-field magnetization steps are not present at the surface while they are relevant in the bulk, strongly suggesting that antiphase boundaries are the responsible of such intriguing features. In addition, a contribution from ordinary magnetoresistance of Pt is identified, which is only distinguishable due to the low resistivity of the $\textit{in-situ}$ grown Pt.

cond-mat.mes-hall↗

arXiv2016

Frequency driven inversion of tunnel magnetoimpedance in magnetic tunnel junctions

Magnetic tunnel junctions (MTJs) are basic building blocks for devices such as magnetic random access memories (MRAMs). The relevance for modern computation of non-volatile high-frequency memories makes ac-transport measurements of MTJs crucial for exploring this regime. Here we demonstrate a frequency-mediated effect in which the tunnel magnetoimpedance reverses its sign in a classical Co/Al{_2}O{_3}/NiFe MTJ, whereas we only observe a gradual decrease of tunnel magnetophase. Such effects are explained by the capacitive coupling of a parallel resistor and capacitor in the equivalent circuit model of the MTJ. Furthermore, we report a positive tunnel magnetocapacitance effect, suggesting the presence of a spin-capacitance at the two ferromagnet/tunnel-barrier interfaces. Our results are important for understanding spin transport phenomena at the high frequency regime, in which the spin-polarized charge accumulation at the two interfaces plays a crucial role.

cond-mat.mes-hall↗

arXiv2016

Hanle Magnetoresistance in Thin Metal Films with Strong Spin-Orbit Coupling

We report measurements of a new type of magnetoresistance in Pt and Ta thin films. The spin accumulation created at the surfaces of the film by the spin Hall effect decreases in a magnetic field because of the Hanle effect, resulting in an increase of the electrical resistance as predicted by Dyakonov [PRL 99, 126601 (2007)]. The angular dependence of this magnetoresistance resembles the recently discovered spin Hall magnetoresistance in Pt/Y3Fe5O12 bilayers, although the presence of a ferromagnetic insulator is not required. We show that this Hanle magnetoresistance is an alternative, simple way to quantitatively study the coupling between charge and spin currents in metals with strong spin-orbit coupling.

cond-mat.mes-hall↗

arXiv2015

Reliable determination of the Cu/n-Si Schottky barrier height by using in-device hot-electron spectroscopy

We show the operation of a Cu/Al_2O_3/Cu/n-Si hot-electron transistor for the straightforward determination of a metal/semiconductor energy barrier height even at temperatures below carrier-freeze out in the semiconductor. The hot-electron spectroscopy measurements return a fairly temperature independent value for the Cu/n-Si barrier of 0.66 $\pm$ 0.04 eV at temperatures below 180 K, in substantial accordance with mainstream methods based on complex fittings of either current-voltage (I-V) and capacitance-voltage (C-V) measurements. The Cu/n-Si hot-electron transistors exhibit an OFF current of ~2 * 10^-13 A, an ON/OFF ratio of ~10^5 and an equivalent subtreshold swing of ~96 mV/dec at low temperatures, which are suitable values for potential high frequency devices.

cond-mat.mes-hall↗

arXiv2015

Modulation of pure spin currents with a ferromagnetic insulator

We propose and demonstrate spin manipulation by magnetically controlled modulation of pure spin currents in cobalt/copper lateral spin valves, fabricated on top of the magnetic insulator Y$_3$Fe$_5$O$_{12}$ (YIG). The direction of the YIG magnetization can be controlled by a small magnetic field. We observe a clear modulation of the non-local resistance as a function of the orientation of the YIG magnetization with respect to the polarization of the spin current. Such a modulation can only be explained by assuming a finite spin-mixing conductance at the Cu/YIG interface, as it follows from the solution of the spin-diffusion equation. These results open a new path towards the development of spin logics.

cond-mat.mes-hall↗

arXiv2014

Spin Hall magnetoresistance at Pt/CoFe2O4 interfaces and texture effects

We report magnetoresistance measurements on thin Pt bars grown on epitaxial (001) and (111) CoFe2O4 (CFO) ferrimagnetic insulating films. The results can be described in terms of the recently discovered spin Hall magnetoresistance (SMR). The magnitude of the SMR depends on the interface preparation conditions, being optimal when Pt/CFO samples are prepared in situ, in a single process. The spin-mixing interface conductance, the key parameter governing SMR and other relevant spin-dependent phenomena such as spin pumping or spin Seebeck effect, is found to be different depending on the crystallographic orientation of CFO, highlighting the role of the composition and density of magnetic ions at the interface on spin mixing.

cond-mat.mes-hall↗

arXiv2014

Hybrid Interface States and Spin Polarization at Ferromagnetic Metal-Organic Heterojunctions: Interface Engineering for Efficient Spin Injection in Organic Spintronics

Ferromagnetic metal-organic semiconductor (FM-OSC) hybrid interfaces have shown to play an important role for spin injection in organic spintronics. Here, 11,11,12,12-tetracyanonaptho-2,6-quinodimethane (TNAP) is introduced as an interfacial layer in Co-OSCs heterojunction with an aim to tune the spin injection. The Co/TNAP interface is investigated by use of X-ray and ultraviolet photoelectron spectroscopy (XPS/UPS), near edge X-ray absorption fine structure (NEXAFS) and X-ray magnetic circular dichroism (XMCD). Hybrid interface states (HIS) are observed at Co/TNAP interface resulting from chemical interaction between Co and TNAP. The energy level alignment at Co/TNAP/OSCs interface is also obtained, and a reduction of the hole injection barrier is demonstrated. XMCD results confirm sizeable spin polarization at the Co/TNAP hybrid interface.

cond-mat.mtrl-sci↗