arXiv subjects

C. I. L. de Araujo

Publications and source records attributed to C. I. L. de Araujo.

At least 19 recordsLinked to original sources

arXiv2026

Impact of magnetic field direction on anti-dot-based superconducting diodes

The superconducting diode effect (SDE) is a fundamental building block for dissipationless nonreciprocal electronics, yet its microscopic origins in thin films often involve competing mechanisms that remain debated. Here, we demonstrate that the SDE can be engineered in niobium films by patterning macroscopic asymmetric antidots, revealing distinct control mechanisms under in-plane and out-of-plane magnetic fields. We identify two dominant contributions to nonreciprocal transport: edge flux pinning, which governs the low-field and in-plane field regimes via surface-barrier asymmetry, and bulk flux pinning, which drives the high-field response and correlates directly with the geometric asymmetry of the antidots. Supported by time-dependent Ginzburg-Landau simulations and an analytical model, we provide a unified description of these regimes, linking the diode efficiency to the specific pinning landscape. These findings establish a flexible design principle for engineering superconducting diodes with tunable functionality, paving the way for their integration into next-generation quantum and cryogenic circuits.

cond-mat.supr-con↗

arXiv2025

Interface-Induced Stability of Nontrivial Topological Spin Textures: Unveiling Room-Temperature Hopfions and Skyrmions

Topological spin configurations, such as soliton-like spin texture and Dirac electron assemblies, have emerged in recent years in both fundamental science and technological applications. Achieving stable topological spin textures at room-temperature is crucial for enabling these structures as long-range information carriers. However, their creation and manipulation processes have encountered difficulties due to multi-step field training techniques and competitive interactions. Thus, a spontaneous ground state for multi-dimensional topological spin textures is desirable, as skyrmions form swirling, hedgehog-like spin structures in two dimensions, while hopfions emerge as their twisted three-dimensional counterparts. Here, we report the first observation of robust and reproducible topological spin textures of hopfions and skyrmions observed at room temperature and in zero magnetic field, which are stabilized by geometric confinement and protected by interfacial magnetism in a ferromagnet/topological insulator/ferromagnet trilayer heterostructure. These skyrmion-hopfion configurations are directly observed at room temperature with Lorenz transmission electron microscopy. Using micromagnetic modelling, the experimental observations of hopfion-skyrmion assemblies are reproduced. Our model reveals a complete picture of how spontaneously organized skyrmion lattices encircled by hopfion rings are controlled by surface electrons, uniaxial anisotropy and Dzyaloshinskii-Moriya interaction, all at ambient temperature. Our study provides evidence that topological chiral spin textures can facilitate the development of magnetically defined information carriers. These stable structures hold promise for ultralow-power and high-density information processing, paving the way for the next generation of topologically defined devices.

cond-mat.mes-hall↗

arXiv2024

Evidence of thermodynamics and magnetic monopole plasma formation by photon-magnon interaction in artificial spin ice

Artificial spin ices (ASI), containing magnetic monopole quasi-particles emerging at room temperature, have been investigated as a promising system to be applied in alternative low-power information technology devices. However, restrictions associated with the intrinsic energetic connections between opposing magnetic monopoles in conventional ASI need to be overcome to achieve this purpose. Here, photon-magnon scattering in nanomagnets is examined as an approach to locally activate the collective dynamics of interacting magnetic systems at the nanoscale. Low-power white and polarized light were employed as a new tool to manipulate magnetic monopole intensity, leading to tuning on the particles response to external magnetic field and spontaneous magnetization flipping without external field (thermodynamics). Our findings showing evidence of magnetic monopole plasma formation in a regular square ASI system are explained by an analytical model of photon-magnon conversion acting directly on the ASI nanomagnet dipole. Micromagnetic simulations based on the samples parameters and values obtained from the model present a very good qualitative correspondence between theory and observations for the investigated ASI system.

cond-mat.mes-hall↗

arXiv2023

Quantifying ground state degeneracy in planar artificial spin ices: the magnetic structure factor approach

Magnetic structure factor (MSF) is employed to investigate the ground state degeneracy in rectangular-like artificial spin ices. Our analysis considers the importance of nanoislands size via dumbbell model approximation. Pinch points in MSF and residual entropy are found for rectangular lattices with disconnected nanoislands, signalizing an emergent gauge field, through which magnetic monopoles interact effectively. Dipole-dipole interaction is also used and its predictions are compared with those obtained by dumbbell model.

cond-mat.mes-hall↗

arXiv2022

Direct observation of magnetic monopole freedom in two-dimensional artificial spin ice

Magnetic monopole unpairing as a function of external magnetic fields is presented as a fingerprint of this emergent quasiparticles freedom in a two-dimensional artificial spin ice system. Such freedom, required for example for further application in magnetricity, is only possible due to ground-state degeneracy, which causes a decreasing of the string energy in rectangular geometries, designed to allow highest equidistance among nanomagnets. We show by simulations that spin correlation in different rectangular artificial spin ices evolves from antiferromagnetic ordered magnetic structure to a ferromagnetic one, passing through an ice regime were pinch points related to Coulomb phase are observed. By measurements of magnetic force microscopy, we observe magnetic monopole creation, transport and annihilation in such systems with free monopoles created and transported throughout the sample without strings attached, as is commonly observed in conventional artificial spin ice systems.

cond-mat.mes-hall↗

arXiv2021

Room temperature two terminal tunnel magnetoresistance in lateral graphene transistor

We investigate the behavior of both pure spin and spin-polarized currents measured with four probe non-local and two probe local configurations up to room temperature and under external gate voltage in a lateral graphene transistor, produced using a standard large-scale microfabrication process. The high spin diffusion length of pristine graphene in the channel, measured both directly and by the Hanle effect, and the tuning of relation between electrode resistance area present in the device architecture, allowed us to observe local tunnel magnetoresistance at room temperature, a new finding for this type of device. Results also indicate that while pure spin currents are less sensitive to temperature variations, spin-polarized current switching by external voltage is more efficient, due to a combination of the Rashba effect and change in carrier mobility by Fermi level shift

cond-mat.mes-hall↗

arXiv2020

Traps for pinning and scattering of antiferromagnetic skyrmions via magnetic properties engineering

Micromagnetic simulations have been performed to investigate the controllability of the skyrmion position in antiferromagnetic nanotracks with their magnetic properties modified spatially. In this study we have modeled magnetic defects as local variations on the material parameters, such as the exchange stiffness, saturation magnetization, perpendicular magnetocrystalline anisotropy and Dzyaloshinskii-Moriya constant. Thus, we have observed not only pinning (potential well) but also scattering (potential barrier) of antiferromagnetic skyrmions, when adjusting either a local increase or a local reduction for each material parameter. In order to control of the skyrmion motion it is very important to impose certain positions along the nanotrack where the skyrmion can stop. Magnetic defects incorporated intentionally in antiferromagnetic racetracks can be useful for such purpose. In order to provide guidelines for experimental studies, we vary both material parameters and size of the modified region. The found results show that the efficiency of skyrmion trap depends on a suitable combination of magnetic defect parameters. Furthermore, we discuss the reason why skyrmions are either attracted or repelled by a region magnetically modified.

physics.comp-ph↗

arXiv2020

Suppression of the skyrmion Hall effect in planar nanomagnets by the magnetic properties engineering: Skyrmion transport on nanotracks with magnetic strips

Micromagnetic simulations have been performed to investigate the suppression of the skyrmion Hall effect in nanotracks with their magnetic properties strategically modified. In particular, we study two categories of magnetically modified nanotracks. One of them, repulsive edges have been inserted in the nanotrack and, in the other, an attractive strip has been placed exactly on the longest axis of the nanotrack. Attractive and repulsive interactions can be generated from the engineering of magnetic properties. For instance, it is known that the skyrmion can be attracted to a region where the exchange stiffness constant is decreased. On the other hand, the skyrmion can be repelled from a region characterized by a local increase in the exchange stiffness constant. In order to provide a background for experimental studies, we vary not only the magnetic material parameters (exchange stiffness, perpendicular magnetocrystalline anisotropy and the Dzyaloshinskii-Moriya constant) but also the width of the region magnetically modified, containing either a local reduction or a local increase for each one of these magnetic properties. In the numerical simulations, the skyrmion motion was induced by a spin-polarized current and the found results indicate that it is possible to transport skyrmions around the longest axis of the nanotrack. In practice, the skyrmion Hall effect can be completely suppressed in magnetic nanotracks with strategically modified magnetic properties. Furthermore, we discuss in detail 6 ways to suppress the skyrmion Hall effect by the usage of nanotracks with repulsive edges and nanotracks with an attractive strip.

physics.comp-ph↗

arXiv2019

Naked-eye visualization of geometric frustration effects in macroscopic spin ices

We study planar rectangular-like arrays composed by macroscopic dipoles (magnetic bars with size around a few centimeters) separated by lattice spacing a and b along each direction. Physical behavior of such macroscopic artificial spin ice (MASI) systems are shown to agree much better with theoretical prediction than their micro- or nano-scaled counterparts, making MASI "almost ideal prototypes" for readily naked-eye visualization of geometrical frustration effects.

cond-mat.mes-hall↗

arXiv2019

Investigation of domain wall pinning by square anti-notches and its applications in three terminals MRAM

In this work we perform investigations of the competition between domain-wall pinning and attraction by anti-notches and finite device borders. The conditions for optimal geometries, which can attain a stable domain-wall pinning, are presented. This allow us the proposition of a three-terminals device based on domain-wall pinning. We obtain, with very small pulses of current applied parallel to the nanotrack, a fast motion of the domain-wall between anti-notches. In addition to this, a swift stabilization of the pinned domain-wall is observed with a high percentage of orthogonal magnetization, enabling high magnetoresistive signal measurement. Thus, our proposed device is a promising magnetoresistive random access memories with good scalability, duration, and high speed information storage.

physics.app-ph↗

arXiv2018

Experimental and theoretical evidences for the ice regime in planar artificial spin ices

In this work, we explore a kind of geometrical effect in the thermodynamics of artificial spin ices (ASI). In general, such artificial materials are athermal. Here, We demonstrate that geometrically driven dynamics in ASI can open up the panorama of exploring distinct ground states and thermally magnetic monopole excitations. It is shown that a particular ASI lattice will provide a richer thermodynamics with nanomagnet spins experiencing less restriction to flip precisely in a kind of rhombic lattice. This can be observed by analysis of only three types of rectangular artificial spin ices (RASI). Denoting the horizontal and vertical lattice spacings by a and b, respectively, then, a RASI material can be described by its aspect ratio $γ$=a/b. The rhombic lattice emerges when $γ$=$\sqrt{3}$. So, by comparing the impact of thermal effects on the spin flips in these three appropriate different RASI arrays, it is possible to find a system very close to the ice regime.

cond-mat.mtrl-sci↗

arXiv2017

Dirac-Surface-State-Dominated Spin to Charge Current Conversion in the Topological Insulator $(Bi_{0.22}Sb_{0.78})_2Te_3$ Films at Room Temperature

We report the spin to charge current conversation in an intrinsic topological insulator (TI) $(Bi_{0.22}Sb_{0.78})_2Te_3$ film at room temperature. The spin currents are generated in a thin layer of permalloy (Py) by two different processes, spin pumping (SPE) and spin Seebeck effects (SSE). In the first we use microwave-driven ferromagnetic resonance of the Py film to generate a SPE spin current that is injected into the TI $(Bi_{0.22}Sb_{0.78})_2Te_3$ layer in direct contact with Py. In the second we use the SSE in the longitudinal configuration in Py without contamination by the Nernst effect made possible with a thin NiO layer between the Py and $(Bi_{0.22}Sb_{0.78})_2Te_3$ layers. The spin-to-charge current conversion is attributed to the inverse Edelstein effect (IEE) made possible by the spin-momentum locking in the electron Fermi contours due to the Rashba field. The measurements by the two techniques yield very similar values for the IEE parameter, which are larger than the reported values in the previous studies on topological insulators.

cond-mat.mes-hall↗

arXiv2017

Realization of Rectangular Artificial Spin Ice and Direct Observation of High Energy Topology

In this letter, we have constructed and experimentally investigated frustrated arrays of dipoles forming two-dimensional artificial spin ices with different lattice parameters (rectangular arrays with horizontal and vertical lattice spacings denoted by $a$ and $b$ respectively). Arrays with three different ratios $γ=a/b = \sqrt{2}$, $\sqrt{3}$ and $\sqrt{4}$ are studied. Theoretical calculations of low-energy demagnetized configurations for these same parameters are also presented. Experimental data for demagnetized samples confirm most of the theoretical results. However, the highest energy topology (doubly-charged monopoles) does not emerge in our theoretical model, while they are seen in experiments for large enough $γ$. Our results also insinuate that magnetic monopoles may be almost free in rectangular lattices with a critical ratio $γ= γ_{c} = \sqrt{3}$, supporting previous theoretical predictions.

cond-mat.stat-mech↗

arXiv2015

Ferromagnetic resonance and magnetoresistive measurements evidencing magnetic vortex crystal in nickel thin film with patterned antidot array

Ferromagnetic vortices deliver robust out-of-plane magnetization at extremely small scales. Their handling and creation therefore has high potential to become a necessary ingredient for future data storage technologies in order to keep up with the pace of growing information density demands. In this study we show that by using one step nanolithography method, we are able to create ferromagnetic vortex lattices in thin nickel films. The necessary control of the magnetic stray field at the domain edges was achieved by actively modifying the ferromagnetic thin film anisotropic properties at nanometer scale. We present experimental evidence using ferromagnetic resonance and magnetoresistance measurements supporting simulations based on the theoretical prediction of the proclaimed vortex structures.

cond-mat.mes-hall↗

arXiv2015

Low-field microwave absorption and magnetoresistance in iron nanostructures grown by electrodeposition on n-type lightly-doped silicon substrates

In this study we investigate magnetic properties, surface morphology and crystal structure in iron nanoclusters electrodeposited on lightly-doped (100) n-type silicon substrates. Our goal is to investigate the spin injection and detection in the Fe/Si lateral structures. The samples obtained under electric percolation were characterized by magnetoresistive and magnetic resonance measurements with cycling the sweeping applied field in order to understand the spin dynamics in the as-produced samples. The observed hysteresis in the magnetic resonance spectra, plus the presence of a broad peak in the non-saturated regime confirming the low field microwave absorption (LFMA), were correlated to the peaks and slopes found in the magnetoresistance curves. The results suggest long range spin injection and detection in low resistive silicon and the magnetic resonance technique is herein introduced as a promising tool for analysis of electric contactless magnetoresistive samples.

cond-mat.mtrl-sci↗

arXiv2014

Order and thermalized dynamics in Heisenberg-like square and Kagomé spin ices

Thermodynamic properties of a spin ice model on a Kagomé lattice are obtained from dynamic simulations and compared with properties in square lattice spin ice. The model assumes three-component Heisenberg-like dipoles of an array of planar magnetic islands situated on a Kagomé lattice. Ising variables are avoided. The island dipoles interact via long-range dipolar interactions and are restricted in their motion due to local shape anisotropies. We define various order parameters and obtain them and thermodynamic properties from the dynamics of the system via a Langevin equation, solved by the Heun algorithm. Generally, a slow cooling from high to low temperature does not lead to a particular state of order, even for a set of coupling parameters that gives well thermalized states and dynamics. Some suggestions are proposed for the alleviation of the geometric frustration effects and for the generation of local order in the low temperature regime.

cond-mat.str-el↗

arXiv2014

Realization of magnetic monopoles current in an artificial spin ice device: A step towards magnetronics

Magnetricity- the magnetic equivalent of electricity- was recently verified experimentally for the first time. Indeed, just as the stream of electric charges produces electric current, emergent magnetic monopoles have been observed to roam freely (generating magnetic current) in geometrically frustrated magnets known as spin ice. However, this is realized only by considering extreme physical conditions as a single crystal of spin ice has to be cooled to a temperature of $0.36 K$. Candidates to overcome this difficulty are artificial analogues of spin ice crystals, the so-called artificial spin ices. Here we show that, by tuning geometrical frustration down, a peculiar type of these artificial systems is an excellent candidate. We produce this material and experimentally observe the emergent monopoles; then, we calculate the effects of external magnetic fields, illustrating how to generate controlled magnetic currents. This potential nano-device for use in magnetronics can be practical even at room temperature and the relevant parameters (such as magnetic charge strength etc) for developing this technology can be tuned at will.

cond-mat.mes-hall↗

arXiv2013

Vortex-crystal pattern in an anti-artificial spin ice system

We have proposed in this work an original system composed by anti-dots nanopatterned in a ferromagnetic thin film, mimicking negatively the structure of an articial spin ice. In the hysteresis loop we notice the emergency of an anisotropy in the magnetization saturation and in the micromagnetic simulations, in the beginning of the hysteresis loop (relaxation), the formation of a vortex crystal array with vortices in diferent positions possessing random polarization and chirality. The crystal of vortices in this electrically connected sample could be most eficient than those observed in non-connected nanodiscs for current-driven or magnetic vortices switching by electric currents.

cond-mat.mes-hall↗