SELECTED PAPERS
2019
Periodic Néel skyrmion transport
In this work, we have used the MuMax3 software to simulate devices consisting of a ferromagnetic thin film placed over a heavy metal thin film. The devices are two interconnected partial-disks where a Néel domain wall is formed in the disks junction. In our simulations we investigate devices with disk radius R=50 nm and different distance d between the disks centers (from 12 nm to d=2R=100 nm). By applying strong sinusoidal external magnetic fields, we find a mechanism able to create, annihilate and even manipulate a skyrmion in each side of the device. This mechanism is discussed in terms of interactions between skyrmion and domain wall. The Néel domain wall formed in the center of the device interacts with the Néel skyrmion, leading to a process of transporting a skyrmion from one disk to the other periodically. Our results have relevance for potential applications in spintronics such as logical devices.
Phy. Lett. A
2018
THIONINE SELF-ASSEMBLY STRUCTURES ON GRAPHENE: FORMATION, ORGANIZATION AND DOPING
The association of organic molecules with two-dimensional (2D) materials, creating hybrid systems with mutual influences, constitutes an important testbed for both basic science self-assembly studies and perspective applications. Following this concept, in this work, we show a rich phenomenology that is involved in the interaction of thionine with graphene, leading to a hybrid material formed by well-organized self-assembled structures atop graphene. This composite system is investigated by atomic force microscopy, electric transport measurements, Raman spectroscopy, and first principles calculations, which show (1) an interesting time evolution of thionine self-assembled structures atop graphene; (2) a highly oriented final molecular assembly (in accordance with the underlying graphene surface symmetry); and (3) a strong n-type doping effect introduced in graphene by thionine. The nature of the thionine–substrate interaction is further analyzed in experiments using mica as a polar substrate. The present results may help pave the way to achieve tailored 2D material hybrid devices via properly chosen molecular self-assembly processes.
Langmuir, 34, (23) 6903
QUANTUM CORRECTIONS TO CONDUCTIVITY IN GRAPHENE WITH VACANCIES
In this work, different regions of a graphene device were exposed to a 30 keV helium ion beam creating a series of alternating strips of vacancy-type defects and pristine graphene. From magnetoconductance measurements as function of temperature, density of carriers and density of strips we show that the electron-electron interaction is important to explain the logarithmic quantum corrections to the Drude conductivity in graphene with vacancies. It is known that vacancies in graphene behave as local magnetic moments that interact with the conduction electrons and leads to a logarithmic correction to the conductance through the Kondo effect. However, our work shows that it is necessary to account for the non-homogeneity of the sample to avoid misinterpretations about the Kondo physics due the difficulties in separating the electron-electron interaction from the Kondo effect.
Physica E: Low-dimensional Systems and Nanostructures, 100, 40
STABLE ROOM TEMPERATURE MAGNETOCURRENT IN ELECTRODEPOSITED PERMEABLE N-TYPE METAL BASE TRANSISTOR
We investigated a permeable metal base transistor consisting of a ZnO/NiFe/Si heterostructure. Both ZnO and NiFe layers were grown by electrodeposition techniques, using only adhesive tape masks to define deposition regions. The base permeability can thus be controlled by varying the NiFe deposition time. We report here our best results obtained for the permeable NiFe base close to the electrical percolation threshold, which gives reasonable sensitivity to the device. Magnetocurrent measurements carried out at room temperature show that this permeable metal base transistor is stable and sensitive under applied magnetic fields of low intensities, ∼ 100 Oe, required for electronics integration.
Appl. Phys. Lett. 112, 092405
2017
REALIZATION OF RECTANGULAR ARTIFICIAL SPIN ICE AND DIRECT OBSERVATION OF HIGH ENERGY TOPOLOGY
In this work, 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 aspect ratios γ = a/b = √2, √3 and √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 the string tension connecting two magnetic monopoles in a pair vanishes in rectangular lattices with a critical ratio γ = γc = √3, supporting previous theoretical predictions.
Sci. Rep. 7, 13982
DIRAC-SURFACE-STATE-DOMINATED SPIN TO CHARGE CURRENT CONVERSION IN THE TOPOLOGICAL INSULATOR (Bi0.22Sb0.78)2Te3 FILMS AT ROOM TEMPERATURE
We report the spin-to-charge current conversion in an intrinsic topological insulator (TI) (Bi0.22Sb0.78)2Te3 film at room temperature. The spin currents are generated in a thin layer of permalloy (Py) by two different processes, the spin pumping effect (SPE) and the spin Seebeck effect (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 (Bi0.22Sb0.78)2Te3 layer in direct contact with Py. In the second, we use the SSE in the longitudinal configuration in Py without contamination by the anomalous Nernst effect, which was made possible with a thin NiO layer between the Py and (Bi0.22Sb0.78)2Te3 layers. The spin-to-charge current conversion is dominated by the TI surface states and is attributed to the inverse Edelstein effect (IEE), which is 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.
Phys. Rev. B 96, 180415(R)
CREATION, TRANSPORT AND DETECTION OF IMPRINTED MAGNETIC SOLITONS STABILIZED BY SPIN-POLARIZED CURRENT
With the recent proposition of skyrmion utilization in racetrack memories at room temperature, skyrmionics has become a very attractive field. However, for the stability of skyrmions, it is essential to incorporate the Dzyaloshinskii–Moriya interaction (DMI) and the out-of-plane magnetic field into the system. In this work, we explore a system without these interactions. First, we propose a controlled way for the creation of magnetic skyrmions and skyrmioniums imprinted on a ferromagnetic nanotrack via a nanopatterned nanodisk with the magnetic vortex state. Then we investigate the detachment of the imprinted spin textures from the underneath of the nanodisk, as well as its transport by the spin-transfer torque imposed by spin-polarized current pulses applied in the nanotrack. A prominent feature of the moving imprinted spin texture is that its topological number Q is oscillating around the averaged value of Q0 as if it is a resonant state between the skyrmions with Q=+1 ou -1 and the bubble with . We may call it a resonant magnetic soliton (RMS). A RMS moves along a straight line since it is free from the skyrmion Hall effect. In our studied device, the same electrodes are employed to realize the imprinted spin texture detachment and its transport. In addition, we have investigated the interaction between the RMS and a magnetic tunnel junction sensor, where the passing of the RMS in the nanotrack can be well detected. Our results would be useful for the development of novel spintronic devices based on moveable spin textures.
J. of Mag. and Mag. Mat. xxx
INVERSE SPIN HALL EFFECT IN THE SEMICONDUCTOR (Ga,Mn)As AT ROOM TEMPERATURE
We report experiments demonstrating that a spin current can be converted into a charge current in a film of the paramagnetic semiconductor (Ga,Mn)As at room temperature by means of the inverse spin Hall effect. The spin currents are generated in a thin layer of permalloy (Py) by two different processes: spin-pumping effect (SPE) and spin Seebeck effect (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 (Ga,Mn)As film either in direct contact with Py or through a thin layer of insulating antiferromagnetic NiO. In the second, we use the SSE in the longitudinal configuration in Py with no contamination by the Nernst effect made possible with the use of a thin layer of NiO between the Py and (Ga,Mn)As layers. The results of the two measurements are consistent with each other, and from them, we obtain a spin Hall angle for (Ga,Mn)As at room temperature of θSH=(1.5±0.5)×10−3, which is one order of magnitude larger than the values reported for p-Si and n-Ge at room temperature.
Phys. Rev. B 95, 214405
2016
MULTILEVEL THERMALLY ASSISTED MAGNETORESISTIVE RANDOM-ACCESS MEMORY BASED ON EXCHANGE-BIASED VORTEX CONFIGURATIONS
A concept of multilevel thermally assisted magnetoresistive random-access memory is proposed and investigated by micromagnetic simulations. The storage cells are magnetic tunnel junctions in which the storage layer is exchange biased and in a vortex configuration. The reference layer is an unpinned soft magnetic layer. The stored information is encoded via the position of the vortex core in the storage layer. This position can be varied along two degrees of freedom: the radius and the in-plane angle. The information is read out from the amplitude and phase of the tunnel magnetoresistance signal obtained by applying a rotating field on the cell without heating the cell. Various configurations are compared in which the soft reference layer consists of either a simple ferromagnetic layer or a synthetic antiferromagnetic sandwich (SAF). Among those, the most practical one comprises a SAF reference layer in which the magnetostatic interaction between the SAF and storage layer is minimized. This type of cell should allow one to store at least 40 different states per cell representing more than five bits per cell.
Phys. Rev. Applied 6, 024015
HALLMARKS OF THE KARDAR-PARISI-ZHANG UNIVERSALITY CLASS ELICITED BY SCANNING PROBE MICROSCOPY
Scanning probe microscopy is an fundamental technique widespread in the quantitative analysis of rough surfaces. We analyzed the interface statistics of surfaces obtained with a probe tip in systems that do not belong to Kardar-Parisi-Zhang (KPZ) universality class. We show that the height, local interface roughness and extremal height distributions for surfaces scanned with a probe tip quantitatively agree with the KPZ class in a data resolution similar or better than experimental evidences of KPZ class in thin lm growth. The underlying mechanism behind this arti cial KPZ class is the nite size of the probe tip not permitting full resolution of neither deep valleys or border of sharp plateaus. The net result is a scanned pro le higher than the original one implying an excess lateral growth, the major characteristic of the KPZ universality class. We proposed that the universality in spatial correlation functions is a robust measure provided that the actual, theoretically accepted, KPZ exponents are imposed in the analysis.
New J. Phys. 18 093018
SPIN-CURRENT TO CHARGE-CURRENT CONVERSION AND MAGNETORESISTANCE IN A HYBRID STRUCTURE OF GRAPHENE AND YTTRIUM IRON GARNET
The use of graphene in spintronic devices depends, among other things, on its ability to convert a spin excitation into an electric charge signal, a phenomenon that requires a spin-orbit coupling (SOC). Here we report the observation of two effects that show the existence of SOC in large-area CVD grown single-layer graphene deposited on a single crystal film of the ferrimagnetic insulator yttrium iron garnet (YIG). The first is a magnetoresistance of graphene induced by the magnetic proximity effect with YIG. The second is the detection of a dc voltage along the graphene layer resulting from the conversion of the spin current generated by spin pumping from microwave driven ferromagnetic resonance into a charge current, which is attributed to the inverse Rashba-Edelstein effect.
ALTERNATIVE USES OF A MEGAVOLT TANDEM ACCELERATOR FOR FEW-KEV STUDIES WITH ION-SOURCE SIMS MONITORING
We increase the versatility of a tandem electrostatic accelerator by implementing simple modifications to the standard operation procedure. While keeping its ability to deliver MeV ion beams, we show that the experimental setup can (i) provide good quality ion beams in the few-keV energy range and (ii) be used to study ion-beam surface modification with simultaneous secondary ion mass spectrometry. This latter task is accomplished without using any chamber connected to the accelerator exit. We perform mass spectrometry of the few-keV anions produced in the ion source by measuring their neutral counterparts at the accelerator exit with energies up to 1.7 MeV. With an additional modification, a high-current few-keV regime is obtained, using the ion source as an irradiation chamber and the accelerator itself only as a mass spectrometer. As an example of application, we prepare a sample for the study of ion-beam assisted dewetting of a thin Au film on a Si substrate.
Review of Scientific Instruments 87, 063305
EMERGENCE AND MOBILITY OF MONOPOLES IN A UNIDIRECTIONAL ARRANGEMENT OF MAGNETIC NANOISLANDS
Magnetricity, the magnetic equivalent of electricity, was recently verified experimentally for the first time. Indeed, like the stream of electric charges that produces electric current, emergent magnetic monopoles have been observed to roam freely in geometrically frustrated magnets known as spin ice. However, such phenomena demand extreme physical conditions, say, a single spin ice crystal has to be cooled to very low temperature, around 0.36 K. Candidates to overcome this difficulty are their artificial analogues, the so-called artificial spin ices. Here, we demonstrate that a specific unidirectional arrangement of nanoislands yields a peculiar system where magnetic monopoles emerge and are constrained to move along aligned dipoles, providing an ordered flow of magnetic charges at room temperature.
INVESTIGATION OF FERROMAGNETIC RESONANCE AND MAGNETORESISTANCE IN ANTI-SPIN ICE STRUCTURES
In this work, we report experimental and theoretical investigations performed in anti-spin ice structures, composed by square lattice of elongated antidots, patterned in nickel thin film. The magnetic vortex crystal state was obtained by micromagnetic simulation as the ground state magnetization, which arises due to the magnetic stray field at the antidot edges inducing chirality in the magnetization of platters among antidots. Ferromagnetic resonance (FMR) and magnetoresistance (MR) measurements were utilized to investigate the vortex crystal magnetization dynamics and magnetoelectric response. By using FMR, it was possible to detect the spin wave modes and vortex crystal resonance, in good agreement with dynamic micromagnetic simulation results. The vortex crystal magnetization configuration and its response to the external magnetic field, were used to explain the isotropic MR behaviour observed.
J. Phys.: Condens. Matter 28 456002
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 transport 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 eld 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 con rming the low eld microwave absorption (LFMA), were correlated to the peaks and slopes found in the magnetoresistance curves. The results suggest long range spin polarized current in low resistive silicon and the magnetic resonance technique is herein introduced as a promising tool for analysis of electric contactless magnetoresistive samples.
Journal of Magnetism and Magnetic Materials 395, 130
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. At very low temperature, however, square ice is more likely to reach states near the ground state than Kagomé ice, for the same island coupling parameters.