C. I. L. de Araujo

Zinc oxide thin films on silicon carbide substrates (ZnO/SiC): electro-optical properties and electrically active defects

The electrical and optical properties of heterojunctions formed by thermally deposited ZnO thin films on n-type 4H-SiC substrates have been investigated. Current–voltage characteristics of the fabricated light emitting devices revealed excellent rectifying behaviors with a typical leakage current lower than 1 nA at a reverse bias of −3 V, and with a forward current at 3 V in the range of 2 mA. A study of the electroluminescent characteristics of ZnO/SiC heterojunctions over the temperature range of 50–450 K showed an emission peak around 410 nm and a broad defect-related electroluminescence at room temperature in the visible range for a forward current of 300 mA. Electrically active deep level centers in ZnO and n-type 4H-SiC epilayers have been investigated by deep level transient spectroscopy (DLTS) and high-resolution Laplace DLTS (LDLTS). Additionally, LDLTS has successfully been employed to resolve the closely spaced hole trap energy levels.

Magnetic vortex crystal formation in the antidot complement of square artificial spin ice

We have studied ferromagnetic nickel thin films patterned with square lattices of elongated antidots that are negative analogues of square artificial spin ice. Micromagnetic simulations and direct current magnetic moment measurements reveal in-plane anisotropy of the magnetic hysteresis loops, and the formation of a dense array of magnetic vortices with random polarization and chirality. These multiply-connected antidot arrays could be superior to lattices of disconnected nanodisks for investigations of vortex switching by applied electric current.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.

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.

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.

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.

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

Abstract 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.

Creation, transport and detection of imprinted magnetic solitons stabilized by spin-polarized current

Abstract 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 Q = 0 as if it is a resonant state between the skyrmions with Q = ± 1 and the bubble with Q = 0 . 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.

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.

Effect of the dipolar coupling on the precessional magnetization switching in two-dimensional arrays of single-domain nano-ellipses

Various spintronic devices use single-domain magnetic nanoparticles as unit cells. Herein, we investigated interparticle dipole-dipole interactions in arrays of Permalloy single-domain nano-ellipses through micromagnetic simulations. In this study, the variation is introduced not only to the aspect ratio and the spacing between ellipses but to the magnetization distribution and the 2D lattice type as well. When integrating the Landau-Lifshitz-Gilbert equation with zero external magnetic field, equilibrium magnetic configurations were obtained for each array. For small values of the spacing between ellipses, they interact strongly, such that the shape anisotropy is locally modified by the distribution of the magnetization. Moreover, the effect of the dipolar coupling on the precessional magnetization reversal is also studied. The minimum field strength required to switch the magnetization depends on the magnetization distribution in a strongly interacting magnetic system. Consequently, we have assessed the minimum spacing between particles in which single-domain nano-ellipses becomes a non-interacting magnetic system.Various spintronic devices use single-domain magnetic nanoparticles as unit cells. Herein, we investigated interparticle dipole-dipole interactions in arrays of Permalloy single-domain nano-ellipses through micromagnetic simulations. In this study, the variation is introduced not only to the aspect ratio and the spacing between ellipses but to the magnetization distribution and the 2D lattice type as well. When integrating the Landau-Lifshitz-Gilbert equation with zero external magnetic field, equilibrium magnetic configurations were obtained for each array. For small values of the spacing between ellipses, they interact strongly, such that the shape anisotropy is locally modified by the distribution of the magnetization. Moreover, the effect of the dipolar coupling on the precessional magnetization reversal is also studied. The minimum field strength required to switch the magnetization depends on the magnetization distribution in a strongly interacting magnetic system. Consequently, we have assessed the...