https://uvadoc.uva.es/handle/10324/1270 Dpto. Electricidad y Electrónica - Artículos de revista 2026-04-29T00:51:15Z https://uvadoc.uva.es/handle/10324/83812 The usefulness of modeling magnetocaloric materials expands from the understanding of their behavior to the prediction of new materials, playing a fundamental role in the opti- mization of their performance. In contrast with other areas of magnetic materials research, micromagnetic simulations of magnetocaloric materials are scarce due to the difficulty of modeling the material in the vicinity of the transition. To solve this limitation, we propose to use the Landau–Lifshitz–Bloch micromagnetic simulations to study the mag- netocaloric effect associated with a second-order ferromagnetic↔paramagnetic transition. Following our proposed methodology and considering material parameters in a mean-field framework, we obtain reliable isothermal entropy change curves for monocrystalline and polycrystalline configurations, where we consider different anisotropic contributions. The robustness of the method was evaluated, yielding results that agreed with previous ex- perimental and theoretical observations. Our study shows that micromagnetic simulations are a powerful tool for analyzing second-order magnetocaloric materials with complex microstructures. 2026-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/83785 Optical transmission changes in ferrofluids when exposed to magnetic fields are difficult to predict: under similar conditions, exposure to a magnetic field may cause increasing, decreasing or even non monotonous optical transmission evolution in different samples. Absence or presence of coalescence has been conjectured as the key phenomenon that causes the different behaviors, but to our knowledge without any theoretical support. In the first part of this work, experimental data are provided in order to illustrate the different possible trends that may be observed. In the second part, a set of some thousands of particles is considered in different arrangements: distributed at random, grouped in single chains or with aggregates of several chains. The optical transmission of each arrangement is obtained by means of the CST software. The numerical results obtained provide a theoretical connection between the optical transmission experimental trends observed and the processes of chain formation and aggregation by coalescence, which facilitates a much deeper understanding of the phenomena observed. 2020-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/81368 The radial basis function generatedfinite difference (RBFFD) method is applied to the analysis of homogenous waveguides. To this end, the Helmholtz equation and the boundary conditions are collocated on the waveguide cross section. At each collocation node, derivatives are locally approximated by RBFFD formulas based on polyharmonic splines supplemented with highdegree polynomials. As a result, a sparse matrix eigenvalue problem is obtained which allows cutoff wavenumbers and axial fields to be calculated. To illustrate the accuracy of the method, we consider a semicircular and an eccentric circular waveguides. 2020-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/81365 The calculation of dispersion diagrams and field patterns of metallic rectangular waveguides filled with an inhomogeneous dielectric whose permittivity varies continuously along the broad size of the guide is considered. In general, this problem has no exact solution, thus numerical techniques should be used. In this article, the pseudospectral frequency-domain (PSFD) method is proposed to address the problem. Starting from the Helmholtz equation, a matrix eigenvalue problem is obtained by applying the collocation technique with Chebyshev polynomials as basis functions. The results obtained are compared with those calculated by the conventional finite-difference frequency-domain method showing that the PSFD technique provides an excellent accuracy. 2020-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/81364 The Runge–Kutta finite-difference time-domain (RK FDTD) method is an extension of the conventional finite-difference time-domain (FDTD) technique to include graphene sheets. According to this method, the relationship between the current density and the electric field for graphene is discretizedby applying an explicit second-order Runge–Kutta(RK) scheme. It has recently been concluded that the RK-FDTD method is subject to the same Courant–Friedrichs–Lewy (CFL) stability limit as the conventional FDTD method. This communication revisits the stability analysis of the RK-FDTD method. To this end, the von Neumann method is combined with the Routh–Hurwitz (RH) criterion. As a result, closed-form stability conditions are obtained. It is shown that in addition to the CFL stability limit, the RK-FDTD method must also satisfy new conditions involving graphene parameters. Unfortunately, the RK-FDTD method becomes unstable for commonly used values of these parameters. The theoretical results are confirmed with numerical examples. 2025-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/80859 This letter introduces anunconditionally stable finite‐difference time domain (FDTD) method, based on the locally one‐dimensional (LOD) technique, for the solution of the two‐dimensional scalar wave equation (WE) inhomogeneous media. The second spatial derivatives in the WE are discretized by using a three‐pointcompact (implicit) finite‐difference formula with a free parameter. This formula has second‐order accuracy and becomes fourth‐order by properly selecting the parameter value. Moreover, the resulting algorithm only involves tridiagonalmatrices, as when using standard (explicit) second‐order finite differences. Additionally, a stability analysis is performed and the numerical dispersion relation of the method is derived.The proposed compact LOD‐WE‐FDTD technique has been applied to the calculation of resonant frequencies in a metallic ridge cavity. The accuracy of the results obtained has been studied as a function of the parameter value. 2024-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/80676 Domain wall motion along ferrimagnets is evaluated using micromagnetic simulations and a collective-coordinates model, both considering two sublattices with independent parameters. Analytical expressions are derived for strips on top of either a heavy metal or a substrate with negligible interfacial Dzyaloshinskii-Moriya Interaction. The work focuses its findings in this latter case, with a field-driven domain wall motion depicting precessional dynamics which become rigid at the angular momentum compensation temperature, and a current-driven dynamics presenting more complex behavior, depending on the polarization factors for each sublattice. Importantly, our analyses provide also novel interpretation of recent evidence on current-driven domain wall motion, where walls move either along or against the current depending on temperature. Besides, our approach is able to substantiate the large non-adiabatic effective parameters found for these systems. 2020-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/77892 We performed classical molecular dynamics simulations to investigate, from an atomistic point of view, the formation of dislocations during the epitaxial growth of Ge on Si. We show that simulations at 900 and 1000 K with deposition rates of 10 monolayers per second provide a good compromise between computational cost and accuracy. In these conditions, the ratio between the Ge deposition rate and the ad-atom jump rate is analogous to that of out-of-equilibrium experiments. In addition, the main features of the grown film (intermixing, critical film thickness, dislocation typology, and surface morphology) are well described. Our simulations reveal that dislocations originate in low-density amorphous regions that form under valleys of the rough Ge film surface. Atoms are squeezed out of these regions to the surface, releasing the stress accumulated in the film and smoothing its roughness. Amorphous regions grow until atoms begin to rearrange in dislocation half-loops that propagate throughout the Ge film. The threading arm ends of the dislocation half-loops move along the surface following valleys and avoiding islands. The film surface morphology affects the propagation path of the dislocation half-loops and the resulting dislocation network. 2026-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/77889 A Deep Learning approach is devised to estimate the elastic energy density at the free surface of an undulated stressed film. About 190000 arbitrary surface profiles are randomly generated by Perlin noise and paired with the corresponding elastic energy density profiles , computed by a semi-analytical Green’s function approximation, suitable for small-slope morphologies. The resulting dataset and smaller subsets of it are used for the training of a Fully Convolutional Neural Network. The trained models are shown to return quantitative predictions of , not only in terms of convergence of the loss function during training, but also in validation and testing, with better results in the case of the larger dataset. Extensive tests are performed to assess the generalization capability of the Neural Network model when applied to profiles with localized features or assigned geometries not included in the original dataset. Moreover, its possible exploitation on domain sizes beyond the one used in the training is also analyzed in-depth. The conditions providing a one-to-one reproduction of the “ground-truth” profiles computed by the Green’s approximation are highlighted along with critical cases. The accuracy and robustness of the deep-learned are further demonstrated in the time-integration of surface evolution problems described by simple partial differential equations of evaporation/condensation and surface diffusion. 2025-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/77885 Antiferromagnetic magnonics has become the focus of intense scientific research because of the advantages of these materials compared to ferromagnets. However, ferrimagnetic materials have received much less attention despite exhibiting similar dynamical features at the angular momentum compensation point. In this paper, we present analytical expressions describing the dispersion relation of forward volume spin waves in ferrimagnetic materials near the angular momentum compensation point. We benchmark the derived dispersion relations against full micromagnetic simulations showing an excellent agreement between both approaches. We predict two different branches for forward volume spin waves in ferrimagnetic materials merging into a single branch at the angular momentum compensation point. Our results can assist in the design of magnonic devices built on ferrimagnetic materials. 2025-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/73799 Memristive devices have shown a great potential for non-volatile memory circuits and neuromorphic computing. For both applications it is essential to know the physical mechanisms behind resistive switching; in particular, the time response to external voltage signals. To shed light in these issues we have studied the role played by the applied voltage ramp rate in the electrical properties of TiN/Ti/HfO2/W metal–insulator–metal resistive switching devices. Using an ad hoc experimental set-up, the current–voltage characteristics were measured for ramp rates ranging from 100 mV s−1–1 MV s−1. These measurements were used to investigate in detail the set and reset transitions. It is shown that the highest ramp rates allow controlling the resistance values corresponding to the intermediate states at the very beginning of the reset process, which is not possible by means of standard quasistatic techniques. Both the set and reset voltages increase with the ramp rate because the oxygen vacancies movement is frequency dependent so that, when the ramp rate is high enough, the conductive filaments neither fully form nor dissolve. In agreement with Chua's theory of memristive devices, this effect causes the device resistance window to decrease as the ramp rate increases, and even to vanish for very high ramp rates. Remarkably, we demonstrate that the voltage ramp rate can be straightforwardly used to control the conductance change of the switching devices, which opens up a new way to program the synaptic weights when using these devices to mimic synapses for neuromorphic engineering applications. Moreover, the data obtained have been compared with the predictions of the dynamic memdiode model. 2023-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/73797 TiN/Ti/HfO2/Pt resistive switching devices have been fabricated, measured, and modeled. After programming the devices in the low resistance state, the current–voltage characteristic below the reset switching voltage was measured at different temperatures (from 90 to 350 K). A weak but complex temperature dependence was obtained for several voltage regimes. These memristors belong to a wider set known as valence change memories, whose conductance is determined by the formation of conductive filaments (CFs) linked to a high density of oxygen vacancies in a dielectric sandwiched between two metal electrodes. This usually leads to ohmic conduction in the low resistance state. However, a non-linear current dependence has been also observed in the measured devices, in addition to symmetric current–voltage curves for positive and negative biases in the 0–0.6 V voltage range. Three different thermal dependences have been considered for explaining the whole set of experimental data. Two of them are linked to ohmic filamentary conduction; the CF shows a conductivity enhancement due to thermally activated mechanisms at low temperatures; on the contrary, a CF conductivity degradation is observed at the higher temperatures. Finally, an additional slightly higher value for the non-linear current component as the temperature rises has also been taken into account. A semiempirical compact model has been implemented including these conduction mechanisms and their corresponding temperature dependences, the device has been simulated in LT-Spice and the experimental currents have been correctly reproduced. 2022-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/73756 Reliable data storage technologies able to operate at cryogenic temperatures are critical to implement scalable quantum computers and develop deep-space exploration systems, among other applications. Their scarce availability is pushing towards the development of emerging memories that can perform such storage in a non-volatile fashion. Resistive Random-Access Memories (RRAM) have demonstrated their switching capabilities down to 4K. However, their operability at lower temperatures still remain as a challenge. In this work, we demonstrate for the first time the forming and resistive switching capabilities of CMOS-compatible RRAM devices at 1.4K. The HfO2-based devices are deployed following an array of 1-transistor-1-resistor (1T1R) cells. Their switching performance at 1.4K was also tested in the multilevel-cell (MLC) approach, storing up to 4 resistance levels per cell. 2024-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/70425 In this study, the resistive switching phenomena in TiN/Ti/HfO2/Ti metal–insulator–metal stacks is investigated, mainly focusing on the analysis of set and reset transitions. The electrical measurements in a wide temperature range reveal that the switching transitions require less voltage (and thus, less energy) as temperature rises, with the reset process being much more temperature sensitive. The main conduction mechanism in both resistance states is Space-charge-limited Conduction, but the high conductivity state also shows Schottky emission, explaining its temperature dependence. Moreover, the temporal evolution of these transitions reveals clear differences between them, as their current transient response is completely different. While the set is sudden, the reset process development is clearly non-linear, closely resembling a sigmoid function. This asymmetry between switching processes is of extreme importance in the manipulation and control of the multi-level characteristics and has clear implications in the possible applications of resistive switching devices in neuromorphic computing. 2024-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/70142 M-type hexagonal ferrites have been getting considerable attention owing to their promising application in electronic fields. Though, the growth of nanosized M-type hexagonal ferrites is still a big challenge. Herein the fabrication of M-type hexaferrite with nominal composition Ca1-xBixFe12-xCoxO19 (x=0.00, 0.05, 0.10, 0.15, 0.20) with high quality is reported by the sol-gel auto combustion route. The objective of the study is to improve the structural, spectral, dielectric, and magnetic characteristics of M-type hexagonal ferrite which was achieved through the variation of concentration of Cobalt and Bismuth. X-ray diffraction (XRD) patterns confirmed the single-phase M-type hexagonal structure. The crystallite size of all samples was found to be in the range of 42–49 nm. Other parameters such as lattice parameters a & c, unit cell volume, crystallite size, X-ray density, Bulk density, and porosity were also calculated. The doping contents were found to decrease the bulk and X-ray densities while increasing the porosity. Fourier-transform infrared (FTIR) spectra showed the formation of metaloxygen stretching vibrations that confirmed the formation of hexagonal ferrites. The scanning electron microscopy (SEM) images revealed a regular platelet hexagonal structure and homogeneously distributed grains were examined. The dielectric constant was high at low frequency and then decreased with increasing frequency, while the dielectric loss was decreased appreciably with doping. The saturation magnetization ranged from 15.51 to 38.27 emu/g, coercivity increased from 207.93 to 1359.69 Oe, and the squareness ratio was found to be in the range of 0.19–0.78. The dielectric and magnetic properties of Ca1- BixFe12-xCoxO19 (x=0.00, 0.05, 0.10, 0.15, 0.20) with the variation of Co and Bi revealed that these materials are good candidates for modern devices 2024-01-01T00:00:00Z https://uvadoc.uva.es/handle/10324/70067 The Y-type hexaferrite and its nanocomposite with carbon dots have been made by using a hydrothermal approach, whereas BaSrZn2-xMnxFe12-2ySiyNiyO22 (x = 0.0, 0.25, 0.5, 0.75, and 1.0) was made using a conventional microemulsion process. A variety of approaches are used to explore the structural features, Surface area, and morphology of the materials. As indicated by on-site substitutions and super-exchange interaction, saturation magnetization (Ms) and magnetic remanence (Mr) rise from 18 to 6 to 47.4 emu g- 1 and 6.7–18.3 emu g- 1, respectively with x,y = 0.75, although coercivity (Hc) declines from 1.4 to 0.39 kOe. The ferrite material’s electrical resistivity enhanced from 25.85 × 106 to 49.13 × 106 Ω-cm. The increased saturation magnetization (Ms), magnetic remanence (Mr), and electrical resistance of ferrite material make it suitable for both high-density recording and microwave devices. The photocatalyst (composite) for the degradation of Rhodamine B in the presence of visible light was made from SrBaZn1.25Mn0.75Fe10.5Si0.75Ni0.75O22/CDs with different ratios. Photocatalysts with a modest CDs concentration (2.5 wt%) successfully degraded Rhodamine B (RhB) up to 94%. After eight rounds, the composite showed excellent structural stability and good reusability. The trapping experiments confirmed that OH radical species are the major contributor in the degradation process. The composite material could be used to catch visible light and purify the environment from various pollutants in the future. 2023-01-01T00:00:00Z