arXiv subjects

Javier Tejada

Publications and source records attributed to Javier Tejada.

12 recordsLinked to original sources

arXiv2024

Radar shielding experiments by composite bilayer magnetic systems

An experimental strong increase of the reflection loss (from 25 up to 35 dBs) and an extension of the absorption bandwidth up to 20% is measured in a set of novel functional bilayer systems. We focus our work on studying the samples in an anechoic chamber under far-field real radar conditions. Each layer consists of a composite material, typically a dielectric matrix filled with random anisotropy hexaferrite and soft metallic materials (powder or wires). Combining the two types of materials into a submillimetric bilayer structure has shown unprecedented improvements in microwave absorption capacities compared to the former absorption of each layer. The capacity to improve the shielding behavior is strongly related to each layer permittivity, permeability, and thickness leading, therefore, to a strong control over the design of novel materials for stealth applications.

cond-mat.mtrl-sci↗

arXiv2024

Experimental radar absorption in high-filling factor magnetic composites

The electromagnetic properties and microwave absorption capabilities are studied in an anechoic chamber under real radar conditions for mono and bilayer composite samples consisting of a polymeric matrix and a magnetic powder filler, either metallic or ceramic. The effect of the filler type and the filling factor is investigated. The results demonstrate exceptional broadband microwave absorption, making these materials highly suitable for stealth technology applications. The experimentally measured absorptions reach -40 dB, while model-based predictions suggest that these systems could overpass the barrier of -50 dB. The experimental results are supported by models, both for single and bilayer systems.

cond-mat.mtrl-sci↗

arXiv2024

Microwave absorption in barium hexaferrite nanocomposites with random anisotropy

This work reports experimental evidence of random magnetic behavior observed in modified barium hexagonal ferrites. We observe a significant transition in the magnetic properties of this system when divalent cations (Ni2+, Cu2+, Mn2+) are incorporated into the structure and give rise to a magnetic nanocomposite. Such introduction randomly occurs throughout each sample and creates conditions for such materials to behave as random anisotropy magnets. We verify the occurrence of such behavior in our samples by fitting the magnetization in approaching saturation to the corresponding theoretical model. We therefore analyze the microwave absorption capacities of random anisotropy magnets in the GHz range and predict large and broad absorption signals under certain conditions. The findings presented here postulate, for the first time, ceramic materials as promising random anisotropy magnets and underline their potential as microwave absorbers, in good agreement with recent theoretical models.

cond-mat.mtrl-sci↗

arXiv2023

New approach to designing functional materials for stealth technology: Radar experiment with bilayer absorbers and optimization of the reflection loss

Microwave power absorption by a two-layer system deposited on a metallic surface has been studied in the experimental setup emulating the response to a radar signal. Layers containing hexaferrite and iron powder in a dried paint of thickness under 1mm have been used. The data is analyzed within a theoretical model derived for a bilayer system from the transmission line theory. A good agreement between experimental and theoretical results is found. The advantage of using a bilayer system over a single-layer system has been demonstrated. How the maximum microwave absorption (minimum reflection loss) can be achieved through the optimization of the filling factors and thicknesses of the two layers is shown.

physics.app-ph↗

arXiv2018

Manipulating Quantum Spins by a Spin-Polarized Current: An Approach Based Upon PT-Symmetric Quantum Mechanics

We propose a quantum processor based upon single-molecule magnets and spin transfer torque described by PT-symmetric quantum mechanics. In recent years PT-symmetric Hamiltonians have been used to obtain stability thresholds of various systems out of equilibrium. One such problem is the magnetization reversal due to the spin transfer torque generated by a spin-polarized current. So far the studies of this problem have mostly focused on a classical limit of a large spin. In this work we are discussing spin tunneling and quantum dynamics of a small spin induced by a spin polarized current within a PT-symmetric theory. This description can be used for manipulating spin qubits by electric currents.

cond-mat.mes-hall↗

arXiv2013

Quantum Forces in Molecular Magnets

Two-state systems may exhibit mechanical forces of purely quantum origin that have no counterpart in classical physics. We show that the such forces must exist in molecular magnets due to quantum tunneling between classically degenerate magnetic states. They can be observed in the presence of a microwave field when the magnet is placed in a static magnetic field with a gradient.

cond-mat.mes-hall↗

arXiv2013

Magnetic properties of double exchange biased diluted magnetic alloy/ferromagnet/antiferromagnet trilayers

The magnetic properties of trilayers consisting of a diluted magnetic alloy, CuMn (Cu0.99Mn0.01), a soft ferromagnet, Py(Ni0.8Fe0.2), and an antiferromagnet, alpha-Fe2O3, were investigated. The samples, grown by UHV magnetron sputtering, were magnetically characterized in the temperature range T = 3-100 K. Typical exchange bias features, namely clear hysteresis cycle shifts and coercivity enhancements, were observed. Moreover the presence of an inverse bias, which had been already reported for spin glass-based structures, was also obtained in a well defined range of temperatures and CuMn thicknesses.

cond-mat.mtrl-sci↗

arXiv2012

Excitation modes of vortices in sub-micron magnetic disks

Classical and quantum theory of spin waves in the vortex state of a mesoscopic sub-micron magnetic disk has been developed with account of the finite mass density of the vortex. Oscillations of the vortex core resemble oscillations of a charged string in a potential well in the presence of the magnetic field. Conventional gyroscopic frequency appears as a gap in the spectrum of spin waves of the vortex. The mass of the vortex has been computed that agrees with experimental findings. Finite vortex mass generates a high-frequency branch of spin waves. Effects of the external magnetic field and dissipation have been addressed.

cond-mat.mes-hall↗

arXiv2012

Microwave Spectrometry for the Evaluation of the Dimensions of Coronary Stents

We study microwave scattering spectra of metallic stents in open air. We show that they behave like dipole antennas in terms of microwave scattering and they exhibit characteristic resonant frequencies for a given nominal size. We obtain a fair agreement between measured frequencies and the values provided by a theoretical model for dipole antennas. This fact opens the door to obtaining methods to detect structural distortions of stents within in vitro conditions. Finally we discuss the in vivo applicability of the suggested method in terms of our theoretical model and the skin depth of microwaves in biological tissues.

physics.med-ph↗

arXiv2011

Quantum depinning of the magnetic vortex core in micron-size permalloy disks

The vortex state, characterized by an in-plane closed flux domain structure and an out-of-plane magnetization at its centre (the vortex core), is one of the magnetic equilibria of thin soft ferromagnetic micron-size dots. In the last two decades many groups have been working on the dynamics of the magnetic moment in nanomagnetic materials at low temperatures, it giving rise to the observation of quantum relaxations and quantum hysteresis cycles. For the first time, we report experimental evidence of quantum dynamics of the vortex core of micron-size permalloy (Fe$_{19}$Ni$_{81}$) disks induced by the application of an in-plane magnetic field. It is attributed to the quantum tunneling of the vortex core through pinning barriers, which are associated to structural defects in the dots, towards its equilibrium position.

cond-mat.mes-hall↗

arXiv2011

The role of temperature in the magnetic irreversibility of type-I Pb superconductors

Evidence of how temperature takes part in the magnetic irreversibility in the intermediate state of a cylinder and various disks of pure type-I superconducting lead is presented. Isothermal measurements of first magnetization curves and magnetic hysteresis cycles are analyzed in a reduced representation that defines an equilibrium state for flux penetration in all the samples and reveals that flux expulsion depends on temperature in the disks but not in the cylinder. The magnetic field at which irreversibility sets in along the descending branch of the hysteresis cycle and the remnant magnetization at zero field are found to decrease with temperature in the disks. The contributions to irreversibility of the geometrical barrier and the energy minima associated to stress defects that act as pinning centers on normal-superconductor interfaces are discussed. The differences observed among the disks are ascribed to the diverse nature of the stress defects in each sample. The pinning barriers are suggested to decrease with the magnetic field to account for these results.

cond-mat.supr-con↗

arXiv2011

Magnetic field dependence of the quantum tunneling of normal-superconductor interfaces in a type-I Pb superconductor

We report experimental evidence of the effect of an applied magnetic field on the non-thermal magnetic relaxation in a disk-shaped type-I lead superconductor. The time evolution of the irreversible magnetization proves to be logarithmic for a wide range of temperatures and magnetic field values along the descending branch of the hysteresis cycle. When the intensity of the magnetic field increases, the crossover temperature separating the thermal and non-thermal regimes of magnetic relaxation is found to decrease, whereas the rate at which such relaxation occurs is observed to increase. These results are discussed in the framework of a recent model for quantum tunneling of normal-superconductor interfaces through the distribution of pinning energy barriers generated by structural defects in the sample, considering that the strength of the barriers decreases with the magnetic field. A phase diagram describing the dynamics of interfaces during flux expulsion in the intermediate state as a function of temperature and magnetic field is constructed.

cond-mat.supr-con↗