arXiv subjects

P. Nieves

Publications and source records attributed to P. Nieves.

15 recordsLinked to original sources

arXiv2024

VelCrys: Interactive web-based application to compute acoustic wave velocity in crystals and its magnetic corrections

We present VelCrys, a web-based interactive tool, that allows to perform further post-processing of the elastic tensor in order to compute and plot the group velocity of the acoustic waves for any crystal symmetry. We also implemented the calculation of effective magnetic corrections to the elastic tensor and corresponding fractional change in group velocity under a magnetic field. We apply it to dry sandstone, cubic CoPt and hcp Co to show some of the program features. In the analysis of magnetic corrections, we find complex landscapes of fractional change in group velocity as a function of ray direction, as well as a field dependence consistent with Simon effect.

cond-mat.mtrl-sci↗

arXiv2022

Automated calculations of exchange magnetostriction

We present a methodology based on deformations of the unit cell that allows to compute the isotropic magnetoelastic constants, isotropic magnetostrictive coefficients and spontaneous volume magnetostriction associated to the exchange magnetostriction. This method is implemented in the python package MAELAS (v3.0), so that it can be used to obtain these quantities by first-principles calculations and classical spin-lattice models in an automated way. We show that the required reference state to obtain the spontaneous volume magnetostriction combines the equilibrium volume of the paramagnetic state and magnetic order of the ground state. We identify an error in the theoretical expression of the isotropic magnetostrictive coefficient $\lambda^{\alpha 1,0}$ for uniaxial crystals given in previous publications, which is corrected in this work. The presented computational tool may be helpful to provide a better understanding and characterization of the relationship between the exchange interaction and magnetoelasticity.

cond-mat.mtrl-sci↗

arXiv2022

First-principles calculations of the spontaneous volume magnetostriction based on the magnetoelastic energy

We present a simple methodology to compute the spontaneous volume magnetostriction with first-principles calculations on the basis of the magnetoelastic energy. This method makes use of deformations of the unit cell only at the ferromagnetic state. Hence, it does not require the difficult first-principles calculation of the equilibrium volume at the paramagnetic state. To validate this methodology, we apply it to body-centered cubic Fe and face-centered cubic Ni single crystals, finding consistent results with experiment and previous first-principles calculations. The simplicity and reliability of this approach could be exploited in the high-throughput screening of spontaneous volume magnetostriction, as well as associated quantities like isotropic magnetoelastic constants and isotropic magnetostrictive coefficients.

cond-mat.mtrl-sci↗

arXiv2022

Atomistic simulations of magnetoelastic effects on sound velocity

In this work, we leverage atomistic spin-lattice simulations to examine how magnetic interactions impact the propagation of sound waves through a ferromagnetic material. To achieve this, we characterize the sound wave velocity in BCC iron, a prototypical ferromagnetic material, using three different approaches that are based on the oscillations of kinetic energy, finite-displacement derived forces, and corrections to the elastic constants, respectively. Successfully applying these methods within the spin-lattice framework, we find good agreement with the Simon effect including high order terms. In analogy to experiments, morphic coefficients associated with the transverse and longitudinal waves propagating along the [001] direction are extracted from fits to the fractional change in velocity data. The present efforts represent an advancement in magnetoelastic modelling capabilities which can expedite the design of future magneto-acoustic devices.

cond-mat.mtrl-sci↗

arXiv2021

MAELAS 2.0: A new version of a computer program for the calculation of magneto-elastic properties

MAELAS is a computer program for the calculation of magnetocrystalline anisotropy energy, anisotropic magnetostrictive coefficients and magnetoelastic constants in an automated way. The method originally implemented in version 1.0 of MAELAS was based on the length optimization of the unit cell, proposed by Wu and Freeman, to calculate the anisotropic magnetostrictive coefficients. We present here a revised and updated version (v2.0) of MAELAS, where we added a new methodology to compute anisotropic magnetoelastic constants from a linear fitting of the energy versus applied strain. We analyze and compare the accuracy of both methods showing that the new approach is more reliable and robust than the one implemented in version 1.0, especially for non-cubic crystal symmetries. This analysis also help us to find that the accuracy of the method implemented in version 1.0 could be improved by using deformation gradients derived from the equilibrium magnetoelastic strain tensor, as well as potential future alternative methods like the strain optimization method. Additionally, we clarify the role of the demagnetized state in the fractional change in length, and derive the expression for saturation magnetostriction for polycrystals with trigonal, tetragonal and orthorhombic crystal symmetry. In this new version, we also fix some issues related to trigonal crystal symmetry found in version 1.0.

cond-mat.mtrl-sci↗

arXiv2021

Influence of grain morphology and orientation on saturation magnetostriction of polycrystalline Terfenol-D

In this work we computationally study the effect of microstructure on saturation magnetostriction of Terfenol-D (Tb$_{0.27}$Dy$_{0.73}$Fe$_{2}$) by means of Finite Element Method. The model is based on the equilibrium magnetoelastic strain tensor at magnetic saturation, and shows that the crystal orientation might play a more significant role on saturation magnetostriction than the morphology of the grains. We also calculate the dependence of saturation magnetostriction on the dispersion angle of the distribution of grains in the oriented growth crystal directions $<011>$ and $<111>$, finding that not highly oriented grain distributions reduce saturation magnetostriction significantly. This result evinces the importance of high-quality control of grain orientation in the synthesis of grain-aligned polycrystalline Terfenol-D, and provides a quantitative estimation for the range of acceptable values for the dispersion angle of the distribution of the oriented grains.

cond-mat.mtrl-sci↗

arXiv2021

Spin-lattice model for cubic crystals

We present a methodology based on the Néel model to build a classical spin-lattice Hamiltonian for cubic crystals capable of describing magnetic properties induced by the spin-orbit coupling like magnetocrystalline anisotropy and anisotropic magnetostriction, as well as exchange magnetostriction. Taking advantage of the analytical solutions of the Néel model, we derive theoretical expressions for the parameterization of the exchange integrals and Néel dipole and quadrupole terms that link them to the magnetic properties of the material. This approach allows to build accurate spin-lattice models with the desire magnetoelastic properties. We also explore a possible way to model the volume dependence of magnetic moment based on the Landau energy. This new feature can allow to consider the effects of hydrostatic pressure on the saturation magnetization. We apply this method to develop a spin-lattice model for BCC Fe and FCC Ni, and we show that it accurately reproduces the experimental elastic tensor, magnetocrystalline anisotropy under pressure, anisotropic magnetostrictive coefficients, volume magnetostriction and saturation magnetization under pressure at zero-temperature. This work could constitute a step towards large-scale modeling of magnetoelastic phenomena.

cond-mat.mtrl-sci↗

arXiv2020

MAELAS: MAgneto-ELAStic properties calculation via computational high-throughput approach

In this work, we present the program MAELAS to calculate magnetocrystalline anisotropy energy, anisotropic magnetostrictive coefficients and magnetoelastic constants in an automated way by Density Functional Theory calculations. The program is based on the length optimization of the unit cell proposed by Wu and Freeman to calculate the magnetostrictive coefficients for cubic crystals. In addition to cubic crystals, this method is also implemented and generalized for other types of crystals that may be of interest in the study of magnetostrictive materials. As a benchmark, some tests are shown for well-known magnetic materials.

cond-mat.mtrl-sci↗

arXiv2019

Computational Design of the Rare-Earth Reduced Permanent Magnets

Multiscale simulation is a key research tool for the quest for new permanent magnets. Starting with first principles methods, a sequence of simulation methods can be applied to calculate the maximum possible coercive field and expected energy density product of a magnet made from a novel magnetic material composition. Fe-rich magnetic phases suitable for permanent magnets can be found by adaptive genetic algorithms. The intrinsic properties computed by ab initio simulations are used as input for micromagnetic simulations of the hysteresis properties of permanent magnets with realistic structure. Using machine learning techniques, the magnet's structure can be optimized so that the upper limits for coercivity and energy density product for a given phase can be estimated. Structure property relations of synthetic permanent magnets were computed for several candidate hard magnetic phases. The following pairs (coercive field (T), energy density product (kJ/m3)) were obtained for Fe3Sn0.75Sb0.25: (0.49, 290), L10 FeNi: (1, 400), CoFe6Ta: (0.87, 425), and MnAl: (0.53, 80).

cond-mat.mtrl-sci↗

arXiv2019

The influence of antiphase boundary of the MnAl $τ$-phase on the energy product

In this work we use a multiscale approach toward a realistic design of a permanent magnet based on MnAl $τ$-phase and elucidate how the antiphase boundary defects present in this material affect the energy product. We show how the extrinsic properties of a microstructure depend on the intrinsic properties of a structure with defects by performing micromagnetic simulations. For an accurate estimation of the energy product of a realistic permanent magnet based on the MnAl $τ$-phase with antiphase boundaries, we quantify for the first time the exchange interaction strength across the antiphase boundary defect with a simple approach derived from the first-principles calculations. These two types of calculations performed at different scales are linked via atomistic spin dynamic simulations performed at an intermediate scale.

cond-mat.mtrl-sci↗

arXiv2019

Database of novel magnetic materials for high-performance permanent magnet development

This paper describes the open Novamag database that has been developed for the design of novel Rare-Earth free/lean permanent magnets. The database software technologies, its friendly graphical user interface, advanced search tools and available data are explained in detail. Following the philosophy and standards of Materials Genome Initiative, it contains significant results of novel magnetic phases with high magnetocrystalline anisotropy obtained by three computational high-throughput screening approaches based on a crystal structure prediction method using an Adaptive Genetic Algorithm, tetragonally distortion of cubic phases and tuning known phases by doping. Additionally, it also includes theoretical and experimental data about fundamental magnetic material properties such as magnetic moments, magnetocrystalline anisotropy energy, exchange parameters, Curie temperature, domain wall width, exchange stiffness, coercivity and maximum energy product, that can be used in the study and design of new promising high-performance Rare-Earth free/lean permanent magnets. The results therein contained might provide some insights into the ongoing debate about the theoretical performance limits beyond Rare-Earth based magnets. Finally, some general strategies are discussed to design possible experimental routes for exploring most promising theoretical novel materials found in the database.

cond-mat.mtrl-sci↗

arXiv2016

Magnetization switching of FePt nanoparticle recording medium by femtosecond laser pulses

Manipulation of magnetization with ultrashort laser pulses is promising for information storage device applications. The dynamic of the magnetization response depends on the energy transfer from the photons to the spins during the initial laser excitation. A material of special interest for magnetic storage is FePt nanoparticles , on which optical writing with optical angular momentum was demonstrated recently by Lambert et al., although the mechanism remained unclear. Here we investigate experimentally and theoretically the all-optical switching of FePt nanoparticles. We show that the magnetization switching is a stochastic process. We develop a complete multiscale model which allows us to optimize the number of laser shots needed to write the magnetization of high anisotropy FePt nanoparticles in our experiments. We conclude that only angular momentum induced optically by the inverse Faraday effect will provide switching with one single femtosecond laser pulse.

cond-mat.mes-hall↗

arXiv2015

Quantum Landau-Lifshitz-Bloch equation and its comparison with the classical case

The detailed derivation of the quantum Landau-Lifshitz-Bloch (qLLB) equation for simple spin-flip scattering mechanisms based on spin-phonon and spin-electron interactions is presented and the approximations are discussed. The qLLB equation is written in the form, suitable for comparison with its classical counterpart. The temperature dependence of the macroscopic relaxation rates is discussed for both mechanisms. It is demonstrated that the magnetization dynamics is slower in the quantum case than in the classical one.

cond-mat.mtrl-sci↗

arXiv2015

Multiscale modeling of ultrafast element-specific magnetization dynamics of ferromagnetic alloys

A hierarchical multiscale approach to model the magnetization dynamics of ferromagnetic ran- dom alloys is presented. First-principles calculations of the Heisenberg exchange integrals are linked to atomistic spin models based upon the stochastic Landau-Lifshitz-Gilbert (LLG) equation to calculate temperature-dependent parameters (e.g., effective exchange interactions, damping param- eters). These parameters are subsequently used in the Landau-Lifshitz-Bloch (LLB) model for multi-sublattice magnets to calculate numerically and analytically the ultrafast demagnetization times. The developed multiscale method is applied here to FeNi (permalloy) as well as to copper- doped FeNi alloys. We find that after an ultrafast heat pulse the Ni sublattice demagnetizes faster than the Fe sublattice for the here-studied FeNi-based alloys.

cond-mat.mtrl-sci↗

arXiv2012

The Landau-Lifshitz-Bloch equation for ferrimagnetic materials

We derive the Landau-Lifshitz-Bloch (LLB) equation for a two-component magnetic system valid up to the Curie temperature. As an example, we consider disordered GdFeCo ferrimagnet where the ultrafast optically induced magnetization switching under the action of heat alone has been recently reported. The two-component LLB equation contains the longitudinal relaxation terms responding to the exchange fields from the proper and the neighboring sublattices. We show that the sign of the longitudinal relaxation rate at high temperatures can change depending on the dynamical magnetization value and a dynamical polarisation of one material by another can occur. We discuss the differences between the LLB and the Baryakhtar equation, recently used to explain the ultrafast switching in ferrimagnets. The two-component LLB equation forms basis for the largescale micromagnetic modeling of nanostructures at high temperatures and ultrashort timescales.

cond-mat.mtrl-sci↗