arXiv subjects

I. P. Miranda

Publications and source records attributed to I. P. Miranda.

11 recordsLinked to original sources

arXiv2025

Spin-lattice couplings in $3d$ ferromagnets: analysis from first-principles

Magnetoelasticity plays a crucial role in numerous magnetic phenomena, including magnetocalorics, magnon excitation via acoustic waves, and ultrafast demagnetization/Einstein-de Haas effect. Despite a long-standing discussion on anisotropy-mediated magnetoelastic interactions of relativistic origin, the exchange-mediated magnetoelastic parameters within an atomistic framework have only recently begun to be investigated. As a result, many of their behaviors and values for real materials remain poorly understood. Therefore, by using a proposed simple modification of the embedded cluster approach that reduces the computational complexity, we critically analyze the properties of exchange-mediated spin-lattice coupling parameters for elemental $3d$ ferromagnets (bcc Fe, fcc Ni, and fcc Co), comparing methods used for their extraction and relating their realistic values to symmetry considerations and orbitally-decomposed contributions. Additionally, we investigate the effects of noncollinearity (spin temperature) and applied pressure on these parameters. For Fe, we find that single-site rotations, associated with spin temperatures around $\sim100$ K, induce significant modifications, particularly in Dzyaloshinskii-Moriya-type couplings; in contrast, such interactions in Co and Ni remain almost configuration independent. Moreover, we demonstrate a notable change in the exchange-mediated magnetoelastic constants for Fe under isotropic contraction. Finally, the conversion between atomistic, quantum-mechanically derived parameters and the phenomenological magnetoelastic theory is discussed, which can be an useful tool towards larger and more realistic dynamics simulations involving coupled subsystems.

cond-mat.mtrl-sci↗

arXiv2025

Proximity-induced flipped spin state in synthetic ferrimagnetic Pt/Co/Gd heterolayers

To develop new devices based on synthetic ferrimagnetic heterostructures, understanding the material's physical properties is pivotal. Here, the induced magnetic moment (IMM), magnetic exchange coupling, and spin textures are investigated in Pt(1 nm)/Co(1.5 nm)/Gd(1 nm) multilayers using a multiscale approach. The magnitude and direction of the IMM are interpreted in the framework of both X-ray magnetic circular dichroism and density functional theory. The IMM transferred by Co across the Gd paramagnetic thickness leads to a nontrivial flipped spin state (FSS) within the Gd layers, in which their magnetic moments couple antiparallel/parallel with the ferromagnetic Co near/far from the Co/Gd interface, respectively. The FSS depends on the magnetic field, which, on average, reduces the Gd magnetic moment as the field increases. For the Pt, in both Pt/Co and Gd/Pt interfaces, the IMM follows the same direction as the Co magnetic moment, with negligible IMM in the Gd/Pt interface. Additionally, zero-field spin spirals were imaged using scanning transmission X-ray microscopy, whereas micromagnetic simulations were employed to unfold the interactions, stabilizing the ferrimagnetic configurations, where the existence of a sizable Dzyaloshinskii-Moriya interaction is demonstrated to be crucial.

cond-mat.mtrl-sci↗

arXiv2025

Design of 2D Skyrmionic Metamaterial Through Controlled Assembly

Despite extensive research on magnetic skyrmions and antiskyrmions, a significant challenge remains in crafting nontrivial high-order skyrmionic textures with varying, or even tailor-made, topologies. We address this challenge, by focusing on a construction pathway of skyrmionic metamaterials within a monolayer thin film and suggest several skyrmionic metamaterials that are surprisingly stable, i.e., long-lived, due to a self-stabilization mechanism. This makes these new textures promising for applications. Central to our approach is the concept of 'simulated controlled assembly', in short, a protocol inspired by 'click chemistry' that allows for positioning topological magnetic structures where one likes, and then allowing for energy minimization to elucidate the stability. Utilizing high-throughput atomistic-spin-dynamic simulations alongside state-of-the-art AI-driven tools, we have isolated skyrmions (topological charge Q=1), antiskyrmions (Q=-1), and skyrmionium (Q=0). These entities serve as foundational 'skyrmionic building blocks' to form the here reported intricate textures. In this work, two key contributions are introduced to the field of skyrmionic systems. First, we present a a novel combination of atomistic spin dynamics simulations and controlled assembly protocols for the stabilization and investigation of new topological magnets. Second, using the aforementioned methods we report on the discovery of skyrmionic metamaterials.

cond-mat.mtrl-sci↗

arXiv2024

Chemical disorder effects on the Gilbert damping of FeCo alloys

The impact of the local chemical environment on the Gilbert damping in the binary alloy Fe$_{100-x}$Co$_{x}$ is investigated, using computations based on density functional theory. By varying the alloy composition x as well as Fe/Co atom positions we reveal that the effective damping of the alloy is highly sensitive to the nearest neighbor environment, especially to the amount of Co and the average distance between Co-Co atoms at nearest neighbor sites. Both lead to a significant local increase (up to an order of magnitude) of the effective Gilbert damping, originating mainly from variations of the density of states at the Fermi energy. In a global perspective (i.e., making a configuration average for a real material), those differences in damping are masked by statistical averages. When low-temperature explicit atomistic dynamics simulations are performed, the impact of short-range disorder on local dynamics is observed to also alter the overall relaxation rate. Our results illustrate the possibility of local chemical engineering of the Gilbert damping, which may stimulate the study of new ways to tune and control materials aiming for spintronics applications.

cond-mat.mtrl-sci↗

arXiv2024

Probing $Ca_3Ti_2O_7$ crystal structure at the atomic level: Insights from $^{111m}Cd/^{111}Cd$ PAC spectroscopy and ab-initio studies

Perturbed angular correlation spectroscopy combined with $ab-initio$ electronic structure calculations is used to unravel the structural phase transition path from the low-temperature polar structure to the high-temperature structural phase in $Ca_3Ti_2O_7$, a hybrid improper ferroelectric. This procedure explores the unique features of a local probe environment approach by monitoring the evolution of the electric field gradient tensor at the calcium sites. The local environments, observed above 1057 K, confirm a structural phase transition from the $A2_1am$ symmetry to an orthorhombic $Acaa$ symmetry in the $Ca_3Ti_2O_7$ crystal lattice, disagreeing with the frequently reported avalanche structural transition from the polar $A2_1am$ phase to the aristotype $I4/mmm$ phase. Moreover, the EFG temperature dependency, within the $A2_1am$ temperature stability, is shown to be sensitive to the recently proposed $Ca_3Ti_2O_7$ ferroelectric polarization decrease within the 500-800~K temperature range.

cond-mat.mtrl-sci↗

arXiv2023

Heat-conserving three-temperature model for ultrafast demagnetization of 3d ferromagnets

We study the ultrafast magnetization dynamics of bcc Fe and fcc Co using the recently suggested heat-conserving three-temperature model (HC3TM), together with atomistic spin- and lattice dynamics simulations. It is shown that this type of Langevin-based simulation is able to reproduce observed trends of the ultrafast magnetization dynamics of fcc Co and bcc Fe, in agreement with previous findings for fcc Ni. The simulations are performed by using parameters that to as large extent as possible are obtained from electronic structure theory. The one parameter that was not calculated in this way, was the damping term used for the lattice dynamics simulations, and here a range of parameters were investigated. It is found that this term has a large influence on the details of the magnetization dynamics. The dynamics of iron and cobalt is compared with previous results for nickel and similarities and differences in the materials' behavior are analysed following the absorption of a femtosecond laser pulse. Importantly, for all elements investigated so far with this model, we obtain a linear relationship between the value of the maximally demagnetized state and the fluence of the laser pulse, which is in agreement with experiments.

cond-mat.mtrl-sci↗

arXiv2023

Metaheuristic conditional neural network for harvesting skyrmionic metastable states

We present a metaheuristic conditional neural-network-based method aimed at identifying physically interesting metastable states in a potential energy surface of high rugosity. To demonstrate how this method works, we identify and analyze spin textures with topological charge $Q$ ranging from 1 to $-13$ (where antiskyrmions have $Q<0$) in the Pd/Fe/Ir(111) system, which we model using a classical atomistic spin Hamiltonian based on parameters computed from density functional theory. To facilitate the harvest of relevant spin textures, we make use of the newly developed Segment Anything Model (SAM). Spin textures with $Q$ ranging from $-3$ to $-6$ are further analyzed using finite-temperature spin-dynamics simulations. We observe that for temperatures up to around 20\,K, lifetimes longer than 200\,ps are predicted, and that when these textures decay, new topological spin textures are formed. We also find that the relative stability of the spin textures depend linearly on the topological charge, but only when comparing the most stable antiskyrmions for each topological charge. In general, the number of holes (i.e., non-self-intersecting curves that define closed domain walls in the structure) in the spin texture is an important predictor of stability -- the more holes, the less stable is the texture. Methods for systematic identification and characterization of complex metastable skyrmionic textures -- such as the one demonstrated here -- are highly relevant for advancements in the field of topological spintronics.

physics.comp-ph↗

arXiv2022

Influence of non-local damping on magnon properties of ferromagnets

We study the influence of non-local damping on magnon properties of Fe, Co, Ni and Fe$_{1-x}$Co$_{x}$ ($x=30\%,50\%$) alloys. The Gilbert damping parameter is typically considered as a local scalar both in experiment and in theoretical modelling. However, recent works have revealed that Gilbert damping is a non-local quantity that allows for energy dissipation between atomic sites. With the Gilbert damping parameters calculated from a state-of-the-art real-space electronic structure method, magnon lifetimes are evaluated from spin dynamics and linear response, where a good agreement is found between these two methods. It is found that non-local damping affects the magnon lifetimes in different ways depending on the system. Specifically, we find that in Fe, Co, and Ni the non-local damping decreases the magnon lifetimes, while in $\rm Fe_{70}Co_{30}$ and Fe$_{50}$Co$_{50}$ an opposite, non-local damping effect is observed, and our data show that it is much stronger in the former.

cond-mat.mtrl-sci↗

arXiv2022

Band filling effects on the emergence of magnetic skyrmions: Pd/Fe and Pd/Co bilayers on Ir(111)

Structurally similar transition metal systems can have widely differing magnetic properties. A prime example of this is found for bilayers on Ir(111), where Pd/Fe/Ir(111) ground state has a well-established noncollinear spin texture, while Pd/Co/Ir(111) present a ferromagnetic (FM) single-domain. To unravel the origins of these different magnetic behaviors, an investigation of Pd/Fe and Pd/Co bilayers on Ir(111), is here performed. Based on the obtained \textit{ab-initio} electronic structure, exchange coupling parameters ($J_{ij}$) and Dzyaloshinskii-Moriya interactions (DMI), we demonstrate that, although in Pd/Co/Ir(111) the DMI is significant, two ingredients play a key role on the origin of noncollinearity in Pd/Fe/Ir(111): the presence of magnetic frustrations and a much more in-plane DMI, both with a long-range influence. The $J_{ij}$ and DMI behaviors in both systems can be explained in terms of a simple rigid-band-like model. Also, by performing spin-dynamics simulations with the magnetic parameters tuned from the Pd/Co/Ir(111) to Pd/Fe/Ir(111) \textit{ab-initio} values, we could find conditions for the emergence of skyrmionic phases in the originally FM Pd/Co/Ir(111).

cond-mat.mtrl-sci↗

arXiv2021

Complex magnetic textures in Ni/Ir$_{n}$/Pt(111) ultrathin films

A combined approach using first-principles calculations and spin dynamics simulations is applied to study Ni/Ir$_{n}$/Pt(111) ($n=0,1,2$) films. The lowest-energy states are predicted to be spin-spirals but with a minute (of the order of a few $μ$eV/atom) energy difference with skyrmionic states. The spontaneous low temperature skyrmions, with $\sim15$ nm to $\sim35$ nm size, arise from a large Dzyaloshinskii-Moriya (DM) and Heisenberg exchange interactions ratio and, in particular, from a large in-plane DM vector component for nearest neighbors. The skyrmions become larger and more dispersed with the enhancement of the Ir buffer thickness. Also, with increasing $n$, the skyrmions stability decrease when an external magnetic field is applied or the temperature is raised. For $n=0$ and $n=1$, we found that metastable skyrmioniums can occur, which are characterized by a slightly lower stability with respect to the external fields and larger critical currents, compared to skyrmions.

cond-mat.mtrl-sci↗

arXiv2021

Mechanisms behind large Gilbert damping anisotropies

A method with which to calculate the Gilbert damping parameter from a real-space electronic structure method is reported here. The anisotropy of the Gilbert damping with respect to the magnetic moment direction and local chemical environment is calculated for bulk and surfaces of Fe$_{50}$Co$_{50}$ alloys from first principles electronic structure in a real space formulation. The size of the damping anisotropy for Fe$_{50}$Co$_{50}$ alloys is demonstrated to be significant. Depending on details of the simulations, it reaches a maximum-minimum damping ratio as high as 200%. Several microscopic origins of the strongly enhanced Gilbert damping anisotropy have been examined, where in particular interface/surface effects stand out, as do local distortions of the crystal structure. Although theory does not reproduce the experimentally reported high ratio of 400% [Phys. Rev. Lett. 122, 117203 (2019)], it nevertheless identifies microscopic mechanisms that can lead to huge damping anisotropies.

cond-mat.mtrl-sci↗