arXiv subjects

Joanna Marciniak

Publications and source records attributed to Joanna Marciniak.

8 recordsLinked to original sources

arXiv2026

Combining Frozen Trajectory Excitation and TACAW for in silico Time-Resolved Vibrational Electron Energy Loss/Gain Spectroscopy

Seeing that ultrafast (picosecond timescale) vibrational electron energy loss spectroscopy (EELS) should soon be experimentally realizable, we present in silico approach capable of providing insight from the computational physics perspective. We present a framework that combines frozen trajectory excitation (FTE) with time auto-correlation of auxiliary wavefunctions (TACAW) to study the time-dependent spectral response of non-equilibrium lattice dynamics in a way comparable directly to experiment - (scanning) transmission electron microscope EELS, (S)TEM-EELS. In this approach, a selected phonon excitation is first introduced into an equilibrium molecular dynamics trajectory using FTE, after which the atomic positions during subsequent relaxations are treated with short-time TACAW analysis performed at different pump-probe delays. This yields momentum- and energy-resolved electron-scattering signals bearing a phonon imprint during the relaxation process, going beyond time-dependent diffuse-scattering intensities alone. We demonstrate the approach for fcc-Ni and 3C-SiC and discuss the observed phonon mode coupling and spectral redistribution during phonon relaxation.

cond-mat.mtrl-sci↗

arXiv2026

DFT calculations of magnetocrystalline anisotropy energy with fixed spin moment

The development of new-generation permanent magnets is based on experimental efforts and innovative theoretical tools for modeling magnetic properties. Magnetocrystalline anisotropy energy (MAE) - one of the main intrinsic properties of permanent magnets - can be calculated using density functional theory (DFT). However, MAEs determined with different exchange-correlation potentials can vary widely. We show how these seemingly contradictory results can be reconciled using the fully relativistic fixed spin moment (FR-FSM) method. This is because the equilibrium pairs [MAE, $m_s$] calculated with different exchange-correlation potentials overlap with the MAE($m_s$) curve determined from the FR-FSM method ($m_s$ denotes the spin magnetic moment). The FR-FSM method also enables the hypothetical maximum MAE value for a given material to be estimated. In the case of magnetic alloys, MAE(FSM) analysis allows the optimal alloying additions to be determined in order to improve the MAE value. The high independence of the MAE($m_s$) works well for exchange-correlation functionals such as LDA and GGA. However, it is not a universal measure and varies with changes in the value of the U parameter when using LDA+U or GGA+U functionals. Concluding, the framework we describe for MAE versus FSM calculations can be a useful tool in the design of new permanent magnets.

cond-mat.mtrl-sci↗

arXiv2025

Mode-Dependent Phonon Relaxation in fcc Ni: Insights from Molecular Dynamics Simulations with Frozen-Trajectory Excitations

We present a computational method and apply it to study phonon relaxation in face-centered cubic (fcc) nickel (Ni). The phonons are excited beyond their thermal equilibrium population, and the relaxation behavior is analyzed as a function of both the wave vector $\vec{q}$ and the phonon frequency $ω$. To efficiently investigate these excitations, we introduce a trajectory post-processing technique, the frozen-trajectory excitation, which facilitates the $(\vec{q},ω)$-resolved analysis. Molecular dynamics simulations combined with frozen-phonon multislice calculations predict relaxation signatures observable with time-resolved transmission electron microscopy (TEM) at 10--20 fs resolution. Our findings indicate mode dependence in the relaxation processes, highlighting the importance of considering phonon-specific behavior in ultrafast dynamics.

cond-mat.mtrl-sci↗

arXiv2024

Giant magnetocrystalline anisotropy energy in Fe--Co alloy under uniaxial compression: first-principles prediction

Uniaxially strained Fe--Co disordered alloys have emerged as promising candidates for cost-effective rare-earth-free permanent magnets due to their high magnetocrystalline anisotropy energy (MAE). Using first-principles, fully relativistic calculations within the coherent potential approximation and PBE exchange-correlation potential, we explore the MAE of tetragonal Fe--Co alloys under uniaxial compression. Our results reveal a previously uncharted high-MAE region, distinct from known structures and accessible through uniaxial compression.

cond-mat.mtrl-sci↗

arXiv2024

Magnetic anisotropy of L1$_0$ FeNi (001), (010), and (111) ultrathin films: A first-principles study

In previous experiments, thin films of L1$\mathrm{_0}$ FeNi with different surfaces, including (001), (110) and (111), were produced and studied. Each surface defines a different alignment of the crystallographic tetragonal axis with respect to the film's plane, resulting in different magnetic anisotropies. In this study, we use density functional theory calculations to examine three series of L1$\mathrm{_0}$ FeNi films with surfaces (001), (010), and (111), and with thicknesses ranging from 0.5 to 3 nm (from 4 to 16 atomic monolayers). Our results show that films (001) have perpendicular magnetic anisotropy, while (010) favor in-plane magnetization, with a clear preference for the tetragonal axis [001]. We propose calling this type of in-plane anisotropy fixed in plane. A film with surface (111) and a thickness of four atomic monolayers has the magnetization easy axis almost perpendicular to the plane of the film. As the thickness of the (111) film increases, the direction of magnetization rotates towards a tetragonal axis [001], positioned at an angle of about 45$^{o}$ to the plane of the film. Furthermore, the magnetic moment of ultrathin films increases by a maximum of 5%, and the most significant changes in spin and orbital magnetic moments occur at a depth of about three near-surface atomic monolayers. The presented results could be useful for experimental efforts to synthesize ultrathin L1$\mathrm{_0}$ FeNi films with different surfaces. Ultrathin L1$\mathrm{_0}$ FeNi films with varying magnetic anisotropies may find applications in spintronic devices.

cond-mat.mtrl-sci↗

arXiv2023

L1$_0$ FePt thin films with tilted and in-plane magnetic anisotropy: first-principles study

Ultrathin L1$_0$ films with different $c$-axis orientations relative to the film plane are promising candidates for data storage materials. In this work, within the framework of density functional theory, we calculated the magnetic properties of ultrathin L1$_0$ (111) and (010) films with thicknesses ranging from 4 to 16 atomic monolayers (from about 0.8 to 3.5~nm). The highest average magnetic moments are observed for the thinnest films considered, and with increasing film thickness, the values converge towards the magnetic moment for bulk. The observed increase comes mainly from enhanced moments in the two atomic monolayers closest to the surface of the films. The easy axis of magnetization of (111) films prefers an alignment close to the tetragonal axis, an example of tilted magnetic anisotropy. The 6-monolayer (111) film (about 1.3~nm thick) inclines the easy axis of magnetization of about 45° to the film plane, which can find use in applications. The (010) films show an in-plane easy magnetization axis in a unique L1$_0$ tetragonal direction. This is an unusual type of in-plane anisotropy, as the particular direction preference is very strong. The computational results encourage further experimental studies of L1$_0$ systems with tilted and in-plane fixed magnetic anisotropy.

cond-mat.mtrl-sci↗

arXiv2023

First-principles study of the magnetic anisotropy of ultrathin B-, C-, and N-doped FeCo films

Iron-based layered systems are of great interest because of their ability to tune effective material parameters such as magnetic anisotropy energy (MAE). The influence of the crystallographic structure of Fe, its thickness, and the presence of other layers above and below the Fe layer on magnetic parameters, such as the MAE of the studied system, is an intriguing and important topic from an application point of view. Here, we present a density functional theory (DFT) study of the magnetic anisotropy of nine-monolayer Fe, FeCo, and FeCo films with B, C, and N dopants placed in octahedral interstitial positions. The theoretical study is based on calculations using the full-potential local-orbital code FPLO and the generalized gradient approximation. The chemical disorder in the FeCo layers was modeled using the virtual crystal approximation. The structures of the layers were subjected to optimization of the geometry of the interlayer spacings and the neighborhood of the dopant sites. We determined the local magnetic moments and the excess charge at each layer position. We also identified the influence of dopant atoms on the magnetic properties of FeCo layers, such as magnetization and magnetic anisotropy.

cond-mat.mtrl-sci↗

arXiv2022

DFT calculation of intrinsic properties of magnetically hard phase L1$\mathrm{_0}$ FePt

Due to its strong magnetocrystalline anisotropy, FePt L1$\mathrm{_0}$ phase is considered as a promising magnetic recording media material. Although the magnetic properties of this phase have already been analyzed many times using density functional theory (DFT), we decided to study it again, emphasizing on full potential methods, including spin-polarized relativistic Korringa-Kohn-Rostoker (SPR-KKR) and full-potential local-orbital (FPLO) scheme. In addition to the determination of exact values of the magnetocrystalline anisotropy constants K$\mathrm{_1}$ and K$\mathrm{_2}$, the magnetic moments (m), the Curie temperature, and the magnetostriction coefficient, we focused on the investigation of the magnetocrystalline anisotropy energy (MAE) dependence on the magnetic moment values using the fully relativistic fixed spin moment (FSM) method with various exchange-correlation potentials. We present nearly identical MAE(m) curves near the equilibrium point, along with different equilibrium values of MAE and magnetic moments. For a magnetic moment reduced by about 10%, we determined a theoretical MAE maximum in the ground state (0 K) equal to about 20.3 MJ m$\mathrm{^{-3}}$ and independent of the choice of the exchange-correlation potential form. These calculations allow us to understand the discrepancies between the previous MAE results for different exchange-correlation potentials.

cond-mat.mtrl-sci↗