arXiv subjects

Wojciech Marciniak

Publications and source records attributed to Wojciech Marciniak.

9 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↗

arXiv2025

Curie temperature study of the Y(Fe$_{1-x}$Co$_x$)$_2$ and Zr(Fe$_{1-x}$Co$_x$)$_2$ systems using mean-field theory and Monte Carlo method

The cubic Laves phases including YFe$_2$, YCo$_2$, ZrFe$_2$, and ZrCo$_2$ are considered as promising candidates for application in hydrogen storage and magnetic refrigeration. While YFe$_2$ and ZrFe$_2$ are ferromagnets, alloying with Co decreases magnetic moments and Curie temperatures ($T_\mathrm{C}$) of pseudobinary Zr(Fe$_{1-x}$Co$_x$)$_2$ and Y(Fe$_{1-x}$Co$_x$)$_2$ systems, leading to the paramagnetic states of YCo$_2$ and ZrCo$_2$. The following study focus on the investigation of Curie temperature of the Y(Fe$_{1-x}$Co$_x$)$_2$ and Zr(Fe$_{1-x}$Co$_x$)$_2$ system from first principles. To do it, the Monte Carlo (MC) simulations and the mean field theory (MFT) based on the disordered local moments (DLM) calculations are used. The DLM-MFT results agree qualitatively with the experiment and preserve the characteristic features of $T_\mathrm{C}(x)$ dependencies for both Y(Fe$_{1-x}$Co$_x$)$_2$ and Zr(Fe$_{1-x}$Co$_x$)$_2$. However, we have encountered complications in the Co-rich regions due to failure of the local density approximation (LDA) in describing the Co magnetic moment in the DLM state. The analysis of Fe-Fe exchange couplings for YFe$_2$ and ZrFe$_2$ phases indicates that the nearest-neighbor interactions play the main role in the formation of $T_{\mathrm{C}}$.

cond-mat.mtrl-sci↗

arXiv2025

Structural and magnetic properties of Fe-Co-C alloys with tetragonal deformation: a first-principle study

Fe-Co alloys with induced tetragonal strain are promising materials for rare-earth-free permanent magnets. However, as ultrathin-film studies have shown, tetragonal Fe-Co structures tend to a rapid relaxation toward a cubic structure as the thickness of the deposited film increases. One of the main methods of inducing the stable strain in the bulk material is interstitial doping with small atoms, like B, C, or N. In this work, we present a full configuration space analysis in density functional theory approach for (Fe$_{1-x}$Co$_x$)$_{16}$C supercells with a single C impurity in one of the octahedral interstitial positions and for the full range of Co concentrations $x$. We discuss all assumptions and considerations leading to calculated lattice parameters, mixing enthalpies, magnetic moments, and averaged magnetocrystalline anisotropy energies (MAE). We present a comprehensive qualitative analysis of the structural and magnetic properties' dependence on short- and long-range ordering parameters. We analyzed all unique Fe/Co atoms occupancies at all stoichiometric concentrations possible in 2x2x2 supercell based on 2-atom tetragonal representation. We rely on the thermodynamic averaging method and large sample count to obtain accurate MAE values. We reevaluate several chemical disorder approximation methods, including effective medium methods (virtual crystal approximation and coherent potential approximation) and special quasirandom structures method applied to Fe-Co-based alloys. We observe a structural phase transition from the body-centered tetragonal structure above 70% Co concentration and confirm the structural stability of Fe-Co-C alloys in the tetragonal range. We show the presence of a broad MAE maximum of around 50% Co concentration and notably high MAE values for Co content $x$ as low as 25%. In addition, we show a positive correlation between MAE and mixing enthalpy.

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↗

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↗

arXiv2021

Electronic structure of YbFe$_4$Al$_8$ antiferromagnet: A combined X-ray photoelectron spectroscopy and first-principles study

Depending on their chemical composition, Yb compounds often exhibit different valence states. Here we investigate the valence state of YbFe$_4$Al$_8$ using X-ray photoelectron spectroscopy (XPS) and first-principles calculaions. The XPS valence band of YbFe$_4$Al$_8$ consists of two contributions coming from divalent (Yb$^{2+}$) and trivalent (Yb$^{3+}$) configurations. The determined value of the valence at room temperature is 2.81. Divalent and trivalent contributions are also observed for core-level Yb 4$d$ XPS spectra. We study several collinear antiferromagnetic models of YbFe$_4$Al$_8$ from the first-principles and for comparison we also consider LuFe$_4$Al$_8$ with a fully filled 4$f$ shell. We predict that only Fe sublattices of YbFe$_4$Al$_8$ carry significant magnetic moments and that the most stable magnetic configuration is AFM-C with antiparallel columns of magnetic moments. We also present a Mullliken electronic population analysis describing charge transfer both within and between atoms. In addition, we also study the effect of intra-atomic Coulomb U repulsion term applied for 4$f$ orbitals on Yb valence and Fe magnetic moments.

cond-mat.mtrl-sci↗

arXiv2017

Ab initio study of magnetocrystalline anisotropy, magnetostriction, and Fermi surface of L10 FeNi (tetrataenite)

The ordered L1$_0$ FeNi phase (tetrataenite) is recently considered as a promising candidate for the rare-earth free permanent magnets applications. In this work we calculate several characteristics of the L1$_0$ FeNi, where most of the results come form the fully relativistic full potential FPLO method with the generalized gradient approximation (GGA). A special attention deserves the summary of the magnetocrystalline anisotropy energies (MAE's), the full potential calculations of the anisotropy constant $K_3$, and the combined analysis of the Fermi surface and three-dimensional $\mathbf{k}$-resolved MAE. Other calculated parameters presented in this article are the magnetic moments $m_{s}$ and $m_{l}$, magnetostrictive coefficient $λ_{001}$, bulk modulus B$_0$, and lattice parameters. The MAE's summary shows rather big discrepancies between the experimental MAE's from literature and also between the calculated MAE's.

cond-mat.mtrl-sci↗