arXiv subjects

Mirosław Werwiński

Publications and source records attributed to Mirosław Werwiński.

At least 19 recordsLinked to original sources

arXiv2026

Interplay between crystal structure and magnetism in CeCrB$_4$

CeCrB$_4$ belongs to the family of MTB$_4$ ternary metal borides, some of whose magnetic phases have recently been identified as quantum spin dimers. Employing density functional theory within the GGA+$U$ framework, this work investigates the key ground-state properties of this system, including its ferromagnetic ground state, the mixed-valence nature of cerium, and the charge transfer from the cationic cerium--chromium layers to the anionic boron layers. In particular, we focus on elucidating the bonding mechanism within the structurally embedded Cr-Cr dimers. Specifically, we evaluate a previously proposed interpretation that the localized Cr $3d_{z^2}$ states are molecular-like bonding and antibonding states. The calculated dependence of the $3d_{z^2}$ electronic structure on the intra-dimer distance correlates with the level-splitting characteristics known for classical homonuclear diatomic molecules.

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

Normal and inverse magnetocaloric effects in structurally disordered Laves phase Y$_{1-x}$Gd$_{x}$Co$_{2}$ (0 $\leq$ x $\leq$ 1) compounds

Magnetic and magnetocaloric properties of Y$_{1-x}$Gd$_{x}$Co$_{2}$ compounds, where x = 0.2, 0.4, 0.6, 0.8 and 1.0, were investigated experimentally and theoretically. Crystal structures were characterized by X-ray diffraction (Rietveld analysis) and investigated samples possess the MgCu$_{2}$-type single phase with Fd-3m space group. Melt-spinning process introduced a chemical and topological disorder, which directly affected the magnetic properties. Refrigerant capacity (RC), strictly connected to the full width at half maximum $δ$TFWHM of the $Δ$S$_M$(T) curve and the maximum of magnetic entropy changes $Δ$S$_{Mpk}$(T)(T,$Δ$H), increases from 29 to 148 J/kg with replacement of Y by Gd atoms from x = 0.2 to x = 0.8. RC and $δ$TFWHM indicate the presence of disorder. Temperature dependences of magnetic entropy change $Δ$S$_M$(T,$Δ$H) and RC were measured in as-quenched and annealed state for Y$_{0.4}$Gd$_{0.6}$Co$_{2}$. This particular composition was chosen for detailed investigation mainly due to its Curie point (T$_C$ = 282 K), which is close to the room temperature. After isothermal annealing ($τ_a$ = 60 min, Ta = 700$^o$C) RC decreased from 122 to 104 J/kg, which clearly indicates the homogenization of the heat treated sample. Furthermore, observed inverse magnetocaloric effect is associated with the presence of antiferromagnetically coupled Gd and Co magnetic moments. The phase transition temperature increases with increasing Gd content from 74 to 407 K for Y$_{0.8}$Gd$_{0.2}$Co$_{2}$ and GdCo2, respectively. Within the FPLO-LDA DFT method, the non-magnetic ground state for YCo$_{2}$ and the magnetic ground state for GdCo$_{2}$ are predicted in agreement with experiment. The dependence of calculated total and species-resolved magnetic moments on Gd concentration reasonably agrees with available experimental data.

cond-mat.mtrl-sci↗

arXiv2025

Effect of substitution La by Mg on electrochemical and electronic properties in La$_{2-x}$Mg$_x$Ni$_7$ alloys: a combined experimental and ab initio studies

La-Mg-Ni-based alloys are promising negative electrode materials for 3rd generation of Ni-MH$_x$ batteries. In this work, we investigate the effect of Mg substitution on the electrochemical and electronic properties of La$_{2-x}$Mg$_x$Ni$_7$ materials. The mechanical alloying technique is used to produce a series of La$_{2-x}$Mg$_x$Ni$_7$ alloys ($x$ = 0.00, 0.25, 0.50 and 0.75). The X-ray diffraction measurements indicate multiphase character of the samples with majority (La,Mg)$_2$Ni$_7$ phases of hexagonal Ce$_2$Ni$_7$-type and rhombohedral Gd$_2$Co$_7$-type. Electrochemical measurements show how the maximum discharge capacity ($C_{max}$) increases with Mg concentration and that reach the highest value of 304 mAh/g for La$_{1.5}$Mg$_{0.5}$Ni$_7$ ($x$ = 0.5). The experimental efforts are followed by the density functional theory (DFT) calculations performed with the full-potential local-orbital minimum-basis scheme (FPLO). To simulate chemical disorder, we use the coherent potential approximation (CPA). The calculations are focused on the La$_{1.5}$Mg$_{0.5}$Ni$_7$ composition with the highest measured value of $C_{max}$. Additionally, several other structures are considered as reference points. We find that hexagonal and rhombohedral structures of La$_2$Ni$_7$ have almost identical total energies, which is in a good agreement with a coexistence of both phases in the samples. The calculated site preferences of Mg in both Ce$_2$Ni$_7$-type and Gd$_2$Co$_7$-type La$_{1.5}$Mg$_{0.5}$Ni$_7$ phases are consistent with the previous experimental data. Furthermore, the valence band of the nanocrystalline La$_{1.5}$Mg$_{0.5}$Ni$_7$ sample is investigated by X-ray photoelectron spectroscopy (XPS). The experimental XPS are interpreted based on the corresponding spectra calculated with DFT.

cond-mat.mtrl-sci↗

arXiv2025

Effect of Gd and Co content on electrochemical and electronic properties of La$_{1.5}$Mg$_{0.5}$Ni$_7$ alloys: a combined experimental and first-principles study

In this work, we investigate the effect of Gd and Co substitutions on the electrochemical and electronic properties of La$_{1.5}$Mg$_{0.5}$Ni$_7$ alloy. Two series of La$_{1.5-x}$Gd$_x$Mg$_{0.5}$Ni$_7$ ($x$ = 0.0, 0.25, 1.0) and La$_{1.5}$Mg$_{0.5}$Ni$_{7-y}$Co$_y$ ($y$ = 0.0, 0.5, 1.5) alloys are produced using mechanical alloying technique. The X-ray diffraction indicates multiphase character of the samples, with the majority of hexagonal Ce$_2$Ni$_7$-type and rhombohedral Gd$_2$Co$_7$-type structures of (La,Mg)$_2$Ni$_7$ phase. Partial substitutions of La by Gd or Ni by Co in La$_{1.5}$Mg$_{0.5}$Ni$_7$ phase result in increase of cycle stability of the metal hydride (MH$_x$) electrodes. All considered alloys reach the maximum discharge capacity after three charging-discharging cycles. Two optimal compositions, in respect of electrochemical properties, are subsequently investigated by the X-ray photoelectron spectroscopy (XPS). The experimental analysis of the valence band is further extended by the density functional theory (DFT) calculations. We also discuss the effects of alloying and site preference of dopants on the position of the van Hove-type singularity, as observed in the electronic densities of states (DOS) in proximity of the Fermi level.

cond-mat.mtrl-sci↗

arXiv2025

Occupation deficiency in layered structures of UNi$_{x}$Sb$_2$ ($ 0 \leq x \leq 1 $) studied by density functional theory supercell calculations

The five crystal structures of selected UNi$_{x}$Sb$_2$ compositions are investigated by density functional theory supercell calculations. The considered phases are USb$_2$, UNi$_{0.33}$Sb$_2$, UNi$_{0.5}$Sb$_2$, UNi$_{0.66}$Sb$_2$, and UNiSb$_2$ ($x$ = 0, 1/3, 1/2, 2/3, 1). The occupation deficiency of Ni is modeled by removing the Ni layers from constructed supercells, followed by relaxation of the structures. A linear dependence of the lattice parameter $c$ versus Ni concentration $x$ is observed, same, fulfilling the empirical Vegard's law. The agreement between results of our calculations with the empirical data from literature confirms the validity of our approach of supercells with empty Ni layers, at least in predicting of lattice parameters. The calculated with orbital polarization magnetic moments on uranium atoms decrease from 1.70 $μ_{\mathrm{B}}$ to 1.61 $μ_{\mathrm{B}}$ with increasing Ni concentration $x$. In comparison to available empirical data of USb$_2$ and UNi$_{0.5}$Sb$_2$, the magnetic moments calculated with orbital polarization are less than 10% smaller.

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↗

arXiv2025

Perpendicular magnetic anisotropy in Au/FeCo/Au ultrathin films: Combined experimental and first-principles study

Magnetic tunnel junctions with a magnetic layer with perpendicular anisotropy are currently used in computer memories that do not require voltage sustaining. An example of layers with perpendicular magnetic anisotropy are ultrathin FeCo films on Au substrate. Here, we present an experimental and computational study of Fe$_{0.75}$Co$_{0.25}$ layers with a thickness up to two nanometers. The investigated FeCo layers are surrounded on both sides by Au layers. In experiment, we found perpendicular magnetic anisotropy in FeCo polycrystalline films with a thickness below 0.74 nm (five atomic monolayers). We also measured a strong surface contribution to effective magnetic anisotropy. From the density functional theory we determined structural, electronic, and magnetic properties of the FeCo films. The calculations showed an irregular dependence of magnetic anisotropy on FeCo layer thickness. We observed perpendicular anisotropy for one- and three-atom FeCo monolayers and in-plane anisotropy for two- and four-atom monolayers. However, the determination of running averages of magnetic anisotropy (of three successive thicknesses of atomic monolayers) leads to a close linear thickness dependence of magnetic anisotropy, similar to the experimental result. The reason why the properties of the several-atom-thick FeCo layers differ so significantly from those of the bulk parent material is the disappearance of the central region of the layer whose properties approximate those of the bulk, and instead the existence of near-interface regions with properties altered by the presence of discontinuities.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic hardness of hexagonal and orthorhombic Fe$_{3}$C, Co$_{3}$C, (Fe-Co)$_{3}$C, and their alloys with boron, nitrogen, and transition metals: A first-principles study

In this study, we considered a large set of materials that are closely related to orthorhombic Fe$_3$C (cementite) with the aim of characterizing trends in their intrinsic magnetic properties and identifying alloys that are optimal for applications. A comprehensive analysis was conducted on the full concentration ranges of hexagonal ($ε$) and orthorhombic ($θ$) phases of (Fe-Co)$_3$C, (Fe-Co)$_3$(B-C), (Fe-Co)$_3$(C-N), and their alloys with 3$d$, 4$d$ and 5$d$ transition metals. The calculations were performed using the density functional theory implemented in the full-potential local-orbital code (FPLO). Calculated properties included formation energies, Curie temperatures, magnetic moments, magnetocrystalline anisotropy energies (MAE), and magnetic hardnesses. The considered compositions exhibit a range of magnetic properties, including soft, semi-hard, and hard magnetic. The materials most promising for hard-magnetic applications are orthorhombic Co$_3$C compound, together with selected Co-rich orthorhombic (Fe,Co)$_3$C and hexagonal (Fe,Co)$_3$C alloys. The calculation results do not indicate that substituting with transition metals increases the potential of the alloys for permanent magnet applications. A significant drawback of alloying orthorhombic $θ$-Fe$_3$C (cementite) with transition metals is the notable decline in the Curie temperature. We found that a considerable proportion of the orthorhombic Co$_3$(B-C-N) alloys are magnetically hard, of which boron substitution raises the Curie temperature and improves stability. By mapping the dependence of MAE on the concentration of elements covering both the 3$d$ (from Fe to Co) and 2$p$ (from B, through C, to N) positions, we have demonstrated for the first time the near isoelectronic nature of MAE. The latter observation may be particularly useful in designing compositions of new magnetically hard materials.

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↗

arXiv2024

Searching for magnetically hard monoborides (and finding a few): A first-principles investigation

New hard magnetic materials with zero or low rare earth content are in demand due to the high prices of the rare earth metals. Among the candidates for such materials, we consider MnB, FeB and their alloys, because previous experiments suggest that FeB has a relatively high magnetic hardness of about 0.83 at room temperature. Using first-principles calculations, we examine the full range of alloys from CrB, through MnB, FeB, to CoB. Furthrmore, we consider alloys of MnB and FeB with substitutions of 3$d$, 4$d$ and 5$d$ transition metals. For the above ninety compositions, we determine magnetic moment, magnetocrystalline anisotropy energy and magnetic hardness. For (Fe-Co)B alloys, the calculated values of magnetic hardness exceed five, which is an exceptionally high. While these values are inflated by the virtual crystal approximation used, we still expect actual magnetic hardnesses well above unity. Furthermore, we classify considered MnB alloys substituted with transition metals as magnetically soft or semi-hard and FeB alloys with Sc, Ti, V, Zr, Nb, Mo, Hf, Ta or W as magnetically hard (with magnetic hardness exceeding unity).

cond-mat.mtrl-sci↗

arXiv2024

Boundary-induced phase in epitaxial iron layers

We report the discovery of a boundary-induced body-centered tetragonal (bct) iron phase in thin films deposited on MgAl$_{2}$O$_{4}$ ($001$) substrates. We present evidence for this phase using detailed x-ray analysis and ab-initio density functional theory calculations. A lower magnetic moment and a rotation of the easy magnetisation direction are observed, as compared to body-centered cubic (bcc) iron. Our findings expand the range of known crystal and magnetic phases of iron, providing valuable insights for the development of heterostructure devices using ultra-thin iron layers.

cond-mat.mtrl-sci↗

arXiv2023

Iron and gold thin films: first-principles study

Using density functional theory, we carried out systematic calculations for a series of ultrathin iron layers with thicknesses ranging from one atomic monolayer to eleven monolayers (up to about 1.5 nm). We considered three cases: (1) iron layers both on a gold substrate and coated with gold, (2) iron layers on a gold substrate but without coverage, and (3) freestanding iron layers adjacent to a vacuum. For our models, we chose initial bcc Fe(001) surfaces and fcc Au(001) substrates. Based on the calculations, we determined the details of the geometry and magnetic properties of the systems. We calculate lattice parameters, magnetic moments, Curie temperatures and magnetocrystalline anisotropy energies. From the thickness dependence, we determined the volume and surface contributions to the magnetic anisotropy constant. The further analysis allowed us to determine the thickness ranges of the occurrence of perpendicular magnetic anisotropy, as well as the effect of thickness and the presence of a substrate and cap layer on the direction of the magnetization easy axis.

cond-mat.mtrl-sci↗

arXiv2023

Magnetic properties and structural phase transition in ultrathin fcc Fe (111) and bcc Fe (111) films: first-principles study

The aim of this work is to investigate the structural and magnetic characteristics of Fe thin films with a triangular (hexagonal) lattice surfaces (fcc (111) and bcc (111)). The properties of these structures have been calculated using density functional theory (DFT) implemented in the full-potential local-orbital(FPLO) code. The results indicate a structural phase transition from fcc to bcc structure when the film thickness exceeds 23 Fe atomic monolayers. The considered fcc films prefer the low-spin ferromagnetic state with an average magnetic moment of about 1.0 $μ_{B}$ per atom. This moment decreases with increasing film thickness until the critical thickness, where, after the structural transition to the bcc phase, it reaches a value close to that of bulk bcc Fe. Moreover, the values of the magnetic anisotropy energy are positive (perpendicular magnetic anisotropy) for the entire thickness range of films with fcc structure (in ferromagnetic low-spin state) and systematically decrease with increasing film thickness. The presented computational results explain the experimentally observed structural transition and may help to select appropriate substrates with suitable lattice parameters for the deposition of ultrathin Fe(111) films.

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↗