scholarly journals An Ab Initio Study of Magnetism in Disordered Fe-Al Alloys with Thermal Antiphase Boundaries

Nanomaterials ◽  
2019 ◽  
Vol 10 (1) ◽  
pp. 44
Author(s):  
Martin Friák ◽  
Miroslav Golian ◽  
David Holec ◽  
Nikola Koutná ◽  
Mojmír Šob
Keyword(s):  
Magnetic Moment ◽  
Nearest Neighbor ◽  
Computational Study ◽  
Magnetic Moments ◽  
Magnetic Flux Density ◽  
Random Structure ◽  
Antiphase Boundaries ◽  
Thermally Induced ◽  
B2 Phase ◽  
Experimental Findings

We have performed a quantum-mechanical study of a B2 phase of Fe 70 Al 30 alloy with and without antiphase boundaries (APBs) with the {001} crystallographic orientation of APB interfaces. We used a supercell approach with the atoms distributed according to the special quasi-random structure (SQS) concept. Our study was motivated by experimental findings by Murakami et al. (Nature Comm. 5 (2014) 4133) who reported significantly higher magnetic flux density from A2-phase interlayers at the thermally-induced APBs in Fe 70 Al 30 and suggested that the ferromagnetism is stabilized by the disorder in the A2 phase. Our computational study of sharp APBs (without any A2-phase interlayer) indicates that they have moderate APB energies (≈0.1 J/m 2 ) and cannot explain the experimentally detected increase in the ferromagnetism because they often induce a ferro-to-ferrimagnetic transition. When studying thermal APBs, we introduce a few atomic layers of A2 phase of Fe 70 Al 30 into the interface of sharp APBs. The averaged computed magnetic moment of Fe atoms in the whole B2/A2 nanocomposite is then increased by 11.5% w.r.t. the B2 phase. The A2 phase itself (treated separately as a bulk) has the total magnetic moment even higher, by 17.5%, and this increase also applies if the A2 phase at APBs is sufficiently thick (the experimental value is 2–3 nm). We link the changes in the magnetism to the facts that (i) the Al atoms in the first nearest neighbor (1NN) shell of Fe atoms nonlinearly reduce their magnetic moments and (ii) there are on average less Al atoms in the 1NN shell of Fe atoms in the A2 phase. These effects synergically combine with the influence of APBs which provide local atomic configurations not existing in an APB-free bulk. The identified mechanism of increasing the magnetic properties by introducing APBs with disordered phases can be used as a designing principle when developing new magnetic materials.

scholarly journals Impact of Disorder on Properties of Vacancies: A Case Study of B2 and A2 Polymorphs of Non-Stoichiometric Fe2CoAl

Crystals ◽  
2021 ◽  
Vol 11 (10) ◽  
pp. 1207
Author(s):  
Martin Friák ◽  
Jana Pavlů ◽  
Mojmír Šob
Keyword(s):  
Magnetic Moment ◽  
Single Phase ◽  
Local Magnetic Moment ◽  
Random Structure ◽  
Ab Initio Study ◽  
B2 Phase ◽  
Structural And Magnetic Properties ◽  
Induced Changes ◽  
Formation Energies

We have performed an ab initio study of vacancy-induced changes in thermodynamic, structural and magnetic properties of single-phase ferromagnetic Fe2CoAl with a chemically disordered (i) two-sublattice B2 phase or (ii) single-sublattice A2 phase. The two polymorphs of slightly non-stoichiometric Fe2CoAl (Fe27Co14Al13) were modeled by two different 54-atom supercells with atoms distributed according to the special quasi-random structure (SQS) concept. Both the lower-energy B2 phase and a higher-energy A2 phase possess elastic constants that correspond to an auxetic material that is mechanically stable. The properties of vacancies were computed by systematically removing different atoms (one at a time) from the supercells and quite wide ranges of values of vacancy-related characteristics were obtained. The increase in the level of disorder (when changing from the B2 to the A2 phase) results in an increase in the scatter of calculated values. The Fe and Co vacancies have lower vacancy formation energies than the Al ones. The total magnetic moment of the supercell decreases when introducing Fe and Co vacancies but it increases due to Al ones. The latter findings can be partly explained by an increase of the local magnetic moment of Fe atoms when the number of Al atoms in the first neighbor shell of Fe atoms is reduced, such as due to Al vacancies.

FORMATION OF MAGNETIC MOMENTS IN INTERMETALLIC COMPOUNDS AND ALLOYS

1993 ◽  
Vol 07 (01n03) ◽  
pp. 756-759 ◽  
Author(s):  
T. BEUERLE ◽  
K. HUMMLER ◽  
M. FÄHNLE
Keyword(s):  
Transition Metals ◽  
Ab Initio ◽  
Intermetallic Compounds ◽  
Magnetic Moment ◽  
Nearest Neighbor ◽  
Physical Mechanism ◽  
Magnetic Moments ◽  
Lmto Method ◽  
Minimum Number ◽  
Alloy Systems

Jaccarino and Walker proposed that in certain alloy systems a considered kind of atom attains a constant value of the magnetic moment if it is surrounded by at least a minimum number of nearest-neighbor atoms of a certain kind, and that it is nonmagnetic otherwise. It is shown by ab initio supercell calculations within the LMTO method that the system Nbi−xMox with 1% iron approximately fulfills the Jaccarino-Walker hypothesis, whereas the alloy FexRh1−x does not. The physical mechanism for the destruction of the Fe moment in bcc alloys with 4d and 5d transition metals is elucidated.

scholarly journals A Quantum-Mechanical Study of Antiphase Boundaries in Ferromagnetic B2-Phase Fe2CoAl Alloy

2021 ◽  
Vol 7 (10) ◽  
pp. 137 ◽  
Author(s):  
Martin Friák ◽  
Josef Gracias ◽  
Jana Pavlů ◽  
Mojmír Šob
Keyword(s):  
Computational Models ◽  
Poisson Ratio ◽  
Magnetic Moments ◽  
Quantum Mechanical ◽  
Random Structure ◽  
Antiphase Boundaries ◽  
Mechanical Study ◽  
Quantum Mechanical Study ◽  
Similar Formation ◽  
Type Lattice

In this study, we performed a quantum mechanical examination of thermodynamic, structural, elastic, and magnetic properties of single-phase ferromagnetic Fe2CoAl with a chemically disordered B2-type lattice with and without antiphase boundaries (APBs) with (001) crystallographic orientation. Fe2CoAl was modeled using two different 54-atom supercells with atoms on the two B2 sublattices distributed according to the special quasi-random structure (SQS) concept. Both computational models exhibited very similar formation energies (−0.243 and −0.244 eV/atom), B2 structure lattice parameters (2.849 and 2.850 Å), magnetic moments (1.266 and 1.274 μB/atom), practically identical single-crystal elastic constants (C11 = 245 GPa, C12 = 141 GPa, and similar C44 = 132 GPa) and auxetic properties (the lowest Poisson ratio close to −0.1). The averaged APB interface energies were observed to be 199 and 310 mJ/m2 for the two models. The studied APBs increased the total magnetic moment by 6 and 8% due to a volumetric increase as well as local changes in the coordination of Fe atoms (their magnetic moments are reduced for increasing number of Al neighbors but increased by the presence of Co). The APBs also enhanced the auxetic properties.

scholarly journals The Impact of Vibrational Entropy on the Segregation of Cu to Antiphase Boundaries in Fe3Al

2021 ◽  
Vol 7 (8) ◽  
pp. 108
Author(s):  
Martin Friák ◽  
Miroslav Černý ◽  
Mojmír Šob
Keyword(s):  
Magnetic Moments ◽  
Energy Difference ◽  
Extended Defects ◽  
Vibrational Entropy ◽  
Phonon Modes ◽  
Antiphase Boundaries ◽  
Related Energy ◽  
Quantum Mechanical Study ◽  
Nonlinear Trends ◽  
The Impact

We performed a quantum mechanical study of segregation of Cu atoms toward antiphase boundaries (APBs) in Fe3Al. The computed concentration of Cu atoms was 3.125 at %. The APBs have been characterized by a shift of the lattice along the ⟨001⟩ crystallographic direction. The APB energy turns out to be lower for Cu atoms located directly at the APB interfaces and we found that it is equal to 84 mJ/m2. Both Cu atoms (as point defects) and APBs (as extended defects) have their specific impact on local magnetic moments of Fe atoms (mostly reduction of the magnitude). Their combined impact was found to be not just a simple sum of the effects of each of the defect types. The Cu atoms are predicted to segregate toward the studied APBs, but the related energy gain is very small and amounts to only 4 meV per Cu atom. We have also performed phonon calculations and found all studied states with different atomic configurations mechanically stable without any soft phonon modes. The band gap in phonon frequencies of Fe3Al is barely affected by Cu substituents but reduced by APBs. The phonon contributions to segregation-related energy changes are significant, ranging from a decrease by 16% at T = 0 K to an increase by 17% at T = 400 K (changes with respect to the segregation-related energy difference between static lattices). Importantly, we have also examined the differences in the phonon entropy and phonon energy induced by the Cu segregation and showed their strongly nonlinear trends.

scholarly journals Magnetic and Electronic Properties of Weyl Semimetal Co2MnGa Thin Films

Nanomaterials ◽  
2021 ◽  
Vol 11 (1) ◽  
pp. 251
Author(s):  
Peter Swekis ◽  
Aleksandr S. Sukhanov ◽  
Yi-Cheng Chen ◽  
Andrei Gloskovskii ◽  
Gerhard H. Fecher ◽  
...  
Keyword(s):  
Thin Films ◽  
Magnetic Properties ◽  
Electronic Properties ◽  
Photoelectron Spectroscopy ◽  
Magnetic Moments ◽  
Field Orientation ◽  
Weyl Semimetal ◽  
Uniform Magnetization ◽  
Experimental Findings ◽  
High Degree

Magnetic Weyl semimetals are newly discovered quantum materials with the potential for use in spintronic applications. Of particular interest is the cubic Heusler compound Co2MnGa due to its inherent magnetic and topological properties. This work presents the structural, magnetic and electronic properties of magnetron co-sputtered Co2MnGa thin films, with thicknesses ranging from 10 to 80 nm. Polarized neutron reflectometry confirmed a uniform magnetization through the films. Hard x-ray photoelectron spectroscopy revealed a high degree of spin polarization and localized (itinerant) character of the Mn d (Co d) valence electrons and accompanying magnetic moments. Further, broadband and field orientation-dependent ferromagnetic resonance measurements indicated a relation between the thickness-dependent structural and magnetic properties. The increase of the tensile strain-induced tetragonal distortion in the thinner films was reflected in an increase of the cubic anisotropy term and a decrease of the perpendicular uniaxial term. The lattice distortion led to a reduction of the Gilbert damping parameter and the thickness-dependent film quality affected the inhomogeneous linewidth broadening. These experimental findings will enrich the understanding of the electronic and magnetic properties of magnetic Weyl semimetal thin films.

scholarly journals Spin Gapless Semiconductor–Nonmagnetic Semiconductor Transitions in Fe-Doped Ti2CoSi: First-Principle Calculations

2018 ◽  
Vol 8 (11) ◽  
pp. 2200 ◽  
Author(s):  
Yu Feng ◽  
Zhou Cui ◽  
Ming-sheng Wei ◽  
Bo Wu ◽  
Sikander Azam
Keyword(s):  
Magnetic Moment ◽  
Electronic Structures ◽  
Magnetic Moments ◽  
Total Magnetic Moment ◽  
First Principle ◽  
Half Metallic ◽  
Heusler Compound ◽  
First Principle Calculations ◽  
Metallic Ferromagnet

Employing first-principle calculations, we investigated the influence of the impurity, Fe atom, on magnetism and electronic structures of Heusler compound Ti2CoSi, which is a spin gapless semiconductor (SGS). When the impurity, Fe atom, intervened, Ti2CoSi lost its SGS property. As TiA atoms (which locate at (0, 0, 0) site) are completely occupied by Fe, the compound converts to half-metallic ferromagnet (HMF) TiFeCoSi. During this SGS→HMF transition, the total magnetic moment linearly decreases as Fe concentration increases, following the Slate–Pauling rule well. When all Co atoms are substituted by Fe, the compound converts to nonmagnetic semiconductor Fe2TiSi. During this HMF→nonmagnetic semiconductor transition, when Fe concentration y ranges from y = 0.125 to y = 0.625, the magnetic moment of Fe atom is positive and linearly decreases, while those of impurity Fe and TiB (which locate at (0.25, 0.25, 0.25) site) are negative and linearly increase. When the impurity Fe concentration reaches up to y = 1, the magnetic moments of Ti, Fe, and Si return to zero, and the compound is a nonmagnetic semiconductor.

scholarly journals Origin of the Low Magnetic Moment in Fe2AlTi: An Ab Initio Study

Materials ◽  
2018 ◽  
Vol 11 (9) ◽  
pp. 1732 ◽  
Author(s):  
Martin Friák ◽  
Anton Slávik ◽  
Ivana Miháliková ◽  
David Holec ◽  
Monika Všianská ◽  
...  
Keyword(s):  
Ab Initio ◽  
Magnetic Moment ◽  
Energy Surface ◽  
Magnetic Moments ◽  
Local Magnetic Moment ◽  
Spin Configuration ◽  
Spin Effects ◽  
Order Of Magnitude ◽  
Huge Impact ◽  
Insight Into

The intermetallic compound Fe 2 AlTi (alternatively Fe 2 TiAl) is an important phase in the ternary Fe-Al-Ti phase diagram. Previous theoretical studies showed a large discrepancy of approximately an order of magnitude between the ab initio computed magnetic moments and the experimentally measured ones. To unravel the source of this discrepancy, we analyze how various mechanisms present in realistic materials such as residual strain effects or deviations from stoichiometry affect magnetism. Since in spin-unconstrained calculations the system always evolves to the spin configuration which represents a local or global minimum in the total energy surface, finite temperature spin effects are not well described. We therefore turn the investigation around and use constrained spin calculations, fixing the global magnetic moment. This approach provides direct insight into local and global energy minima (reflecting metastable and stable spin phases) as well as the curvature of the energy surface, which correlates with the magnetic entropy and thus the magnetic configuration space accessible at finite temperatures. Based on this approach, we show that deviations from stoichiometry have a huge impact on the local magnetic moment and can explain the experimentally observed low magnetic moments.

scholarly journals An Ab Initio Study of Li/Ni-doped NaxMeO2 Cathode Material for Na-Ion Batteries

2020 ◽  
Vol 03 (02) ◽  
pp. 1-1
Author(s):  
Arianna Massaro ◽  
◽  
Ana B. Muñoz-García ◽  
Mariarosaria Tuccillo ◽  
Michele Pavone ◽  
...  
Keyword(s):  
Cathode Material ◽  
State Of The Art ◽  
Magnetic Moments ◽  
Electronic Conduction ◽  
Random Structure ◽  
Cell Parameters ◽  
Adsorption Sites ◽  
Current State ◽  
Partial Charges ◽  
Na Ions

The current state-of-the-art quantum mechanics methodologies were applied to derive information on the bulk and surface properties of the P2-type layered oxide Na0.85Li0.17Ni0.21Mn0.64O2 (NLNMO), a cathode material. The special quasi-random structure (SQS) approach was employed to identify the arrangement of Li, Ni, and Mn ions in a supercell containing 115 atoms. Both the cell parameters and atomic positions were determined from DFT-PBE+U calculations to highlight specific distortions induced by the dopants (Ni and Li). The analysis of atomic partial charges and atomic magnetic moments revealed that Li has a purely structural role, while Ni and Mn actively participate in both redox processes and electronic conduction. Using a new surface slab model, the interaction between the layered Na0.85Li0.17Ni0.21Mn0.64O2 (001) surface and the Na ions was examined to identify the most favorable adsorption sites and the possible paths for the migration of the Na ions on the electrode surface.

scholarly journals Impact of Nano-Scale Distribution of Atoms on Electronic and Magnetic Properties of Phases in Fe-Al Nanocomposites: An Ab Initio Study

Nanomaterials ◽  
2018 ◽  
Vol 8 (12) ◽  
pp. 1059 ◽  
Author(s):  
Ivana Miháliková ◽  
Martin Friák ◽  
Yvonna Jirásková ◽  
David Holec ◽  
Nikola Koutná ◽  
...  
Keyword(s):  
Ab Initio ◽  
Density Functional ◽  
Nearest Neighbor ◽  
Magnetic Moments ◽  
Generalized Gradient ◽  
Functional Theory ◽  
Α Phase ◽  
Ordered Intermetallic ◽  
Random Structures ◽  
The Impact

Quantum-mechanical calculations are applied to examine magnetic and electronic properties of phases appearing in binary Fe-Al-based nanocomposites. The calculations are carried out using the Vienna Ab-initio Simulation Package which implements density functional theory and generalized gradient approximation. The focus is on a disordered solid solution with 18.75 at. % Al in body-centered-cubic ferromagnetic iron, so-called α -phase, and an ordered intermetallic compound Fe 3 Al with the D0 3 structure. In order to reveal the impact of the actual atomic distribution in the disordered Fe-Al α -phase three different special quasi-random structures with or without the 1st and/or 2nd nearest-neighbor Al-Al pairs are used. According to our calculations, energy decreases when eliminating the 1st and 2nd nearest neighbor Al-Al pairs. On the other hand, the local magnetic moments of the Fe atoms decrease with Al concentration in the 1st coordination sphere and increase if the concentration of Al atoms increases in the 2nd one. Furthermore, when simulating Fe-Al/Fe 3 Al nanocomposites (superlattices), changes of local magnetic moments of the Fe atoms up to 0.5 μ B are predicted. These changes very sensitively depend on both the distribution of atoms and the crystallographic orientation of the interfaces.

Magnetic Properties and Structural Transformation in Copper-304 Stainless Steel Multilayer Materials

1995 ◽  
Vol 382 ◽  
Author(s):  
K. Parvin ◽  
S.P. Weathersby ◽  
T.W. Barbee ◽  
T.P. Weihs ◽  
M.A. Wall
Keyword(s):  
Stainless Steel ◽  
Magnetic Moment ◽  
Magnetic Moments ◽  
High Volume ◽  
304 Stainless Steel ◽  
X Ray Diffraction ◽  
Multilayer Foils ◽  
Large Period ◽  
Fcc Phase ◽  
Bcc Phase

ABSTRACTMultilayer foils of Cu-304 stainless steel (304SS) with equal layer thicknesses in the range t=5.0-500 Å and total thicknesses 10-20 μm have been synthesized using magnetron sputtering at ambient substrate temperature. The x-ray diffraction data of as-deposited films show two structural regimes: small thickness (t=5-10 Å) which is characterized by epitaxial FCC growth of 304SS on copper, and large thickness (t=13.5-500 Å) which shows epitaxial FCC 304SS growth near the interface and BCC 304SS growth away from the interface. FCC structured films show very small magnetic moments at room temperature similar to bulk 304SS stable FCC phase. However, a strong magnetic moment is observed for thicker samples due to ferromagnetic metastable 304SS BCC phase. Two opposing transformations occur in the 304 layers as the samples are heated. The first transformation is from the metastable BCC 304SS to the stable FCC phase. This transformation produces a strong drop in magnetic moment and is clearly visible in the large period multilayers which contain high volume fractions of BCC 304SS. The second transformation is from the original FCC phase to a new stable BCC phase in the 304SS near the copper-304SS interfaces.The transformation is produced by diffusion of nickel from the 304SS into the surroundingcopper and the chemical destabilization of the FCC phase which starts near 400 ºC.This transformation produces a sharp increase in magnetic moment. The magnetic signal drops to zero near 675 ºC which is the Curie temperature of ferromagnetic BCC Fe.75 Cr25..

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