arXiv subjects

Martin Sahlberg

Publications and source records attributed to Martin Sahlberg.

17 recordsLinked to original sources

arXiv2026

Chemical short-range order controls deformation pathways in a complex concentrated alloy

Chemical short-range order (CSRO) is an intrinsic feature of complex concentrated alloys (CCAs), yet its influence on deformation mechanisms is controversial because of the inconclusive state of concurrent CSRO quantification during deformation. Here, we provide experimental evidence that CSRO acts as an intrinsic thermodynamic state variable governing stacking-fault energetics and deformation pathways in a Co30Cr40Ni30 alloy. By comparing quenched (CSRO-lean) and aged (CSRO-enriched) conditions with equivalent grain structure and phase constitution, we isolate the influence of atomic-scale chemical ordering on mechanical behavior. Calorimetry confirms reversible CSRO formation, while synchrotron X-ray diffraction and electron microscopy reveal that CSRO suppresses deformation-induced fcc-hcp martensitic transformation at both room and cryogenic temperatures. Despite differences in transformation dynamics, the macroscopic tensile response is still broadly similar. Atomistic simulations show that CSRO increases both stable and unstable stacking-fault energies, raising the energetic barrier for partial-dislocation activity and stabilizing the fcc lattice against transformation. Together, the experimental and computational results establish CSRO as an added degree of freedom for tuning stacking-fault energetics and controlling deformation pathways in complex concentrated alloys.

cond-mat.mtrl-sci↗

arXiv2025

Magnetic properties of the Fe$_5$SiB$_2$-Fe$_5$PB$_2$ system

The magnetic properties of the compound Fe$_5$Si$_{1-x}$P$_{x}$B$_2$ have been studied, with a focus on the Curie temperature $T_\textrm{C}$, saturation magnetization $M_\textrm{S}$, and magnetocrystalline anisotropy. Field and temperature dependent magnetization measurements were used to determine $T_\textrm{C}\left(x\right)$ and $M_\textrm{S}\left(x\right)$. The saturation magnetization at 10 K (300 K) is found to monotonically decrease from $1.11~\mathrm{MA/m}$ ($1.03~\mathrm{MA/m}$) to $0.97~\mathrm{MA/m}$ ($0.87~\mathrm{MA/m}$), as $x$ increases from zero to one. The Curie temperature is determined to be 810 K and 615 K in Fe$_5$SiB$_2$ and Fe$_5$PB$_2$, respectively. The highest $T_\textrm{C}$ is observed for $x=0.1$, while it decreases monotonically for larger $x$. The Curie temperatures have also been theoretically determined to be 700 K and 660 K for Fe$_5$SiB$_2$ and Fe$_5$PB$_2$, respectively, using a combination of density functional theory and Monte Carlo simulations. The magnitude of the effective magnetocrystalline anisotropy was extracted using the law of approach to saturation, revealing an increase with increasing phosphorus concentration. Low--field magnetization vs. temperature results for $x = 0, 0.1, 0.2$ indicate that there is a transition from easy--axis to easy--plane anisotropy with decreasing temperature.

cond-mat.mtrl-sci↗

arXiv2025

Design of thermal hysteresis in nonstoichiometric alloys with giant magnetocaloric effect

The non-stoichiometric Fe$_2$P-type (FeMnP$_{0.5}$Si$_{0.5}$)$_{1-x}$(FeV)$_{x}$ alloys ( $x=0, 0.01$, $0.02$, and $0.03$) have been investigated as potential candidates for magnetic refrigeration near room temperature. The magnetic ordering temperature decreases with increasing FeV concentration, $x$, which can be ascribed to decreased ferromagnetic coupling strength between the magnetic atoms. The strong magnetoelastic coupling in these alloys results in large values of the isothermal entropy change ($ΔS_M$); $15.7$ J/kgK, at $2$ T magnetic field for the $x = 0$ alloy. $ΔS_M$ decreases with increasing $x$. Results from M{ö}ssbauer spectroscopy reveal that the average hyperfine field (in the ferromagnetic state) and average center shift (in the paramagnetic state) have the same decreasing trend as $ΔS_M$. The thermal hysteresis ($ΔT_{hyst}$) of the magnetic phase transition decreases with increasing $x$, while the mechanical stability of the alloys improves due to the reduced lattice volume change across the magnetoelastic phase transition. The adiabatic temperature change $ΔT_{ad}$, which highly depends on $ΔT_{hyst}$, is $1.7$ K at $1.9$ T applied field for the $x = 0.02$ alloy.

cond-mat.mtrl-sci↗

arXiv2023

Giant magnetocaloric effect in the (Mn,Fe)NiSi-system

The search for energy-efficient and environmentally friendly cooling technologies is a key driver for the development of magnetic refrigeration based on the magnetocaloric effect (MCE). This phenomenon arises from the interplay between magnetic and lattice degrees of freedom that is strong in certain materials, leading to a change in temperature upon application or removal of a magnetic field. Here we report on a new material, Mn$_{1-x}$Fe$_x$NiSi$_{0.95}$Al$_{0.05}$, with an exceptionally large isothermal entropy at room temperature. By combining experimental and theoretical methods we outline the microscopic mechanism behind the large MCE in this material. It is demonstrated that the competition between the Ni$_2$In-type hexagonal phase and the MnNiSi-type orthorhombic phase, that coexist in this system, combined with the distinctly different magnetic properties of these phases, is a key parameter for the functionality of this material for magnetic cooling.

cond-mat.mtrl-sci↗

arXiv2022

Site-specific atomic substitution in a giant magnetocaloric Fe$_2$P-type system

Giant magnetocaloric (GMC) materials constitute a requirement for near room temperature magnetic refrigeration. (Fe,Mn)$_2$(P,Si) is a GMC compound with strong magnetoelastic coupling. The main hindrance towards application of this material is a comparably large temperature hysteresis, which can be reduced by metal site substitution with a nonmagnetic element. However, the (Fe,Mn)$_2$(P,Si) compound has two equally populated metal sites, the tetrahedrally coordinated $3f$ and the pyramidally coordinated $3g$ sites. The magnetic and magnetocaloric properties of such compounds are highly sensitive to the site specific occupancy of the magnetic atoms. Here we have attempted to study separately the effect of $3f$ and $3g$ site substitution with equal amounts of vanadium. Using formation energy calculations, the site preference of vanadium and its influence on the magnetic phase formation are described. A large difference in the isothermal entropy change (as high as 44\%) with substitution in the $3f$ and $3g$ sites is observed. The role of the lattice parameter change with temperature and the strength of the magnetoelastic coupling on the magnetic properties are highlighted.

cond-mat.mtrl-sci↗

arXiv2022

Tuning skyrmions in B20 compounds by 4d and 5d doping

Skyrmion stabilization in novel magnetic systems with the B20 crystal structure is reported here, primarily based on theoretical results. The focus is on the effect of alloying on the 3d sublattice of the B20 structure by substitution of heavier 4d and 5d elements, with the ambition to tune the spin-orbit coupling and its influence on magnetic interactions. State-of-the-art methods based on density functional theory are used to calculate both isotropic and anisotropic exchange interactions. Significant enhancement of the Dzyaloshinskii-Moriya interaction is reported for 5d-doped FeSi and CoSi, accompanied by a large modification of the spin stiffness and spiralization. Micromagnetic simulations coupled to atomistic spin-dynamics and ab initio magnetic interactions reveal a helical ground state and field-induced skyrmions for all these systems. Especially small skyrmions $\sim$50 nm are predicted for Co$_{0.75}$Os$_{0.25}$Si, compared to $\sim$148 nm for Fe$_{0.75}$Co$_{0.25}$Si. Convex-hull analysis suggests that all B20 compounds considered here are structurally stable at elevated temperatures and should be possible to synthesize. This prediction is confirmed experimentally by synthesis and structural analysis of the Ru-doped CoSi systems discussed here, both in powder and in single-crystal forms.

cond-mat.mtrl-sci↗

arXiv2021

Data-driven design of a new class of rare-earth free permanent magnets

A new class of rare-earth-free permanent magnets is proposed. The parent compound of this class is Co$_3$Mn$_2$Ge, and its discovery is the result of first principles theory combined with experimental synthesis and characterisation. The theory is based on a high-throughput/data-mining search among materials listed in the ICSD database. From ab-initio theory of the defect free material it is predicted that the saturation magnetization is 1.71 T, the uniaxial magnetocrystalline anisotropy is 1.44 MJ/m$^3$, and the Curie temperature is 700 K. Co$_3$Mn$_2$Ge samples were then synthesized and characterised with respect to structure and magnetism. The crystal structure was found to be the MgZn$_2$-type, with partial disorder of Co and Ge on the crystallographic lattice sites. From magnetization measurements a saturation polarization of 0.86 T at 10 K was detected, together with a uniaxial magnetocrystalline anisotropy constant of 1.18 MJ/m$^3$, and the Curie temperature of $T_{\rm C}$ = 359 K. These magnetic properties make Co$_3$Mn$_2$Ge a very promising material as a rare-earth free permanent magnet, and since we can demonstrate that magnetism depends critically on the amount of disorder of the Co and Ge atoms, a further improvement of the magnetism is possible. From the theoretical works, a substitution of Ge by neighboring elements suggest two other promising materials - Co$_3$Mn$_2$Al and Co$_3$Mn$_2$Ga. We demonstrate here that the class of compounds based on $T_3$Mn$_2$X (T = Co or alloys between Fe and Ni; X=Ge, Al or Ga) in the MgZn$_2$ structure type, form a new class of rare-earth free permanent magnets with very promising performance.

cond-mat.mtrl-sci↗

arXiv2020

Interstitial Carbon in bcc HfNbTiVZr high entropy alloy from first principles

The remarkable mechanical properties of high entropy alloys can be further improved by interstitial alloying. In this work we employ density functional theory calculations to study the solution energies of dilute carbon interstitial atoms in tetrahedral and octahedral sites in bcc HfNbTiVZr. Our results indicate that carbon interstitials in tetrahedral sites are unstable, and the preferred octahedral sites present a large spread in the energy of solution. The inclusion of carbon interstitials induces large structural relaxations with long-range effects. The effect of local chemical environment on the energy of solution is investigated by performing a local cluster expansion including studies of its correlation with the carbon atomic Voronoi volume. However, the spread in solution energetics can not be explained with a local environment analysis only pointing towards a complex, long-range influence of interstitial carbon in this alloy.

cond-mat.mtrl-sci↗

arXiv2018

Magnetic and mechanical effects of Mn substitutions in AlFe2B2

The mechanical and magnetic properties of the newly discovered MAB-phase class of materials based upon AlFe2B2 were investigated. The samples were synthesised from stoichiometric amounts of all constituent elements. X-ray diffraction shows that the main phase is orthorhombic with an elongated b-axis, similar to AlFe2B2. The low hardness and visual inspection of the samples after deformation indicate that these compounds are deformed via a delamination process. When substituting iron in AlFe2B2 with manganese, the magnetism in the system goes from being ferro- to antiferromagnetic via a disordered ferrimagnetic phase exhibited by AlFeMnB2. Density functional theory calculations indicate a weakening of the magnetic interactions among the transitions metal ions as iron is substituted by manganese in AlFe2B2. The Mn-Mn exchange interactions in AlMn2 B2 are found to be very small.

cond-mat.mtrl-sci↗

arXiv2018

Magnetocrystalline anisotropy of Fe5PB2 and its alloys with Co and 5d elements: a combined first-principles and experimental study

The Fe$_5$PB$_2$ compound offers tunable magnetic properties via the possibility of various combinations of substitutions on the Fe and P-sites. Here, we present a combined computational and experimental study of the magnetic properties of (Fe$_{1-x}$Co$_{x}$)$_5$PB$_2$. Computationally, we are able to explore the full concentration range, while the real samples were only obtained for 0 <= x <= 0.7. The calculated magnetic moments, Curie temperatures, and magnetocrystalline anisotropy energies (MAEs) are found to decrease with increasing Co concentration. Co substitution allows for tuning the Curie temperature in a wide range of values, from about six hundred to zero kelvins. As the MAE depends on the electronic structure in the vicinity of Fermi energy, the geometry of the Fermi surface of Fe$_5$PB$_2$ and the k-resolved contributions to the MAE are discussed. Low temperature measurements of an effective anisotropy constant for a series of (Fe$_{1-x}$Co$_{x}$)$_5$PB$_2$ samples determined the highest value of 0.94 MJ m$^{-3}$ for the terminal Fe$_5$PB$_2$ composition, which then decreases with increasing Co concentration, thus confirming the computational result that Co alloying of Fe$_5$PB$_2$ is not a good strategy to increase the MAE of the system. However, the relativistic version of the fixed spin moment method reveals that a reduction in the magnetic moment of Fe$_5$PB$_2$, by about 25%, produces a fourfold increase of the MAE. Furthermore, calculations for (Fe$_{0.95}$X$_{0.05}$)$_5$PB$_2$ (X = 5$d$ element) indicate that 5% doping of Fe$_5$PB$_2$ with W or Re should double the MAE. These are results of high interest for, e.g., permanent magnet applications, where a large MAE is crucial.

cond-mat.mtrl-sci↗

arXiv2016

AlM2B2 (M=Cr, Mn, Fe, Co, Ni): a group of nanolaminated materials

Combining theory with experiments, we study the phase stability, elastic properties, electronic structure and hardness of layered ternary borides AlCr2B2, AlMn2B2, AlFe2B2, AlCo2B2, and AlNi2B2. We find that the first three borides of this series are stable phases, while AlCo2B2 and AlNi2B2 are metastable. We show that the elasticity increases in the boride series, and predict that AlCr2B2, AlMn2B2, and AlFe2B2 are more brittle, while AlCo2B2 and AlNi2B2 are more ductile. We propose that the elasticity of AlFe2B2 can be improved by alloying it with cobalt or nickel, or a combination of them. We present evidence that these ternary borides represent nanolaminated systems. Based on SEM measurements, we demonstrate that they exhibit the delamination phenomena, which leads to a reduced hardness compared to transition metal mono- and diborides. We discuss the background of delamination by analyzing chemical bonding and theoretical work of separation in these borides.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic properties of Fe$_5$SiB$_2$ and its alloys with P, S, and Co

Fe$_5$SiB$_2$ has been synthesized and magnetic measurements have been carried out, revealing that M$_{\text{sat}}$ = 0.92 MA/m at T = 300 K. The M vs T curve shows a broad peak around T = 160 K. The anisotropy constant, K$_1$, estimated at T = 300 K, is 0.25 MJ/m$^3$. Theoretical analysis of Fe$_5$SiB$_2$ system has been carried out and extended to the full range of Fe$_5$Si$_{1-x}$P$_x$B$_2$, Fe$_5$P$_{1-x}$S$_x$B$_2$, and (Fe$_{1-x}$Co$_x$)$_5$SiB$_2$ compositions. The electronic band structures have been calculated using the Full-Potential Local-Orbital Minimum-Basis Scheme (FPLO-14). The calculated total magnetic moments are 9.20, 9.15, 9.59 and 2.42$μ_B$ per formula units of Fe$_5$SiB$_2$, Fe$_5$PB$_2$, Fe$_5$SB$_2$, and Co$_5$SiB$_2$, respectively. In agreement with experiment, magnetocrystalline anisotropy energies (MAE's) calculated for T = 0 K changes from a negative (easy-plane) anisotropy -0.28 MJ/m$^3$ for Fe$_5$SiB$_2$ to the positive (easy-axis) anisotropy 0.35 MJ/m$^3$ for Fe$_5$PB$_2$. Further increase of the number of p-electrons in Fe$_5$P$_{1-x}$S$_x$B$_2$ leads to an increase of MAE up to 0.77 MJ/m$^3$ for the hypothetical Fe$_5$P$_{0.4}$S$_{0.6}$B$_2$ composition. Volume variation and fixed spin moment calculations (FSM) performed for Fe$_5$SiB$_2$ show an inverse relation between MAE and magnetic moment in the region down to about 15\% reduction of the spin moment. The alloying of Fe$_5$SiB$_2$ with Co is proposed as a practical realization of magnetic moment reduction, which ought to increase MAE. MAE calculated in virtual crystal approximation (VCA) for a full range of (Fe$_{1-x}$Co$_x$)$_5$SiB$_2$ compositions reaches the maximum value of 1.16 MJ/m$^3$ at Co concentration x = 0.3, with the magnetic moment 7.75$μ_B$ per formula unit. Thus, (Fe$_{0.7}$Co$_{0.3}$)$_5$SiB$_2$ is suggested as a candidate for a rare-earth free permanent magnet.

cond-mat.mtrl-sci↗

arXiv2016

Magnetic structure of the magnetocaloric compound AlFe2B2

The crystal and magnetic structures of AlFe2B2 have been studied with a combination of X-ray and neutron diffraction and electronic structure calculations. The magnetic and magnetocaloric properties have been investigated by magnetisation measurements. The samples have been produced using high temperature synthesis and subsequent heat treatments. The compound crystallises in the orthorhombic crystal system Cmmm and it orders ferromagnetically at 285 K through a second order phase transition. At temperatures below the magnetic transition the magnetic moments align along the crystallographic $a$-axis. The magnetic entropy change from 0 to 800 kA/m was found to be -1.3 J/K kg at the magnetic transition temperature.

cond-mat.mtrl-sci↗

arXiv2011

The crystal and magnetic structure of the magnetocaloric compound FeMnP0.5Si0.5

The crystal and magnetic structure of the magnetocaloric compound FeMnP0.5Si0.5 have been studied by means of neutron and X-ray powder diffraction. Single phase samples of nominal composition FeMnP0.5Si0.5 have been prepared by the drop synthesis method. The compound crystallizes in the Fe2P-type structure (P-62m) with the magnetic moments aligned along the aaxis. It is found that the Fe atoms are mainly situated in the tetrahedral 3g site while the Mn atoms prefer the pyramidal 3f position. The material is ferromagnetic (TC = 382 K) and at 296 K the total magnetic moment is 4.4 μB/f.u. It is shown that the magnetic moment in the 3f site is larger (2.5 μB) than in the 3g site (1.9 μB).

cond-mat.mtrl-sci↗

arXiv2011

Strongly enhanced magnetic moments in ferromagnetic FeMnP$_{0.50}$Si$_{0.50}$

The compound FeMnP$_{0.50}$Si$_{0.50}$ has been studied by magnetic measurements, Mössbauer spectroscopy and electronic structure and total energy calculations. An unexpected high magnetic hyperfine field for Fe atoms located at the tetrahedral Me(1) site in the Fe$_2$P structure is found. The saturation moment derived from magnetic measurements corresponds to 4.4 $μ_B$/f.u. at low temperatures, a value substantially higher than previously reported, but in accord with the results from our electron structure calculations. This high saturation moment, a first order nature of the ferromagnetic transition and a tunable transition temperature make the Fe$_{2-x}$Mn$_{x}$P$_{1-y}$Si$y$ system promising for magnetocaloric applications.

cond-mat.mtrl-sci↗

arXiv2009

A new material for hydrogen storage, ScAl0.8Mg0.2

A novel aluminium rich alloy for hydrogen storage has been discovered, ScAl0.8Mg0.2, which has superior properties regarding hydrogen storage capacity, kinetics and stability towards air oxidation in comparison to hydrogen absorption in state-of-the-art intermetallic compounds. Detailed analysis of the hydrogen absorption in ScAl0.8Mg0.2 has been performed using in situ synchrotron radiation powder X-ray diffraction, neutron powder diffraction and quantum mechanical calculations. The results from calculations and experiments are in good agreement with each other.

cond-mat.mtrl-sci↗

arXiv2009

Order--disorder induced magnetic structures of FeMnP$_{0.75}$Si$_{0.25}$

We report on the synthesis and structural characterization of the magnetocaloric FeMnP$_{0.75}$Si$_{0.25}$ compound. Two types of samples (as quenched and annealed) were synthesized and characterized structurally and magnetically. We have found that minute changes in the degree of crystallographic order causes a large change in the magnetic properties. The annealed sample, with higher degree of order is antiferromagnetic with a zero net moment. The as-quenched sample has a net moment of 1.26 $μ_B$/f.u. and ferrimagnetic-like behavior. Theoretical calculations give rather large values for the Fe and Mn magnetic moments, both when occupied on the tetrahedral and pyramidal lattice site. The largest being the Mn moment for the pyramidal site reaches values as high as 2.8 $μ_B$/atom.

cond-mat.mtrl-sci↗