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.Comment: 18 pages, 21 figures, 9 table

Variants of the X-phase in the Mn-Co-Ge system

We report two new variants of the X-phase (orthorhombic, space group Pnnm) derived from the Mn-Co-Ge system. Two compositionally related crystals were investigated by means of single-crystal X-ray diffraction (XRD) and energy dispersive spectroscopy (EDS). The Mn14.9Co15.5Ge6.6 and Mn14Co16.2Ge6.8 intermetallic compounds are part of the homogeneity region of the X-phase and adopt the Mn-14(Mn0.11Co0.64Si0.25)(23) structure type. The composition obtained from refinement of the XRD data is in agreement with the EDS results. In the present study, chemical disorder was only detected on the 8h positions. The ordering is compared with other members of the X-phase family and shows that the degree of disordering depends on the chemical composition. No completely ordered variants of the X-phase have yet been reported

TbMgNi4-xCox-(H,D)2 System. II : Correlation between Structural and Magnetic Properties

The magnetic properties of TbMgNi4-xCox intermetallic compounds and selected hydrides and deuterides of this system have been studied by various techniques, including magnetic measurements, in situ X-ray and neutron powder diffraction. The intermetallic compounds crystallize in a SnMgCu4-type structure and magnetically order below a Curie temperature (T-C), which increases exponentially with the Co content. This can be due to the ordering of the Co sublattice. On the other hand, the insertion of D or H in TbMgNiCo3 strongly decreases T-C. The X-ray diffraction measurements versus temperature reveal cell volume minima at T-C for the compounds with x = 1-3 without any hints of the structure change. The analysis of the neutron diffraction patterns for the intermetallics with x = 2 and 3 indicates a slightly canted ferrimagnetic structure below T-C. The Tb moments refined at 16 K are 4.1(2) (mu B)/Tb for x = 2, and 6.2(1) mu B/Tb for x = 3, which are smaller than the free ion value (9.5 mu B/Tb). This reduction can be due to the influence of temperature but also reveals the crystal field effect. As Ni and Co occupy statistically the same Wyckoff site, an average Ni/Co moment was refined, leading to 1.7(2) (mu B)/atom for x = 2 and 1.8(1) mu B/atom for x = 3 at 16 K. This moment is slightly canted compared to the Tb moment

Effect of small cation occupancy and anomalous Griffiths phase disorder in nonstoichiometric magnetic perovskites

The structural, magnetic, magnetocaloric and Griffiths phase (GP) disorder of non-stoichiometric perovskite manganites La0.8-xSr0.2-yMn1+x+yO3 are reported here. Determination of valence states and structural phases evidenced that the smaller cations Mn2+ and Mn3+ will not occupy the A-site of a perovskite under atmospheric synthesis conditions. The same analysis also supports that the vacancy in the A-site of a perovskite induces a similar vacancy in the B-site. The La3+ and Sr2+ cation substitutions in the A-site with vacancy influences the magnetic phase transition temperature (TC) inversely, which is explained in terms of the electronic bandwidth change. An anomalous non-linear change of the GP has been observed in the Sr substituted compounds. The agglomeration of Mn3+-Mn4+ pairs (denoted as dimerons), into small ferromagnetic clusters, has been identified as the reason for the occurrence of the GP. A threshold limit of the dimeron formation explains the observed non-linear behaviour of the GP formation. The Sr-substituted compounds show a relatively large value of isothermal entropy change (maximum 3.27 J/kgK at mu H-0 = 2T) owing to its sharp magnetic transition, while the broad change of magnetization in the La-substituted compound enhances the relative cooling power (maximum 98 J/kg at mu H-0 = 2T)

Structural and magnetic properties of new members of the 3:29 phase from the Ce-Fe-Mn system and 1:11 from the Ce-Co-Mn

The Ce–Fe–Mn and Ce–Co–Mn systems have been re-visited with the intent of finding new potential phases for application as permanent magnets. Two new ternary compounds, Ce3(Fe0.638Mn0.362)29 (Nd3(Fe,Ti)29-type, space group P21/c, No. 14, Pearson Symbol mP128) and CeCo8Mn3 (Ce(Ni,Mn)11-type, space group P4/mbm, No. 127, Pearson Symbol tP24) have been discovered in the compositional range where the Ce2(T,Mn)17 (T = Fe, Co) phases are expected to exist with a (H)–Th2Ni17-type structure (space group P63/mmc, No. 194, Pearson Symbol hP38). Detailed investigations of the crystal structures have been performed using X-ray powder diffraction (XRPD) with supporting energy-dispersive X-ray (EDS) analysis. Compositions of the new compounds have been defined based on the EDS analysis as follows: Ce9.7Fe57.5Mn32.8 and Ce9.2Co65.2Mn25.6. A short discussion on the crystal structure peculiarities of the 1:5, 1:11, 1:12, 2:17 and 3:29 compounds in the Ce–T–Mn (T = Fe, Co, Ni, Cu) systems has been made. We present magnetic measurements on selected representatives of the studied phases. The most interesting being the Ce3(Fe0.638Mn0.362)29 phase which has a transition temperature well above room temperature. CeNi4.95Mn6.05 and CeCo8Mn3 exhibits properties characteristic of a canted antiferromagnetic state

Influence of Mn content on the magnetic properties of the hexagonal Mn (Co,Ge)2 phase

Herein, we report on the effect of Mn content on the magnetic properties of the hexagonal Mn(Co,Ge)2 with composition Mn36+xCo49-xGe15.This compound was previously described as Mn2Co3Ge (MgZn2-type structure), but later as Mn(Co,Ge)2 with its own structure type, all samples in this work follow the same superstructure model. Samples were synthesized by induction melting, the crystal structures were evaluated using a combination of X-ray diffraction together with scanning electron microscopy equipped and an energy dispersive X-ray spectroscopy detector. The Curie temperature (TC) is shifted towards lower temperature as the Mn content is increased. On the other hand, the spin reorientation temperature (TSRT) increases and the magnetic moment decreases as the Mn content is increased. The magnetocaloric properties were investigated for the x = 1 alloy, Mn37Co48Ge15. It was found that the isothermal entropy change is 2 J kg−1 K−1 at room temperature for an applied field of 5 T

Direct and indirect magnetocaloric effects near room temperature related to structural transitions in Y0.9Pr0.1Fe2D3.5 deuteride

The structural and magnetic properties of Y0.9Pr0.1Fe2D3.5 deuteride have been investigated by synchrotron and neutron diffraction, magnetic measurements, and differential scanning calorimetry. Deuterium insertion induces a 23.5% cell volume increase and a lowering of crystal symmetry compared to the cubic C15 Y0.9Pr0.1Fe2 parent compound (Fd-3m SG). The deuteride is monoclinic (P21/c SG) below 330 K and undergoes a first-order transition between 330 and 350 K toward a pseudo-cubic structure (R-3 m SG) with TO–D = 342(2) K. In both structures, the D atoms are located in 96% R2Fe2 and 4% RFe3 tetrahedral interstitial sites (R = Y0.9Pr0.1). The compound is ferromagnetic, accompanied by a magnetostrictive effect below TC = 274 K. The analysis of the critical exponents indicates a second-order type transition with a deviation from the isotropic 3D Heisenberg model toward the 3D XY model. This implies an easy plane of magnetization in agreement with cell parameter variation showing planar magnetic orientation. A weak magnetic peak is even observed at the order–disorder transition with a maximum at 343 K. Magnetic entropy variations are characteristic of direct and inverse magnetocaloric effects at TC and TO–D, respectively.This work was done thanks to the synchrotron beam time allocated by the Synchrotron SOLEIL (No. BAG 20201440) and the neutron beam time allocated by LLB (Nos. ID 232 and 233). We are also grateful to Florence Porcher and Francoise Damay for their help as local contact for neutron diffraction measurements at LLB. A. Herrero acknowledges the Department of Education of the Basque Government as grantee of the “Programa Predoctoral de Formación de Personal Investigador No Doctor” and “Estancia Predoctoral” programs. Part of the results were obtained by V. Shtender within the postdoctoral stay at ICMPE, Project ID No. PRESTIGE-2017-2-0002. The authors have no conflicts to disclose

Impact of the R and Mg on the structural, hydrogenation and magnetic properties of R3-XMgXCo9 (R = Pr, Nd, Tb and Y) compounds

International audienceR2MgCo9 (R = Pr, Nd, Tb and Y) compounds have been synthesized by a powder sintering method and thecorresponding hydrides have been prepared by a solid gas method. Their crystal structures and magneticproperties have been systematically studied. X-ray diffraction analysis showed that all R2MgCo9 compoundsbelong to the PuNi3-type structure. The elements Tb, Y, Nd, Pr yield a lowering of the equilibrium pressure whichcorrelates well with the increase in cell volume. The R2MgCo9H(D)x (R = Pr, Nd, Tb and Y; (9.4 ≤ x ≤ 12))hydrides (deuterides) preserve the PuNi3-type structure with hydrogenation-induced volume expansion rangingfrom 14.7 to 19.6%. The substitution of deuterium for hydrogen in R2MgCo9–(H,D)2 (R = Tb and Y) prevents fastdesorption at room temperature and ambient pressure. As for the magnetic properties, all the studied intermetalliccompounds show ferromagnetic or ferrimagnetic behavior, and in some cases a temperature dependent spinreorientation. Hydrogen insertion reduces the magnetization and decreases the magnetic ordering temperature(TC), whereas Mg for R substitution increases TC

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Hydrogen production by a fully de novo enzyme

Molecular catalysts based on abundant elements that function in neutral water represent an essential component of sustainable hydrogen production. Artificial hydrogenases based on protein-inorganic hybrids have emerged as an intriguing class of catalysts for this purpose. We have prepared a novel artificial hydrogenase based on cobaloxime bound to a de novo three alpha-helical protein, α3C, via a pyridyl-based unnatural amino acid. The functionalized de novo protein was characterized by UV-visible, CD, and EPR spectroscopy, as well as MALDI spectrometry, which confirmed the presence and ligation of cobaloxime to the protein. The new de novo protein produced hydrogen under electrochemical, photochemical and reductive chemical conditions in neutral water solution. A change in hydrogen evolution capability of the de novo enzyme compared with native cobaloxime was observed, with tunover numbers around 80% for that of cobaloxime, and hydrogen evolution rates of 40% of that of cobaloxime. We discuss these findings in the context of existing literature, and our study contributes important information about the functionality of cobaloxime HER catalysts in protein environments, and the feasibility of artificial enzymes to the field of artificial metalloenzymes. Small de novo proteins as enzyme scaffolds have the potential to function as upscaleable bioinspired catalysts thanks to their efficient atom economy, and the findings presented here show that this type of novel enzymes are a possible product