B20 Phase Research Articles

Structural and thermal stability of B20-type high-pressure phases FeGe and MnGe

Stability and phase transformations in the high-pressure-synthesized phases of FeGe and MnGe with a noncentrosymmetric B20-type crystal structure are studied theoretically using ab initio density-functional calculations and experimentally by means of differential scanning calorimetry. For both germanides in magnetic state, the evolutionary genetic search yields the same set of preferred equiatomic polymorphs. In case of FeGe, this result is consistent with available theoretical and experimental information, whereas for MnGe we find new hypothetical phases, in addition to the known metastable B20 phase. The ground state of nonmagnetic MnGe is unexpectedly represented by a tetragonal structure quite unusual for the pd-bonded AB-type compounds. It follows from our calculations that, for a given input chemical composition, the existence of a structure stable at finite temperatures (and pressures) can be efficiently established by zero-temperature evolutionary search, if this structure appears in the output short list of lowest-energy states. The presented calorimetric study of the metastable FeGe and MnGe phases shows that their behavior upon heating notably depends on preparation method and sample history.

B20-type FeGe on Ge(1 0 0) prepared by pulsed laser melting

B20-type FeGe is one of the noncentrosymmetric materials hosting magnetic skymions. In this work, we have prepared B20-type FeGe films by pulsed laser melting of metal Fe deposited on Ge(100). The formation of the B20 phase is confirmed by X-ray diffraction. The FeGe samples show a superparamagnetic behavior and their blocking temperatures increase with increasing the pulsed laser energy density. We conclude that this phenomenon is due to the increased grain size of the B20-type FeGe with increasing laser energy density. The presented method can be used to obtain different B20-type transition metal germanides and silicides, which can be magnetic skyrmion-hosting materials for spintronics.

Structural stability, electronic and thermoelectric properties of ruthenium silicide

The structural phase transition, electronic and transport properties of RuSi intermetallic compound has been studied using density functional theory (DFT). The pressure induced phase transition from B20 to B2 phase is observed at 6.4 GPa and compared with other theoretical results. The calculated ground state, electronic and transport properties are in good agreement with the previously reported results. The stability of RuSi at 0 and high pressure is also confirmed by phonon dispersion curve [see supplementary information].The band structure and density of states show that RuSi is metallic in B2 phase while semiconductor in B20 phase with indirect band gap of 0.34 eV. We have also calculated the transport properties of RuSi in terms of electrical conductivity (σ/τ), electronic thermal conductivity (κe/τ), Seebeck coefficient (S) and power factor (S2σ/τ) in B20 phase. Our results show that RuSi is p-type semiconductor and is in good agreement with the experimental data. Bonding nature is discussed using Fermi surface, band structure and charge density difference plots.

Ab initio analysis of magnetic properties of the prototype B20 chiral magnet FeGe

FeGe in the B20 phase is an experimentally well-studied prototypical chiral magnet exhibiting helical spirals, skyrmion lattices and individual skyrmions with a robust length of 70~nm. While the helical spiral ground state can be verified by first-principles calculations based on density functional theory, this feature size could not be reproduced even approximately. To develop a coherent picture of the discrepancy between experiment and theory, we investigate in this work the magnetic properties of FeGe from first-principles using different electronic-structure methods. We study atomistic as well as micromagnetic parameters describing exchange and Dzyaloshinskii-Moriya interactions, and discuss their subtle dependence on computational, structural, and correlation parameters. In particular, we quantify how these magnetic properties are affected by changes of the lattice parameter, different atomic arrangements, exchange and correlation effects, finite Fermi-function broadening, and momentum-space sampling. In addition, we use the obtained atomistic parameters to determine the corresponding Curie temperature, which agrees well with experiments. Our results indicate that the well-known and well-accepted relation between the micromagnetic parameters and the period of the helical structure, is not valid for FeGe. This calls for new experiments exploring the relation by measuring independently the spin stiffness, the spiralization and the period of the helical spin spiral.

Chemical Pressure Stabilization of the Cubic B20 Structure in Skyrmion Hosting Fe1–xCoxGe Alloys

Iron monogermanide (FeGe) with the noncentrosymmetric cubic B20 structure is a well-known helimagnet and a magnetic skyrmion host with a relatively high ordering temperature (∼280 K). FeGe and related metal monogermanide compounds, such as CoGe and MnGe, have several structural polymorphs and typically require high pressure (∼4 GPa) and high temperature (∼1000 °C) to synthesize in the cubic B20 structure. Here, we report that the cubic B20 phase of both FeGe and alloys of Fe1–xCoxGe could in fact be formed without the application of high pressure by simply reacting elemental powders at modest temperatures (550 °C). Furthermore, the incorporation of Co into Fe1–xCoxGe (0.05 ≤ x ≤ 0.1) stabilizes the cubic B20 structure up to 650 °C, which we propose is caused by chemical pressure induced by the incorporation of Co into the lattice. Interestingly, chemical vapor transport reactions using the Fe1–xCoxGe alloys as precursors yield plentiful growth of large (0.1 to 1 mm) single crystals of pure FeGe. Magnetic ...

Formation of Fe-Co-Si Structure in Fe and Co Implanted Si Substrate

ABSTRACTTernary Fe-Co-Si B20 phase structure was formed by implanting Fe and Co ions consecutively into Si(100) substrate at 50 keV energy, each with a fluence of 1.0 × 1017 atoms/cm2 and post-thermal vacuum annealing at 500 oC for 60 minutes. An in-situ magnetic field was used to enhance the formation of the ternary phase in the Si substrate during the implantation process. The magnetic field of 0.05 T was applied perpendicular to the incoming ion beam direction and parallel to the substrate surface to form elongated clusters in the transverse direction of the sample. Prior to the implantation of ions, the implant ions depth profiles were simulated using a dynamic ion-solid interaction code (TRIDYN). The TRIDYN simulation predicted a saturation in the peak concentration of the Fe and Co ions at a fluence of 1.0 × 1017 atoms/cm2. XPS measurement at the peak concentration depth (40 nm) showed the presence of Fe (23 %) and Co (32 %) in the Si matrix. XRD characterization confirmed the presence of stable Fe-Co-Si B20 phase structure in the annealed samples implanted with the in-situ magnetic field.

Robust Zero-Field Skyrmion Formation in FeGe Epitaxial Thin Films.

B20 phase magnetic materials have been of significant interest because they enable magnetic Skyrmions. One major effort in this emerging field is the stabilization of Skyrmions at room temperature and zero magnetic field. We grow phase-pure, high crystalline quality FeGe epitaxial films on Si(111). Hall effect measurements reveal a strong topological Hall effect after subtracting the ordinary and anomalous Hall effects, demonstrating the formation of high density Skyrmions in FeGe films between 5 and 275K. In particular, a substantial topological Hall effect was observed at a zero magnetic field, showing a robust Skyrmion phase without the need of an external magnetic field.

Electronic structure and the origin of the Dzyaloshinskii-Moriya interaction in MnSi

The metallic helimagnet MnSi has been found to exhibit skyrmionic spin textures when subjected to magnetic fields at low temperatures. The Dzyaloshinskii-Moriya (DM) interaction plays a key role in stabilizing the skyrmion state. With the help of first-principles calculations, crystal field theory, and a tight-binding model we study the electronic structure and the origin of the DM interaction in the B20 phase of MnSi. The strength of the $\stackrel{P\vec}{D}$ parameter is determined by the magnitude of the spin-orbit interaction and the degree of orbital mixing, induced by the symmetry-breaking distortions in the B20 phase. Our calculations suggest strong coupling between Mn-$d$ and Si-$p$ states, which is consistent with a mixed valence ground state $|{d}^{7\ensuremath{-}x}{p}^{2+x}\ensuremath{\rangle}$ configuration. Consistent with previous calculations, we find that DFT+U leads to the experimental magnetic moment of $0.4\phantom{\rule{0.28em}{0ex}}{\ensuremath{\mu}}_{B}$, which redistributes electrons between the majority and minority spin channels. We derive the magnetic interaction parameters $J$ and $\stackrel{P\vec}{D}$ for Mn-Si-Mn superexchange paths using Moriya's theory assuming the interaction to be mediated by ${e}_{g}$ electrons near the Fermi level. Using parameters from our calculations, we get reasonable agreement with the observations.

Magnetic, thermodynamic, and electrical transport properties of the noncentrosymmetricB20germanides MnGe and CoGe

We present magnetization, specific heat, resistivity, and Hall effect measurements on the cubic B20 phase of MnGe and CoGe and compare to measurements of isostructural FeGe and electronic structure calculations. In MnGe, we observe a transition to a magnetic state at $T_c=275$ K as identified by a sharp peak in the ac magnetic susceptibility, as well as second phase transition at lower temperature that becomes apparent only at finite magnetic field. We discover two phase transitions in the specific heat at temperatures much below the Curie temperature one of which we associate with changes to the magnetic structure. A magnetic field reduces the temperature of this transition which corresponds closely to the sharp peak observed in the ac susceptibility at fields above 5 kOe. The second of these transitions is not affected by the application of field and has no signature in the magnetic properties or our crystal structure parameters. Transport measurements indicate that MnGe is metal with a negative magnetoresistance similar to that seen in isostructural FeGe and MnSi. Hall effect measurements reveal a carrier concentration of about 0.5 carriers per formula unit also similar to that found in FeGe and MnSi. CoGe is shown to be a low carrier density metal with a very small, nearly temperature independent diamagnetic susceptibility.

Magnetoresistance in polycrystalline and epitaxial Fe1−xCoxSi thin films

Thin films of Fe${}_{1\ensuremath{-}x}$Co${}_{x}$Si were grown using molecular beam epitaxy on Si(111). These 20-nm-thick films, with compositions $x=0$ or 0.5, were produced by two methods: the first produced large (111)-textured crystallites of the B20 phase; the second produced phase-pure B20 (111) epilayers. The lattice mismatch with the substrate causes biaxial tensile strain in the layers, greater in the epilayers, that distorts the (111)-oriented material to a rhombohedral form. Magnetotransport measurements show that a combination of additional scattering arising from crystal grain boundaries and strain-free polycrystalline films results in a higher resistivity than for the epitaxial films. Magnetometry for $x=0.5$ suggests an increase in the ordering temperature in strained films relative to the polycrystalline films of $15\ifmmode\pm\else\textpm\fi{}4$ K. Moreover, the characteristic linear magnetoresistance, typical of bulk single-crystal material of this composition, is retained in the polycrystalline film but reduced in the epitaxial film. While the bulk properties of these materials are reproduced qualitatively, there are small quantitative modifications, due to the strain, to properties such as band gap, Curie temperature, and magnetoresistance.

Spectroscopic ellipsometric evidence of the solid-state reactions in Ni/Si multilayered films, induced by ion-beam mixing and thermal annealing

Solid-state reactions, induced by ion-beam mixing (IBM) and thermal annealing, in Ni/Si multilayered films (MLF) with an overall stoichiometry of Ni2Si, NiSi and NiSi2, while keeping the nominal thickness of Ni sublayer constant (3.0 nm), were studied by using spectroscopic ellipsometry as well as X-ray diffraction (XRD). The mixing was performed with Ar+ ions of an energy of 80 keV and a dose of 1.5× 1016 Ar+/cm2. Unlike the results of our previous study on Fe/Si MLF [Y.V. Kudryavtsev et al., Phys. Rev. B 65, 104417 (2002)], it was shown that an amorphous phase of NiSi in the B20 phase was formed during deposition independent of the overall stoichiometry of MLF, i.e., the nominal thickness of Si sublayer. IBM leads to some structural changes in the Ni/Si MLF, which cannot be detected by XRD but are confidently recognized by optical tools. A thermal annealing at 673 K of the Ni/Si MLF with an overall stoichiometry of NiSi and NiSi2 causes formation of the η-NiSi phase. The first trace of NiSi2 phase on the background of the η-NiSi phase was detected by XRD after an annealing at 1073 K, while, according to the optical results, NiSi2 turned out to be the dominant phase for the annealed Ni/Si MLF with an overall stoichiometry of NiSi2.

Interface structure and stabilization of metastable B2-FeSi/Si(111) studied withlow-energy electron diffraction and density functional theory

We present a combined experimental and theoretical investigation of the interfacebetween a B2-type FeSi film and Si(111). Using an ultra-thin B2-FeSi film grownon Si(111), the interface is still reached by electrons, so quantitative low-energyelectron diffraction (LEED) could be applied to determine the bonding geometryexperimentally. As a result, the local configuration at the shallow buriedinterface is characterized by near-substrate Fe atoms being 8-fold coordinatedto Si atoms and by the silicide unit cell being rotated by 180° withrespect to the Si unit cell (B8 configuration). The interface energetics wereexplored by total-energy calculations using density functional theory (DFT). TheB8-type interface proves to be the most stable one, consistent with theexperimental findings. The atomic geometries obtained experimentally (LEED)and theoretically (DFT) agree within the limits of errors. Additionally, thecalculations explain the stabilization of the B2 phase, which is unstable as bulkmaterial: the analysis of the elastic behaviour reveals a reversed energy hierarchyof B2 and the bulk stable B20 phase when epitaxial growth on Si(111) is enforced.

Structural and electronic structure properties of FeSi: the drivingforce behind the stability of the B20 phase

We present the results of a first-principles pseudopotential plane-wave studyfor the structural properties of the ε-FeSi (B20), NaCl (B1) andCsCl (B2) structures of FeSi. The calculations were performed using the localdensity and the generalized gradient approximations (LDA and GGA), for theexchange-correlation potential. The electronic structures of the B1and B2 phases have been similarly investigated. These calculations haveenabled us to identify the driving force behind the crystallization of FeSi inthe B20 phase. Both the B1 and B2 phases are found to be semimetallic, withthe Fermi energy lying in a pseudo-band-gap. The B20 structure is predicted tobecome unstable with respect to the B2 phase at a moderate pressure, of 13.5and 10.9 GPa according to the GGA and LDA calculations, respectively.

Diffusion in the B20-type phase FeSi

Abstract Self-diffusion of Fe and impurity diffusion of Ge in the B20 phase FeSi have been studied as a function of temperature using the radiotracer method with isotopes 59Fe and 71Ge. 71Ge has been used to simulate Si self-diffusion. Both diffusivities are very small, and their values for Fe are about one to two orders of magnitude smaller than for Ge. This uncoupled diffusion may be explained by vacancy-based diffusion on the Fe and Si sublattice. Both diffusivities reach a value of about D(T M) ≈ 10−14m2s−1 at T M = 1683 K. which is more typical for semiconductors than for metals. Results are discussed in comparison with other diffusion data on phases of the Fe-Si system and on other transition metal silicides.