Mn Antisites Research Articles

Tuning magnetism and band topology through antisite defects in Sb-doped MnBi4Te7

The fine control of magnetism and electronic structure is crucial since the interplay between magnetism and band topology can lead to various novel magnetic topological states including axion insulators, magnetic Weyl semimetals and Chern insulators etc. Through crystal growth, transport, thermodynamic, neutron diffraction measurements, we show that with Sb-doping, the newly-discovered intrinsic antiferromagnetic topological insulator MnBi4Te7 evolves from antiferro-magnetic to ferromagnetic and then ferrimagnetic. We attribute this to the formation of Mn(Bi,Sb) antisites upon doping, which result in additional Mn sublattices that modify the delicate interlayer magnetic interactions and cause the dominant Mn sublattice to go from antiferromagnetic to ferro-magnetic. We further investigate the effect of antisites on the band topology using the first-principles calculations. Without considering antisites, the series evolves from antiferromagnetic topological insulator (x = 0) to ferromagnetic axion insulators. In the exaggerated case of 16.7% of periodic antisites, the band topology is modified and type-I magnetic Weyl semimetal phase can be realized at intermediate dopings. Therefore, this doping series provides a fruitful platform with continuously tunable magnetism and topology for investigating emergent phenomena, including quantum anomalous Hall effect, Fermi arc states, etc.

Intrinsic Defects in LiMn2O4: First-Principles Calculations.

Spinel LiMn2O4 has attracted wide attention due to its advantages of a high-voltage plateau, good capacity, environmental friendliness, and low cost. Due to different experimental synthesis methods and conditions, there are many intrinsic point defects in LiMn2O4. By means of first-principles calculations based on a reasonable magnetic configuration, we studied the formation energies, local structures, and charge compensation mechanism of intrinsic point defects in LiMn2O4. The formation energies of defects under the assumed O-rich equilibrium conditions were examined. It was found that O, Li, and Mn vacancies, Mn and Li antisites, and Li interstitial could appear in the lattice at some equilibrium conditions, but Mn interstitial is hard to form. The charge was compensated mainly by adjusting the oxidation state of Mn around the defect, except for the defects at the 8a Wyckoff site. The binding energies between point defects were calculated to shed light on the clustering of point defects. Furthermore, the diffusion of Li ions around the defects was discussed. Cation antisites led to a decrease of the Li diffusion barrier but O vacancy caused an increase of the barrier. This study provides theoretical support for understanding point defects in spinel LiMn2O4.

Antisite-induced half-metallicity and fully-compensated ferrimagnetism in Co–Mn–V–Al alloy

We predicted that Co–Mn–V–Al alloy is a fully-compensated half-metallic ferrimagnet in CoVMnAl-type atomic arrangement with 25% ∼ 50% Co–Mn antisites using band structure calculations. The CoVMnAl-type atomic arrangement with about 30% Co–Mn antisites was successfully synthesized. The measurements of magnetic properties and electronic transport properties confirmed the half-metallicity and fully-compensated ferrimagnetism. We argued that the half-metallicity and ferrimagnetism in Co–Mn–V–Al compounds originate from the antisite between the Co and Mn atoms, which implies a new way to search for half-metallic material in Heusler alloys.

The Electronic Structure and Magnetic Properties of Transition Metal Anti-sites-Doped ZnO: the First-Principles Calculations

The electronic structures and magnetic properties of transition metal (Mn, Fe, Co) anti-sites-doped ZnO are investigated systematically by the first-principles calculations. It is found that the Co and Mn anti-sites-doped ZnO systems are ferromagnetic, while the Fe-doped ZnO are antiferromagnetic. The Mn and Co anti-sites may be the main origin of ferromagnetism in Mn- and Co-doped ZnO systems. The magnetic exchange interactions of these systems are discussed.

Surface spin polarization of the nonstoichiometric Heusler alloy Co2MnSi

Using a combined approach of spin-resolved photoemission spectroscopy, band structure and photoemission calculations we investigate the influence of bulk defects and surface states on the spin polarization of Co2Mn(alpha)Si thin films with bulk L21 order. We find that for Mn-poor alloys the spin polarization at EF is negative due to the presence of Co_Mn antisite and minority surface state contributions. In Mn-rich alloys, the suppression of Co(Mn) antisites leads to a positive spin polarization at the Fermi energy, and the influence of minority surface states on the photoelectron spin polarization is reduced.

Polarization reduction in half-metallic Heusler alloys: the effect of point defects andinterfaces with semiconductors

Half-metallic full-Heusler alloys represent a promising class of materials for spintronicapplications. However, (i) intrinsic point defects in Heusler compounds can be detrimentalwith respect to their predicted 100% spin polarization at the Fermi level and (ii) whenjoined to mainstream semiconductors the presence of interface states—whichdestroys half-metallicity—can degrade their performance. Here, we present anoverview of recent first-principles calculations performed to explore both theseissues. In particular, we focus on ab initio FLAPW calculations performed forCo2MnGe and Co2MnSi in the presence of intrinsic defects (such as stoichiometric atomic swaps as well asnon-stoichiometric antisites) and when interfaced with GaAs and Ge. Our findings showthat Mn antisites, due to their low formation energies, can easily occur, in excellentconsistency with experimental observations, and that they do not destroy half-metallicity.On the other hand, Co antisites, which also show a modest formation energy, give rise todefect states at the Fermi level. As for the [001]-ordered interfaces, we show that the stronghybridization in proximity to the junction gives rise to rather broad interfacestates that locally destroy half-metallicity. However, the bulk gaps (both in theminority spin channel for the Heusler alloy and for both spin channels in thesemiconducting side) are fully recovered within a few layers away from the junction.

Role of the presence of transition-metal atoms at the antisites in CrAs, CrSe and VAs zinc-blende compounds

In a recent publication [I. Galanakis, K. Özdog˜an, E. Şaşıog˜lu, B. Aktaş, Phys. Rev. B 74 (2006) 140408(R)] we have shown that in the case of CrAs and related transition-metal chalcogenides and pnictides, crystallizing in the zinc-blende structure, the excess of the transition-metal atoms leads to half-metallic ferrimagnetism. The latter property is crucial for spintronic applications with respect to ferromagnets due to the lower stray fields created by these materials. We extend this study to cover the case where the transition-metal atoms sitting at antisites are not identical to the ones in the perfect sites. In Cr-based compounds, the creation of Mn antisites keeps the half-metallic ferrimagnetic character produced also by the Cr antisites. In the case of VAs, Cr and Mn antisites keep the half-metallic character of VAs (contrary to V antisites) due to the larger exchange splitting exhibited by these atoms.

Ab initiostudy of ground-state properties of the Laves-phase compoundZrMn2

By an ab initio density functional approach the structural and phase stability, electronic and magnetic properties, elastic constants, phonon dispersion, and defect formation of the Laves-phase compound $\mathrm{Zr}{\mathrm{Mn}}_{2}$ for the C15, C14, and C36 crystal structures were investigated. In order to study the stability of magnetic phases, nonmagnetic and spin polarized calculations for ferro- and antiferromagnetic orderings were performed. At low temperatures, the ferromagnetic cubic C15 phase was obtained as the ground state, with the ferromagnetic hexagonal C14 and C36 phases being almost degenerate in energy. From the calculated temperature-dependent free energies a structural transformation from C15 to C14 at about ${T}_{tr}=200\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ is predicted, confirming the experimentally observed C14 structure at elevated temperatures. Elastic properties were investigated for the nonmagnetic and ferromagnetic C14 and C15 phases. Structural stability studies based on the calculated temperature-dependent free vibrational energies very strongly favor the existence of ferromagnetic phases. Point defect formation properties for vacancies and antiste defects were calculated by combining a supercell approach with a statistical mechanics model. Mn antisites are the most favorable defects broadening the homogeneity range of the $\mathrm{Zr}{\mathrm{Mn}}_{2}$ phase toward the Mn-rich side. The existence of ordered Mn-rich compounds is predicted. Large magnetic moment of $3\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{B}$ for Mn antisite defects are derived.

Role of structural defects on the half-metallic character ofCo2MnGeandCo2MnSiHeusler alloys

Heusler alloys, such as ${\mathrm{Co}}_{2}\mathrm{MnSi}$ and ${\mathrm{Co}}_{2}\mathrm{MnGe},$ have been predicted from first--principles to be half metallic and potential candidates for spintronic applications. However, spin polarizations of only 50--60 % were experimentally obtained for these compounds---a decrease attributed to defects in the Mn and Co sublattices. Accurate ab initio full-potential linearized augmented plane wave calculations are performed in order to determine the effects of several types of defects (such as antisites and atomic swaps) on the electronic and magnetic properties of the bulk Heusler compounds. Our findings, in general agreement with experiments, show that Mn antisites have the lowest formation energy and retain the half-metallic character. On the other hand, Co antisites have a slightly higher formation energy and a dramatic effect on the electronic properties: the defect states that locally destroy half metallicity are energetically localized and are screened out in a couple of atomic shells. In this case, the spin polarization at the Fermi level is strongly reduced, and the spin polarization due to the s electrons, responsible for the tunneling current, is in excellent agreement with experiment. Finally, both Mn-Si and Mn-Co atomic swaps have very high formation energies, keeping however the half-metallic character.

Role of Mn antisite defects on half-metallicity in Co 2MnSi Heusler alloy

Accurate first-principles FLAPW calculations performed for Mn antisite defects in Co 2MnSi alloy show that (i) the low defect formation energy results in a high concentration of Mn antisites during growth, in excellent quantitative agreement with experiments and (ii) half-metallicity – typical of bulk Co 2MnSi – is kept, with the disorder resulting in a small increase of the spin-gap. These findings indicate that other kinds of defects or surface effects should be invoked in order to explain the observed reduced spin-polarization.

Spin injection at Heusler/semiconductor interfaces: First-principles determination of potential discontinuity and half-metallicity

Technologically relevant properties, such as potential discontinuity and half-metallic behavior, are determined by means of an accurate first-principles approach for Co2MnGe/GaAs and Co2MnGe/Ge interfaces. Interface states appear in both sides of the junctions, so that half-metallicity, typical of bulk Co2MnGe, is locally lost. As for the potential discontinuity, the character of the contact is dramatically affected by the semiconductor side: In Co2MnGe/GaAs, irrespective of the atomic termination, the Fermi level is pinned within the energy band gap, whereas it lies close to the valence-band maximum in the case of Co2MnGe/Ge. This gives rise to a rectifying contact in the first case and ohmic for holes in the second case, both potentially useful for spin–injection purposes. Finally, we investigate the effects of possible Co–Mn antisites in bulk Co2MnGe and find that half-metallicity is only locally destroyed, since defect-induced gap states are shown to be screened in a couple of nearest-neighbor atomic shells.