Substitutional Mn Atoms Research Articles

Comparative Study of Magnetic Properties of (Mn1−xAxIV)Bi2Te4 AIV = Ge, Pb, Sn

We investigated the magnetic properties of the antiferromagnetic (AFM) topological insulator MnBi2Te4 with a partial substitution of Mn atoms by non-magnetic elements (AIV = Ge, Pb, Sn). Samples with various element concentrations (10–80%) were studied using SQUID magnetometry. The results demonstrate that, for all substitutes the type of magnetic ordering remains AFM, while the Néel temperature (TN) and spin-flop transition field (HSF) decrease with an increasing AIV = Ge, Pb, Sn concentration. The rate of decrease varies among the elements, being highest for Pb, followed by Sn and Ge. This behavior is attributed to the combined effects of the magnetic dilution and lattice parameter increase on magnetic properties, most prominent in (Mn1−xPbx)Bi2Te4. Besides this, the linear approximation of the experimental data of TN and HSF suggests higher magnetic parameters for pure MnBi2Te4 than observed experimentally, indicating the possibility of their non-monotonic variation at low concentrations and the potential for enhancing magnetic properties through doping MnBi2Te4 with small amounts of nonmagnetic impurities. Notably, the (Mn1−xPbx)Bi2Te4 sample with 10% Pb substitution indeed exhibits increased magnetic parameters, which is also validated by local-probe analyses using ARPES. Our findings shed light on tailoring the magnetic behavior of MnBi2Te4-based materials, offering insights into the potential applications in device technologies.

Influences of C, Si and Mn on the wear resistance of coiled tubing steel

Coiled tubing steels of different grades with close compositions and similar microstructures may have a large difference in wear performance. Although contents of alloying elements, e.g., C, Si and Mn, in the steels are low, slightly adjusting their concentrations could markedly modify the wear resistance of the coiled tubing steel. In this work, atomic simulations were carried out to understand influences of the minor elements on the wear resistance of coiled tubing steel. Molecular dynamics simulations were conducted to study the wear behavior of steels with different contents of the elements, and first-principles calculations were implemented to investigate influences of the elements on atomic bonding based on their electronic properties. It is demonstrated that interstitial C atoms increase the local lattice size, and their bonding to Fe atoms shows hybrid covalent and ionic characteristics. Substitutional Mn atoms weaken the overall metallic bond but enhance the surrounding electron localizations; and substitutional Si atoms decrease local lattice size and are covalently bonded to surrounding Fe atoms. Corresponding changes in the electronic states caused by the elements strongly affect the atomic bonding. The three alloying elements have different solid-solution strengthening mechanisms, and their synergy strongly affects the wear resistance of the steel. The revealed mechanisms would help selecting and adjusting alloying elements to enhance the wear resistance of coiled tubing steels.

Investigations of the Mn–Ni–Sm ternary system by means of anomalous X-ray diffraction, X-ray absorption and DFT calculations

Various applications benefit from the use of compounds combining lanthanides and 3d metals because of their fundamental physicochemical properties. To tune compositions for dedicated applications and to overcome the criticality of raw materials or geopolitical tension, a better understanding of the binary, ternary and even quaternary systems is needed. The present work focuses on the ternary Mn-Ni-Sm system, which is particularly interesting for energy storage applications as it forms hydrogen absorbing intermetallic compounds. The fruitful combination of standard X-ray diffraction, anomalous X-ray diffraction close to the Mn 3d edge, X-ray absorption near edge structure, electron probe micro-analysis and first-principles calculations provides a better understanding of the physicochemical properties and phase equilibrium of this ternary system. In particular, the atomic substitution of Mn atoms in the Ni sites has been investigated for the stacking AB2/AB5 Sm2MnyNi7-y system. The Mn atoms substitute preferentially in the AB5 sub-units, whereas Sm vacancies are observed in the AB2 sub-units inducing an antagonist variation in cell volume units. In the SmMnyNi12-y system, the preferential localization of Mn atoms has been highlighted in site 8i for the SmMn4Ni8 compound.

Mn diffusion in the ferritic Fe-25 %Cr Alloy: A First-principles study

The doping manganese (Mn) in the ferritic Fe-Cr alloy could effectively improve its high temperature performance when applied as metallic interconnects of solid oxide fuel cells (SOFCs). A theoretical study of Mn diffusion property in such alloy is implemented. It is found that the substitutional Mn atom prefers to diffuse along the 〈111〉 crystal direction with activation energy of about 0.40 eV. The interstitial Mn atom diffuses across the crystal between two tetrahedral interstitials with activation energy of about 1.37 eV. Under the working temperature of SOFCs at 600℃-800℃, the magnitude of diffusion coefficients mainly varies from 10-15-10-10m2/s which is qualitatively consistent with the experimental result that diffusion rate following the order of DMn > DCr. Our results reveal the diffusion mechanism of Mn atom in the ferritic Fe-Cr alloy at the atomic scale, which provide theoretical guidance for further applications of metallic interconnects in SOFCs.

Thermal Annealing of Graphene Implanted with Mn at Ultralow Energies: From Disordered and Contaminated to Nearly Pristine Graphene

Ultralow-energy (ULE) ion implantation is increasingly being explored as a method to substitutionally dope graphene. However, complex implantation-related effects such as defect creation and surface contamination, and how they can be minimized by thermal annealing, remain poorly understood. Here, we address these open questions taking as the model case epitaxial graphene grown on Cu(111), which was subsequently ULE implanted with Mn at 40 eV and then studied as a function of annealing temperature under ultrahigh vacuum. While significant surface cleaning occurs at annealing temperatures as low as 200 °C, recovery from the implantation-induced disorder requires at least 525 °C. Upon high-temperature annealing, in the 600–700 °C range, the Mn atoms that were incorporated upon implantation as intercalated species (between graphene and the Cu surface) experience diffusion into the Cu layer, creating a subsurface alloy. Annealing at 700 °C restored implanted graphene to a nearly pristine state, with a well-ordered graphene lattice with substitutional Mn atoms and a well-defined Dirac cone. In addition to the insight into the complex physicochemical effects induced by thermal annealing, our results provide useful guidelines for future experimental studies on graphene that is modified (e.g., substitutionally doped) by using ULE ion implantation.

Topological properties of Sb(111) surface: A first-principles study

First-principles calculations based on the density functional theory were performed to systematically study the electronic properties of the thin film of antimony in (111) orientation. By considering the spin–orbit interaction, for stoichiometric surface, the topological states keep robust for six-bilayer case, and can be recovered in the three-bilayer film, which are guaranteed by time-reversal symmetry and inverse symmetry. For reduced surface doped by non-magnetic Bi or magnetic Mn atom, localized three-fold symmetric features can be identified. Moreover, band structures show that the non-trivial topological states stand for non-magnetic substitutional Bi atom, while can be eliminated by adsorbed or substitutional magnetic Mn atom.

The Al Doping Effect on Epitaxial (In,Mn)As Dilute Magnetic Semiconductors Prepared by Ion Implantation and Pulsed Laser Melting.

One of the most attractive characteristics of diluted ferromagnetic semiconductors is the possibility to modulate their electronic and ferromagnetic properties, coupled by itinerant holes through various means. A prominent example is the modification of Curie temperature and magnetic anisotropy by ion implantation and pulsed laser melting in III–V diluted magnetic semiconductors. In this study, to the best of our knowledge, we performed, for the first time, the co-doping of (In,Mn)As diluted magnetic semiconductors by Al by co-implantation subsequently combined with a pulsed laser annealing technique. Additionally, the structural and magnetic properties were systematically investigated by gradually raising the Al implantation fluence. Unexpectedly, under a well-preserved epitaxial structure, all samples presented weaken Curie temperature, magnetization, as well as uniaxial magnetic anisotropies when more aluminum was involved. Such a phenomenon is probably due to enhanced carrier localization introduced by Al or the suppression of substitutional Mn atoms.

Doping Graphene with Substitutional Mn.

We report the incorporation of substitutional Mn atoms in high-quality, epitaxial graphene on Cu(111), using ultralow-energy ion implantation. We characterize in detail the atomic structure of substitutional Mn in a single carbon vacancy and quantify its concentration. In particular, we are able to determine the position of substitutional Mn atoms with respect to the Moiré superstructure (i.e., local graphene-Cu stacking symmetry) and to the carbon sublattice; in the out-of-plane direction, substitutional Mn atoms are found to be slightly displaced toward the Cu surface, that is, effectively underneath the graphene layer. Regarding electronic properties, we show that graphene doped with substitutional Mn to a concentration of the order of 0.04%, with negligible structural disorder (other than the Mn substitution), retains the Dirac-like band structure of pristine graphene on Cu(111), making it an ideal system in which to study the interplay between local magnetic moments and Dirac electrons. Our work also establishes that ultralow-energy ion implantation is suited for substitutional magnetic doping of graphene. Given the flexibility, reproducibility, and scalability inherent to ion implantation, our work creates numerous opportunities for research on magnetic functionalization of graphene and other two-dimensional materials.

Magnetism and DFT calculations for understanding magnetic ground state of Fe doped Mn2O3

In the present work, we have carried out experimental analysis along with first-principles density functional theory (DFT) calculations to understand the magnetic ground state of Fe doped Mn2O3. The analysis of structural properties show that the orthorhombic type of crystal structure with space group Pcab is preserved, but the unit cell volume decreases with an increase in Fe concentration. Magnetic susceptibility measurements show that two antiferromagnetic transitions (TN1 = 25 K, TN2 = 80 K) for undoped Mn2O3 merged into one at around 35 K with increasing concentration of Fe doping (Mn2−xFexO3; x = 0; 0.20; 0.50; 0.75). M-H curve at 5 K exhibits small hysteresis around the origin. The magnitude of magnetization increases with the increasing concentration of Fe. M-H curve at 100 K shows the linear behavior of M concerning H for x = 0.20 and x = 0.50, indicating the paramagnetic state of the sample. As a complement to the experimental analysis, first-principles calculations using DFT were carried out. Fe doping was simulated by the corresponding substitution of Mn atoms to reproduce stoichiometric features of Mn2−xFexO3. The agreement between the two approaches suggests that the magnetic ground state of Fe doped Mn2O3 is tunable with Fe concentration.

Determining the crystal and electronic structures of the magnesium secondary battery cathode material MgCo2−xMnxO4 using first-principles calculations and a quantum beam during discharge

The stable structures of the spinel compounds MgCo2O4 and MgCo1.5Mn0.5O4 following the insertion of Mg atoms into vacant 16c sites during discharge were investigated using first-principles calculations. During this insertion, Mg atoms at 8a sites were found to migrate to other vacant 16c sites, such that the spinel form transitioned to a rock salt structure. The structural change from the standard spinel phase to a rock salt form was minimal in the case of MgCo2O4, since this change required the insertion of numerous Mg atoms. In contrast, a more pronounced structural change from the normal spinel to a rock salt form occurred in the MgCo1.5Mn0.5O4, as this change required fewer Mg atom insertions. The data suggest that the electron density and bond length between Mg atoms at 8a sites and O atoms in MgCo1.5Mn0.5O4 are both reduced compared to that in MgCo2O4. The Mg atoms in MgCo1.5Mn0.5O4 were determined to readily undergo intercalation as a result of the substitution of Mn atoms.

First-principles study of Zn-doping effects on phase stability and magnetic anisotropy of Ni-Mn-Ga alloys

The effect of Zn doping on Ni-Mn-Ga magnetic shape memory alloy was studied by the first-principles calculations using exact muffin-tin orbital method in combination with the coherent-potential approximation and projector augmented-wave method. Trends in martensitic transformation temperature TM and Curie temperature TC were predicted from calculated energy differences between austenite and nonmodulated martensite, ΔEA−NM, and energy differences between paramagnetic and ferromagnetic state, ΔEPM−FM. Doping upon the Ga-sublattice results in stabilization of martensitic phase which indicates the increase in TM. TC is affected only weakly or slightly decreases, because ΔEPM−FM of martensite does not change significantly with doping. The substitution of Mn atoms by Zn causes the decrease in both TM and TC. Comparing to Cu-doped Ni-Mn-Ga alloys, we predict that doping with Zn results in smaller decrease in TC but also in smaller increase in TM. Moreover, Cu doping upon the Ga-sublattice strongly decreases the magnetic anisotropy energy of martensite, whereas such strong effect was not observed for Zn doping. Based on the calculations of Zn-doped Ni-Mn-Ga alloys we suggest that simultaneous doping with Zn and an element increasing TC can result in significant increase in both transformation temperatures without strong decrease of magnetic anisotropy.

Effect of Mn doping in mechanical properties of Cd1−xMnxTe2

We have investigated the structural and elastic properties of Cd1−xMnxTe2 compound using the DFT based FP-LAPW method within a framework of GGA proposed by Perdew–Burke–Ernzerhof. These compounds are found to be stable in the pyrite phase, in agreement with the available experimental reports. The results of the equilibrium lattice constants and bulk moduli are in comparison with experimental and theoretical studies of CdTe2. The analysis of elastic constant at relaxed positions reveals the ductile nature of the compound with ionic contribution in the inter atomic bonding, whereas as the concentration of Mn increases the sample material starts to show a brittle nature with covalent bonding. The substitution of Mn atoms on the Cd site affect their mechanical properties (bulk modulus, young modulus, shear modulus) of the compound which have been reported. The formation energy was also calculated which predicted that the compound is thermodynamically stable and the stability increases with concentration of the Mn.

Defect induced ferromagnetic ordering in epitaxial Zn0.95Mn0.05O films on sapphire (0 0 0 1)

We report the influence of Mn dopant on magnetic properties of Zn0.95Mn0.05O (ZMO)/Al2O3(0 0 0 1) hetero-epitaxial systems grown by using pulsed-laser deposition. The room temperature (RT) intrinsic ferromagnetic (FM) ordering verified by superconducting quantum interference device magnetometer and x-ray magnetic circular dichroism spectrum of Mn L2,3 edges is ascribed to the substitutional Mn atoms in the Zn site of ZnO. Mn in ZMO has a tetrahedral local symmetry instead of the octahedral symmetry of MnO, after verifying the absence of the Mn-related impurities or clusters in ZMO epitaxial film by Mn K-edge and Zn K-edge x-ray absorption spectroscopy spectrum, as well as the analysis of long-range structural ordering on Renninger scan of forbidden (0 0 0 5) reflection in x-ray diffraction, transmission electron microscopy and Raman spectrum. Comparison of x-ray absorption spectra of ZMO with those of ZnO epilayers at O K-, Zn K-, and L3-edges indicates that the substitution of the Zn site with Mn enhances the charge-transfer (CT) transition and the presence of Zn vacancies (VZn) also dominate the photoluminescence (PL) spectrum, implying that the formation of numerous VZn defects plays an important role in activating FM interactions. The strong CT effect and the existence of high-density VZn suggest that the intrinsic RT FM ordering of insulating ZMO is a result of the formation of the bound magnetic polarons (BMPs) that interact with each other via intermediate magnetic impurities.

P -type codoping effect in (Ga,Mn)As: Mn lattice location versus magnetic properties

In the present work, we perform a systematic investigation on p-type codoping in (Ga,Mn)As. Through gradually increasing Zn doping concentration, the hole concentration increases, which should theoretically lead to an increase of the Curie temperature (TC) according to the p-d Zener model. Unexpectedly, although the film keeps its epitaxial structure, both TC and the magnetization decrease. The samples present a phase transition from ferromagnetism to paramagnetism upon increasing hole concentration. In the intermediate regime, we observe a signature of antiferromagnetism. By using channeling Rutherford backscattering spectrometry and particle-induced x-ray emission, the substitutional Mn atoms are observed to shift to interstitial sites, while more Zn atoms occupy Ga sites, which explains the observed behavior. This is also consistent with first-principles calculations, showing that the complex of substitutional Zn and interstitial Mn has the lowest formation energy.

Influence of Mn-substitution on the magnetic and thermal properties of TbCrO3

Studies on the Mn-substitution induced modification in the structural, thermal, magnetic and magnetocaloric properties of TbCrO3 orthochromite are presented. The phase formation and purity of the samples were examined by analyzing X-ray diffraction (XRD) patterns. Magnetization measurements as a function of temperature reveal the antiferromagnetic ordering of Cr3+ spins (TN1) at ∼ 157 K and Tb3+ spins (TN2) ∼ 5 K for TbCrO3 sample. The substitution of Mn-atoms at Cr-site results in a decrease in TN1 whereas an increase in TN2. The isothermal magnetization measurements reveal the presence of metamagnetic transition at 5 K and 2 K. Results of, ac and dc magnetization and isothermal remnant magnetization suggest a spin-glass like ordering below TN2. The heat capacity measurements show peaks corresponding to the magnetic ordering of Cr3+ and Tb3+ spins. The temperature variation of magnetic entropy change (-ΔSm) for different ΔH suggests the presence of the magnetocaloric effect in the vicinity of TN2. It is found that values of -ΔSm decreases with the increase in Mn-substitution. Present results have been understood in terms of modifications in magnetic exchange interactions introduced by the Mn-substitution at Cr-site.

Pure dephasing of single Mn spin in semiconductor quantum dots

We present comprehensive analytical and numerical studies on the pure dephasing of a single Mn spin in a semiconductor quantum dot due to (i) its sp-d exchange interaction with an electronic environment, and (ii) its hyperfine interaction with the nuclear spin environment. For (i), by modeling the electronic environment by an open two-level system, we provide exact analytical expressions and present detailed analysis for the Mn spin pure dephasing in both the Markovian and non-Markovian regimes. This provides a clear physical picture and a general theoretical framework based on which we estimate the Mn spin pure dephasing due to various fluctuations (such as thermal excitation, optical pumping, tunneling, or electron/hole spin relaxation) of the electronic environment and reveals a series of interesting behaviors, such as thermal, optical, and electrical control of the crossover between the Markov and non-Markov regimes. In particular, we find rapid Mn spin pure dephasing on a nanosecond time scale by the thermal fluctuation and optical pumping, but these mechanisms can be strongly suppressed by shifting the electron envelope function relative to the Mn atom with an external electric field through the quantum-confined Stark effect. The thermal fluctuation mechanism is also exponentially suppressed at low temperature. For (ii), we find that the Mn spin dephasing time is limited by the thermal fluctuation of the nuclear spins to a few microseconds even at low temperature and its value varies from sample to sample, depending on the distribution of spinful isotopes on the nearest-neighbor sites surrounding the substitutional Mn atom. Our findings may be useful to understand and suppress the Mn spin pure dephasing for its applications in quantum information processing.

Variable electronic transport properties of double carbon-nanotube junctions via substitutional Fe, Co, and Mn atom doping

Electronic and transport properties of carbon-nanotube (CNT) hybrid junction CNT(3,3)/CNT(6,0)/CNT(3,3) are investigated and compared with substitutionally doped junction with transition metal atoms, iron, cobalt, and manganese; using density functional theory combined with non-equilibrium Green's function formalism. Transmission coefficient and current–voltage (I–V) characteristics are calculated for all configurations. The results indicate a smooth increase of current with voltage in pristine junction up to 0.75 V while in the case of higher voltages, the increase accelerates significantly and the conductance for the voltages between 1 and 1.5 becomes almost constant, about 40μA V. The replacement of carbon atoms with Fe, Co, and Mn in the scattering region causes rather substantial reduction of the transmission and conductance of the hybrid junction. It causes the current to reduce nearly 25% at the voltage of 1.5 V compared to the pristine one. It also appears that, there is no serious difference in the I–V characteristics for different impurity atoms, except some deviation in manganese conductance from the other two impurities’ curves. These results can be useful for future CNTs based nano-electronic applications.

Screening the bulk properties and reducibility of Fe-doped Mn2O3 from first principles calculations

Manganese oxides, particularly Mn2O3, have demonstrated great potential for oxygen carrier materials in chemical looping applications. The application of these materials in the industrial scale is hindered by thermodynamic restrictions related to the reoxidation process. This disadvantage can be overcome by doping the oxide with a guest cation. Iron is one of the most promising dopants, but the atomic-level understanding of its effects on the properties of α-Mn2O3 is incomplete. Herein, we report a systematic GGA+U study of the bulk properties and reducibility of FexMn2-xO3 (0≤x≤2) as a function of Fe dopant concentration. In particular, we focus on a representative set of 20 models with different Fe content, generated by screening several thousand structures. Our results indicate that substitution of Mn atoms with Fe stabilizes FexMn2-xO3, which is visible through negative values of doping energies, decreasing oxide formation energies, and higher oxygen vacancy formation energies with increasing Fe concentration. Similar to Fe, the presence of an oxygen vacancy increases the band gap in the major spin channel of FexMn2-xO3. Oxygen transport in FexMn2-xO3 is found to depend on Fe content and distribution in the lattice. All in all, our findings provide atomic-level insight into the properties of FexMn2-xO3 and generally agree with experimental observations. Obtained information can be applied to investigate the reactivity of FexMn2-xO3.

Electronic structure of (In,Mn)As quantum dots buried in GaAs investigated by soft-x-ray ARPES

Electronic structure of a molecular beam epitaxy-grown system of (In,Mn)As quantum dots (QDs) buried in GaAs is explored with soft-x-ray angle-resolved photoelectron spectroscopy (ARPES) using photon energies around 1 keV. This technique, ideally suited for buried systems, extends the momentum-resolving capabilities of conventional ARPES with enhanced probing depth as well as elemental and chemical state specificity achieved with resonant photoexcitation. The experimental results resolve the dispersive energy bands of the GaAs substrate buried in ∼2 nm below the surface, and the impurity states (ISs) derived from the substitutional Mn atoms in the (In,Mn)As QDs and oxidized Mn atoms distributed near the surface. An energy shift of the Mn ISs in the QDs compared to (In,Mn)As DMS is attributed to the band offset and proximity effect at the interface with the surrounding GaAs. The absence of any ISs in the vicinity of the VBM relates the electron transport in (In,Mn)As QDs to the prototype (In,Mn)As diluted magnetic semiconductor. The SX-ARPES results are supported by measurements of the shallow core levels under variation of probing depth through photon energy. X-ray absorption measurements identify significant diffusion of interstitial Mn atoms out of the QDs towards the surface, and the role of magnetic circular dichroism is to block the ferromagnetic response of the (In,Mn)As QDs. Possible routes are drawn to tune the growth procedure aiming at practical applications of the (In,Mn)As based systems.

Correlation between the magnetic and thermoelectric properties in Mg2−xMnxSi

Single crystals of Mg2−xMnxSi (x = 0, 0.1, 0.2, 0.3, and 0.4) were prepared using a vertical Bridgman method. The formation of desired materials was confirmed using single-crystal and powder X-ray diffraction. The thermoelectric and magnetic properties were investigated for various Mn contents in the temperature range between 2 and 300 K and in magnetic fields up to 70 kOe. For various x values, Mg2−xMnxSi with x = 0.2 possesses the highest figure of merit. The experimental results revealed that the substitutional Mn atoms exhibit mixed valences of +3 (majority) and +2, giving rise to dramatic changes of carrier density and magnetic interaction. At the same time, the Seebeck coefficient and magnetic susceptibility show a sudden change at the same temperature. These results imply that the thermoelectric properties are correlated with the magnetic properties in the Mg2−xMnxSi crystals.