Interstitial Mn Atoms Research Articles

Thermoelectric performance of lead-free manganese telluride via alkaline Mg doping for mid-temperature application

Manganese telluride (MnTe) is a potential p- type semiconducting thermoelectric material in the mid temperature range. Herein, magnesium (Mg) is incorporated with Manganese telluride (MnTe) in the stoichiometric ratio of Mn1.12−xMgxTe (x = 0, 0.025, 0.05, 0.075, 0.1) via vacuum sealing and hot press densification methods. The interstitial Mn atoms and Mg atoms create point defects and mass fluctuation, simultaneously. Stacking faults, twin boundaries and lattice dislocations are distinguished from HRTEM (High resolution transmission electron microscope). These defects are acting as a phonon scattering centers results in reduced total thermal conductivity of 0.827 W/mK at 653 K for Mn1.02Mg0.1Te. The excess Mn atoms in MnTe matrix and the substituted Mg atoms firmly contribute to improve the electrical conductivity by producing more acceptor levels, which significantly enhance the power factor of 439 µW/mK2 at 753 K for Mn1.07Mg0.05Te. Consequently, the maximum zT of 0.33 is achieved at 803 K for Mn1.07Mg0.05Te due to the synergistic effects of increased electrical property and reduced thermal conductivity.

Theoretical study on the origin of anomalous temperature-dependent electric resistivity of ferromagnetic semiconductor

Employing Korringa–Kohn–Rostoker Green’s function methodology, our investigation elucidates the previously obscure origins of the anomalous temperature-dependent electrical resistivity behavior of (Ga,Mn)As ferromagnetic semiconductors. Phonon and magnon excitations induced by temperature effects are addressed via the coherent potential approximation, while the Kubo–Greenwood formula is employed to compute transport properties. Consequently, the anomalous temperature-dependent electrical resistivity arising from the ferromagnetic–paramagnetic transition is successfully replicated. Our examination of electronic structures and magnetic interactions reveals pivotal roles played by antisite defects and interstitial Mn atoms in governing this behavior. As this approach enables both the estimation of temperature-dependent transport properties and the assessment of underlying mechanisms from a microscopic standpoint, it holds significant potential as a versatile tool across diverse fields.

Influence of N-type doping sites on electronic transport properties of Mg3Sb1.5Bi0.5 alloys

In order to clarify the doping influence of different donor dopants (Co, Mn, and Te) in Mg3Sb1.5Bi0.5 crystals, defect formation energy and electronic structures are successfully determined by first-principles calculations. Also, its effect on thermoelectric properties parameters is analyzed using the crystals prepared via the directional solidification method. It is determined that Co atoms substitute Mg2 sites, while Mn atoms are more likely to be doped at interstitial sites. Both Sb and Bi sites are susceptible to the substitution of Te atom. Te- and Co-doped crystals can achieve higher carrier concentration and carrier effective mass, respectively. Moreover, interstitial Mn atoms render a balanced influence on carrier concentration and carrier effective mass, resulting in an optimal electronic transport performance. The maximum of PF is 2.31 W m-1K−2, which is comparable to the previously reported codoped Mg3Sb1.5Bi0.5 crystals. A theoretical reference is presented to further optimize the properties of Mg3Sb1.5Bi0.5 crystals.

Magnetic properties of Mn-doped Bi2Se3 topological insulators: Ab initio calculations

Doping Bi$_2$Se$_3$ by magnetic ions represents an interesting problem since it may break the time reversal symmetry needed to maintain the topological insulator character. Mn dopants in Bi$_2$Se$_3$ represent one of the most studied examples here. However, there is a lot of open questions regarding their magnetic ordering. In the experimental literature different Curie temperatures or no ferromagnetic order at all are reported for comparable Mn concentrations. This suggests that magnetic ordering phenomena are complex and highly susceptible to different growth parameters, which are known to affect material defect concentrations. So far theory focused on Mn dopants in one possible position, and neglected relaxation effects as well as native defects. We have used ab initio methods to calculate the Bi$_2$Se$_3$ electronic structure influenced by magnetic Mn dopants, and exchange interactions between them. We have considered two possible Mn positions, the substitutional and interstitial one, and also native defects. We have found a sizable relaxation of atoms around Mn, which affects significantly magnetic interactions. Surprisingly, very strong interactions correspond to a specific position of Mn atoms separated by van der Waals gap. Based on the calculated data we performed spin dynamics simulations to examine systematically the resulting magnetic order for various defect contents. We have found under which conditions the experimentally measured Curie temperatures ${T_{\rm{C}}}$ can be reproduced, noticing that interstitial Mn atoms appear to be important here. Our theory predicts the change of ${T_{\rm{C}}}$ with a shift of Fermi level, which opens the way to tune the system magnetic properties by selective doping.

Abnormal magnetic properties of MnBi thin film synthesized by MBE

MnBi has attracted much attention due to its unusual magnetic properties. It has the potential as an alternative to the permanent magnets containing rare-earth elements. In this paper, the MnBi thin film was grown by plasma-assisted molecular beam epitaxial technique. The as-grown film shows ferromagnetic property and an anomalous magnetization dependence on temperature. The X-ray photoelectron spectrum reveals that there are regular and interstitial sites Mn atoms with two bonding states. The observed decrease of magnetization at ∼230 K would be ascribed to the movements of Mn atoms to the interstitial sites leaving behind Mn vacancies. Also at ∼90 K, it exhibits a spin reorientation due to the magnetization change from c-axis to ab-plane which is related to the abnormal temperature dependence of the lattice parameters.

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.

Magnetic properties of single-phase MnBi grown from MnBi49 melt

The single-phase NiAs-type MnBi, embedded in Bi matrix, was grown from homogeneous MnBi49 melt at low temperatures to prevent the formation of Mn1.08Bi. An abrupt magnetization change was observed at ∼240 K. The origin of this change was ascribed to the movement of the Mn atoms between the regular sites and the interstitial sites in the MnBi lattices. The splitting of the x-ray photoelectron lines of MnBi indicates the presence of two binding states of Mn atoms, one of which was ascribed to interstitial Mn atoms. A large coercivity up to 1.79 T at 400 K was observed in the as-grown bulk isotropic MnBi alloys.

Magnetic, electrical and structural properties of annealed ferromagnetic (Zn,Sn)As2:Mn thin films on InP substrates: comparison with undoped ZnSnAs2

We report the magnetic and electrical properties in Mn-doped and undoped (Zn,Sn)As2 epilayers, that were annealed at slightly higher temperatures than the growth temperature. (Zn,Sn)As2 :Mn were epitaxially grown on InP (001) substrates at 300°C, and showed room-temperature ferromagnetism. The hole concentration, saturation magnetization and Curie temperature were measured and evaluated as a function of annealing temperature. The Curie temperature had a tendency to slightly increase at annealing temperatures up to 340°C, and completely disappeared at 400°C. The ferromagnetism could be attributed to hole-mediated ferromagnetism resulting from Mn ion substitutions at both the II-group Zn and IV-group Sn sites, especially from the large solubility of Mn2+ substitution at Zn sites. The disappearance of ferromagnetism may be explained by several types of mechanisms : migration of mobile interstitial Mn atoms, diffusion of substitutional Mn ions to the surface, substitution of interstitial Mn atoms on Zn vacancies, and formation of MnAs clusters. It is noteworthy that the growth of magnetic semiconductor thin films from substrate lattice matching is essential for avoiding magnetic secondary phases such as MnAs clusters.

Mn incorporation into the GaAs lattice investigated by hard x-ray photoelectron spectroscopy and diffraction

(Received 22 December 2010; revised manuscript received 31 March 2011; published 15 June 2011) Photoelectron spectroscopy and diffraction have been used to investigate structural changes during the annealing process of Ga1−xMnxAs samples. Hard x-ray radiation helped in observing photoelectron core-level spectra and electron diffraction from the bulk underlying the oxidized surface layer. High electron-energy resolution enabled us to separate the components due to substitutional and interstitial Mn atoms in the intrinsic Mn 2p3/2 photoemission profile, resulting in two peaks at 638.8 and 639.5 eV binding energy, respectively. The peaks display the known characteristic behavior after annealing, that is, an almost complete reduction of the interstitial component and preservation of the substitutional component. In the photoelectron diffraction, a sensitivity of high-energy polar plots to the incorporation sites of photoemitting atoms into the atomic lattice has been shown. As a consequence, the experimental polar plots from substitutional and interstitial Mn atoms, which are supported theoretically, show characteristic features that provide structural information. From the similarities and differences of the polar plots for Mn and Ga, we have confirmed the assignment of components within the intrinsic part of the photoemission Mn 2p3/2 signal suggested by photoelectron spectroscopy.

Spin correlations and electron transport in MnBi:Au films

The structural, magnetic, and electron transport properties of Mn55−xAuxBi45 (x = 0, 4.5) thin films prepared by magnetron sputtering have been investigated. The magnetization of the MnBi films decreases and the coercivity increases due to Au doping. The temperature dependence of resistivity between 2 to 300 K shows that the films are metallic but the 4.5% Au-doped film shows a Kondo behavior with resistance minimum at 10.2 K. The magnetoresistance is anisotropic and the positive transverse magnetoresistance is significantly enhanced (16.3% at 70 kOe) by Au doping. We interpret these data in terms of a model in which Au atoms preferentially substitute for Mn atoms on the Mn lattice, and some Mn atoms are displaced to interstitial sites in the NiAs structure. These interstitial Mn atoms are coupled antiferromagnetically to the Mn atoms on the original Mn lattice leading to the large decrease in magnetization, Kondo effect, and the positive magnetoresistance.

The source of room temperature ferromagnetism in granular GaMnAs layers with zinc blende clusters

AbstractGranular GaAs:(Mn,Ga)As films were prepared by annealing the Ga0.985Mn0.015As/GaAs layers at 500 °C or 600 °C. It is commonly accepted that this processing should result in the formation of cubic or hexagonal MnAs clusters, respectively. We demonstrate that such a priori assumption is not justified. If in the as grown sample there are not many defects with the interstitial Mn atoms, only small cubic clusters can be formed even after annealing at 600 °C. Moreover, in a sample containing solely cubic GaMnAs clusters, the Mn ions are ferromagnetically coupled at room temperature. This fact was explained by the existence of GaMnAs solid solution in the clusters, with content of Mn close to 20% (higher than ever found in the layers) as was confirmed by experiment and theory. Extended X‐ray absorption spectroscopy studies excluded the possibility of formation of the hypothetic zinc blende MnAs clusters. Not more than one Mn atom was detected in the second shell around central Mn atom. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

X-ray magnetic dichroism in III-V diluted magnetic semiconductors: First-principles calculations

The electronic structure of the (Ga,Mn)As, (Ga,Mn)N, and (Ga,Gd)N, diluted magnetic semiconductors (DMSs) were investigated theoretically from first principles, using the fully relativistic Dirac linear muffin-tin orbital band-structure method. The electronic structure is obtained with the local spin-density approximation (LSDA), as well as the $\text{LSDA}+U$ method. The x-ray magnetic circular and linear dichroism (XMCD and XMLD) spectra at the Mn, As, Ga, and $\text{N}\text{ }K$ and Gd, Mn, As, and $\text{Ga}\text{ }{L}_{2,3}$ edges were investigated theoretically from first principles. The origin of the XMCD spectra in these compounds was examined. The effect of interstitial Mn atoms was found to be crucial for the x-ray magnetic dichroism at the Mn and $\text{As}\text{ }K$ and ${L}_{2,3}$ edges in the (Ga,Mn)As DMS. The influence of the exchange splitting and spin-orbit coupling strength on each of the constituent atoms were furthermore analyzed. The orientation dependence of the XMCD and XMLD at the $\text{Mn}\text{ }{L}_{2,3}$ edges in the (Ga,Mn)As DMS was investigated by calculating the XMCD and XMLD spectra for the $⟨001⟩$ and $⟨110⟩$ magnetization axis. We found a quite small anisotropy in the XMCD and a giant anisotropy in the XMLD at the $\text{Mn}\text{ }{L}_{2,3}$ edges. The exchange splitting of the initial $\text{Mn}\text{ }2p$ core level was found to be responsible for huge magnetocrystalline anisotropy of the XMLD at the $\text{Mn}\text{ }{L}_{2,3}$ edges. The calculated results are compared with available experimental data.

Element-specific electronic structure of Mn dopants and ferromagnetism of (Zn,Mn)O thin films

Element-specific electronic structure of (Zn,Mn)O thin films with various Mn concentrations has been investigated using X-ray absorption and emission spectroscopy. According to comparison between the experimental spectra and the density functional theory calculations (partial density of states and exchange interactions for various Mn defect configurations), the substitutional Mn impurities do not induce ferromagnetism in (Zn,Mn)O samples. The ferromagnetic properties can be obtained when defect configurations consisting of both substitutional and interstitial Mn atoms are present. The ferromagnetism in ZnO-based magnetic semiconductors is favored to be Ruderman–Kittel–Kasuya–Yoshida type and the established theoretical model is in a good agreement with the X-ray spectroscopic measurements.

Structural characterization of Mn doped SiC magnetic thin films

Mn-doped SiC magnetic thin films prepared by co-deposited molecular beam epitaxy (MBE) method on Si (111) substrates at 950 ℃ have been investigated by reflection high energy diffraction (RHEED), X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) techniques. RHEED results reveal that the SiC thin films doped with Mn are of the cubic structure. XRD and XANES results show that in the thin films with Mn doping concentrations of 0.5％ and 18％ , almost all the Mn atoms react with Si atoms, forming Mn4Si7 compound embedded in the SiC matrix, and no substitutional or interstitial Mn atoms exist in the SiC lattice. Furthermore, we hold that the ferromagnetism of the Mn doped SiC thin films originates mainly from the Mn4Si7 secondary phase.

Experimental and theoretical investigations on ferromagnetic nature of Mn-doped dilute magnetic semiconductors

X-ray absorption fine structure (XAFS) and first-principles calculations are effectively combined to establish correlations of fabrications, atomic and electronic structures as well as ferromagnetism for the Mn-doped dilute magnetic semiconductors (DMS), and to shed light on the magnetism origin for a variety of ZnO-, GaN-, and Si-based DMSs. The results of Mn:ZnO and (Mn,N):ZnO thin films reveal that either the existence of Zn vacancy or N substitution of O sites can stabilize the ferromagnetic interactions between neighboring Mn-Mn pairs, and enhance the magnetic moment per Mn. In a 2.5at.% Mn-doped GaN film, a part of substantial Mn ions is found to locate at the interstitial sites near the substitutional Mn ions, forming Mn-Mn dimers that possess unique electronic and magnetic properties. Similar phenomena have also been found in the Mn-doped Si system in which the interstitial Mn atoms intend to assemble together via an intervening substitutional Mn ion. We have proposed a pathway to understand the microscopic origin of ferromagnetism in the DMS materials from the viewpoint of experimental determination and theoretical calculations.

GaMnAs with high Mn composition grown in Stranski–Krastanov mode

GaMnAs diluted magnetic semiconductor (DMS) thin films with high Mn compositions were grown on (001) GaAs substrate by low-temperature molecular beam epitaxy (LT-MBE) after inserting an InAs wetting layer onto the GaAs buffer. The growth follows the Stranski–Krastanov mode, which brings about special magnetic characteristics of Ga 0.88Mn 0.12As. The insulating-DMS-like M( T) relation of the as-grown sample indicates the existence of a high proportion of interstitial Mn atoms. The magnetization is greatly improved after annealing, and the low Curie temperature is suggested to be due to the surface hole-localization effect. The investigation on the magnetic anisotropy shows an almost isotropic characteristic of the magnetic properties, which is ascribed to the S–K growth mode.

Optimal Doping Control of Magnetic Semiconductors via Subsurfactant Epitaxy

"Subsurfactant epitaxy" is established as a conceptually new approach for introducing manganese as a magnetic dopant into germanium. A kinetic pathway is devised in which the subsurface interstitial sites on Ge(100) are first selectively populated with Mn, while lateral diffusion and clustering on or underneath the surface are effectively suppressed. Subsequent Ge deposition as a capping layer produces a novel surfactantlike phenomenon as the interstitial Mn atoms float towards newly defined subsurface sites at the growth front. Furthermore, the Mn atoms that failed to float upwards are uniformly distributed within the Ge capping layer. The resulting doping levels of order 0.25 at. % would normally be considered too low for ferromagnetic ordering, but the Curie temperature exceeds room temperature by a comfortable margin. Subsurfactant epitaxy thus enables superior dopant control in magnetic semiconductors.

Structural study of MnxSi1-x magnetic semiconductor thin films

The structure of MnxSi1-x magnetic semiconductor thin films prepared by molecular beam epitaxy(MBE) on Si(100) substrate at 600 ℃ has been studied by X-ray diffraction (XRD) and X-ray absorption near edge structure (XANES) technique. The XRD results show that in the MnxSi1-x thin films with high Mn doping concentrations (x=0.08 and 0.17), only diffraction peaks of crystalline Mn4Si7 are observed. XANES results indicate that all the Mn K-edge XANES spectra of MnxSi1-x thin films with different Mn doping concentrations (x=0.007, 0.03, 0.08 and 0.17) show the similar feature. XANES calculation based on multiple-scattering theory further reveals that the experimental spectra for samples with different Mn doping concentrations are reproduced by the calculated Mn4Si7 spectrum. These results reveal that for the MnxSi1-x magnetic semiconductor thin films, Mn atoms mainly exist in the Si thin film substrate in the form of Mn4Si7 nanocrystalline grains, the substitutional or interstitial Mn atoms scarcely exist.

Interstitial Mn in (Ga,Mn)As: Hybridization with Conduction Band and Electron Mediated Exchange Coupling

We investigate theoretically the possibility of n-type DMS based on III{V materials with Mn impurities in interstitial instead of substitutional positions, and discuss some situations when this can happen. We show that the d-states at interstitial Mn atoms in (Ga,Mn)As hybridize with both valence and conduction bands. The hybridization is strong enough to establish an indirect ferromagnetic coupling of the Mn magnetic moments mediated either by holes or by conduction electrons. Moreover, the Curie temperatures estimated within the mean-fleld theory are comparable with Tc obtained for conventional materials with the same concentration of MnGa.

Enhanced magnetization by modulated Mn delta doping in GaAs

The magnetic property of GaMnAs can be enhanced by increasing the Mn concentration as well as hole concentration. The interstitial Mn is inevitably formed when Mn concentration is very high even if the growth condition is carefully controlled. They act as a double donor and compensates for Mn acceptors in GaAs. The interstitial Mn atoms also tend to be bound by those occupying the substitutional Ga site. The combination of interstitial and substitial Mn atom produces antiferromagnetic pairs. In order to reduce the interstitial Mn and increase hole concentration, we introduce Mn delta-doped layers separated by undoped GaAs layers. The magnetic properties are investigated as a function the spacing between Mn delta-doped layers. It is found that magnetization characteristics such as the magnetic susceptibility, coercive force and Curie temperature are enhanced when the spacing between Mn delta-doped layers is optimized.