Mn Layers Research Articles

Strong magnetic correlations to 900 K in single crystals of the trigonal antiferromagnetic insulatorsSrMn2As2andCaMn2As2

Crystallographic, electronic transport, thermal and magnetic properties are reported for SrMn2As2 and CaMn2As2 single crystals grown using Sn flux. Rietveld refinements of powder x-ray diffraction data show that the two compounds are isostructural and crystallize in the trigonal CaAl2Si2-type structure (space group P-3m1), in agreement with the literature. Electrical resistivity rho versus temperature T measurements demonstrate insulating ground states for both compounds with activation energies of 85 meV for SrMn2As2 and 61 meV for CaMn2As2. In a local-moment picture, the Mn^{+2} 3d^5 ions are expected to have high-spin S = 5/2 with spectroscopic splitting factor g = 2. Magnetic susceptibility chi and heat capacity measurements versus T reveal antiferromagnetic (AFM) transitions at TN = 120(2) K and 62(3) K for SrMn2As2 and CaMn2As2, respectively. The anisotropic chi(T < TN) data indicate that the hexagonal c axis is the hard axis and hence that the ordered Mn moments are aligned in the ab plane. The chi(T) for both compounds and Cp(T) data for SrMn2As2 show strong dynamic short-range AFM correlations from TN up to at least 900 K, likely associated with quasi-two-dimensional connectivity of strong AFM exchange interactions between the Mn spins within the corrugated honeycomb Mn layers parallel to the ab plane.

Novel Quaternary Dilute Magnetic Semiconductor (Ga,Mn)(Bi,As): Magnetic and Magneto-Transport Investigations

Magnetic and magneto-transport properties of thin layers of the (Ga,Mn)(Bi,As) quaternary dilute magnetic semiconductor grown by the low-temperature molecular-beam epitaxy technique on GaAs substrates have been investigated. Ferromagnetic Curie temperature and magneto-crystalline anisotropy of the layers have been examined by using magneto-optical Kerr effect magnetometry and low-temperature magneto-transport measurements. Post-growth annealing treatment of the layers has been shown to enhance the hole concentration and Curie temperature in the layers. A significant increase in the magnitude of magneto-transport effects caused by incorporation of a small amount of Bi into the (Ga,Mn)As layers, revealed in the planar Hall effect (PHE) measurements, is interpreted as a result of enhanced spin-orbit coupling in the (Ga,Mn)(Bi,As) layers. A two-state behaviour of the planar Hall resistance at zero magnetic field provides its usefulness for applications in non-volatile memory devices.

Structural analysis of PrBaMn2O5+δ under SOFC anode conditions by in-situ neutron powder diffraction

Abstract The crystal structure and oxygen stoichiometry of the proposed double perovskite solid oxide fuel cell (SOFC) anode material PrBaMn2O5+δ were determined under SOFC anode conditions via in-situ neutron diffraction. Measurements were performed in reducing atmospheres between 692 K and 984 K. The structure was fit to a tetragonal (space group P4/mmm) layered double perovskite structure with alternating Pr and Ba A-site cation layers. Under all conditions examined, the oxygen sites in the Ba and Mn layers were fully occupied, while the sites in the Pr layer were close to completely vacant. The results of the neutron diffraction experiments are compared to previous thermogravimetric analysis experiments to verify the accuracy of both experiments. PrBaMn2O5+δ was shown to be stable over a wide range of reducing atmospheres similar to anode operating conditions in solid oxide fuel cells without significant structural changes.

Passivation and Potential Fluctuation of Mg-Alloy AZ31B in Alkaline Environments

The pH effect on the passivation and corrosion behavior of Mg alloy AZ31B was investigated in chloride-free NaOH solutions in the range of 10 to 14. The electrochemical behavior of α-Mg matrix, secondary (β-phase) Mg17Al12, and constituent intermetallic particles (AlxMny) were compared in these solutions using various techniques such as open circuit potential (OCP) monitoring, potentiodynamic and potentiostatic polarization, scanning vibration electrode technique (SVET), microcapillary cell technique as well as electrochemical atomic force microscope (EC-AFM) technique. Optical microscope (both in-situ and ex-situ) was utilized to characterize corrosion morphology of AZ 31B alloy in different pH NaOH solutions and yield information on the cathodic and anodic behavior of different phases and particles. Microscopic examination of the passive film formation and composition on different phases/particles was carried out via field emission scanning electron microscope (FE-SEM) equipped with energy dispersive spectroscopy (EDS). Gallium focused ion beam (FIB) was used to examine the cross section of different phases after exposure. The results indicate that significant potential fluctuations (from -1.6 VSCE to -0.4 VSCE) occurred to AZ31B in high pH (≥ 13) NaOH solutions with the presence of oxygen. Removal of oxygen by argon sparging diminished the fluctuation range (from -1.6 VSCE to -1.3 VSCE). Oxide nodules and associated cracks were found at the interface between α-matrix and β-phase. Thick oxide film was found to form on the α-matrix and the thickness decreased as the distance increased from the α/β interface (~1500 nm) into the α-matrix (~700 nm). In contrast, oxide film thickness of <100 nm was determined on β-phase. AlxMny particles were found to preferentially dissolve in high pH NaOH solutions and crevice was identified at the interface between Mn oxide layer and α-matrix. In relatively low pH (≤ 11) NaOH solution, OCP remained constant at -1.4 VSCE during long term exposure with the presence of oxygen. No significant potential fluctuation was observed. SEM/FIB examination indicated that corrosion occurred on the α-matrix in these NaOH only environment. In comparison, β-phase remained intact with thin passive film. Mg(OH)2 dome formed on AlxMny particles suggesting cathodic nature of the intermetallic particles. This was also corroborated by the in-situ microscopic examination of the gas formation on the AlxMny particles. OCP and polarization experiments suggest that hydrogen evolution reaction is the dominant cathodic reaction at these pH levels and the bubbles were found to initiate and grow preferentially on the AlxMny particles. At pH 12, thin oxide film (<100 nm) formed across the entire sample surface without any preferential corrosion and/or passivation on different phases/particles. The OCP remained constant at -1.3 VSCE during long term exposure. This was approximately 0.1 V higher than the value observed in pH 11 solution and no significant potential rise and drop was observed. Both potentiodynamic and potentiostatic polarization indicated that anodic current density decreased with increasing pH from 10 to 12 but increased with increasing pH from 12 to 14. Lowest passivation current density was identified at pH 12, which agrees with SEM/FIB and optical microscopic examinations. It is hypothesized that below pH 12, microgalvanic effect exists between α-matrix (anode) and AlxMny particles (cathode), which leads to the preferential dissolution of α-matrix. This was verified by SVET technique and AlxMny particles were found to be the cathode compared to α-matrix. At pH 13 and above, better passive film formation on β-phase may lead to the significant increase in OCP where the dominant cathodic reaction switched to oxygen reduction reaction, which in turn caused significant oxidation of α-matrix by microgalvanic coupling. AlxMny particles were anodic compared to the α-matrix and preferential dissolution of Al is suggested by SEM/EDS.

HfMnSb2 : A Metal-Ordered NiAs-type Pnictide with a Conical Spin Order.

The NiAs-type structure is one of the most common structures in solids, but metal order has been almost exclusively limited to chalcogenides. The synthesis of HfMnSb2 is reported with a novel metal-ordered NiAs-type structure. HfMnSb2 undergoes a conical spin order below 270 K, in marked contrast to conventional magnetic order observed in NiAs-type pnictides. We argue that the layered arrangement of Hf and Mn makes it a quasi 2D magnet, where the Mn layers with localized magnetic moments (Mn(2+) ; S=5/2) can interact only through RKKY interactions, instead of metal-metal bonding that is otherwise dominant for typical NiAs-type pnictides. This result suggests that controlling order-disorder in NiAs-type pnictides enables a study of 2D-to-3D crossover behavior in itinerant magnetic system.

HfMnSb2: A Metal‐Ordered NiAs‐type Pnictide with a Conical Spin Order

AbstractThe NiAs‐type structure is one of the most common structures in solids, but metal order has been almost exclusively limited to chalcogenides. The synthesis of HfMnSb2 is reported with a novel metal‐ordered NiAs‐type structure. HfMnSb2 undergoes a conical spin order below 270 K, in marked contrast to conventional magnetic order observed in NiAs‐type pnictides. We argue that the layered arrangement of Hf and Mn makes it a quasi 2D magnet, where the Mn layers with localized magnetic moments (Mn2+; S=5/2) can interact only through RKKY interactions, instead of metal–metal bonding that is otherwise dominant for typical NiAs‐type pnictides. This result suggests that controlling order–disorder in NiAs‐type pnictides enables a study of 2D‐to‐3D crossover behavior in itinerant magnetic system.

Structural changes induced spin-reorientation of ultrathin Mn films grown on Ag(001)

The strained body centered tetragonal (bct) Mn ultrathin film from lattice parameter a=2.89Å to lattice value of 2.73Å induces anti-ferromagnetic behavior between Mn layers. The magnetic easy axis of Mn film was demonstrated theoretically to switch from the in-plane to out-of-plane by magneto-optical Kerr effect investigation. By including spin–orbit coupling in full potential linearized augmented plane waves and linearized muffin-tin orbitals methods, manganese ultrathin film displays different magnetic behaviors and the spin-reorientation transition is shown to be correlated to these structural changes. The calculated magnetic moment of manganese planes are enhanced and reach a value of ~4.02μB. The polar magneto-optical Kerr effect is calculated for a photon energy range extended to 15eV. It shows a pronounced peak in visible light.

Impact of embedded Mn nanodots on resistive switching characteristics of Si-rich oxides as measured in Ni-electrode metal–insulator–metal diodes

We studied the formation of high-density Mn nanodots (NDs) by remote H2 plasma (H2-RP) treatment and investigated how the embedding of Mn NDs affects the resistive switching properties of Si-rich oxides (SiOx) because it is expected that NDs will trigger the formation of the conductive filament path in SiOx. We fabricated Mn NDs with different ND areal densities by exposing an ultrathin Mn layer on the SiOx/Ni bottom electrode to H2-RP without external heating. After the deposition of SiOx as a resistive switching layer and the fabrication of Ni top electrodes, resistive switching behaviors of metal–insulator–metal (MIM) diodes were characterized from current–voltage (I–V) curves and compared with the results obtained from samples of SiOx without Mn NDs and with an ultrathin Mn layer instead. The embedding of Mn NDs in SiOx was found to be effective in increasing the ON/OFF ratio in resistance and reducing the variation in operation voltage.

Collapse and control of theMnAu2spin-spiral state through pressure and doping

MnAu$_{2}$ is a spin-spiral material with in-plane ferromagnetic Mn layers that form a screw-type pattern around a tetragonal $c$ axis. The spiral angle $\theta$ was shown using neutron diffraction experiments to decrease with pressure, and in later studies it was found to suffer a collapse to a ferromagnetic state above a critical pressure, although the two separate experiments did not agree on whether this phase transition is first or second order. To resolve this contradiction, we use density functional theory calculations to investigate the spiral state as a function of pressure, charge doping, and also electronic correlations via a Hubbard-like $U$. We fit the results to the one-dimensional $J_{1} - J_{2} - J_{3} - J_{4}$ Heisenberg model, which predicts either a first- or second-order spiral-to-ferromagnetic phase transition for different regions of parameter space. At ambient pressure, MnAu$_{2}$ sits close in parameter space to a dividing line separating first- and second-order transitions, and a combination of pressure and electron doping shifts the system from the first-order region into the second-order region. Our findings demonstrate that the contradiction in pressure experiments regarding the kind of phase transition are likely due to variations in sample quality. Our results also suggest that MnAu$_{2}$ is amenable to engineering via chemical doping and to controlling $\theta$ using pressure and gate voltages, which holds potential for integration in spintronic devices.

Evaluation of the Corrosion Protection Performance of Epoxy-Coated High Manganese Steel by SECM and EIS Techniques

The corrosion protection performances of epoxy-coated Mn steel and carbon steel were evaluated by electrochemical impedance spectroscopy (EIS) and scanning electrochemical microscopy (SECM) analysis. EIS was performed on coated Mn steel with a scratch in a 0.1 M NaCl solution after a wet/dry cyclic corrosion test. The charge transfer resistance (R ct) and film resistance (R f) of the coated Mn steel displayed a higher value than the coated carbon steel. The increase in the charge transfer resistance and film resistance of the coated steel is due to the presence Mn in steel. SECM was conducted to estimate the corrosion protection performance of the epoxy-coated Mn steel immersed in a 0.1 M NaCl solution. It was found that dissolution of Fe2+ was suppressed at the scratch on the coated Mn steel due to the higher resistance for anodic dissolution of the substrate. SEM/EDX analysis showed that Mn was enriched in corrosion products at a scratched area of the coated steel after corrosion testing. FIB-TEM analysis confirmed the presence of the nanoscale oxide layer of Mn in the rust of the steel, which had a beneficial effect on the corrosion resistance of the coated steel by forming protective corrosion products in the wet/dry cyclic test.

Visible-light emission at room temperature in Mn-doped Si light-emitting diodes

We demonstrate Si-based light-emitting diodes that continuously emit reddish-yellow visible light at room temperature by utilizing optical transitions between the p-d hybrid orbitals of Mn atoms doped in Si. Our light-emitting diodes show clear visible-light electroluminescence with two peaks at ${E}_{1}=1.75$ and ${E}_{2}=2.30\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$, corresponding to optical transitions between p-d hybrid orbitals of Mn atoms. The electrons at the p-d hybrid orbitals of Mn in Si are excited by hot holes that are accelerated by an intensive electric field in the depletion layer of reverse biased Si $p\text{\ensuremath{-}}n$ junctions containing a Mn-doped Si (Si:Mn) layer. The observed two peaks at ${E}_{1}=1.75$ and ${E}_{2}=2.30\phantom{\rule{0.16em}{0ex}}\mathrm{eV}$ are redshifted and blueshifted by 0.14 eV, respectively, from those of GaAs:Mn or ZnS:Mn. Our observations are consistent with the $p\text{\ensuremath{-}}d$ hybridized electronic structure of Mn atoms doped in Si as predicted by first-principles calculations.

Mn Adsorption on the GaAs(111)–(2×2)B Surface: First Principles Studies

Abstract Mn adsorption on the GaAs(111)–(1×1)B surface electronic and magnetic properties are investigated using first principles total energy calculations within the periodic spin polarized density functional theory. Results show that one Mn atom adsorption on top of the surface drives to an interstitial Mn atom. The interstitial atom is bonded to three first monolayer As atoms forming a chain-like structure. This stable structure has a ferromagnetic behavior with a Mn magnetic moment of ∼ 3.98 μ B . The Mn coverage increase yields a stable structure with the creation of an interstitial Mn layer, which in turn gives rise to a GaMnAs like alloy at the surface. The layer displays a ferromagnetic characteristic with magnetic moment of ∼ 4.40 μ B . A surface formation energy analysis of all these stable structures shows that one Mn interstitial monolayer and a Mn interstitial atom may coexist. Density of states shows that there is an important contribution of the Mn atoms to the occupied and unoccupied states. The DOS of one Mn monolayer is metallic with the most important contribution around the Fermi level coming from the Ga-p and Mn-d orbitals.

Cycloid spirals and cycloid cone transition in the HoMn6−xCrxGe6 (T, x) magnetic phase diagramm by neutron diffraction

The structures and magnetic properties of the antiferromagnetic hexagonal pseudo ternary compounds HoMn6−xCrxGe6 (x=1, 1.5 and 2) are studied by neutron diffraction in the temperature range 1.5–300K. The substitution of nonmagnetic Cr for Mn greatly affects the magnetic properties of HoMn6Ge6 by reducing the ordering temperature from 466K to 278K, 205K and 130K for (x=0, 1, 1.5 and 2) respectively, increasing the c/a ratio, suppressing the high temperature (HT) commensurate phase with q2=(0, 0, 1/2) and changing the high temperature (LT) q1=(0, 0, q1z) skew spiral rotation plane.HoMn5CrGe6 and HoMn4Cr2Ge6 display in the entire magnetically ordered regime cycloid spiral structures with the wave vector: q=(0, 0, qz), qz≈0.18(3) r.l.u. and Φs≈64.8° turn angle. The Ho and Mn/Cr (001) layers have ferromagnetic arrangements coupled antiferromagnetically. The Ho moments in the z=0 layer are oriented in a direction opposite to the line bisecting the angle 2φMn≈2×28° of the Mn layers at z=±∼0.25. This triple unit changes orientation collectively in the direction of q within the (b, c) plane containing the wave vector. Above 50K, the wave vector length increases linearly from 0.24 to 0.28(1) r.l.u. below TN.The HT HoMn4.5Cr1.5Ge6 cycloid spiral, is stable in the range Tt<T<TN (q1=(0, 0, q1z), q1z=0.1711(3) r.l.u., Φs =61.6° at 40K). At variance to the other compounds, crossing Tt≈36K from above, it undergoes a first order magnetoelastic transition from a cycloid HT q1 to a LT longitudinal cone structure: q2=(0, 0, q2z), q2z=0.149r.l.u., Φs=53.6° and cone angle ΦC=47°. The q1 and q2 phases coexist in the LT range down to 1.5K≤T≤Tt in varying amounts with T. This transition is directly manifested by the splitting of the HT magnetic satellites and the increase of characteristic nuclear intensities. The results are summarised in a (T, x) magnetic phase diagram.

Short- and long-range magnetic order in LaMnAsO

The magnetic properties of the layered oxypnictide LaMnAsO have been revisited using neutron scattering and magnetization measurements. The present measurements identify the N\'eel temperature ${T}_{N}=360(1)$ K. Below ${T}_{N}$ the critical exponent describing the magnetic order parameter is $\ensuremath{\beta}=0.33--0.35$, consistent with a three-dimensional Heisenberg model. Above this temperature, diffuse magnetic scattering indicative of short-range magnetic order is observed, and this scattering persists up to ${T}_{\text{SRO}}=650(10)$ K. The magnetic susceptibility shows a weak anomaly at ${T}_{\text{SRO}}$ and no anomaly at ${T}_{N}$. Analysis of the diffuse scattering data using a reverse Monte Carlo algorithm indicates that above ${T}_{N}$ nearly two-dimensional, short-range magnetic order is present with a correlation length of 9.3(3) \AA{} within the Mn layers at 400 K. The inelastic scattering data reveal a spin gap of 3.5 meV in the long-range ordered state, and strong, low-energy (quasielastic) magnetic excitations emerging in the short-range ordered state. Comparison with other related compounds correlates the distortion of the Mn coordination tetrahedra to the sign of the magnetic exchange along the layer-stacking direction, and suggests that short-range order above ${T}_{N}$ is a common feature in the magnetic behavior of layered Mn-based pnictides and oxypnictides.

Structural and magnetic features of solid-phase transformations in Mn/Bi and Bi/Mn films

Solid-phase transformations at different annealing temperatures in Mn/Bi (Mn on Bi) and Bi/Mn (Bi on Mn) films have been studied using X-ray diffraction, electron microscopy, and magnetic measurements. It has been shown that the synthesis of the α-MnBi phase in polycrystalline Mn/Bi films begins at a temperature of ~120°C and the Mn and Bi layers react completely at 300°C. The resulting α-MnBi(001) samples have a large perpendicular magnetic anisotropy (Ku ≃ 1.5 × 107 erg/cm3) and a coercive force H > HC ~ 3 kOe. In contrast to Mn/Bi, the ferromagnetic α-MnBi phase in Bi/Mn films is not formed even at annealing processes up to 400°C and Mn clusters are formed in a Bi melt. This asymmetry in phase transformations occurs because chemosorbed oxygen existing on the surface of the Mn film in Bi/Mn films suppresses a solid-phase reaction between Mn and Bi. The analysis of the results obtained implies the existence of new low-temperature (~120°C) structural transformation in the Mn–Bi system.

Termination layer compensated tunnelling magnetoresistance in ferrimagnetic Heusler compounds with high perpendicular magnetic anisotropy.

Although high-tunnelling spin polarization has been observed in soft, ferromagnetic, and predicted for hard, ferrimagnetic Heusler materials, there has been no experimental observation to date of high-tunnelling magnetoresistance in the latter. Here we report the preparation of highly textured, polycrystalline Mn3Ge films on amorphous substrates, with very high magnetic anisotropy fields exceeding 7 T, making them technologically relevant. However, the small and negative tunnelling magnetoresistance that we find is attributed to predominant tunnelling from the lower moment Mn–Ge termination layers that are oppositely magnetized to the higher moment Mn–Mn layers. The net spin polarization of the current reflects the different proportions of the two distinct termination layers and their associated tunnelling matrix elements that result from inevitable atomic scale roughness. We show that by engineering the spin polarization of the two termination layers to be of the same sign, even though these layers are oppositely magnetized, high-tunnelling magnetoresistance is possible.

Exchange bias in epitaxial and polycrystalline thin film Ru2MnGe/Fe bilayers

We report on thin film bilayers of the antiferromagnetic Heusler compound Ru2MnGe and Fe, as well as the resulting exchange bias field at low temperatures and its temperature dependence. Epitaxial Ru2MnGe/Fe bilayers show an exchange bias field up to 680 Oe at 3 K. For increasing temperatures, a linearly decreasing exchange bias field is found, which vanishes at 130 K. Furthermore, we grew polycrystalline Ru2MnGe showing an exchange bias field up to 540 Oe, which vanishes around 30 K. By adding a very thin intermediate layer of Mn, the exchange bias field for polycrystalline samples has been increased by about 40%. We discuss differences between the epitaxial and polycrystalline films regarding magnetic and crystallographic properties and compare our results to already published work on this system.

The effect of atomic structure on interface spin-polarization of half-metallic spin valves: Co2MnSi/Ag epitaxial interfaces

Using density functional theory calculations motivated by aberration-corrected electron microscopy, we show how the atomic structure of a fully epitaxial Co2MnSi/Ag interfaces controls the local spin-polarization. The calculations show clear difference in spin-polarization at Fermi level between the two main types: bulk-like terminated Co/Ag and Mn-Si/Ag interfaces. Co/Ag interface spin-polarization switches sign from positive to negative, while in the case of Mn-Si/Ag, it is still positive but reduced. Cross-sectional atomic structure analysis of Co2MnSi/Ag interface, part of a spin-valve device, shows that the interface is determined by an additional layer of either Co or Mn. The presence of an additional Mn layer induces weak inverse spin-polarisation (−7%), while additional Co layer makes the interface region strongly inversely spin-polarized (−73%). In addition, we show that Ag diffusion from the spacer into the Co2MnSi electrode does not have a significant effect on the overall Co2MnSi /Ag performance.

Ferromagnetism in GaAs/InAs/GaAs Quantum Dot Layer Delta-Doped with Mn

Transport, magnetotransport and magnetic properties of structures GaAs/InAs/GaAs with a InAs quantum dot (QD) layer have been investigated in the temperature interval 4.2&lt;T&lt;300 K. The structures were delta-doped by Mn from a one side to provide magnetic properties and by carbon from the other side to enhance a p-type conductivity. The ferromagnetic phase up to 400 K was detected by SQUID magnetometer. Anomalous Hall-effect was observed at low T. The role of additional disorder in conducting channel due to the QD layer was investigated. It is shown a principal role of a fluctuation potential of Mn layer separated from conducting QD layer by a spacer in anomalous transport properties of structures. The negative magnetoresistance was observed at low T due to the reduction of the spin-flip scattering by aligning spins by magnetic field.

Role of nitrogen on the magnetic properties of MBE grown Mn0.04Ge0.96 films

We report the role of nitrogen on the distribution of Mn atoms and ferromagnetism in Mn0.04Ge0.96 films. High quality Mn0.04Ge0.96 films are obtained by sequential evaporation of stacked Mn and Ge layers on Si (100) substrate in N2 environment. The experimental results reveal that the films grown under 5×10−5Torr N2 partial pressure exhibit improved crystallinity and magnetic properties when compared to vacuum deposited films. Correlating the results of electron spin resonance (ESR) and x-ray photoelectron spectroscopy (XPS) studies with magnetic measurements, we have found that introduction of N2 during growth effects the distribution of Mn atoms in Ge. Incorporation of nitrogen into the films and degree of magnetic ordering was obtained from XPS spectra and line shape analysis of ESR spectra. In the presence of nitrogen, enhancement in Curie temperature is associated with the increase in the number of active Mn atoms, however promoted Mn diffusion leads to formation of Mn5Ge3 clusters.