Increasing Mn Composition Research Articles

Phase equilibria of the Co-Cr-Mn ternary system at 700 ℃

The isothermal phase diagram of the Co-Cr-Mn ternary system at 700 ℃ was experimentally determined via field-emission electron probe microanalysis/wavelength dispersive X-ray spectrometry and electron backscatter diffraction analyses. Three-phase regions of αCo(para)+ βMn+ σ and αMn+ βMn+ σ were confirmed via equilibrated microstructures; however, three-phase microstructures related to αCo(para)+ εCo+ σ and α’+ (Cr)+ σ were not observed in our study. The three-phase region of αCo(para)+ εCo+ σ was estimated to be very narrow because the phase boundaries between the αCo(para) and εCo phases were different within 2 at% for the Co, Cr, and Mn compositions. Furthermore, the α’ phase, presumed to be a nitride, was not observed in the microstructure equilibrated under a strictly controlled high-purity Ar gas (99.999%) atmosphere. The σ phase indicated a wide single-phase region similar to a previous study reported at temperatures between 900 ℃ and 1200 ℃. The αMn phase, which is stable below 727 ℃ in the Cr-Mn binary system and below 900 ℃ in the Co-Mn binary system, had a large Co solubility of 9.8 at% in the Co-Cr-Mn ternary system. The composition dependences on the lattice constants of the σ phase in the Co-Cr-Mn ternary system were examined via X-ray diffraction analysis; this analysis showed that the lattice constants a and c changed significantly with increasing Mn composition in the Co and Mn substitution, while the lattice constant of a was constant and c was slightly increased by increasing Mn composition in Cr and Mn substitution. From first-principles calculations, it was deduced that increases or decreases in bonding lengths between the nearest atoms occupying the five sublattice sites were associated with small changes in the lattice constant when Mn replaced Cr. The hardness of the σ phase decreased with increasing c/a, and the hardness in the crystal orientation of (100) was more dependent on the Mn composition than that in the crystal orientation of (001).

Effect of Manganese Addition on the Structure, Magnetic Properties and Microwave Absorption of La0.8Ba0.2MnxFe½(1-x)Ti½(1-x)O3

We have carried out modification of La0.8Ba0.2MnxFe½(1-x)Ti½(1-x)O3 (x = 0.1 – 0.8) magnetic materials by wet milling method. Raw materials of La2O3, BaCO3, Fe2O3, TiO2 and MnCO3 were mixed according to stoichiometry calculation for each composition. The mixture was milled for 5 hours and then sintered at 1000 °C for 5 hours. The refinement results by X-ray diffraction pattern shows that the increasing Mn composition enhances the mass fraction of La0.8Ba0.2MnxFe½(1-x)Ti½(1-x)O3 phase which has the same structure as LaMnO3. For x = 0.8 a single phase of LaMnO3 was formed. The single phase has a crystal monoclinic crystal structure with space group of I 1 2 / a 1, with lattice parameters given by a = 5.519(5) Å, b = 5.5537(5) Å and c = 7.8176(9) Å, α = γ = 90o and β = 90.345(6)o, V = 239.64(3) Å3, ρ = 6.463 gr.cm-3, wRp = 5.96, and χ2 (chi-squared) = 1.17. The hysteresis curve shows that the sample with composition x = 0.8 produces ferromagnetic behaviour at room temperature. The ferromagnetic properties arise due to the mixed valence of Mn3+ and Mn4+ ions through a double exchange mechanism. The results of the microwave absorption indicated that there was a broadening of absorption peak frequency at 9.9 GHz. The reflection loss (RL) increases with the increasing of LaMnO3 phase. For x = 0.8 we have the best of RL where the microwave absorption was calculated reaching 95% at the highest peak frequency with a thickness of 1.5 mm. Thus we have been successful in creating a single phase of La0.8Ba0.2MnxFe½(1-x)Ti½(1-x)O3 with application as a microwave absorber.

Electronic and magnetic properties of off-stoichiometric Co2MnβSi/MgO interfaces studied by x-ray magnetic circular dichroism

We have studied the electronic and magnetic states of Co and Mn atoms at the interface of the Co2MnβSi (CMS)/MgO (β = 0.69, 0.99, 1.15, and 1.29) magnetic tunnel junction (MTJ) by means of x-ray magnetic circular dichroism. In particular, the Mn composition (β) dependences of the Mn and Co magnetic moments were investigated. The experimental spin magnetic moments of Mn, mspin(Mn), derived from x-ray magnetic circular dichroism weakly decreased with increasing Mn composition β in going from Mn-deficient to Mn-rich CMS films. This behavior was explained by first-principles calculations based on the antisite-based site-specific formula unit (SSFU) composition model, which assumes the formation of only antisite defect, not vacancies, to accommodate off-stoichiometry. Furthermore, the experimental spin magnetic moments of Co, mspin(Co), also weakly decreased with increasing Mn composition. This behavior was consistently explained by the antisite-based SSFU model, in particular, by the decrease in the concentration of CoMn antisites detrimental to the half-metallicity of CMS with increasing β. This finding is consistent with the higher tunnel magnetoresistance ratios which have been observed for CMS/MgO/CMS MTJs with Mn-rich CMS electrodes.

Magneto-optical properties of Mn3+ substituted Fe3O4 nanoparticles

MnxFe3−xO4 (0.0≤x≤1.0) nanoparticles were synthesized by the polyol synthesis method and the effect of Mn3+ substitution on structural, magnetic and optical properties of Fe3O4 was studied. X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), UV–visible spectroscopy and vibrating sample magnetometer (VSM) were used to study the physical properties. The crystallite (from XRD) and particle sizes (from TEM and SEM) are in close agreement with each other. Lattice parameter increases with increasing Mn3+ concentration, due to the respective larger ionic radius of Mn3+ ion compared with the Fe3+ ion. The magnetic hysteresis (M–H) curves revealed superparamagnetic characteristics of the products. The extrapolated specific saturation magnetization (σs) values decreased from maximum value of 47.3emu/g to the minimum value of 25.6emu/g by increasing Mn composition. The particle size dependent Langevin function was applied to determine the magnetic particle dimensions (Dmag) around 15nm. The observed magnetic moments of NPs are in range of (1.06–1.96)µB and significantly less than 4µB of bulk Fe3O4. Magnetic anisotropy was offered as uniaxial and calculated effective anisotropy constants (Keff) are between 34.47×104Erg/g and 21.83×104Erg/g. The size-dependent saturation magnetization suggests the existence of a magnetically inactive layer as 1.638nm on FeMnxFe2−xO4 NPs. The UV–vis diffuse reflectance spectroscopy (DRS) and Kubelka−Munk theory were applied to determine the optical properties. The estimated optical band gap (Eg) values dropped almost linearly from 2.05eV to 1.17eV with increasing Mn composition.

Spin-glass-like freezing in spin-chain compounds Ca3Co2−xMnxO6: Effect of disorder

We have illustrated the presence of spin-glass-like freezing in the Mn-doped spin-chain compounds Ca3Co2−xMnxO6, which was not reported previously. With increasing Mn composition, the magnetic relaxation due to spin freezing is gradually suppressed. Interestingly, the magnetization in the intermediate Mn compositions relaxes more slowly (characteristic time is significantly enhanced) than those in x = 0.0 and 1.0. This could be understood within the framework of disorder with randomness or imperfections, which plays an important role in the formation of spin-glass-like feature in intermediate Mn compositions, quite different from the cases in x = 0.0 with strong spin frustration and in x ∼ 1.0 with perfect Co/Mn ionic order.

Manganese doping effects on interband electronic transitions, lattice vibrations, and dielectric functions of perovskite-type Ba0.4Sr0.6TiO3 ferroelectric ceramics

The Ba0.4Sr0.6−x Mn x TiO3 (BSMT) ceramics with different Mn composition (from 1% to 10%) have been prepared via the conventional solid-state reaction sintering. The X-ray diffraction analysis shows that the ceramics are polycrystalline with the single perovskite phase. The lattice vibrations and optical properties have been investigated using Raman scattering, spectroscopic ellipsometry (SE), and infrared reflectance spectra. It was found that the optical bandgap for the BSMT ceramics is varied between 3.40 and 3.65 eV. The three first-order Raman-active phonon modes can be observed, and the frequency of the A 1(LO3)/E(LO) mode shows a blue shift of 8 cm−1 with the Mn composition, which can be attributed to the distortion of the TiO6 octahedron. With increasing Mn composition, the frequency of the infrared-active TO4 mode decreases from 532 to 520 cm−1, owing to the local variation of the lattice constant induced by the Mn incorporation. Moreover, the optical functions of the ceramics from the far-infrared to ultraviolet region are obtained based on the SE and reflectance spectra, which is useful for the potential applications in ferroelectric-based optoelectronic devices.

Temperature dependence of electronic band transition in Mn-doped SnO 2 nanocrystalline films determined by ultraviolet-near-infrared transmittance spectra

Mn-doped SnO 2 (SMO) nanocrystalline films with the composition from 2.5 to 12.5% have been prepared on quartz substrates by pulsed laser deposition. The temperature dependence of electronic structures and optical constants in the SMO films have been investigated by transmittance spectra from 5.3 to 300 K. Optical response functions have been extracted by fitting the transmittance spectra in the photon energy range of 0.5–6.5 eV with the Adachi's model. It was found that the absorption edge presents a red-shift trend with increasing Mn composition, and the optical band gap (OBG) is varied between 4.22 and 3.44 eV. Moreover, as the Mn composition increases, the temperature dependence of OBG becomes weaker. The band gap narrowing value [(5.3 K)–(300 K)] has been reduced from 98 to 3 meV and linearly decreases with the Mn composition. The phenomena could be attributed to the transition from low doping level SnO 2 band-like states to Mn-related localized states. Moreover, the Urbach energy shows the degree of the structural disorder, which could be explained by an empirical formulas in different temperature regimes.

New cubic Cd1−xMnxS epilayers grown on GaAs (100) substrates by hot-wall epitaxy

The new cubic Cd1−xMnxS/GaAs(100) epilayers were grown by hot-wall epitaxy, and their structural and optical properties were investigated by x-ray diffraction (XRD) and photoluminescence (PL). The cubic structure spectra of the Cd1−xMnxS epilayers were only observed in the Mn composition range of 0.000≤x≤0.050 from the XRD spectra and the x-ray ϕ-scan curves. The lattice constants of the Cd1−xMnxS epilayers decreased linearly with increasing Mn composition and were consistent with Vegard’s law. The Cd1−xMnxS epilayers from x-ray reciprocal space mapping were found to be in a partially biaxial compressive strain state. In the cubic Cd1−xMnxS epilayers, only yellow emission PL peaks at around 2.0 eV without other peaks such as the appreciable near-edge emission peak and self-activated peak were observed.

Far-Infrared−Ultraviolet Dielectric Function, Lattice Vibration, and Photoluminescence Properties of Diluted Magnetic Semiconductor Sn1−xMnxO2/c-Sapphire Nanocrystalline Films

Optical properties of Sn1−xMnxO2 (x from 0.0 to 0.15) nanocrystalline films grown on c-plane sapphire substrates have been investigated at room temperature by ultraviolet-infrared transmittance, far-infrared reflectance, and photoluminescence spectra. The X-ray diffraction analysis indicate that the films are of tetragonal rutile structure except for 5% Mn doping, in which the slight orthorhombic phase appears due to the presence of defects and strain. The dielectric functions are successfully determined from 0.025 to 6.5 eV using the Adachi and Lorentz multioscillator dispersion models in the high and low photon energy regions, respectively. The fundamental absorption edge is found to shift toward a lower energy side with increasing Mn composition. The refractive index of pure SnO2 film is estimated to be the lowest among the Sn1−xMnxO2 system. On the other hand, the low Eu transverse optical (TO) phonon frequencies slightly increase with the Mn composition. However, the highest Eu(TO) and A2u(TO) vibration modes present an opposite change trend. Compared with SnO2 single crystal, four corresponding longitudinal optical (LO) phonon frequencies decrease for the films owing to the variation of the lattice constants and destruction of the crystal symmetry. Photoluminescence spectra of doped SnO2 films show the remarkable intensity changes and a blue-shift trend compared to pure SnO2 film. Moreover, a novel emission peak of about 1.56 eV associated with the Mn dopant can be observed. It can be concluded that the Mn incorporation effects are the main contributions because the replacement of Sn with Mn ion can induce the 2p−3d hybridization and result in the electronic band structure modification of the Sn1−xMnxO2 films.

Magnetic properties and phase stability of Co2(Ti1−xMnx)Ga Heusler alloys

Magnetic properties and phase stability of the L21 phase in Co2(Ti1−xMnx)Ga alloys were investigated by magnetic and DSC measurements. The order-disorder phase transition temperature from the L21 to the B2 phase TtL21/B2 was found to increase linearly with increasing Ti composition and its linear extrapolation to x = 0.0 accords with the reported value of TtL21/B2 for the Co2TiGa alloy. The Curie temperature TC increases with increasing Mn composition, exhibiting an upward convexity, but is rather independent of composition in the high Mn region. It is concluded that the Co2(Ti1−xMnx)Ga alloys located in the middle composition range possess both a high TC and a high TtL21/B2.

Optical and Electrical Characterizations of ZnMnO Thin Films on c-Al2O3

We have studied the manganese (Mn) composition dependence of the optical and electrical properties of ZnMnO thin films, which are grown on c-Al2O3 substrates by plasma-assisted molecular beam epitaxy (PAMBE). The lattice constant and grain size are estimated by X-ray diffraction (XRD), and it is found that the lattice constant increases and the grain size decreases with increasing Mn composition. When more Mn is incorporated into the ZnMnO thin films, a blue shift of the absorption edges and photoluminescence emission peaks are observed. Hall measurement shows n-type conduction behavior for all the samples. The decrease in mobility might be related with the increase in the number of impurity scattering centers, and the decrease in the carrier concentration can be attributed to the carrier quenching effect induced by the deep-level defects. The ac electrical response of ZnMnO is studied by impedance spectroscopy (IS). The equivalent RC circuit and parameters of the grain and grain boundary are determined.

A comprehensive study of temperature-dependent reflectance and photoluminescence of Zn1−xMnxO thin films grown on c-Al2O3

We present a systematic study of temperature-dependent reflectance (R) and photoluminescence (PL) measurements on ZnMnO films grown by plasma-assisted molecular beam epitaxy. For the first time, the three free-exciton transitions FXA (Γ7c-Γ7vu), FXB (Γ7c-Γ9v), and FXC (Γ7c-Γ7vl) and the longitudinal-optical phonon replicas of FXB and FXC of ZnMnO films have been clearly observed in the R spectra. The parameters describing the activation energy and the temperature dependence of the transition energy and broadening have been extracted by fitting the experimental R and PL spectra. The spectral data of ZnMnO films not only show the deterioration of crystalline quality with increasing Mn composition fraction but also indicate the Mn clustering caused by Mn atom segregation.

Structural, optical and magnetic properties of Ga1−xMnxN films grown by MOCVD

In this paper, we have investigated the structural, optical and magnetic properties of Ga1−xMnxN films grown by MOCVD. The valence band structure was analysed by the x-ray photoelectron spectroscopy (XPS) measurements. A different Fermi level shift behaviour was observed with increasing Mn composition in the GaN. This shift behaviour was attributed to the donor-like defects induced by the Mn doping in the heavily doped samples. The presence of these defects was further confirmed by the calthodoluminescence spectra. The additional peak around 2.0 eV in the heavily doped samples corresponds to the intra-d-shell transitions of Mn2+. This Mn2+ state was formed by trapping the electrons released from the donor-like defects, which is quite consistent with the XPS results. Finally, the magnetic properties were also discussed in combination with the above results.

A spectroscopic ellipsometric investigation of new critical points of Zn 1− xMn xS epilayers

Zn 1− x Mn x S epilayers were grown on GaAs (1 0 0) substrates by hot-wall epitaxy. X-ray diffraction (XRD) patterns revealed that all the epilayers have a zincblende structure. The optical properties were investigated using spectroscopic ellipsometry at 300 K from 3.0 to 8.5 eV. The obtained data were analyzed for determining the critical points of pseudodielectric function spectra, 〈 ɛ( E)〉 = 〈 ɛ 1( E)〉 + i〈 ɛ 2( E)〉, such as E 0, E 0 + Δ 0, and E 1, and three E 2 (Σ, Δ, Γ) structures at a lower Mn composition range. These critical points were determined by analytical line-shapes fitted to numerically calculated derivatives of their pseudodielectric functions. The observation of new peaks, as well as the shifting and broadening of the critical points of Zn 1− x Mn x S epilayers, were investigated as a function of Mn composition by ellipsometric measurements for the first time. The characteristics of the peaks changed with increasing Mn composition. In particular, four new peaks were observed between 4.0 and 8.0 eV for Zn 1− x Mn x S epilayers, and their characteristics were investigated in this study.

Spectroscopic ellipsometry and absorption study of Zn1−xMnxO∕Al2O3 (0⩽x⩽0.08) thin films

We grow Zn1−xMnxO∕Al2O3 (0⩽x⩽0.08) thin films on sapphire (0001) using radio-frequency sputtering deposition method with Ar and various N2 flow rates. We examine the effect of N2 codoping on the band gap and Mn-related midgap absorption of (Zn,Mn)O. Using spectroscopic ellipsometry, we measure pseudodielectric functions in the spectral range between 1 and 4.5eV. Using the model of Holden et al. [T. Holden et al., Phys. Rev. B 56, 4037 (1997)], we determine the uniaxial (Zn,Mn)O dielectric function and the E0 band-gap energy. The fitted band gap does not change appreciably with increasing Mn composition up to 2%. We find a very large broadening of both the E0 band gap and its exciton partner E0x peaks even for less than 2% of optically determined Mn composition. In ellipsometric spectra, we also find Mn-related 3eV optical structure. In particular, optical absorption spectra with varying N2 gas flow rate show that the Mn-related peak intensity decreases with increasing N2 flux. The decrease of the 3eV Mn-related peak intensity is attributed to increasing N2 flow rate and Mn–N hybridization.

Lattice vibrations in hexagonal Ga1−xMnxN epitaxial films on c-plane sapphire substrates by infrared reflectance spectra

The lattice vibrations of undoped hexagonal Ga1−xMnxN (x from 0.0% to 1.5%) epitaxial films grown on c-plane sapphire substrates by metalorganic chemical vapor deposition have been investigated using infrared reflectance spectra in the frequency region of 200–2000cm−1 (5–50μm) at room temperature. The experimental reflectance spectra were analyzed using the Lorentz oscillator model for infrared-active phonon observed. The E1(LO) phonon frequency slightly decreases with increasing Mn composition. However, the E1(TO) phonon frequency linearly increases with the Mn composition, which can be well expressed by (558.7+350x)cm−1 and the broadening values are found to be larger than that of the GaN film. It indicates that Mn incorporation decreases the peak values (from the E1 phonon) of the infrared dielectric functions due to the local variation in the lattice constants and to the destruction of the crystal translational symmetry.

Ellipsometric investigation of optical constant and energy band gap of Zn 1− xMn xSe/GaAs (1 0 0) epilayers

Zn 1− x Mn x Se/GaAs (1 0 0) epilayers were grown using a hot-wall epitaxy method. The spectroscopic ellipsometry was used to determine the optical dielectric constant. The obtained pseudodielectric function spectra revealed the distinct structures at energies of E 0, E 0 + Δ 0, E 1, E 1 + Δ 1, E 2 and E 0 ′ + Δ 0 critical points (CPs) at lower Mn composition range. These critical points were determined by analytical line-shapes fitted to numerically calculated derivatives of their pseudodielectric functions. The peak characteristics were changed with the change in Mn composition. The spectral dependence of pseudodielectric function 〈 ɛ〉 was used to obtain the fundamental energy gaps E 0 including a unique relation with Mn composition. Also, the shifting and broadening of the CPs were observed with increasing Mn composition.

Effect of Mn composition on characterization of Zn1−xMnxSe epilayers

Zn1−xMnxSe epilayers have been grown on GaAs (100) substrates by hot-wall epitaxy. The structure of the epilayers was found to be zinc-blende over a large Mn composition range, 0⩽x⩽0.75. Exciton-related near-edge emissions and the intra-Mn2+ transition were observed in the 5 K photoluminescence (PL) spectrum. The composition dependence of PL peak energy was also studied. The ZnSe-like longitudinal optical phonon peak was observed in the Raman scattering spectrum and the Raman frequency was blueshifted with increasing Mn composition. The variation of the dielectric constants e1 and e2 as a function of Mn composition x was measured by spectroscopic ellipsometry in the 3.0–8.5 eV range at 300 K. The dielectric function spectra reveal distinct structures at each critical point.

MBE growth and magnetic properties of InMnN diluted magnetic semiconductor

A new type nitride-based diluted magnetic semiconductors In 1− x Mn x N films was prepared on Al 2O 3 substrate at different temperatures by RF-plasma-assisted molecular beam epitaxy. Reflection high-energy electron diffraction, X-ray diffraction and atomic force microscope show that Mn can be homogeneously incorporated into InN up to 4% and 10% respectively at 300°C and 200°C. The average c-axis lattice constant decreases with increasing Mn composition. The homogeneous In 1− x Mn x N films grown at low temperature (200°C) with low Mn composition x=0.04 displayed a paramagnetic behavior at low temperature, whereas a paramagnetic to spin-glass transition was observed for the sample with x=0.1 at about 3 K . Room temperature ferromagnetism was observed in the highly doped inhomogeneous In 1− x Mn x N film grown at higher temperature (300°C).

Ferromagnetic properties of IV–VI diluted magnetic semiconductor Ge1−xMnxTe films prepared by radio frequency sputtering

IV–VI diluted magnetic semiconductor Ge1−xMnxTe films with various Mn compositions up to x=0.53 were prepared on glass substrates by a rf sputtering method. Magnetic measurements indicate that all the Ge1−xMnxTe films (0.07⩽x⩽053) show ferromagnetic order at low temperatures since hysteretic behavior is clearly observed. The effective spin Seff’s of Mn ions deduced from the spontaneous magnetization at 4.2 K are less than 1.0, indicating that Mn ions are partially aligned ferromagnetically. The value of the Seff depends on the carrier concentration as well as the Mn composition. The magnetic order of the Ge1−xMnxTe films is thus caused by the competition between the ferromagnetic Ruderman–Kittel–Kasuya–Yosida (RKKY) interaction and the antiferromagnetic superexchange mechanism. The Curie temperature based on the mean field approximation increases linearly with increasing Mn composition up to x=0.3 and above this point tends to saturate around 80 K. On the basis of a simple RKKY mechanism, the spin exchange integral Jpd between holes and Mn ions has also been investigated. Compared to the experimental data, the Jpd shows a clear change from 0.62 eV (x⩽0.3) to 0.59 eV (x⩾0.4). This would be attributed to inhomogeneities in Mn distribution at high Mn compositions.