Electronic excitations in hole-doped La1−xSrxMnO3 studied by resonant inelastic X-ray scattering

We report a resonant inelastic X-ray scattering (RIXS) study on perovskite manganese oxides La_1−xSr_xMnO₃ (x=0, 0.2, and 0.4) at Mn K-absorption edge. Hole-doping effect on the electronic excitations in the strongly correlated electron systems is elucidated by comparing with undoped LaMnO₃. The scattering spectra of metallic La_0.6Sr_0.4MnO₃ show that a salient peak appears in low energies indicating the persistence of the Mott gap. At the same time, the energy gap is partly filled by doping holes and the spectral weight shifts toward lower energies. Though the peak position of the excitations shows weak dispersion in momentum dependence, RIXS intensity changes as a function of the scattering angle (2θ), which is related to the anisotropy. Furthermore, anisotropic temperature dependence is observed in La_0.8Sr_0.2MnO₃ which shows a metal-insulator transition associated with a ferromagnetic transition. We consider that the anisotropy in the RIXS spectra is possibly attributed to the correlation of the orbital degrees of freedom. The anisotropy is large in LaMnO₃ with long-range orbital order, while it is small but finite in hole-doped La_1−xSr_xMnO₃ which indicates persistence of short-range orbital correlation.     

Strain induced phase separation on La0.5Ca0.5MnO3 thin films

The effect of epitaxial strain on La_0.5Ca_0.5MnO₃ films of various thicknesses grown on SrTiO₃, SrLaAlO₄, and SrLaGaO₄ substrates is studied by Raman spectroscopy, magnetic, and resistivity measurements. The transport and magnetic properties as well as Raman spectra of the films are affected by epitaxial strains. The energy of the A_g(2) mode and the tilting angle of the MnO₆ octahedra is affected by the strain imposed by the substrate. In the spectra of the films deposited on the (1 0 0) SrTiO₃ substrate strong Jahn-Teller (JT) modes appear, which couple with charge-ordering. In all other films these modes are suppressed and no additional Raman lines are present at low temperatures contrary to the bulk compound. The low frequency continuum scattering decreases at low temperatures indicating a coupling with both the charge and orbital transitions. Comparison of the Raman spectra with the magneto-transport properties suggests an interpretation in terms of a strain induced phase separation between ferromagnetic metallic and antiferromagnetic insulating states.     

Optical evidence for lattice polarons in lightly doped lanthanum manganites in paramagnetic state

The reflection and absorption spectra and the resistivity data for La_0.93Sr_0.07MnO₃, La_0.9Sr_0.1MnO₃, La_0.85Sr_0.15MnO₃, La_0.92Ca_0.08MnO₃ and La_0.85Ba_0.15MnO₃ single crystals have been collected and analyzed by using small lattice polaron theory. The activation energies of the polaron hop are determined. The lattice polarons are found to be dominated in the electronic transport and optical properties in the paramagnetic state in La_0.93Sr_0.07MnO₃, La_0.92Ca_0.08MnO₃ and La_0.85Ba_0.15MnO₃ crystals. It is shown that lattice polaron involves more than one Mn site.     

Temperature dependence of infrared conductivity of manganites (x = 0, 0.05 and 0.2)

The temperature dependence of the reflectivity spectra of three manganites ceramics with compositions Pr_0.7Ca_0.3MnO₃, Pr_0.7Ca_0.25Sr_0.05MnO₃ and Pr_0.7Ca_0.1Sr_0.2MnO₃ has been investigated by infrared reflectivity spectroscopy in the wave number range 0.005-1.1 eV. The compound Pr_0.7Ca_0.25Sr_0.05MnO₃ which shows the largest conductivity jump at the ferromagnetic-paramagnetic phase transition has been studied in details. The optical conductivity of this compound is deduced from the best fit to reflectivity spectra of a “double-damping Drude” model, itself derived from the factorized form of the dielectric function. Excellent agreement with Kramers-Kronig transformation is reported. The model allows in particular to discriminate the contributions to the optical conductivity of trapped charges (polarons) and mobile charge carriers. Received 20 July 1999 and Received in final form 15 October 1999     

Relationship between the structural distortion and the Mn electronic state in La1−xCaxMnO3: a Mn K‐edge XANES study

A theoretical study of the X‐ray absorption near‐edge structure (XANES) spectra at the Mn K‐edge in the La_1?xCa_xMnO₃ series is reported. The relationship between the edge shift, the Ca–La substitution and the distortion of the MnO₆ octahedra in these systems has been studied. It is shown that, by correctly considering these effects simultaneously, the experimental XANES data are consistent with the presence of two different Mn local environments in the intermediate La_1?xCa_xMnO₃ compounds. By taking into account the energy shift associated with the modification of the MnO₆ distortion as Ca substitutes for La, it is possible to reproduce the XANES spectra of the intermediate‐doped compounds starting from the experimental spectra of the end‐members LaMnO₃ and CaMnO₃. These results point out the need to re‐examine the conclusions derived in the past from the simple analysis of the Mn K‐edge XANES edge‐shift in these materials. In particular, it is shown that the modification of the Mn K‐edge absorption through the La_1?xCa_xMnO₃ series is well reproduced by considering the simultaneous presence of both distorted and undistorted octahedra and, consequently, that the existence of charge‐ordering phenomena cannot be ruled out from the XANES data.     

铁磁-d波超导结中的粗糙界面散射和自旋反转效应对隧道谱的影响

考虑到铁磁层中的自旋极化效应、以及界面的粗糙散射和自旋反转效应,利用推广了的Blonder Tinkham Klapwijk理论模型,计算铁磁d波超导结中的自旋极化隧道谱.研究表明1)自旋反转效应能使零偏压电导峰变得尖锐;2)粗糙的界面散射除了能压低零偏压电导峰的高度,还能使零偏压凹陷处感应出一中心峰.结果能定性地解释最近的两篇关于La_2/3Ba_1/3 MnO₃/DyBa₂Cu₃O_{7关键词： 自旋极化效应 自旋反转效应 粗糙界面散射效应 隧道谱}     

Structural,thermal and magnetic investigations of heavily Mn-doped ZnO nanoparticles

We present a systemic study on the structural, thermal and magnetic properties of Zn_1?xMn_xO nanoparticles synthesized by a combustion method with heavily Mn doping concentrations x=0.05, 0.15 and 0.25. The structural evolutions in relation to the possible variation of the Mn oxidation state and dopant induced tiny Zn₂MnO₄/Mn₂O₃ and ZnMnO₃/MnO₂ impurities, which have not been detected by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM), were investigated by means of X-ray photoelectron spectroscopy (XPS), Raman scattering spectra, thermo-gravimetric analysis (TGA) and magnetic measurements. It is evidenced that the optimal Mn concentration x in ZnO to grow single phase Zn_1?xMn_xO should be below the cation percolation threshold x_p (about 0.125), which is the basis to form real diluted magnetic semiconductors (DMSs).     

Mössbauer, X-ray, electrical and magnetic properties of lithium borosilicate glasses containing iron and manganese Part I. Mössbauer measurements

The effect of the replacement of MnO₂ by Fe₂O₃ on the oxidation states of iron in some lithium borosilicate glasses were studied using Mössbauer spectroscopy. DTA and infrared measurements showed the presence of two glassy phases, one of them containing iron and the other manganese. The Mössbauer spectra were measured at room temperature for all the glass samples. The spectra revealed the presence of Fe ions only in ferric state in both tetrahedral and octahedral coordination. Also, it was observed that the ratio between the number of iron ions in both coordination states has not changed with the increase of MnO₂ content.     

Interpretation of temperature-dependent thermoelectric power behaviour of La0.67Ba0.33MnO3 manganites

The temperature dependence of the thermoelectric power S(T) in polycrystalline La_0.67Ba_0.33MnO₃ has been investigated. In the ferromagnetic regime, the phonon thermoelectric power is evaluated by incorporating the scattering of phonons with impurities, grain boundaries, charge carriers and phonon. The Mott expression is used to compute the electron diffusive thermoelectric power (S_c^diff.) using Fermi energy as electron-free parameter. The S_c^diff infers linear temperature dependence and S_ph^drag increases exponentially with temperature, which is an artefact of various operating scattering mechanisms. The behaviour of the S(T) is determined by competition among the several operating scattering mechanisms for the heat carriers and a balance between carrier diffusion and phonon drag contributions in the La_0.67Ba_0.33MnO₃. Numerical analysis of thermoelectric power of the present model shows similar results as those revealed from experiments.     

Synthesis of Manganese Oxide Microspheres by Ultrasonic Spray Pyrolysis and Their Application as Supercapacitors

Manganese oxide (MnO₂) microspheres are prepared using an ultrasonic spray pyrolysis (USP) process. A mixed solution of potassium permanganate and hydrochloric acid is nebulized into microsized droplets, which are then carried by air flow through a furnace tube. Each microdroplet serves as one microreactor and produces one microsphere. Upon heating, KMnO₄ is decomposed into MnO₂ microspheres; this synthetic process can easily be scaled up. Characterization of the MnO₂ microspheres by scanning electron microscopy, transmission electron microscopy, powder X‐ray diffraction, Raman spectroscopy, and X‐ray photoelectron spectra is described. Different morphologies of MnO₂ microspheres can be controlled by tuning the precursor concentrations (and ratios) and furnace temperatures. Microspheres synthesized at 150 °C give amorphous MnO₂ while synthesis at 500 °C yields crystalline α‐MnO₂. The electrochemical properties investigated by cyclic voltammetry give specific capacitance as high as 320 F g^?1, demonstrating promising properties as supercapacitors. In addition, these microspheres can be directly sprayed on conductive substrates, such as carbon fiber paper, and may have useful applications as a supercapacitor electrode coating. The supercapacitive properties of MnO₂ microspheres at higher charge and discharge rates can be improved by increasing the surface area coverage or coating them with a thin layer of conductive polymer.     

Synthesis and characterization of ZrO2/MnO2/carbon clusters nanocomposite materials

Nano-sized ZrO₂/MnO₂/carbon clusters composite materials has been successfully obtained by the calcination of a Zr(acac)₄/Mn(acac)₃/epoxy resin complex under an oxygen atmosphere. The compositions of the resulting composite materials were determined using inductively coupled plasma (ICP) spectroscopy, elemental analysis and surface characterization by X-ray diffraction (XRD), scanning electron microscopy and transmission electron microscopy (TEM). The ultraviolet–visible (UV–VIS), X-ray photoelectron spectra (XPS) and electron spin resonance (ESR) spectra of the composites were also measured. ESR spectral examinations suggest the possibility of an electron transfer in the process of MnO₂ → carbon clusters → ZrO₂. The visible light-responsive oxidation–reduction ability of the calcined material was also investigated.     

Effect of an interface boundary on the magnetooptical and magnetotransport properties of La<Subscript>0.67</Subscript>Ca<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript>/La<Subscript>0.67</Subscript>Sr<Subscript>0.33</Subscript>MnO<Subscript>3</Subscript> heterostructures

The optical, magnetooptical (Kerr effect and magnetotransmission), and magnetotransport properties of La_2/3Ca_1/3MnO₃/La_2/3Sr_1/3MnO₃ and La_2/3Ca_1/3MnO₃/SrTiO₃/La_2/3Sr_1/3MnO₃ heterostructures on SrTiO₃ substrates are studied. The contribution of the interface boundary to the magnetotransmission is typical of a material with a transitional composition. It is found that a 2-nm-thick SrTiO₃ spacer does not influence the shape and position of the magnetotransmission peak in a field normal to the surface of the heterostructure but increases the contribution of the upper layer to the magnetotransmission in the Voigt geometry and also enhances the magnetoresistance that is due to the tunneling of spin-polarized carriers through the spacer. The Kerr spectra taken of the heterostructures are typical of single-layer single-crystal films.     

Structural and magnetic phase transitions occurring in Pr0.7Sr0.3MnO3 manganite at high pressures

The crystal and magnetic structures and the vibrational spectra of Pr_0.7Sr_0.3MnO₃ manganite are studied within the pressure range up to 25 GPa by methods of X-ray diffraction and Raman spectroscopy. Neutron diffraction studies have been performed at pressures up to 4.5 GPa. The magnetic phase transition from the ferromagnetic phase (T _C = 273 K) to the A-type antiferromagnetic phase (T _N = 153 K) is found at P ≈ 2 GPa. This transition is characterized by a broad pressure range corresponding to the phase separation. The Raman spectra of Pr_0.7Sr_0.3MnO₃ measured under high pressures significantly differ from the corresponding spectra of the isostructural doped A_{1 ? x} A′ _x MnO₃ manganites, (where A is a rare-earth ion and A′ is an alkaline-earth ion) with the smaller average ionic radius 〈r _A〉 of A and A′ cations. Namely, the former spectra do not include clearly pronounced stretching phonon modes. At P ～ 7 GPa, there appears the structural phase transition from the orthorhombic phase with the Pnma space group to the orthorhombic high-pressure phase with the Imma symmetry. In the vicinity of the phase transition, anomalies in the pressure dependences of the lattice parameters, unit cell volume, and phonon frequencies corresponding to the characteristic lattice vibration modes are observed.     

Mesoscopic magnetic inhomogeneities in the low-temperature phase and structure of Sm1−x SrxMnO3 (x < 0.5) perovskite

Results are presented of studies of the ¹⁵⁴Sm_1?x Sr_xMnO₃ system using neutron powder diffraction and small-angle polarized neutron scattering. An analysis of the neutron diffraction spectra showed that at T < 180 K these exhibit typical Jahn-Teller distortions of the manganese-oxygen octahedrons which persist under further cooling and on transition of the sample to a metallic magnetically ordered state. The magnetic contribution to the diffraction is satisfactorily described using the (A _x (A _y )F _z ) model and is interpreted as the coexistence of ferromagnetic and antiferromagnetic phases. The exaggerated widths of the diffraction lines indicate an appreciable contribution from microdeformations evidently associated with the inhomogeneity of the system. Small-angle polarized neutron scattering showed that the Sm system for x = 0.4 and 0.25 is magnetically inhomogeneous in the low-temperature phase. Ferromagnetic correlations occur on scales of around 200 Å and having dimensions greater than 1000 Å which, combined with the temperature hysteresis of the magnetic small-angle scattering intensity observed for an x = 0.4 sample in the low-temperature phase, suggests that the transition is of a percolation nature.     

Transport properties and magnetoresistance in La0.8Te0.2MnO3/ZrO2 composites

The transport properties and magnetoresistance of electron-doped manganate / insulator composites (La_0.8Te_0.2MnO₃)_{1 - x}/(ZrO₂)_x (x=0, 0.3, and 0.5) are investigated. It is found that the metal-insulator transition temperature of this system shifts to a lower value as the ZrO₂ content increases. The introduction of ZrO₂ enhances both the domain scattering and electron relative scattering in the metal transport region. In the adiabatic small polaron hopping transport region, the thermal activation energy seems invariable regardless of the ZrO₂ content. The application of a magnetic field promotes the charge transportation capabilities of the composites, and the magnetoresistance is enhanced with an increase of the ZrO₂ content. This could be attributed to the more remarkable modification effect of magnetic field on ordering degree in the composites than in pure La_0.8Te_0.2MnO₃.     

Magnetoimpedance in La0.67Pb0.33MnO3 manganite prepared by sol–gel method

In the present work, manganite La_0.67Pb_0.33MnO₃ was prepared by the sol–gel method. The difference between metal–insulator transition temperature T_MI (217 K) and Curie temperature T_c (342 K) in the sol–gel nanocrystalline manganite is mainly due to the grain boundary effect. The breaking of Mn–O–Mn bonds and strong scattering at the grain boundary cumber the transport. At room temperature 300 K, impedance and resistance increase with increasing frequency of ac currents. The observed dc magnetoresistance in sol–gel La_0.67Pb_0.33MnO₃ is related to the spin-polarized inter-grain tunneling and spin-dependent scattering at grain boundaries. The sol–gel manganite shows the magnetoimpedance characteristics, which are different from those of traditional sintered manganites and metallic giant magnetoimpedance materials. For sol–gel La_0.67Pb_0.33MnO₃ at low frequencies, the impedance experiences a peak under a low longitudinal field. In contrast, at high frequencies the peak phenomenon disappears, and the impedance drops sharply with low fields, which is due to the inter-grain or grain boundary effect. The permeability also sensitively varies with an application of transverse field. The magnetoimpedance effect in sol–gel nanocrystalline manganite is influenced by both field-induced permeability change and dc magnetoresistance.     

Phase separation of the spin system in the La<Subscript>0.93</Subscript>Sr<Subscript>0.07</Subscript>MnO<Subscript>3</Subscript> crystal

The magnetic state of the manganite La_0.93Sr_0.07MnO₃ in the range 4.2–290 K was studied using elastic neutron scattering. The magnetic state of this compound was found to occupy a particular place in the La_1?xSr_xMnO₃ solid-solution system, in which the antiferromagnetic type of order (LaMnO₃, T_N=139.5 K) switches to ferromagnetic ordering (La_0.9Sr_0.1MnO₃, T_C=152 K) with increasing x. In the transition state, this compound contains large-scale spin configurations of two types. A fractional crystal volume of about 10% is occupied by regions of the ferromagnetic phase with an average linear size of 200 Å, while the remainder of the crystal is a phase with a nonuniform canted magnetic structure. Arguments are presented for the phase separation of the La_0.93Sr_0.07MnO₃ spin system being accounted for by Mn⁴⁺ ion ordering.     

Centers of charge nonuniformity in absorption spectra of lanthanum manganites

In order to study the microscopic nature of the phase separation in lanthanum manganites, experimental investigations were made of the optical, electrical, and magnetic properties of La_1?δMnO₃ and La_0.9Sr_0.1MnO₃ lanthanum manganite single crystals. The infrared absorption spectra revealed two bands at 0.14 eV and 0.35 eV whose intensity was sensitive to the magnetic order. The different temperature dependence of the electrical resistivity (semiconducting) and the transmission (metallic) below the Curie ferromagnetic temperature indicates that an insulator-metal transition takes place in various regions of the insulating matrix, i.e., phase separation occurs. Characteristic features of the properties and the nature of the phase separation in these compounds can be explained using a model of polar (hole [MnO ₆ ^8? ] _JT and electron [MnO ₆ ^10? ] _JT ) pseudo-Jahn-Teller clusters which form centers of charge nonuniformity in the crystal.     

Linear expansion, phase separation, and magnetic inhomogeneities in La0.92Ca0.08MnO3

A relation of the thermal expansion with magnetic and magnetotransport properties has been revealed in La_0.92Ca_0.08MnO₃ single crystals in the paramagnetic state. The magnetotransport and lattice properties and the specific features in the neutron scattering characteristics of the La_0.92Ca_0.08MnO₃ single crystals have been explained by the phase separation in the paramagnetic state into magnetic inhomogeneities (clusters) with short-range (～10 Å) and long-range (>10² Å) orders. The performed investigations have demonstrated that the clusters are closely related to the crystal lattice and that the magnetic inhomogeneities in the paramagnetic region are correlated to T ～ 250–300 K ? TC.