Anisotropy of permittivity of the hexagonal multiferroic HoMnO<Subscript>3</Subscript> in the spectral range 0.6–5.0 eV

The anisotropy of the optical properties of a single crystal of the hexagonal manganite HoMnO₃ has been investigated by spectroscopic ellipsometry in the spectral range 0.6–5.0 eV. It has been demonstrated that the optical absorption edge for the polarization E ⊥ c is determined by the intense narrow transition O(2p) → Mn(3d) centered at 1.5 eV, whereas this transition for the polarization E ‖ c is strongly suppressed and shifted toward higher energies by 0.2 eV. It has been revealed that, at the temperature T = 293 K, the spectra for both polarizations E ‖ c and E ⊥ c exhibit a broad absorption band centered at ∼2.4 eV, which was earlier observed in nonlinear spectra during optical second harmonic generation.     

Electric and magnetic properties of PMMA/manganite composites

We present the synthesis and characterization of the La_2/3Sr_1/3MnO₃ manganite in the form of tapes using polymethyl methacrylate (PMMA) as binder. We have studied their electric and magnetic properties as a function of temperature and magnetic field. The magnetization results have been shown to be dominated by the intrinsic magnetic properties of the manganite. Resistivity measurements showed an insulating behavior in the whole range of temperatures measured, indicating that the percolation threshold of manganite grains has not been reached even for the sample with 35% of PMMA relative content. The obtained magnetoresistance is largest in the sample with 35% of PMMA relative content.     

Magnetism of LiMn<Subscript>2</Subscript>O<Subscript>4</Subscript> manganite in structurally ordered and disordered states

The specific features of the crystal structure and the magnetic state of stoichiometric lithium manganite in the structurally ordered Li[Mn₂]O₄ and disordered Li_{1 − δ}Mn_δ[Mn_{2 − δ}Li_δ]O₄ (δ = 1/6) states have been investigated using neutron diffraction, X-ray diffraction, and magnetic methods. The structurally disordered state of the manganite was achieved under irradiation by fast neutrons (E _eff ≥ 1 MeV) with a fluence of 2 × 10²⁰ cm⁻² at a temperature of 340 K. It has been demonstrated that, in the initial sample, the charge ordering of manganese ions of different valences arises at room temperature, which is accompanied by orthorhombic distortions of the cubic spinel structure, and the long-range antiferromagnetic order with the wave vector k = 2π/c(0, 0, 0.44) is observed at low temperatures. It has been established that the structural disordering leads to radical changes in the structural and magnetic states of the LiMn₂O₄ manganite. The charge ordering is destroyed, and the structure retains the cubic symmetry even at a temperature of 5 K. The antiferromagnetic type of ordering transforms into ferrimagnetic ordering with local spin deviations in the octahedral sublattice due to the appearance of intersublattice exchange interactions.     

Investigations on the soft-mode mechanism of the ferroelectric phase transition in

Dielectric and Raman spectroscopic measurements have been performed to investigate the ferroelectric phase transition in . Single crystals were grown by the zone melting method. The frequency dependence of the dielectric permittivity from 1 MHz to 1 GHz has been studied in a temperature range between 265 and 285 K. A Debye like dielectric dispersion was found, showing a critical slowing down around K. Polarized Raman spectra have been taken between 220 and 310 K. Two softening modes have been found, one of A- and another one of B / B g-symmetry. The phase transition mechanism in can be classified as partially order-disorder and partially displacive, confirming former structural results. It resembles strongly that of monoclinic . Received: 7 April 1998 / Revised: 5 June 1998 / Accepted: 16 June 1998     

High pressure raman spectra of crystalline sulfur

Abstract

Well—resolved Raman spectra of crystalline sulfur have been recorded in a diamond anvil cell (DAC) in the pressure range from atmospheric pressure to 50 GPa at room temperature, using an 0.6 m triple spectrograph and a CCD multichannel detector. The spectra indicate two phase transitions in the pressure region between 10 and 15 GPa.     

Properties of the ceramic manganite (La0.65Ca0.35)1 − x Mn1 + x O3 ± Δ (x = 0.2) sintered at temperatures of 1000–1450°C

It has been shown that the ceramics (La_0.65Ca_0.35)_{1 ? x} Mn_{1 + x} O_{3 ± Δ} (x = 0.2) sintered at temperatures up to 1450°C is formed as a composite material consisting of manganite and manganese oxide grains. It has been found that, at a sintering temperature of 1450°C, the manganite grain size abruptly increases, which is accompanied by the formation of a nanometer-sized layered structure. It has been revealed for the first time that the temperature dependence of the magnetoresistance of the ceramic manganite with this structure is characteristic of single crystals.     

Manganite Pr0.7Ca0.3MnO3 films obtained by extraction pyrolysis

The magnetic properties of manganite Pr_0.7Ca_0.3MnO₃ films, obtained for the first time by the extraction pyrolysis method, have been investigated. The effect of the synthesis temperature of the magnetic properties of the manganite films is demonstrated.     

Magnetic transformations and structural disorder in lithium manganite LiMnO<Subscript>2</Subscript>

Investigation of the structural and magnetic characteristics of lithium manganite LiMnO₂ with a variable Li/Mn ratio has been performed. A significant difference in the ⁷Li NMR spectra in the paramagnetic and antiferromagnetic manganite phases is revealed. It is shown that the common feature of the compositions studied is a high degree of structural disorder, which manifests itself in the presence of manganese ions in lithium positions (Mn_Li) and lithium ions in manganese positions (Li_Mn); this effect is also characteristic of the stoichiometric composition.     

The effect of pressure on ftir spectra of ammonium halides

Abstract

FTIR spectra of NH₄Cl, NH₄Br and NH₄I have been measured in the range of 600 - 4800 cm^?1 at pressures up to 30 GPa at 135 K. The spectra show significant changes at the phase transition IV - V, particularly the splitting of v₄ in phase V is clearly demonstrated.     

Singularities of magnetic and elastic characteristics of La2/3Ba1/3MnO3: Analysis of martensitic kinetics

A coordinated temperature behavior of magnetic susceptibility and internal friction has been observed in the La_2/3Ba_1/3MnO₃ manganite in the temperature region of the crystal phase separation 5–340 K. Stepwise temperature behavior of the susceptibility of the single crystal sample and corresponding singular behavior of the internal friction in the polycrystalline manganite have been found. These small-scale features of the temperature dependences of the susceptibility and the internal friction are considered to be a reflection of martensitic kinetics of the structural phase transformation R3¯c↔Imma in the 200 K temperature region.     

Preparation and low field magnetoresistance of double layered manganite La1.4Ca1.6Mn2O7

Double layered manganite of La_1.4Ca_1.6Mn₂O₇ (DLCMO) was prepared using solid state reaction method and had a metal-insulator transition temperature (T_MI) of 125 K. The short range 2D-feerromagnetic ordering (T_C2) starts growing when T<168 K and it gets converted into 3D-ferromagnetic ordering (T_C1) at 114 K. Low field magnetoresistance (MR) behaviour of the DLCMO was investigated and compared with an infinite layered manganite La_0.7Ca_0.3MnO₃ (LCMO). For DLCMO, in the temperature range between T_C1 and T_C2, the MR showed a gradual increase with the magnetic field. The observed MR and R-T behaviour of double layered manganite for T_C1<T<T_C2 has been explained in the frame work of the two phase model [ferromagnetic (FM) domains and paramagnetic (PM) regions] and percolative behaviour of transport in FM-PM mixture.     

Surface modifications of crystalline SiO2 and Al2O3 induced by energetic heavy ions

Abstract

Glasses with composition × CoO.(0.25-x) CaO.0.75 B₂O₃ have been prepared over the range 0 ≤ x ≤ 0.25 containing 2 mol% of V₂O₅. Electron paramagnetic Resonance spectra of VO²⁺ of these glasses has been measured at X-band (ν ≈ 9.15 GHz) at room temperature and at liquid nitrogen temperature. Spin Hamiltonian parameters of the VO²⁺ ions have been calculated. It is found that for x ≤ 0.12 the octahedral symmetry of the V⁴⁺ site is improved. A part of these glasses were annealed at 473 K for 24 hours and EPR spectra were recorded and analysed.     

The Hall effect in La0.67Ba0.33MnO3

The Hall effect in polycrystalline barium-substituted lanthanum manganite La_0.67Ba_0.33MnO₃ has been investigated in the temperature interval 298<T<355 K. It is found that the anomalous Hall coefficient in this material is two orders of magnitude greater than the normal coefficient. At T ₀=333 K the normal Hall coefficient changes sign, which indicates a change in the type of conductivity. The temperature dependence of the normal Hall coefficient, electrical conductivity, and magnetoresistance is explained on the basis of the concept of motion of the mobility edge attendant as the temperature changes. Zh. éksp. Teor. Fiz. 113, 981–987 (March 1998)     

Correlation of electrical, magnetic, and thermal properties of the La0.85Ag0.15MnO3 manganite near the phase transition temperature

The correlation between electrical, magnetic, and lattice properties of manganites has been studied by analyzing the temperature dependence of the electrical resistivity of the La_0.85Ag_0.15MnO₃ manganite. The results have been discussed within the model of a phase-separated manganite in terms of the percolation theory. It has been shown that, from analyzing the temperature and magnetic field dependences of the electrical resistivity, it is possible to predict the behavior of the magnetization and specific heat, as well as the change of magnetic entropy near the phase transition temperature, and that the obtained agreement with experimental data for this analysis is quite reasonable.     

Thermal desorption of carbon monoxide from Mo(110)

Adsorption and desorption of carbon monoxide (CO) from Mo(110) have been investigated by temperature programmed desorption (TPD). The TPD spectra exhibit peaks in two temperature regions: 250-400 and 850-1100 K. The first region correspond to adsorption in molecular form, whereas the latter region corresponds to desorption of CO that has been dissociatively adsorbed. When the surface was saturated by CO, about 35% of the desorption intensity originates from the high-temperature region (recombinative desorption) and the remaining desorption signal is from the low-temperature region (molecular adsorption). Desorption parameters for molecular adsorbed CO were obtained using several different heating rates. Desorption energies were estimated to range from 0.9 eV for low coverages of CO to about 0.65 eV for high coverages. Corresponding prefactors were estimated to be in the range from 1 210¹¹ to 1 210⁹ s^{m 1}. The experimental data have been compared with Monte Carlo simulations of first-order TPD from a bcc (110) lattice, in which partial poisoning of adsorption sites by dissociated carbon and oxygen was modelled. Repulsive near-neighbour interactions were used.     

Inhomogeneous magnetic state in the electron-doped Sr<Subscript>0.98</Subscript>La<Subscript>0.02</Subscript>MnO<Subscript>3</Subscript> Manganite According to <Superscript>55</Superscript>Mn NMR data

⁵⁵Mn NMR spectra in the magnetically ordered state in Sr_0.98La_0.02MnO₃ manganite have been obtained and the magnetic susceptibility has been measured. It has been shown that the microscopic phase separation into the antiferromagnetic matrix and ferromagnetic clusters, which can be presented as magnetic polarons, is observed in the long-range magnetic order region.     

Maxwell–Wagner relaxation and magnetodielectric properties of Bi<Subscript>0.5</Subscript>La<Subscript>0.5</Subscript>MnO<Subscript>3</Subscript> ceramics

The complex permittivity ε = ε′–iε″ of manganite bismuth–lanthanum Bi_0.5La_0.5MnO₃ ceramics has been measured at temperature T = 78 K in the frequency range f = 200–10⁵ Hz and in the magnetic induction range B = 0–5 T. Dielectric relaxation and the pronounced magnetodielectric effect have been detected. The explanation based on the superposition of Maxwell–Wagner relaxation and the magnetoresistance effect has been proposed.     

Phase transformations in HoMnO<Subscript>3</Subscript> oxide at high temperatures

Hexagonal holmium manganite is investigated by means of high-temperature X-ray radiography and thermal analysis. At 1200°C, the manganite transitions from the structural P6₃cm modification into the P6₃/mmc form. Mechanoactivation in a high-energy ball mill results in the emergence of the orthorhombic phase of HoMnO₃, which undergoes the Jahn–Teller transition in the temperature range of 1000–1160°C.     

Low-dimensional exchange interactions,structural deformation,and antiferromagnetic ordering in lithium manganite LiMnO<Subscript>2</Subscript>

Structure deformation in lithium manganite LiMnO₂ has been analyzed. It is shown that suppression of the Jahn-Teller effect with a decrease in temperature is related to striction effects, which are caused by intercluster exchange interactions. It is concluded that low-dimensional interactions play an important role in the formation of magnetic properties of lithium manganite.     

A new phase growth in LiF: Ti and LiF: Ti,Mg

Abstract

ITC measurements, optical absorption spectra in the wavenumber range 5.4 × 10⁴?200 cm^?1, microspectrophotometry, photoluminescence spectra, optical and electron microscopy and X-ray diffraction have been used to study the growth and the nature of a new phase occlusions in LiF: Ti³⁺ and LiF: Mg²⁺, Ti³⁺. They grow along the 100 direction as a consequence of high temperature heat treatments in moist atmosphere.