Papers by Keyword: Manganite

Abstract: In our work the temperature dependencies of magnetization, entropy changes, and the Curie temperature (T_C) of La_1-xCa_xMnO₃ (x = 0.33 and 0.5) were investigated using Monte Carlo method and Heisenberg model. In our simulation, magnetic Mn³⁺ and Mn⁴⁺ ions are described by classical Heisenberg spins, while oxygen, lanthanum and calcium ions are considered as non-magnetic. The Mn magnetic ions are distributed on a simple cubic lattice according to the perovskite structure of the manganite. The theoretical Curie temperatures and magnetocaloric values are in a good agreement well with experimental data.

Abstract: New La0.54Ho0.11Sr0.35Mn1-xCuxO3 manganites were obtained by sol-gel method using oxides and acetates and sintered finally in air at 12000 C for 15 h. The samples were studied by X-ray diffraction, magnetic and electric methods. Space group, lattice constants, average size of the crystalline blocks and positions of cations/anions in the unit cell, were determined using the FullProf code. The samples contain only a perovskite phase with an orthorhombic structure (Pnma space group). Small-angle neutron scattering (SANS) measurements were performed on the SANS-1 setup at the FRG-1 reactor of the GKSS Research Centre (Geesthacht, Germany). The sizes of magnetic clusters have been determined at room temperature.

Abstract: In Sm_0.55Sr_0.45MnO₃ single crystals, grown by the floating-zone method with the cooling in oxygen, has been observed the spontaneous generation of voltage (SGV). Its maximum reaches 60 μV and occurs in the temperature range where simultaneous decay of the CE-type antiferromagnetic order and the charge order take place in some clusters. The SGV peak becomes lower by about 45% if magnetic field 14.2 kOe applied. Also we observed the SGV in La_0.75Ba_0.25MnO₃ single crystals. Maximum value of SGV occurs in a vicinity of the Curie temperature. The SGV value seems to be almost independent from the cooling or heating rate and crystallographic direction, but in La_0.75Ba_0.25MnO₃ it is smaller than in Sm_0.55Sr_0.45MnO₃ by an order of magnitude. La_0.75Ba_0.25MnO₃ compound contains ferromagnetic clusters, in which the electrical charges are localized according to gain in the s-d exchange energy. They are distributed in the paramagnetic lattice, impoverished of an electrical charge. It is shown that SGV stems from the presence of regions with different electrical charges in both samples.

Abstract: The oxygen isotope effect in ordered half-doped manganite PrBaMn₂^16-18O₆ has been investigated. Real and imaginary parts of ac magnetic susceptibility, dc magnetization and magnetoresistance were measured at temperatures from 4.2 to 320 K in applied magnetic field up to 35 kOe. Substantial increase of charge ordering (CO) transition temperature T_CO under oxygen isotope substitution ¹⁶O ¹⁸O and small decrease of ferromagnetic (FM) transition temperature T_FM were found. Small systematic shift of ferromagnetic transition temperature in oxygen reduced manganite PrBaMn₂^{16 18}O₅ is also considered. The obtained results are compared with those observed in other manganites demonstrating the large isotope effect.

Abstract: The hyperfine magnetic interactions of ¹¹⁹Sn probe atoms in the CaCu₃Mn_3.96Sn_0.04O₁₂ double manganite by Mössbauer spectroscopy using magnetic measurements have been investigated. A consistent description of the results obtained in terms of the Weiss molecular field model by taking into account the peculiarities of the local environment of tin atoms has allowed the indirect Cu²⁺OMn⁴⁺ (J_CuMn 51 ± 1 K) and Mn⁴⁺OMn⁴⁺ (J_MnMn 0.6 ± 0.6 K) exchange interaction integrals to be estimated. Based on the KanamoriGoodenoughAnderson model, we show that the magnitude and sign of the intrasublattice exchange integral J_MnMn correspond to both the electronic configuration of the Mn⁴⁺ cations and the geometry of their local crystallographic environment in the compound under study.

Abstract: The crystal growth of electron-doped manganites Ca_1-xEu_xMnO₃ by the floating zone method is reported. The special features of the growth parameters are discussed. Results of x-ray analysis, magnetic and transport characteristics are shown.

Abstract: The theoretical model of the crystal structure of lanthanum manganite doped by non-Jahn-Teller ions is proposed. In order to describe the changes in the crystal structure and orbital state of manganese ions subsystem, we use modified shell model and virtual crystal model. The orbital ordering collapse is explained in terms of dynamical Jahn-Teller effect. The model of superexchange interaction helps to find the values of antiferromagnetic and ferromagnetic exchange parameters for dynamical and static orbital states of interacting ions. The magnetic structure of LaMn_1-xGa_xO₃ is explained and magnetic resonance spectrum is predicted.

Abstract: The effect of point lattice defects on the magnetic properties of manganites LaMnO_3+δ, La_0.67Ca_0.33MnO₃ and La_0.67Ba_0.33MnO₃ has been investigated. Temperature dependences of magnetic susceptibility of the initial and electron-irradiated (F9×1018 e/cm²) samples were measured in the temperature range 80K

Abstract: Multiferroic bismuth manganites (BiMnO₃) possess both ferromagnetic and ferroelectric properties. Their electrical properties can be controlled by doping for useful applications. In this work single-phase cobalt doped bismuth manganite nanoparticles having general formula BiMn_1-xCo_xO₃ (x=0, 0.2, 0.4, 0.6) were synthesized by co-precipitation method. Structural properties like lattice parameters and crystallite size of samples were determined by the data obtained by X-rays diffraction. The dielectric constant (ε) and dielectric loss tangent (tanδ) of samples were investigated as a function of frequency from 20Hz-3MHz using ac measurement data. For all the compositions dielectric constant was decreased with increasing frequency, however it increased with the increase in cobalt content. However cobalt addition causes a decrease in loss tangent as compared to pure BiMnO₃ composition. The origin of this behavior is discussed in terms of Maxwell-Wagner and Koops model. Substitution of Mn with Co, in BiMnO₃-based compounds is supposed to cause better properties in terms of tangent loss.

Abstract: The structural and electrical transport properties of La0.67Ba0.33Mn1-yTiyO3 manganite, with y = 0.00, 0.05, 0.10, 0.15, 0.20, 0.40 and 0.60, prepared using the solid state reaction technique have been investigated. The X-ray diffraction spectra of the Ti substituted samples showed the formation of single phase compound with Pm3m cubic structure except for the y = 0.60 sample, which showed La2Ti2O7 phase formation. Lattice parameter increased with Ti content and then decreased at y = 0.60. Resistivity versus temperature study showed that only samples with y = 0.05 and 0.10 exhibited both metallic and semiconductor-like behaviour with the metal-insulator transition temperature, Tp of 167 K and 43 K, respectively. At higher Ti concentration the samples only showed the semiconducting behaviour. At T < Tp the resistivity curves followed the ρ = ρo1 + ρ1T2 relation and for T > Tp, the curves can be fitted with the nearest neighbour hopping (NNH), variable range hopping (VRH) or/and the small polaron hopping (SPH) models.