Solid State Phenomena Vol. 190

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: A study of the low temperature magnetic state of polycrystalline colossal magnetoresistance perovskite Nd_2/3Ca_1/3MnO₃ has been carried out. The data obtained, such as strongly divergent ZFC and FC static magnetizations and frequency dependent ac susceptibility, are evident of the glassy magnetic state of the system. Well defined maxima Tmax in the in-phase linear ac susceptibility χ curves were observed, indicating a spin-glass transition. Clear frequency dependence of the cusp temperature Tmax was found. The frequency dependence of Tmax was successfully analyzed by the dynamical scaling theory of a three-dimensional spin glass. Slow relaxation process and variety of relaxation times found imply a cluster glass magnetic state of the compound at low temperatures rather than a canonical spin glass state. The cluster glass state, accompanied by the multiple magnetic transitions of Nd_2/3Ca_1/3MnO₃, might exist due to the competing interaction between the FM clusters and the AFM matrix induced by the complex nanophase segregated state of the compound.

Abstract: Results of structural neutron diffraction study and data on the transport and magnetic properties (the linear and nonlinear (second and third order) susceptibilities) are presented for the La_1-xSr_xCoO₃ and La_1-xCa_xCoO₃ at x = 0.15 single crystals. It is founf that these compounds are insulators and exhibit the rhombohedral R3-c space group. According to the magnetic measurements, the development of the ferromagnetic (F) clustered state proceeds into three stages. On cooling, during the first stage, the F clusters nucleate at the preferential sites that are likely produced by variation in oxygen and doping stoichiometry. The second stage is characterized by sharp increasing the concetraion of the isolated F clusters on cooling. This is the process of the homogeneous nucleation. A coalescence of the isolated F clusters into some large-scale complexes containig some ammount of the domains is associated with the third stage. Note that it is the typical behavior observed at a first order phase transition. The data allow one to determined reliably the temperature boundaries of the stages.

Abstract: We present the results of the study of middle infrared absorption (MIR) and X-ray absorption (XAS) spectra of nanostructured CuO and Cu₂O ceramics prepared by the high pressure torsion method from coarse-grain oxide powders. We have found that the MIR bands at ~140 meV in CuO and Cu₂O nanoceramics are associated with the presence of Cu⁺ and Cu²⁺ ions, respectively. X-ray absorption spectra indicate an appearance of extra Cu²⁺ and Cu⁺ ions in nanoCu₂O and nanoCuO ceramics, respectively.

Abstract: The phase transitions and critical phenomena in three-dimensional (3D) site-diluted 3-and 4-state Potts models is investigated by Monte-Carlo method based on the highly efficient Wolff algorithm. The systems with linear sizes L=20-44 at spin concentrations p=1.00, 0.95, 0.90, 0.80, 0.70, 0.65 are explored. The second-order phase transition is shown to occur in the three-dimensional 3-state Potts model with nonmagnetic impurities. In the 4-state Potts model there are observed first-order phase transitions in weakly diluted state, when the model is strongly diluted the first-order phase transitions change to the second-order one. On the basis of the finite-size scaling theory static critical exponents of specific heat α, susceptibility γ , magnetization β, and exponent of correlation radius ν for the systems under study are calculated.

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 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 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: 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: Ultrasonic investigation of ZnSe:Cr²⁺ crystal was carried out at low temperatures in magnetic field applied along the wave vector. The observed anomalies in magnetic field dependences of attenuation and wave number evidence for existence of resonant transitions between the energy levels of the Jahn-Teller tetrahedral complex CrSe₄.