Solid State Phenomena Vols. 152-153

Abstract: Glass-coated Cu-Mn-Ga microwires were fabricated by Taylor-Ulitovsky technique. By means of energy dispersive spectroscopy microanalysis, an average alloy composition of Cu56Ga28Mn16 was determined. The temperature dependence of magnetization measured at a low magnetic field showed the coexistence of two ferromagnetic phases. The Curie temperature of one phase is 125 K and above room temperature for the other one. X-ray diffraction at room temperature and at 100 K reflects the presence of the same three crystalline phases corresponding to the cubic B2 Cu-Mn-Ga structure as a main phase and the minor phases of fcc Cu rich solid solution with Mn and Ga and the monoclinic CuO.

Abstract: La1-x(Sr, Ca)xMnO3 manganites present a special interest due their high Curie and metal-insulator transition temperatures and the possibility to obtain magnetoresistive sensors able to work near room temperature. We have synthesized two rows of manganites, with La0.54Ho0.11Sr0.35-xKxMnO3 and La0.54Ho0.11Ca0.35-xNaxMnO3 chemical compositions. XRD analysis, performed with a Huber diffractometer at room temperature, indicated that the manganites contain perovskite phases, with Pnma structure. The atomic positions, lattice parameters and some microstructure parameters, as average size of coherent blocks and microstrains, were obtained by using GSAS program. The dependence of the resistivity with temperature and the applied magnetic field was determined by means of four probes method, between 77 and 300 K. The influence of K/Na concentration on the disorder degree (at atomic and crystallite level) and on the transport mechanisms was discussed.

Abstract: The mixed perovskites (Bi1-xSrx)FeO3-y (x = 0.07, 0.14, 0.25 and 0.50) were studied by 57Fe Mössbauer spectroscopy in the temperature range 87–700 K. The samples were prepared by conventional ceramic technology with 10% 57Fe isotope enrichment. X–ray diffraction measurements showed that the samples were single-phase and had a rhombohedral structure at x = 0.07 and a cubic one at x = 0.14-0.50. As the strontium content x increases, the Neel temperature TN(x) increases from 640 K (x = 0) to 670 K (x = 0.25) and then decreases to 637 K (x = 0.5). The similar dependence on x was found for the hyperfine field values B(x) at T = 87 K. The experimental Mössbauer spectra are satisfactorily described by three to four Fe3+ states with the room temperature isomer shift values in the range 0.17-0.43 mm/s, which correspond to the iron sites with 4, 5 and 6 oxygen neighbours.

Abstract: 57Fe Mössbauer study of the Bi1-xLaxFeO3 (х=0, 0.1, 0.2, 0.61, 0.9, 1.0) perovskites has been performed at room temperature (RT) to analyze influence of the space spin-modulated magnetic structure (SSMMS) on parameters of hyperfine interactions, the valence and magnetic states of iron ions. SSMMS in perovskites BiFeO3 and Bi0.9La0.1FeO3 leads to a specific distribution of hyperfine fields P(H) with two peaks. Substitution La for Bi more than х = 0.2 destroys SSMMS and P(H) corresponds to a distribution for a homogeneous spin structure. Concentration dependences of hyperfine fields (H), isomer (δ) and quadrupole (ε) shifts were measured. Due to the isomer shift values the iron ions are in the trivalent state. The local magnetic moments μ(Fe) on Fe+3 ions were determined in these perovskites.

Abstract: The investigation of magnetic properties of diluted ferrite NiGa0.8Al0.8Fe0.4O4 is carried out. Anomalous behaviour of spontaneous magnetization σs(T), coercive force Hc(T) and volume magnetostriction ω(T) is revealed for the sample with frustrated magnetic structure. It is found that volume magnetostriction ω reaches very large (that is to say “giant”) values (ω ~ 5•10–4) (for T = 84 K). It is revealed that the derivative dω/dH is equal ≈ 43 • 10–9 Oe–1 (for T = 84 K). The assumption is made that the size of clusters is about 3 nm in the spin glass phase (for Tt < T < TC).

Abstract: Data on the magnetic properties of polycrystalline Pr0.7Ca0.3MnO3 manganite films obtained for the first time by extraction pyrolysis are presented. The effect of synthesis temperature on the magnetic properties of the manganite films is shown.

Abstract: Single crystals of cobalt oxyborates Co3O2BO3 and Co3-xFexO2BO3 were synthesized. The crystal structure and electric properties were investigated. The difference in the electrical resistivity behaviors was found. For parent Co3O2BO3 nor simple activation law, nor Mott variable range hopping (VRH) are acquirable to describe the experimental data in wide temperature region. In contrast for Co3-xFex O2BO3 Mott’s variable-range hopping conductivity clearly dominates.

Abstract: It was found that below the room temperature both parent Nd2/3Ca1/3MnO3 and doped (Nd0.9Y0.1)2/3Ca1/3MnO3 compounds exhibit a sequence of phase transformations: charge ordering, structural transformation of O-O type within the orthorhombic structure, and three magnetic transitions. Three different types of long-range magnetic order co-exist in (Nd0.9Y0.1)2/3Ca1/3MnO3 at low temperatures (as it was earlier found in Nd2/3Ca1/3MnO3): the antiferromagnetic orderings of PCE and DE types existing below ~110 K and ~60 K, respectively, and the ferromagnetic one of B type existing below ~42 K. Charge ordering occurs at 290 K in the doped compound. Diluting of Nd subsystem by Y in the parent perovskite has opposite effects on the temperatures of magnetic and charge orderings: the temperatures of all magnetic transformations are reduced in the doped compound by 20-30 K, while the charge ordering one increases by 80 K. A relationship between evolution of the phase transformation temperatures and crystal and electronic structures of the compound are analyzed.

Abstract: The present work is devoted to the theoretical interpretation of nuclear magnetic resonance (NMR) spectra on nonmagnetic lanthanum ion in charge-ordered La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3 compounds. It is shown that the anisotropic hyperfine interaction, which arises due to the polarization of lanthanum’s external p-shells, plays a crucial role in spectrum forming. Our model allows to explain experimental spectra. The work is partially supported by CRDF REC-005.

Abstract: The oxygen isotope effect in ceramic bilayer manganite (La1 zPrz)1.2Sr1.8Mn216 18O7 (z = 0, 0.3, 0.4, 0.6) 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 40 kOe. Substantial decrease of ferromagnetic (FM) transition temperature TFM under oxygen isotope substitution 16O → 18O was found. This positive isotope effect amounts to more than 20 K. A number of additional transitions of ferromagnetic type were observed at T >170 K, which also depend on oxygen overall mass. The obtained results are compared with those observed in other manganites exhibiting the large isotope effect.