A.M. Balagurov

Performance of the high resolution fourier diffractometer at the IBR-2 pulsed reactor

Abstract The first neutron Fourier diffractometer at the pulsed neutron source, the High Resolution Fourier Diffractometer (HRFD), is described. The HRFD combines both high neutron flux at the sample position, ∼107n/cm2/s, provided by the IBR-2 high flux pulsed reactor, and high resolution, very close to 0.001, over a wide range of d-spacings. The paper deals with the basic principles of HRFD, its design, and performance. Examples of structural experiments are also given.

Neutron scattering studies of ordered gamma -ZrD

The crystal structure and the lattice dynamics of the ordered gamma -ZrD phase were studied by means of neutron diffraction and inelastic neutron scattering (INS). It was found that the actual structure of the gamma phase is orthorhombic, space group Cccm, rather than tetragonal as previously believed. The one-phonon deuterium optical band in the INS spectrum was observed at an energy transfer of 107 meV (with the main feature at 103.4 meV), which is higher than the deuterium vibrational energies for all other phases of the Zr-D system. Distinct features were also observed in the INS spectrum below the free multiphonon bands, which were explained as the bound multiphonon states.

Time-of-flight neutron spectrometer for micro sample studies under high pressure

Abstract A new neutron spectrometer for investigations of elastic and inelastic neutron scattering on polycrystal microsamples under high pressure in diamond and sapphire anvils cells is described. The spectrometer is operating at the IBR-2 pulsed reactor at JINR. The time-of-flight method and ring-shaped multicounter detector are used to register the scattered neutrons. Parameters and methodical peculiarities of the device and the examples of experimental studies are given.

Atomic and magnetic structure of YBa2(Cu1−x57Fex)3O6+y studied by neutron diffraction on isotope enriched samples

Abstract The YBa 2 (Cu 1− x 57 Fe x ) 3 O 6+ y system was studied by neutron diffraction in a wide range of iron concentration in both oxygen-saturated, y ≈1, and oxygen-deficient, y ≈0.5, states. Detailed data about the atomic and magnetic structure were obtained. The dependence of both atomic coordinates and long-range magnetic order displays some specific features which are quite different from those in the Y123-Cu/Co system. In contrast with this system, a marked amount of Fe atoms occupies Cu(2) sites for x ≥0.7, the Cu(2)-O(1) interatomic distance does not depend on iron concentration, and there is no long-range magnetic order in oxygen-saturated samples whatever the iron concentration. A temperature study of 3 deoxygenated samples with x = 0.07, 0.10 and 0.15, confirms the existence of a collinear AF structure, involving a strong coupling between the iron spin and the copper lattice and a rather strong moment induced on the Cu(1) sites. We also observed a non linear variation of the lattice parameters in the T -range 8–400 K, without any abrupt change of slope.

Magnetic ordered states in the oxygen-deficient YBa2(Cu1-xFex)3Oy<6.5 system

Abstract The magnetically ordered states in the oxygen-deficient YBa 2 (Cu 1- x Fe x ) 3 O y system (0.01≤ x ≤0.30) have been studied by Mossbauer spectroscopy. Magnetic ordering of Fe atoms is found at Cu(1) sites as well as at Cu(2) sites. At small iron concentrations x ≤0.05 the Cu(1) and Cu(2) sublattices are not coupled magnetically and two magnetic phase transitions are observed: the low-temperature transition for the Cu(1) sublattice at T m1 ≈20 K and the high-temperature transition for the Cu(2) sublattice at T m2 ≈400 K. With increasing iron concentration at x ≥0.10 a strong coupling between the Cu(1) and Cu(2) sublattices appears and the whole matrix is ordered antiferromagnetically with a single magnetic ordering point T m2 ≈460 K. The phase diagrams showing the regions of existence of different magnetic phases versus iron concentration are plotted for the oxygen-saturated and oxygen-reduced YBa 2 (Cu 1- x Fe x ) 3 O y systems.

Very large magnetoresistance and spin state transition in Ba-doped cobaltites

Structural, magnetization, and magnetotransport measurements have been performed for anion deficient La0.5Ba0.5Co1− x Fe x O3−δ ( x ≤ 0.4 ) and La1− y Ba y CoO3−δ ( 0.5 ≤ y ≤ 0.6 ) perovskites. It has been found that the iron doped compositions with x ≤ 0.23 are predominantly ferromagnetic. The Curie point and magnetization are slightly larger in insulating phase (x = 0.1) than in metallic one (x = 0.05). Magnetoresistance ratio rises with lowering temperature and increasing iron content reaching three orders of magnitude in predominantly antiferromagnetic state ( x ≥ 0.25 ). The rise of the barium content in La1− y Ba y CoO3−δ series above y = 0.5 leads to stabilization of antiferromagnetic phase and strong enhancement of the magnetoresistance. Antiferromagnetic ordering is accompanied by increase in resistivity. The lack of the low field intergrain magnetoresistance is in agreement with a weak spin polarization of the charge carriers. Magnetoresistance is associated with antiferromagnet–ferromagnet (from mixed high/low into intermediate spin state) transition induced by magnetic field. It is suggested that ferromagnetism is originated from superexchange interaction via oxygen.

Neutron powder diffraction on YBa2(Cu1-x58Nix)3O7-δ

Abstract Neutron powder diffraction study of YBa 2 (Cu 1− x Ni x ) 3 O 7− δ with 58 Ni ( x = 0.05. 0.09) as well as with a natural mixture of Ni isotopes ( x = 0.06. 0.15) was carried out for the first time. The results obtained suggest that nickel substitutes for copper at least to concentration of x = 0.07, and for all x studied the structure resembles that of undoped YBa 2 Cu 3 O 7− δ . Relative coordinates of atoms are, in practice, unchanged. The Ni atoms substitute the Cu cations located both in Cu-O chains and in the CuO 2 planes with equal probabilities. Our results confirmed that the solubility of Ni in the structure of YBa 2 Cu 3 O 7− δ phase is limited and much lower than that of Fe.

Neutron diffraction study of the superconductor YBa2(Cu1−xFex)3O7−δ at x = 0.06 and 0.10, δ = 0

Abstract Neutron diffraction patterns of YBa 2 Cu 1− x Fe x ) 3 O 7−δ ( x = 0.0, 0.06, 0.10 and δ = 0) superconducting samples were obtained and refined by the Rietveld technique. For x = 0.06 and 0.10 the structure resembles that of undoped YBa 2 Cu 3 O 7 and may be refined in either orthorhombic Pmmm or tetragonal P4/mmm space groups. The Fe atoms substitute the Cu cations located both in Cu-O chains and in the CuO 2 planes with equal probabilities. The analysis within the Pmmm group leads to some amount of vacancies on the Cul sites of the chains.

The effect of oxygen isotope substitution on the phase diagram of nearly half-doped R1-xSrxMnO3 manganites (R = Sm, NdTb, NdEu)

The effect of isotope substitution on the electrical resistance, magnetic susceptibility and neutron diffraction patterns of nearly half-doped R1?xSrxMnO3 manganites was studied for ceramic samples with R = Sm (x = 0.425?0.525), Nd/Tb (x = 0.45) and Nd/Eu (x = 0.45). In Nd/Tb and Nd/Eu manganites, the composition was chosen to yield the same mean ionic radius as in the Sm compound, but the samples differed in the cation disorder parameter ?. The oxygen isotope substitution leads to fundamental changes in the phase diagram of the manganites studied, favouring enhanced inhomogeneity, stabilizing the insulating antiferromagnetic (AFM) state and even causing a metal?insulator transition (MIT). The MIT induced by the oxygen isotope substitution was found here for the first time in manganites with A-type AFM; earlier this unusual phenomenon was observed only for compounds with the CE-type AFM structure. This suggests that such a MIT is closely related to the ferro?AFM crossover independently on the specific nature of the AFM phase and related orbital order.

Magnetic ordering of Fe atoms in oxygen-saturated and oxygen- reduced YBa2(Cu1−xFex)3Oy

Abstract The magnetic ordering of Fe atoms in the YBa2(Cu1−xFex)3Oy system is studied in the superconducting (SC) and non-SC states by Mossbauer spectroscopy. In the SC samples a single magnetic ordering point Tm1 is observed for all Fe atoms: Tm1 = 10 K and 16 K for x = 0.05 and 0.10, respectively. In the non-SC state for x = 0.05 the Cu(1) and Cu(2) sublattices are magnetically independent and two magnetic ordering points are found: Tm1=20 K and Tm2=405 K. For x=0.10 a strong magnetic coupling between the Cu(1) and Cu(2) sublattices leads to a single magnetic ordering temperature Tm2=435 K.