A. M. Balagurov

Magnetic state of the structural separated anion-deficient La0.70Sr0.30MnO2.85 manganite

The results of neutron diffraction studies of the La0.70Sr0.30MnO2.85 compound and its behavior in an external magnetic field are stated. It is established that in the 4–300 K temperature range, two structural perovskite phases coexist in the sample, which differ in symmetry (groups

Investigation of the crystal and magnetic structures of BaFe12 - xAlxO19 solid solutions (x = 0.1‒1.2)

R\bar 3c

Investigation of the HgBa2CuO4+δ structure under external pressures up to 5 GPa by neutron powder diffraction

and I4/mcm). The reason for the phase separation is the clustering of oxygen vacancies. The temperature (4–300 K) and field (0–140 kOe) dependences of the specific magnetic moment are measured. It is found that in zero external field, the magnetic state of La0.70Sr0.30MnO2.85 is a cluster spin glass, which is the result of frustration of Mn3+-O-Mn3+ exchange interactions. An increase in external magnetic field up to 10 kOe leads to fragmentation of ferromagnetic clusters and then to an increase in the degree of polarization of local spins of manganese and the emergence of long-range ferromagnetic order. With increasing magnetic field up to 140 kOe, the magnetic ordering temperature reaches 160 K. The causes of the structural and magnetic phase separation of this composition and formation mechanism of its spin-glass magnetic state are analyzed.

Study of the crystalline and magnetic structures of BaFe 11.4 Al 0.6 O 19 in a wide temperature range

A structural transition with a reduction in symmetry of the high temperature cubic phase (sp. gr. Fd3m) to the tetragonal phase (sp. gr. I41/amd) and the appearance of a ferrimagnetic structure occur in CuFe2O4 copper ferrite at T ≈ 440°C. It is established by an experiment on a high-resolution neutron diffractometer that the temperature at which long-range magnetic order occurs is higher than that of tetragonal phase formation. When cooling CuFe2O4 spinel from 500°C, the equilibrium coexistence of both phases is observed in a fairly wide temperature range (∼40°C). The composition studied is a completely inverse spinel in the cubic phase, and in the tetragonal phase the inversion parameter does not exceed few percent (x = 0.06 ± 0.04). At the same time, the phase formed upon cooling has a classical value of tetragonal distortion (γ ≈ 1.06). The character of temperature changes in the structural parameters during the transition from cubic to tetragonal phase indicates that this transition is based on the Jahn-Teller distortion of (Cu,Fe)O6 octahedra rather than the mutual migration of copper and iron atoms.

Detector for the FSD Fourier-diffractometer Based on ZnS(Ag)/ 6 LiF Scintillation Screen and Wavelength Shifting Fiber Readout

The structure of barium ferrite BaFe12 - xAlxO19 solid solutions (x = 0.1‒1.2) with iron partially replaced with diamagnetic aluminum ions has been studied by neutron diffraction. Experimental data have been collected at room temperature on a high-resolution diffractometer, which yielded precise information about the changes in the crystal and magnetic structures and data on the behavior of the sample microstructure. Barium hexaferrite retains a magnetoplumbite structure in the entire range of aluminum concentrations under study, and its magnetic structure is described within the Gorter model, with moments orientated along the hexagonal axis. The total magnetic moment per formula unit decreases while diamagnetic aluminum ions substitute for iron ions. Microstrains in crystallites increase with an increase in the diamagnetic ion concentration, which is related to the difference in the ionic radii of iron and aluminum ions.

Ferro-and antiferromagnetic ordering in LaMnO3+δ

The high-resolution Fourier diffractometer (HRFD) was commissioned at the IBR-2 pulsed reactor at FLNP JINR in 1994. The specific feature of the HRFD design is the use of fast Fourier chopper for modulating the primary neutron beam intensity and the correlation method of diffraction data acquisition. This allowed to reach with HRFD extremely high resolution (Δd/d ≈ 0.001) over a wide range of inter-planar spacings at a relatively short flight path between chopper and sample (L = 20 m). Over time, a lot of diffraction experiments on crystalline materials, the main goal of which was to study their atomic and magnetic structures, were performed at HRFD. Successful implementation of the Fourier diffractometry technique at the IBR-2 reactor stimulated the construction of yet another Fourier diffractometer intended for internal mechanical stress studies in bulk materials (FSD, Fourier Stress Diffractometer). In this paper the experience of using this technique at the IBR-2, which is a long-pulse neutron source, is considered, the examples of HRFD studies are given, and possible solutions for existing technical problems of using correlation diffractometry and ways of increasing the intensity and resolution of HRFD are discussed.

Investigation of the structure and properties of quartz in the α-β transition range by neutron diffraction and mechanical spectroscopy

Abstract Neutron powder diffraction investigation of the changes in the structure of the mercury superconductor HgBa 2 CuO 4+δ has been carried out in the pressure range 0–5.07 GPa at room temperature. The compressibility values of the unit cell parameters and several bond distances in the structure have been determined. The relative reductions of the unit cell parameters are close to each other and are approximately 1.7% in the base plane and 2.3% along the c -axis. At the same time, the interatomic distance modulations are strongly inhomogeneous: approximately 4% for the apical CuO bond and only about 0.4% for the apical HgO bond.

Real disordered crystal structure and Curie temperature of intermetallic compounds Y2Fe17−xMx (M=Si or Al)

The structural study of barium hexaferrite (BaFe12O19) whose iron ions are partly replaced by diamagnetic aluminum ions (x = 0.6) is carried out with the help of the neutron-diffraction method. Experimental data are acquired in a wide temperature range of 4–730 K via a high-resolution diffractometer, which makes it possible to obtain both precise information on changes in crystalline and magnetic structures and behavioral data on the sample microstructure. BaFe11.4Al0.6O19 retains the magnetoplumbite structure in the whole temperature range and exhibits the “invar effect,” according to which its bulk thermal-expansion coefficient is close to zero at low temperatures (<150 K). The magnetic structure is defined by the Gorter model with magnetic moments oriented along the hexagonal axis. For all nonequivalent crystallographic positions, the magnetic moment of F3+ is close to 4 μB at 4 K. The total magnetic moment per formula unit is 15.6 μB, i.e., less than the nominal value of 20 μB. This is caused by the fact that diamagnetic Al ions are included in the composition. Crystallite microstrains increase insignificantly with temperature due to increasing influence of the magnetic subsystem.