Small-angle Polarized Neutron Scattering Research Articles

SMALL-ANGLE SCATTERING OF NEUTRONS ON Sr<sub>2</sub>FeMoO<sub>6–δ</sub> SAMPLES WITH DIFFERENT-DEGREE SUPERSTRUCTURAL ORDERING OF Fe/Mo CATIONS

Single-phase Sr2FeMoO6-δ samples with different-degreesuperstructiral ordering Fe/Mo cations superstructural ordering (P, 76, 86 and 93 %) were obtained by the solid-phase technique. Based on the results of measuring the magnetic characteristics in the samples, we found that an increase in magnetization (26.41, 32.36 and 42.66 A·m2·kg–1), magnetic moment (1.33, 3.07 and 3.58 μB/f.u.) and Curie temperatures (422, 428 and 437 K) withparameter P (76, 86 and 93 %) can be explained by the presence of antistructural defects, as well as antiferromagnetic inclusions. This determines the redistribution of electron density, which is accompanied by the change in electronic configuration of a part of Fe/Mo cations. Based on the temperature dependences of the magnetic moment of the samples measured in ZFC and FC modes, and on small-angle polarized neutron scattering (SANS), we found that the samples are in a magnetically inhomogeneous state. An important result to mention is that we detected the difference between the slope of the SANS curves of samples with different oxygen content, which demonstrates a different microstructure of inhomogeneities. The main inhomogeneities are magnetic inclusions with the dimensions depending on the superstructural ordering of Fe/Mo cations. According to the Porod law, it was shown that the Sr2FeMoO6-δ samples with wave vector values 0.1 > q > 0.002 Å–1 contain polydisperse grains with a smooth surface. For q > 0.1 Å–1 a deviation from the Porod law is observed, confirming the presence of magnetic inhomogeneities with a diameter < 6 nm in the grains.

Fe3P impurity phase in high-quality LiFePO4: X-ray diffraction and neutron-graphical studies

High discharge capacity and long life cycles of electrodes fabricated from various LiFePO4 powders have been used to select samples for detailed structural studies. In some samples, the presence of ferromagnetic Fe3P crystallites was revealed by the synchrotron X-ray diffraction analysis, with the integral intensity ratio of the peaks Fe3P (231) and LiFePO4 (112) equal to ∼1/12. Small-angle polarized neutron scattering (SAPNS) detected the presence of magnetic nuclear contrasting regions with size of 17 ± 1 nm. The average diameter of LiFePO4 crystallites is 230 nm, and Fe3P crystallites were found as 17 × 54 nm2 plates. The high quality of these samples was provided by their manufacturer via synthesis of the Fe3P impurity phase. It can be stated that the set of studies, developed in the study, is helpful in a search for new effective impurity phases and in optimization of their parameters.

Small-angle polarized neutron scattering study of the mesostructure of phase precipitates in the steel P91 after heat treatment

This paper reports on the results of the analysis of the mesostructure of phase precipitates formed in the steel P91 during heat treatment in the temperature range of 600–820°C after quenching at 1050°C for 30 min. Measurements have been performed by the small-angle polarized neutron scattering (SAPNS) method with the separation of nuclear-magnetic interference scattering. The characteristic sizes of the phases precipitated during heat treatment have been determined to be in the range of 150–450 A. It has been demonstrated that the method is highly effective for solving problems of this class, which is based on the possibility to separate and investigate nuclear-magnetic contrast scattering against the background of the total nuclear and magnetic small-angle scattering.

Iron nanowires embedded in mesoporous silica: Polarized neutron scattering study

Magnetic iron nanowires embedded in the mesoporous silica matrix have been studied by Small Angle Polarized Neutron Scattering. Magnetic nanocomposites were prepared by a novel variant of the synthesis based on the introduction of the hydrophobic metal compound, Fe(CO) 5, into the hydrophobic part of as-prepared mesoporous silica-surfactant composite. The mesoporous silica represents a highly regular hexagonal structure of mesopores. A diffraction peak in SAPNS at q c≈1.55 nm −1 is observed, which corresponds to a hexagonal structure with a periodicity of a 0≈4.6 nm. The scattering pattern of the pure matrix SiO 2 is well fitted by a sum of the contributions: the scattering on a powder consisting of large particles (1/ q 4) and the scattering on the regular structure of pores (diffraction peak). The samples with iron inside the pores give additional scattering. This scattering depends significantly on the annealing temperature what demonstrates the different degrees of crystallization process. The latter is fitted as a scattering on the bunches of the nanowires with average radius R b and with its spread Δ R . The nuclear–magnetic interference in scattering of polarized neutrons was also studied. We observe no interference scattering on the periodical structure. Instead, a significant interference scattering is detected, which may be directly related to the bunches of the nanowires created during the crystallization process.

SANS study of new magnetic nanocomposites embedded into the mesoporous silica

Magnetic nanocomposites embedded into mesoporous silica have been studied by Small Angle Polarized Neutron Scattering. It is well known that mesoporous silica represents a highly regular hexagonal structure of nanotubes. For the samples under study a diffraction peak in SANS at q c ≈1.7 nm −1 is observed which corresponds to a hexagonal structure with periodicity a≈3.5 nm . We found that the intercalation of iron into the silica matrix leads to some changes of the matrix itself. Additionally, the nuclear–magnetic interference in scattering of polarized neutrons was investigated. The interference reveals the absence of a periodical magnetic structure at room temperature which would be consistent with the hexagonal structure of nanotubes. The correlation between magnetic nanoparticles and pores of the matrix is revealed in the interference scattering at q⪡ q c.

The investigation of Fe–Mn-based alloys with shape memory effect by small-angle scattering of polarized neutrons

The small-angle polarized neutron scattering (SAPNS) technique has been used to study a nuclear and magnetic homogeneity in the distribution of both substituent (Si, Cr, Ni) and interstitial (C, N) alloying elements on the mesoscopic range in Fe–Mn-based alloys with shape memory effect (SME). The four groups of alloys with various basic compositions: FeMn 18 (wt%), FeMn 20Si 6, FeMn 20Cr 9N 0.2 and FeMn 17Cr 9Ni 4Si 6 were investigated. It was found that the small-angle scattering of neutrons and depolarization on these alloys are very small altogether. The scattering did not exceed 1.5% from the incident beam and depolarization ∼2% for all samples. It means that these alloys are well nuclear and magnetically homogeneous on the scale of 10–1000 Å. However, the difference in the homogeneity depending on the compositions still takes place. Thus, the adding of Si in FeMn 18 and FeMn 20Cr 9N 0.2 alloys improves the homogeneity pronouncedly. At once, the effect of the doping by C or N atoms on the homogeneity in FeMn 20Si 6 and FeMn 17Cr 9Ni 4Si 6 alloys is multivalued and depend on the presence of substitutional atoms (Ni and Cr). The capability of SAPNS as a method for the study of mesoscopic homogeneity in materials with SME and testing of the quality of their preparation is discussed.

Small-angle polarized neutron scattering in YBa 2 (Cu 0.9 Fe 0.1 ) 3 O 7-y ceramics at T =290-550 K

Small-angle polarized neutron scattering (SAPNS) measurements were performed on deoxygenated YBa2 (Cu0.9Fe0.1)3O7−y ceramics in vacuum in a temperature range 290 < T < 550 K and magnetic fields 0 < H < 70 Oe. Anomalies in the temperature curves of the scattering inten- sity I(T, q) (where q is the momentum transfer), the polariza- tion P(T ) of the transmitted beam and the magnetic-nuclear interference term ∆(T, q) were observed at 300 < T < 500 K. From analysis of SAPNS and depolarization data it was found that magnetic correlations of ferromagnetic type with a scale of a few hundred angstroms exist in the system. These results agree with Mossbauer-spectroscopy data. The nature of these effects is discussed.

Vector analysis of polarization at small-angle neutron scattering

The three-dimensional (3-D) analysis in small-angle polarized neutron scattering (SAPNS) is presented, and the corresponding experimental device is described. The peculiar properties of 3-D analysis allow us (i) to investigate critical scattering in iron at energy transfer ω∼1– 50 μeV where direct measurement of the magnetic scattering is impossible, (ii) to determine the correlation length of the critical fluctuation very close to the Curie temperature where in the case of the conventional critical scattering the inelasticity and multiple scattering spoil the results.

Small-angle polarized neutron scattering in Sm 1− xSr xMnO 3 ( x

The results of small-angle polarized neutron scattering (SAPNS) measurements on 154 Sm 1−x Sr x MnO 3 ( x=0.25; 0.4 ) perovskite with complete polarized analysis are shown. Ferromagnetic correlations with scale 180–250 Å and magnetic–nuclear interference (MNI) in low-temperature phase are found. SAPNS and depolarization data can be explained by a picture of strong magnetic mesoscale disordering Sm system in low-temperature phase. Furthermore the system shows temperature hysteresis in this phase. The possible scenario of low-temperature phase transition is discussed.

The methods of small-angle scattering of polarized neutrons

The review of experimental methods of small-angle polarized neutron scattering (SAPNS) is presented. The developed methods of SAPNS such as the left-right asymmetry of scattering, the separation of elastic and inelastic magnetic scattering by 3-dimensional analysis and others allow to get the precise information about spin correlations in magnetics that can not be obtained by others methods. Several results are presented for the illustration of SAPNS methods. Among them the critical exponent for correlation radius ( ν = 0.67 ± 0.01 for Fe), the dynamical exponent ( z = 2.617 ± 0.004 for Fe), a temperature behavior of magnitostriction near T C in PdFe(4 at.%), the study of 3-spin dynamics and the confirmation of ‘hard’ variant of dipolar critical dynamics.

The observation of the magnetic correlations in YBa 2(Cu 1− xFe x) 3O 7+ y ceramics by small-angle polarized neutron scattering

The small-angle polarized neutron scattering (SAPNS) technique has been used to observe the magnetic ordering in the system of YBa 2Cu 2.61Fe 0.39O 7+ y ceramics. The anomalies of the temperature dependences of the scattering intensity I( T, q), the polarization P( T, q) and the difference Δ( T, q) in SAPNS intensities of neutrons polarized along with or opposite to the applied magnetic field were found in the temperature region T < 40 K. These effects are interpreted as scattering of neutrons from magnetic fluctuations with characteristic scale lying in the range from 70 to 370 Å.

Precise measurements of spin-wave parameters in ferromagnets by polarized neutrons

By means of small-angle polarized neutron scattering (SAPNS) magnetic excitations were studied in amorphous ferromagnets Fe 50Ni 22Cr 10P 18 ( T c = 355 K) and Fe 48Ni 34P 18 ( T c = 525 K) in the temperature range 50 K ⩽ T ⩽ 293 K and Fe 40Ni 40B 20 ( T c = 669 K). Spin-wave stiffness and damping as well as dipolar energy were measured for the first time for ε q ⋍ 10 −5 eV, q ⋍ 10 −3 A ̊ −1 , and this was carried out with a high statistical accuracy obtained as a result of a special experimental geometry. The experimental method is highly sensitive also to small deviations of SW-spectrum under annealing and tensile stress and this was demonstrated on FeNiB.

Small-angle scattering of polarized neutrons in HTSC ceramics

The results of the first observation of the small-angle polarized neutron scattering (SAPNS) from magnetic flux inhomogeneities in high- T c YBaCuO and LaSrCuO ceramics are reported. The difference between the scattering cross sections for neutrons initially polarized along or opposite to the applied field was found at T < T c and in the range of applied field below H c1 of grain in ceramics. The alternative sign effect was observed in remanent field trapped in grains after applying a field. The effect is explained by nuclear-magnetic interference and is proportional to the Fourier transform of the cross-correlator of the magnetic-nuclear (MN) scattering length density fluctuations (SLD).

Small Angle Polarized Neutron Scattering in the Superconducting Y-Ba-Cu-O Ceramic

Small Angle Polarized Neutron Scattering in the Superconducting Y-Ba-Cu-O Ceramic