Nuclear-magnetic Interference Research Articles

High‐quality structure parameter for magnetic liquids obtained by small‐angle scattering of polarized neutrons

AbstractThe preparation of magnetic liquids with the aid of different aluminium alkyl chains leads to narrow size distributions. A structural investigation of different ferrofluid samples was performed with small‐angle neutron scattering of polarized neutrons (SANSPOL). The use of polarized neutrons extends the known method for composition exploration in nanometre‐scale magnetic systems. For these new possibilities special data‐analysis procedures are indispensable. We use a new simultaneous fitting procedure for the spin‐up, spin‐down and nuclear–magnetic interference term to obtain the structure parameter information. The advantage of this method is the reduction of parameter correlations for the independent model parameters. Copyright © 2004 John Wiley & Sons, Ltd.

Magnetic properties of iron nanoparticles in mesoporous silica

Magnetic properties of nanocomposites based on mesoporous silica SiO 2 with embedded iron particles are studied by small angle polarized neutron scattering at temperatures from 8 to 300 K and magnetic fields from 1 to 350 mT. This mesoporous silica forms highly regular hexagonal structures of nanotubes. A diffraction peak in SANS at q c=1.55 nm −1 is observed, corresponding to a hexagonal structure with periodicity a=4.6 nm. The scattering of the pure matrix Si0 2 is well fitted by a sum of two contributions: Porod scattering from large particles (1/ q 4) plus the scattering on a regular structure of pores (diffraction peak). Additional scattering is observed from samples with iron in the pores. The latter is fitted as scattering from bunches of nanowires with an average radius of R b=4.68 nm and a spread of Δ R b=0.9 nm. The nuclear-magnetic interference in the scattering of polarized neutrons is studied. We observe no interference scattering on the periodic structure that would be consistent with a hexagonal structure of nanotubes/nanowires. Instead, appreciable interference scattering is detected which is directly related with bunches of nanowires created during crystallization. The temperature and magnetic field dependences of this interference scattering demonstrate a transition from superparamagnetic to dipole–glass state at T = 80 K caused by an interplay of magnetic field, dipole–dipole, and thermal interactions.

The full potential of small-angle scattering with polarised neutrons for structure parameter evaluation in magnetic liquids

The use of polarised neutrons in small-angle neutron scattering (SANSPOL) extent the potentialities for composition exploration in nanometre-scale magnetic systems. To use these new possibilities in intricate magnetic structure investigation special data analysis procedures are indispensable. We present a new simultaneous fitting procedure for the spin-up, spin-down and nuclear–magnetic interference term. The advantage of this method is the reduction of parameter correlations for the independent model parameters. Together with confidence intervals and normalised residual properties this can be used to judge the quality and stringency of a chosen model. We illustrate the utilisation of polarised neutrons in SANS in magnetic colloid samples and demonstrate the attained qualitative and quantitative improvements of this technique in detail. The results reveal the necessity of an advanced data analysis in SANSPOL for an optimal use of the experimental endeavours.

Observation of magnetic-nuclear cross-correlations in Ni-Mn-Ga

A study of the non-stoichiometric single crystal Ni 49.1Mn 29.4Ga 21.5 with small-angle polarized neutron scattering was previously reported. This alloy, with a Curie temperature T C≈374 K, has a martensitic transition at T m=306 K. A large increase of the SANS intensity takes place on cooling to T m. An increase of the spin–spin correlation radius at zero magnetic field from ∼40 Å to ∼400 Å was suggested. In this report, we experimentally confirm this assumption by the observation of magnetic-nuclear interference (MNI) for T m< T< T C. The amplitude ratio of nuclear-to-magnetic scattering does not exceed a few percents. The temperature, field, and q-dependencies of MNI are discussed.

Polarised SANS study of microstructural evolution under neutron irradiation in a martensitic steel for fusion reactors

This work presents the results of polarised small-angle neutron scattering (SANS) measurements of modified martensitic steel DIN1.4914, originally developed for application in future fusion reactors (MANET steel). SANS measurements were made using the D22 instrument at the ILL Grenoble using an ad hoc polarised beam set-up. The investigated MANET samples were neutron irradiated and subsequently post-irradiation tempered to reproduce as much as possible the expected service conditions. The results, based on the analysis of the nuclear-magnetic interference, are discussed taking into account both the occurrence of Cr redistribution phenomena with correlated changes in the composition of the precipitate phases, and the growth of non-magnetic defects (He-bubbles or microvoids).

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.

Polarized SANS study of microstructural evolution in a martensitic steel for fusion reactors

The results of a polarized SANS study of a martensitic steel (MANET) developed for fusion-reactor technology are presented. The measurements were carried out to investigate Cr-redistribution phenomena in the martensitic matrix, which can play a crucial role in ductile-to-brittle transition changes under irradiation. The nuclear–magnetic interference term and the ratio of nuclear plus magnetic to nuclear SANS cross sections show that such inhomogeneities, which are present immediately after quenching and give rise to Fe-rich precipitates, dissolve even for short tempering times.

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.

What are the advantages of studying mesoscopic magnetic inhomogeneities in perovskites using polarized neutrons?

Advantages and characteristic features of studying small-angle scattering of polarized neutrons (SAPNS) are discussed, mainly, on the examples of perovskites. However, the peculiarities such as depolarization analysis, magnetic-nuclear interference are general for SAPNS method and can be used for investigation of magnetic correlations in any magnetic disordered systems.

Nuclear-magnetic interference in the inelastic scattering of the polarized neutrons in a dipolar ferromagnet

The problem of the inelastic nuclear-magnetic interference (INMI) in the scattering of the polarized neutrons in isotropic cubic dipolar ferromagnet is considered theoretically. It is shown that using polarized neutrons the contribution of INMI to the cross section can be separated experimentally from two other main contributions of the scattering — nuclear and magnetic. The small-angle and near-Bragg scattering is considered and it is found, that the INMI is related to the longitudinal spin susceptibility which exhibits an infra-red divergence. The expression for the INMI cross section is derived in one-loop approximation.

The spin correlations in YBa 2(Cu 1− xFe x) 3O y ceramics at T=15– [formula omitted] investigated by the small-angle scattering of polarized neutrons

The ceramic YBa 2(Cu 1− x Fe x ) 3O y system was studied by means of the small-angle scattering of polarized neutrons in a wide range of iron concentration 0⩽ x⩽0.2 in both oxygen-rich y⩾7 and oxygen-reduced y⩽6.7 states. The measurements were carried out in the range of temperatures 15≤ T⩽550 K and magnetic fields 0< H⩽4500 Oe. It was obtained that magnetic phase transition of the spin-glass like type occurs for all oxygen-rich samples and oxygen-reduced sample with x=0.1 at T<45 K. Moreover, it was found that in the high-temperature region the magnetic ordering was observed for the oxygen-reduced sample with x=0.1 and y=6.36 at T m2≈100 K and for the oxygen-rich one with x=0.2 and y=7.21 at T m2≈280 K, respectively. The observed anomalies of the temperature dependencies of the scattering intensity I( T, q), polarization P( T) of the transmitted beam and magnetic-nuclear interference term Δ( T, q) (where q is momentum transfer) are in consequence of scattering of neutrons from spin-density fluctuations. The characteristic scale R c of the spin correlations obtained from the fitting of the observed scattering described as: I( q)≈ A/( q 2+ κ 2) 2 (where κ=1/ R c) are estimated as few hundred angstrom. After vacuum annealing of the oxygen-reduced sample with x=0.1 and y=6.36 at T=550 K the magnetic ordering occurred previously were not found but the anomalies of the temperature dependencies of I( T, q) and P( T) were observed in the range of 350< T<500 K.

Polarised neutrons and axial-vector interactions in magnetic materials

Inelastic polarised neutron scattering is analysed when the magnetic interaction includes a part characterised by an axial vector. In this case the dynamical chiral (DC) scattering and inelastic nuclear-magnetic interference (INMI) may appear. As examples we consider magnetic field and antiferromagnets with k AF =0 . Possibilities of other axial-vector interactions such as the Dzialoshinskii–Moriya interaction and elastic torsion are mentioned too. The DC scattering in magnetic field is determined by the three-spin correlations. Their experimental investigation in iron above T c allows to confirm predictions of the Polyakov–Kadanoff–Wilson operator algebra and demonstrate crossover to the dipolar critical dynamics. The INMI in the antiferromagnets with k AF =0 is considered theoretically and appearance of the infra-red divergence in the case of quasi-2D compound BaNi 2(PO 4) 2 was demonstrated.

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 investigation of the spin correlations in YBa 2(Cu 1− xFe x) 3O y ceramics by the small-angle scattering of polarized neutrons

A series of samples of the oxygen-rich YBa 2(Cu 1− x Fe x ) 3O y⩾7 with x=0.1, 0.13, 0.2 and the oxygen-reduced YBa 2(Cu 0.9Fe 0.1) 3O 6.36 ceramics were studied by means of the small-angle scattering of polarized neutrons (SAPNS). It was obtained that magnetic phase transition like spin-glass type occurs for all samples at T<45 K. Moreover, it was found that the magnetic ordering were observed for both the oxygen-reduced and oxygen-rich ( x=0.2) samples in the high-temperature region nearly T ≈ 100 and 280 K, respectively. The observed anomalies of the temperature dependencies of the scattering intensity I( T, q), polarization P( T) of transmitted beam and magnetic-nuclear interference term Δ( T, q) (where q is momentum transfer) are uniquely interpreted as scattering of neutrons from the spin density fluctuations. The characteristic scale R c of the spin correlations is estimated as a few hundred angstrom.

Nuclear-magnetic interference in the inelastic scattering of the polarized neutrons

The problem of the inelastic nuclear-magnetic interference (INMI) at the scattering of the polarized neutrons is considered theoretically. General expressions for this interference are derived and discussed. It is shown that the INMI appears at the presence of some interaction, which connects nuclear and magnetic variables and contributes to both nuclear and magnetic parts of the scattering. However at H=0 , where H is the applied magnetic field or the sample magnetization the INMI takes place if the spin–lattice interaction is characterized by an axial vector. An example of the Dzialoshinskii–Moriya interaction is considered. It is shown that in this case the INMI is connected to the three spin non-diagonal susceptibility with non-local chiral spin operator. Application of the theory to the spin–Peierls compound CuGeO 3 is discussed on the basis of preliminary experimental findings.

Polarized neutron inelastic scattering on the spin–Peierls system CuGeO 3: “three-directional” versus “three-dimensional” polarization analysis

We report and discuss recent results obtained in the inorganic spin–Peierls system CuGeO 3 by using both conventional (“three-directional”) and “three-dimensional” polarization analysis. Quite surprisingly, a relatively strong rotation of the final neutron polarization P f is observed when the incident polarization is set parallel to the scattering vector Q . The largest effect is seen for ( Q, ω ) values corresponding to the dispersion of the spin gap along the b-direction. The qualitative analysis of the P i - and Q -dependencies leads to ascribe this effect to the imaginary part of the inelastic nuclear–magnetic interference terms. The origin of the enhancement of such terms in CuGeO 3 will be discussed.

Microstructural investigation, using polarized neutron scattering, of a martensitic steel for fusion reactors

Small- and wide-angle polarized neutron scattering has been used to investigate the microstructure of modified martensitic steel DIN 1.4914 (“MANET”-type) developed as a potential candidate for the “first wall” of future fusion reactors. The nuclear–magnetic interference term and the comparison of the size distribution functions, obtained from the nuclear and from the magnetic scattering components, show that for quench temperatures lower than 1200°C three kinds of microstructural inhomogeneities can be identified: (a) tiny C–Cr elementary aggregates (1 nm or less in size), (b) larger (1–25 nm) Fe-carbides, (c) much larger inhomogeneities arising either from M 23C 6 precipitates or from fluctuations in the Cr distribution. The scattering data are also compared with those previously obtained on the same samples from a conventional SANS instrument and the influence of the available Q-range on the accuracy of the obtained size distribution functions is discussed.

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).

Polarised neutron scattering from the surface and bulk of magnetic fluid

We have studied the local magnetic properties of a ferrofluid near the surface layer and in the bulk volume, using narrow (∼ 1.5 mm) polarised neutron beam. The polarisation of the transmitted beam showed some peculiarities (found by three-dimensional analysis) near the surface comparing to the bulk region. These peculiarities are better manifested when a magnetic field is applied and an increased spin rotation was observed near the surface. The small angle scattering intensity near the surface region was higher than in the bulk volume. The q dependence of surface section reveals clusters with a spread of size ∼10 2−10 3 A ̊ , which can be connected with a low fractal dimensionality D f ≊ 1 . In bulk material the nuclear-magnetic interference cross-section measured corresponds to the theoretical prediction.

Magnetic Moments of Compound States inEr168

A polarized-neutron transmission method is used to determine the shift of the neutron resonance energies due to the hyperfine interaction of the compound states. Resonances in ${\mathrm{Er}}^{167}$+$n$ at 0.460 and 0.584 eV are investigated. The resonance at 0.584 eV ($J=I\ensuremath{-}\frac{1}{2}$) shows a strong effect, leading to a very large value of the ${g}_{J}$ factor, while ${g}_{J}$ for the 0.460-eV resonance ($J=I+\frac{1}{2}$) is in the order of ${g}_{I}$ for the ${\mathrm{Er}}^{167}$ ground state. The results are compared to some recent predictions based on a statistical model of the compound nucleus. The role of nuclear magnetic interference terms which lead to a strong background transmission effect is discussed.