Neutron Polarization Analysis Research Articles

Local symmetry of the crystal-field Hamiltonian ofCePtSnby polarized neutron scattering

We present the results of a polarized neutron scattering study of crystal-field (CF) excitations in a $\mathrm{CePtSn}$ single crystal and their interpretation, describing the magnetic properties of the metallic Kondo antiferromagnet $\mathrm{CePtSn}$ in its paramagnetic phase. Using neutron polarization analysis, we demonstrate that all terms of the monoclinic site CF Hamiltonian have to be considered in order to account for all the available experimental data, including susceptibility and high-field magnetization measurements. The CF eigenstates, as probed by inelastic polarized neutron scattering, are not affected by cooperative phenomena neither in the vicinity of the Kondo temperature, ${T}_{\mathrm{K}}$, nor below the N\'eel temperature, ${T}_{\mathrm{N}}$.

Effect of thermomagnetic treatment on the magnetic state of a ferrofluid: a polarised neutron study

In magnetic colloids of sufficiently high particle concentration, frustration of dipolar interactions could arise as found in spin-glass systems. To understand this situation, a ferrofluid consisting of magnetite particles was frozen in either a horizontal or a vertical field and then 3D neutron-polarisation analysis was performed around the hysteresis loop in both field configurations. The same was done in the fluid state. A comparison of the data leads to the conclusion that frustration plays a key role in the self-organisation of nanoparticles in a frozen magnetic colloid.

Spherical neutron polarization analysis on the three-axis spectrometer IN22

The recent availability at the ILL of two three-axis spectrometers, IN20B and IN22, offering high flux of polarized neutrons in the thermal-energy range has opened a new field of investigation of magnetic and lattice excitations. In this short article, we will show recent technical developments concerning the installation and the optimization of the spherical neutron polarimetry (SNP) on the three-axis spectrometer IN22.

Nuclear and magnetic short-range order in some dilute β-Mn alloys

Using full XYZ neutron polarization analysis, we have studied the nuclear and magnetic diffuse scattering of various β-Mn alloys. This has enabled the study of not only the magnetic short-range spin-correlations in isolation, but also the crucial role of the interplay between the magnetic and nuclear disorder in the lattice as the impurity atoms are introduced. Using reverse Monte-Carlo methods, we deduce the nuclear short-range order parameters and magnetic correlations of these alloys over a large number of near neighbour shells.

Inelastic neutron polarization analysis in low-dimensional systems

We will review recent results obtained by longitudinal polarization analysis and spherical neutron polarimetry (SNP) on typical low-dimensional strongly correlated electronic systems (spin-Peierls CuGeO 3, high- T c La 0.85Sr 0.15CuO 4 and YBa 2Cu 3O 6+ x , or spin-ladder Sr 14Cu 24O 41), which all demonstrate the importance of polarized neutrons for studies of magnetic and structural inelastic contributions. In such systems, the hybridization between the structural and magnetic degrees of freedom is expected to give rise to non-negligible hybrid correlation functions, which can be detected and accurately investigated by neutron polarimetry. The interest of the SNP in inelastic scattering will be discussed through measurements on CuGeO 3.

Spin-resolved neutron spectroscopy from the heavy Fermion compound CeCu 6

Neutron time-of-flight spectroscopy with neutron polarisation analysis permits the in situ separation of magnetic and lattice vibrational energy spectra. Preliminary experiments on the heavy Fermion compound, CeCu 6, in which the Ce magnetic moment is suppressed by the Kondo effect, allow an indicative separation of a broadened crystal field transition and features due to lattice vibrations. An inelastic spin-flip feature at −12 meV is due to the crystal field while an inelastic non-spin-flip feature at −6 meV is predominantly due to phonon scattering.

Polarized neutron reflectivity studies of magnetic semiconductor superlattices

Polarized neutron reflectivity studies of EuS/PbS, EuS/YbSe and GaMnAs/GaAs superlattices performed at the NIST Center for Neutron Research are presented. Pronounced antiferromagnetic (AFM) interlayer coupling has been found in EuS/PbS superlattices for a very broad range of PbS spacer thicknesses. Similar, but weaker, AFM coupling is also present in EuS/YbSe, although only for relatively thin YbSe layers. Neutron polarization analysis shows distinct in-plane asymmetry of the magnetization directions of EuS layers in both systems under investigation. For GaMnAs/GaAs superlattices, ferromagnetic (FM) interlayer correlations have been observed. Polarized neutron reflectometry investigations of several GaMnAs/GaAs superlattices have revealed that the manganese magnetic moments in individual GaMnAs layers, in spite of low Mn concentration, form a truly long range, that is in certain cases a single domain, ferromagnetic state.

Neutron polarization analysis study of crystal field excitations in CePtSn

We present the study of the Kondo antiferromagnet CePtSn, whose magnetic properties at T> T N can be explained by a strong anisotropy of the crystal-field (CF) potential on the Ce sites, which have an orthorhombic point-group symmetry. Applying a microscopic model with parameters deduced from magnetization and inelastic neutron scattering (INS) data, we predict a strong magnetic dipole moment parallel to the a-axis in the ground state. The dramatically increased count rate at the upgraded ILL three-axis spectrometer IN20 permits now to use the polarized beam even with a small single crystal sample (0.2 cm 3) to separate the CF signal from phonon background and to employ the XYZ polarization analysis to confirm directly this previously deduced anisotropy of the local CF Hamiltonian.

Spherical polarimetry on the three-axis spectrometer IN22

The existence at the ILL of three-axis spectrometers (TAS) offering high flux of polarized neutrons in the thermal range (“CRG” IN22 and IN20B) has open a new field of investigation of magnetic and lattice excitation spectra. In this paper, we will show recent technical developments concerning the installation and the optimization of the spherical neutron polarization analysis on TAS IN22.

Polarization analysis of off-specular neutron scattering from domains and rough interfaces in a FeCoV/TiZr multilayer

Specular neutron reflectivity is commonly used as a powerful tool in determining the scattering length density (SLD) profile perpendicular to the surface of thin films. Information about the lateral fluctuations in the SLD, such as the presence of magnetic domains and rough interfaces between layers, can be obtained by studying the pattern of off-specular scattering. Advancements in neutron-source brilliance, beam-line optics and detector efficiency, as well as advances in the theoretical description of off-specular scattering, have contributed to the increasing importance of off-specular neutron-reflectometry studies. We present specular and off-specular polarized neutron reflectometry experiments on FeCoV/TiZr multilayers, with a detailed description of the orientations of the magnetic moments inside the FeCoV layers and at the FeCoV/TiZr interfaces.

A new thermal neutron spectrometer/diffractometer for polarization analysis (SV30) at the research reactor FRJ-2

A new thermal neutron spectrometer for polarization analysis is under construction at the Research Center Julich. The spectrometer was conceived to be a multiple purpose instrument, which can be used both as a triple-axis spectrometer and a diffractometer for applications of polarized neutron scattering in triple-axis spectroscopy, crystallographic investigations and measurements of magnetic diffuse scattering. Therefore, the spectrometer is equipped with two exchangeable detector modules that carry an analyzer unit with a single detector and an image plate neutron detector. Two double focusing monochromators (Cu(200) and PG(002)) allow for a high flux of neutrons with an energy up to 125 meV at the sample position. The use of 3He filters as a polarizer and an analyzer together with a large-area image plate neutron detector makes possible polarization analysis of high-energy neutrons over a large solid angle of 60°× 60°.

Polarised neutron scattering study of magnetic correlations and spin dynamics in La 0.875 Sr 0.125 MnO 3+?

The rhombohedral La0.875Sr0.125MnO3+δ compound exhibits colossal magnetoresistance in the vicinity of the ferromagnetic transition, Tc (∼250 K). From neutron polarisation analysis measurements, the scattering peak in the forward direction above Tc (Tc>T>1.6Tc), showed a Lorentzian-type q dependence, with magnetic correlation lengths (Ornstein–Zernike form) in the range 11–14 A. This forward scattering peak has both neutron spin-flip and non-spin-flip components and our measurements demonstrate that we have simultaneously observed the lattice and magnetic components of polarons above Tc. From the energy scale of the observed diffusive fluctuation in this compound, the polarons were determined to have a mobility of ∼2.5×10-11 cm2 s-1at room temperature.

A neutron polarisation analysis study of the spin-glass phase of Y(Al 1-x Fe x ) 2

Diffuse neutron scattering with full polarisation analysis has been used to characterise the spin correlations in the extended spin-glass phase of the C15 Laves-phase compound, Y(Al1-xFex)2, at concentrations of x=0.25 and x=0.65. Whilst the nuclear diffuse scattering indicates weak anticlustering of Fe atoms on the Al sublattice, the magnetic scattering can be interpreted as arising from small ferromagnetic clusters of less than 10 A in diameter, with these continuing to exist at much higher temperatures than the spin-glass temperature. The ferromagnetic correlations which give rise to the clusters are short-range, and some evidence is found for weak antiferromagnetic correlations.

Optimising neutron polarisers—measuring a single cross-section

This article is part of a series of works exploring the optimisation of neutron polarisation analysis measurements. It deals with measurements of individual spin flip and non-spin flip neutron scattering cross-sections. An instrumental quality factor is presented. The optimum effective thickness for gaseous spin polarised 3He transmission filters is derived and presented. Cu 2MnAl Heusler alloy polarising monochromators and supermirror devices are considered using the quality factor. Absolute comparisons are made between these different types of polarisers. The effect of instrumental background is calculated for a wide range of experimental situations. Even very small backgrounds can have a very large effect on the quality of measurements achievable indicating that great attention must be paid to background reduction on polarisation analysis instruments.

Neutron-scattering studies of the geometrically frustrated spinelLiMn2O4

The spinel structure oxide ${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4},$ a geometrically frustrated antiferromagnet, was studied by neutron-scattering methods. Diffraction studies with unpolarized neutrons, $\ensuremath{\lambda}=2.52\AA{}$ and $E=12.9\mathrm{meV},$ are consistent with ${T}_{c}=65.5(5)\mathrm{K}.$ In agreement with previous results both Bragg peaks, indicative of long-range order, and a broad Lorentzian feature, centered at $Q=1.36{\AA{}}^{\ensuremath{-}1},$ indicative of short range order, coexist down to 2 K. The correlation length \ensuremath{\xi} associated with the short-range order is 3(1) \AA{}, the order of the nearest-neighbor (nn) Mn-Mn distances. Quantitative estimates of the Bragg and Lorentzian contributions from 80 to 2 K indicate the growth of the Bragg at the expense of the Lorentzian component, that their integrated intensities become essentially equal below 20 K and that the total magnetic scattering is conserved within the E-transfer Q range studied. Further studies with neutron polarization analysis, using neutrons with $\ensuremath{\lambda}=4.8\AA{}$ and 3.55 meV, quantitatively measure the total differential magnetic cross section, $d{\ensuremath{\sigma}}_{\mathrm{mag}}/d\ensuremath{\Omega}$ to be 1.05 b ${\mathrm{sr}}^{\mathrm{\ensuremath{-}}1}$ ${\mathrm{f}.\mathrm{u}.}^{\mathrm{\ensuremath{-}}1}$ at 1.5 K, which is close to the value expected for a static spin model. $d{\ensuremath{\sigma}}_{\mathrm{mag}}/d\ensuremath{\Omega}$ increases by about 20% between 120 and 1.5 K below ${T}_{c},$ suggesting significant spin dynamics for $Tg{T}_{c},$ which lie outside of the energy window of the neutron polarization analysis experiment. In addition, above ${T}_{c}$ the Lorentzian peak broadens slightly and the center shifts from 1.36 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$ at 80 K to 1.29 ${\mathrm{\AA{}}}^{\mathrm{\ensuremath{-}}1}$ at 120 K. Inelastic magnetic scattering was measured using neutron polarization analysis over the energy range +2.0 to -4.9 meV at 1.5 and 80 K. At 1.5 K the scattering is essentially elastic with at most \ensuremath{\sim}23% of the spins fluctuating while at 80 K it is largely inelastic with at least 75% of the spins fluctuating. These results are consistent with the existence of a correlated paramagnetic state above ${T}_{c},$ which evolves with decreasing temperature into a ground state below 20 K in which about half the spins are long-range ordered and the other half are in a spin-glass-like configuration. This is in turn consistent with the presence of partial charge ordering between ${\mathrm{Mn}}^{3+}$ and ${\mathrm{Mn}}^{4+}$ in the low-temperature structure of ${\mathrm{LiMn}}_{2}{\mathrm{O}}_{4}$ and the inherent geometrical frustration of the Mn sublattice.

Investigation of in-chain spin correlations in YBa 2Cu 3O 6+ x by neutron polarization analysis

Spin correlation in Cu–O chains of YBCO are studied. The results were obtained at T=110 K by neutron polarization analysis with 3He filter as analzyer. Experimental data indicate a magnetic peak at k=0.75, which corresponds to the holes concentration in the chain c=0.5. There is another anomaly at k=0.5 ( c=0), which may be connected with the sample inhomogenity and some admixtures of copper oxide. The cross-section in the peak at k=0.75 is 5 mb/sr.

Polarized neutron reflectometry: recent developments and perspectives

Polarized neutron reflectometry (PNR) has an important role in solving non-collinear magnetic structures in thin films. In principle PNR provides the unambiguous determination of multi-axial magnetic depth profiles in systems of scientific and technological importance. However, only a limited number of problems have been solved unimpeachably, because the magnetic depth profiles were obtained by fitting procedures yielding solutions whose main features were not evident in the raw data. A recent formulation of reflectometry, and the concurrent development of “total polarimetry” are now stimulating the design of neutron polarization and polarization analysis geometries capable of determining directly if and how a magnetic system contains a non-collinear structure in the film ( x– y) plane. In the simplest of these geometries the neutrons are polarized and analyzed along the z-axis perpendicular to the film plane.

The study of a critical state in YBa 2Cu 3O 7− δ superconductors with a large demagnetization factor by 3-D neutron polarization analysis

The trapped magnetic flux distribution was studied in thin flat superconducting samples of 1-2-3 compound having both a large demagnetization factor and a high edge barrier for vortex entry and exit. A three-dimensional analysis of neutron polarization was used to obtain the distribution patterns. It is shown that the nature of the critical state in polycrystalline wafers, unlike that in a ceramic wafer, is not described by the Bean model.

SANS polarization analysis with nuclear spin-polarized 3He

A neutron spin filter based on transmission through nuclear-spin-polarized 3He gas has been applied to polarization analysis of small angle neutron scattering (SANS). Such spin filters, which are based on the large spin dependence of the absorption of neutrons by 3He, make SANS polarization analysis possible because of their large angular acceptance. In the present experiment, a 3He-based analyzer was employed to separate nuclear scattering into its coherent and spin-incoherent components. Polarized 3He analyzers were prepared by two different optical pumping methods and installed on the NG3 SANS instrument at the NIST Center for Neutron Research (NCNR). Measurements were taken on cellophane tape and silica gel, for which the scattering is almost completely incoherent and coherent, respectively, and on a combined sample. For the combined sample, separation of the coherent part from the incoherent part was successfully demonstrated using polarization analysis.

Magnetic diffuse scattering in disordered systems studied by neutron polarization analysis (invited)

Neutron polarization analysis studies of both nuclear and magnetic correlations in disordered alloys and compounds provide a unique insight into the character and extent of magnetic disorder in these systems and its dependence on local atomic environment. We present here a review of polarization analysis studies of magnetic and structural defects in a variety of materials, performed using the diffuse scattering spectrometer D7 at the Institut Laue–Langevin. The diverse range of magnetic materials studied using D7 include spin-glasses, frustrated itinerant electron systems, pyrochlores, and quasicrystals.