Spin-polarized Neutron Research Articles

Spin-ordered phase transitions in isospin asymmetric nuclear matter

Spin polarized states in nuclear matter with Skyrme effective forces are studied on the base of a Fermi liquid theory for a wide range of isospin asymmetries and densities. There are a few possible scenarios of spin ordered phase transitions: (a) nuclear matter with SLy4 interaction undergoes at some critical density a phase transition to a spin polarized state with the oppositely directed spins of neutrons and protons; (b) for SkI5 interaction, a spin polarized state with the like-directed neutron and proton spins is formed; (c) nuclear matter with SkI3 interaction under increasing density, at first, undergoes a phase transition to the state with the opposite directions of neutron and proton spins, which goes over at larger density to the state with the same direction of nucleon spins. Spin polarized states at strong isospin asymmetry are accompanied by the long tails in the density profiles of the neutron spin polarization parameter near the critical density, if the energy gain of the transition from the nonpolarized state to a polarized one is the decreasing function of isospin asymmetry (SLy4 force). If the energy gain is increased with isospin asymmetry, there are no long tails in the density distribution of the neutron spin polarization parameter (SkI3, SkI5 forces).

Neutron scattering study of magnetic structure and metamagnetic transition between low- and high-moment states ofNpNiGa5

Systematic neutron scattering experiments have been carried out in order to reveal the magnetic properties of $\mathrm{Np}\mathrm{Ni}{\mathrm{Ga}}_{5}$. We have observed a ferromagnetic transition with the magnetic moment parallel to the tetragonal $c$ axis at ${T}_{\mathrm{C}}=30\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. We have also observed antiferromagnetic reflections with the propagation vector $\mathbit{q}=(1∕2\phantom{\rule{0.3em}{0ex}}1∕2\phantom{\rule{0.3em}{0ex}}1∕2)$, coexisting with the ferromagnetic component below ${T}_{\mathrm{N}}=18\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. The direction of the antiferromagnetic component is perpendicular to the $c$ axis, most likely ${\ensuremath{\mu}}^{\mathrm{AF}}\ensuremath{\Vert}⟨1\phantom{\rule{0.3em}{0ex}}1\phantom{\rule{0.3em}{0ex}}0⟩$. Spin-polarized neutron scattering experiments have revealed that only Np carries a magnetic moment. The polarization of Ni atom is less than $0.03\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}∕\mathrm{Ni}$, judging from our experimental accuracy. Thus, we conclude that the magnetic moments of Np atoms exhibit the simple ferromagnetic ordering, ${T}_{\mathrm{N}}<T<{T}_{\mathrm{C}}$, while a canted antiferromagnetic structure appears where $T<{T}_{\mathrm{N}}$. A cant angle $\ensuremath{\theta}$ of $\ensuremath{\sim}90\ifmmode^\circ\else\textdegree\fi{}$ between two adjacent Np moments and a magnetic moment $\ensuremath{\mu}$ of $\ensuremath{\sim}0.8\phantom{\rule{0.3em}{0ex}}{\ensuremath{\mu}}_{\mathrm{B}}∕\mathrm{Np}$ at $3\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ have been estimated. The temperature and the magnetic field dependences of these two components have also been quantitatively analyzed. (i) It is quite unusual that the Np total moment increases with the transition from the ferromagnetic to canted antiferromagnetic structure. (ii) The metamagnetic transition cannot be explained in terms of a simple spin-flip mechanism with competing interactions and anisotropy. (iii) Furthermore, an unusual reduction of the Np magnetic moment is accompanied with the metamagnetic transition. From these results, we concluded that a remarkable change in the $5f$ electronic state between the ferromagnetic low-moment state and the canted antiferromagnetic high-moment state takes place at ${T}_{\mathrm{N}}$ and metamagnetic transition.

The neutrino response of low-density neutron matter from the virial expansion

Abstract We generalize our virial approach to study spin-polarized neutron matter and the consistent neutrino response at low densities. In the long-wavelength limit, the virial expansion makes model-independent predictions for the density and spin response, based only on nucleon–nucleon scattering data. Our results for the neutrino response provide constraints for random-phase approximation or other model calculations, and we compare the virial vector and axial response to response functions used in supernova simulations. The virial expansion is suitable to describe matter near the supernova neutrinosphere, and this work extends the virial equation of state to predict neutrino interactions in neutron matter.

Investigation of CoFe/TiZr multilayers by polarized neutron reflectometry

CoFe/TiZr is one of the materials for producing polarizers and analyzers with excellent properties of neutron spin polarization. It is necessary to know the relations among the interface structures, reflectivity and producing technology. Specular reflection of polarized neutrons was employed to study the influence of annealing temperatures on reflectivities, polarizing efficiencies and depth profiles of CoFe/TiZr multilayer. The experimental results indicate that the best annealing temperature is about 250 °C.

Platypus: a time-of-flight neutron reflectometer at Australia's new research reactor

A time-of-flight neutron reflectometer is to be built at the new 20 MW research reactor (OPAL) in Sydney. The instrument will be positioned at the end of a curved supermirror neutron guide that will provide optimal transmission of cold neutrons (2–20 A) while removing fast neutrons and high-energy γ-rays. The reflectometer will utilise a white neutron beam that will be pulsed using a series of boron coated disc choppers. Typically three angle settings will be required to collect a complete reflectivity profile (from Q z = 0.005–0.5 A− 1). The new instrument will operate with a vertical scattering plane, making it suitable for both solid and “free-liquid” surfaces. The instrument will also be capable of producing a spin-polarized neutron beam making provision for polarized neutron reflectometry studies. Detection of the reflected neutrons will take place using a 2-dimensional 3He delay-line detector with high-speed data acquisition electronics.

NEUTRON SPIN POLARIZATION IN STRONG MAGNETIC FIELDS

The effects of strong magnetic fields on the inner crust of neutron stars are investigated after taking into account the anomalous magnetic moments of nucleons. The energy spectra and wave functions for protons and neutrons in a uniform magnetic field are provided. The particle spin polarizations and the yields of protons and neutrons are calculated in a free Fermi gas model. Obvious spin polarizations occur when B≥1014 G for protons and B≥1017 G for neutrons, respectively. It is shown that the neutron spin polarization depends solely on the magnetic field strength.

Exact determination of phase information in spin-polarized neutron specular reflectometry

A method is proposed to determine the complex reflection coefficient of any real scattering length density (i.e., in the case where there is no effective absorption, which is a good approximation for neutrons) in neutron specular reflectometry. The method makes use of a magnetic reference layer mediated between the substrate and an unknown sample and measurements of the polarization direction of the reflected beam or a suitable choice of polarization and reflectivity analysis of the reflected beam. It corresponds to the concept proposed recently, but has been derived in the formalism of the transfer matrix. The method is based upon relations between the polarization of the incident and reflected beams, and the transfer matrix elements of the unknown and known layers. Thus, in this manner, only by final polarization orientation measurement can we find the reflection coefficient of any unknown sample which is surrounded on both sides by a uniform medium. Apart from the final polarization orientation measurement of the reflected beam, which is hampered by complications in selecting the physical solution, we show that the reflection coefficient can be determined by more flexible ways using a suitable choice of possible measurements of reflectivity and polarization of the reflected beam. A schematic example is presented to illustrate the method.

Absence of spin-flip transition at the Cr(001) surface: A combined spin-polarized scanning tunneling microscopy and neutron scattering study

The spin-density wave (SDW) on Cr(001) has been investigated at temperatures between $20--300\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ by means of spin-polarized scanning tunneling microscopy (SP-STM). Although neutron-scattering data measured on the same crystal clearly show a spin-flip transition from a transversal (T)-SDW to a longitudinal (L)-SDW at the expected spin-flip (SF) temperature ${T}_{\mathrm{SF}}=123\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, no change was found on the Cr(001) surface with SP-STM. Throughout the entire temperature range the Cr(001) surface maintains a topological antiferromagnetic order with an in-plane magnetization that inverts between adjacent atomically flat terraces separated by monatomic step edges. The experimental results are interpreted by an absence of a spin-flip transition in the near-surface region probably driven by the surface anisotropy. The continuous connection of the surface T-SDW to the bulk L-SDW is accomplished by the formation of a 90\ifmmode^\circ\else\textdegree\fi{} domain wall just below the surface.

Phase determination in spin-polarized neutron specular reflectometry by using a magnetic substrate

The scattering length density profile for a thin film structure can be determined uniquely if both the modulus and phase of the reflection coefficient is known. Here, we describe a method for recovering the phase information which utilize a magnetic substrate and based on polarization analysis of the reflected beam. The method is derived in the formalism of transfer matrix so it is applicable for any unknown real scattering length density of nonmagnetic films (i.e. in the case where there is no effective absorption).

Retrieval of the reflection coefficient in spin-polarized neutron specular reflectometry

A method is proposed which allows a complete determination of the complex reflectioncoefficient for any free unknown real potential (i.e., in the case where there is no effectiveabsorption). It exploits the interference of the spin components of a polarizedneutron beam in the presence of a magnetic reference layer mediated betweensubstrate and sample. Following the recent investigations, the method requirespolarization analysis of the reflected beam instead of reflectivity analysis. However, ourmethod is based on the transfer matrix formalism. In this method we use therelations between the polarization and the reflectivity of the reflected beam andthe transfer matrix elements for the known and unknown samples. Using theserelations we show that instead of fully polarization measurement, some suitablechoice of polarization and reflectivity measurements can be used to determine thereflection coefficient. The method is supplemented with a schematic example.

Dynamically polarized hydrogen target as a broadband, wavelength-independent thermal neutron spin polarizer

A hydrogen-rich sample with dynamically polarized hydrogen nuclei was tested as a wavelength-independent neutron transmission spin polarizer. The experiment used a modified setup of the dynamic nuclear polarization target station at the GKSS research center. The standard solvent sample at the GKSS DNP station was used. It is 2.8 mm thick and consists of 43.4 wt% water, 54.6 wt% glycerol, and 2 wt% of EHBA-Cr(v) complex. The wavelength of the incident neutrons for the transmission experiment was λ = 8.1 Å with Δ λ / λ = 10 % . The polarization of neutron beam after the target sample was analyzed with a supermirror analyzer. A neutron polarization of −52% was achieved at the hydrogen polarization of −69%. Further experiments will test the feasibility of other hydrogen-rich materials, such as methane, as the polarizer. A theoretical calculation shows that a polarized methane target would allow over 95% neutron polarizations with more than 30% transmission.

Noncollinear magnetism in liquid oxygen: A first-principles molecular dynamics study

We perform first-principles molecular dynamics of liquid oxygen in which the magnetic structure evolves according to a generalized density-functional scheme allowing for noncollinear spin configurations. We investigate both structural correlations between the orientations of the molecular axes and magnetic correlations between the orientations of the molecular magnetic moments, demonstrating a clear relation between the local molecular configuration and the relative magnetic arrangement. The nuclear structure factor obtained from the simulation is found to agree well with the experimental one. The calculated magnetic structure factor shows antiferromagnetic correlations between molecules in the first shell, in accord with spin-polarized neutron scattering measurements. We observe the formation of dynamically coupled molecules, known as ${\mathrm{O}}_{4}$ units, in which the molecular moments are aligned in an antiferromagnetic fashion. An analysis based on the life time of such units, revealed that in most cases the ${\mathrm{O}}_{4}$ units occur as transient configurations during collisions. However, we also observed a small fraction of ${\mathrm{O}}_{4}$ units surviving for relatively long periods. To account for electronic excitations which are missed in our density-functional scheme, we complement our description with a mean field model for the thermal fluctuations of the magnetic structure. The combined scheme is found to improve the description of the magnetic neutron structure factor and allows us to study the dependence of the magnetic susceptibility on temperature.

Solid state and conventional neutron optical elements

Neutron optical elements are needed to prepare neutron beams with the phase space volume required for an experiment. In the last years solid state neutron optical devices have been developed in Berlin, especially polarizers and collimators where the neutrons move in channels formed by thin silicon wafers. Such devices are smaller and lighter than their conventional counterparts, have well-defined channels and their lengths are below 10 cm. Solid state straight and radial benders for the spin polarization or analysis of neutrons and straight and radial collimators were built and tested at HMI. Their performance is compared to conventional benders and cavities. Finally, some general aspects of solid state elements are summarized.

Solid state and conventional neutron optical elements

Neutron optical elements are needed to prepare neutron beams with the phase space volume required for an experiment. In the last years solid state neutron optical devices have been developed in Berlin, especially polarizers and collimators where the neutrons move in channels formed by thin silicon wafers. Such devices are smaller and lighter than their conventional counterparts, have well-defined channels and their lengths are below 10 cm. Solid state straight and radial benders for the spin polarization or analysis of neutrons and straight and radial collimators were built and tested at HMI. Their performance is compared to conventional benders and cavities. Finally, some general aspects of solid state elements are summarized.

Neutron reflectometry studies of exchange springs in DyFe2/YFe2 superlattices

Spin-polarised neutron reflectometry has been used to investigate a magnetic exchange spring, formed in a (75 Å DyFe2/150 Å YFe2)×18 multilayer thin film. The existence of an exchange spring is found to enhance the level of neutron spin flipping during reflection, a strong indicator of the formation of transverse magnetic moments within the YFe2 layers.

Spin polarized states in strongly asymmetric nuclear matter

In the framework of a Fermi liquid theory it is considered the possibility of appearance of spin polarized states in strongly asymmetric nuclear matter with Skyrme effective interaction. The zero temperature dependence of neutron and proton spin polarization parameters as functions of density is found for SLy4, SLy5 effective forces. It is shown that at some critical density it will be formed the state with the oppositely directed spins of neutrons and protons, while the state with the same direction of spins does not appear. In comparison with neutron matter, even small admixture of protons strongly decreases the threshold density of spin instability. It is clarified that protons become totally polarized within very narrow density domain while in the density profile of neutron spin polarization parameter their appear long tails near the transition density.

Neutron reflectometry studies of exchange springs in DyFe2/YFe2 superlattices

Spin-polarised neutron reflectometry has been used to investigate a magnetic exchange spring, formed in a (75 A DyFe2/150 A YFe2)×18 multilayer thin film. The existence of an exchange spring is found to enhance the level of neutron spin flipping during reflection, a strong indicator of the formation of transverse magnetic moments within the YFe2 layers.

Distribution of vortices in Nb/Al multilayers studied by spin-polarized neutron reflectivity and magnetization

We present SPNR and DC magnetization studies of non-uniformly distributed vortices in Nb/Al multilayers for fields applied near-parallel to the film surface. Peaks are observed in the M– H curves that are shown to correspond to vortex row-transitions and the field values of the transitions agree well with free energy calculations. An additional peak is observed at an applied field smaller than the first row-transition field and this is shown to arise from the lower critical field parallel to the surface. Demagnetization effects are discussed. SPNR measurements performed at low field give the London penetration length and measurements in the mixed state are consistent with a single row of vortices residing in the film center, but with positional fluctuations amounting to 1/4 of the film thickness. It is also shown that cycling the applied field leads to a surface-induced reorientation of the vortex magnetic field, which points perpendicular to the surface in zero field.

Spin-resolved off-specular neutron scattering from magnetic domain walls using the polarized [formula omitted] gas spin filter

We report on the use of the polarized 3 He gas filter and neutron resonant enhancement techniques for the measurement of spin-polarized diffuse neutron scattering due to ferromagnetic domains. A CoO/Co exchange biased bilayer was grown on a Ti/Cu/Al 2O 3 neutron resonator template. The system is cooled in an applied magnetic field of H a =2000 Oe through the Néel temperature of the antiferromagnet to 10 K where the applied magnetic field is swept as to measure the magnetic hysteresis loop. After the second magnetization reversal at the coercive field H c2 =+230 Oe , the system is supposed to approach the original magnetic configuration. In order to prove that this is not the case for our exchange biased bilayer, we have measured four off-specular maps I++, I+−, I−+, I− − at H a ≈+370 Oe , where the Co magnetic spins were mostly reversed. They show a striking behavior in the total reflection region: while the nonspin-flip scattering exhibits no diffuse reflectivity, the spin-flip scattering shows strong diffuse scattering at incident angles which satisfy the resonance conditions. Moreover the spin-flip off-specular part of the reflectivity is asymmetric. The I−+ intensity occurs at higher exit angles than the specularly reflected neutrons, and the I+− intensity is shifted to lower angles. Their intensities are noticeably different and there is a splitting of the resonance positions for the up and down neutron spins ( α n +≠ α n −). Additionally, a strong influence of the stray fields from magnetic domains to the resonance splitting is observed.

Development of low temperature solid state detectors for ultra-cold neutrons within superfluid 4He

Abstract As part of an R&D programme for the development of a next-generation experiment to measure the neutron electric dipole moment, in which ultra-cold neutrons (UCN) are produced and stored in superfluid 4 He (superthermal source), we have developed cryogenic detectors of UCN that can operate in situ within the superfluid. Surface barrier detectors and PIN diode detectors have been tested and proven to work well at temperatures as low as 80 mK. When combined with a layer of 6 LiF which converts neutrons to charged particles, these detectors form a reliable UCN detection system which has been tested in liquid helium down to 430 mK. The detectors have operated within superfluid helium for periods of up to 30 days with no signs of degradation. The development of this detection system has enabled us to measure the flux of UCN from a superthermal UCN source with no intervening transmission windows which can attenuate the flux. The addition of thin films of magnetically aligned iron also enables these detectors to be used in situ for neutron spin-polarisation analysis.