W. T. Lee

Spin echo scattering angle measurement at a pulsed neutron source

Two experiments were performed to adapt spin echo scattering angle measurement (SESAME) to pulsed neutron sources. SESAME is an interferometric method that provides enhanced resolution of neutron scattering angles without the loss of neutron intensity that results when collimation is used to improve angular resolution. The method uses the neutron equivalent of optical wave plates to produce a phase difference between the two neutron spin components of a polarized neutron beam. Because the wave plate is inclined to the neutron beam, this phase difference depends sensitively on the trajectory of the neutron. In the absence of a sample, a second wave plate, which is parallel to the first, undoes the phase difference introduced by the first wave plate, producing a polarization identical to that of the incident neutron beam. When a scattering sample is placed between the two neutron wave plates, the cancellation of the phase difference between the neutron spin states is not perfect and the resulting neutron-beam polarization is a measure of the distribution of scattering angles. In the first experiment, thin (30 and 60 µm-thick) magnetized Permalloy films were used as neutron wave plates. In a second experiment, current-carrying solenoids with triangular cross sections were used as birefringent prisms for neutrons. The arrangement of these prisms was such that they mimicked the effect of the neutron wave plates in the first experiment. In both experiments, correlation lengths in the scattering sample of about 1000 A were probed using very simple and inexpensive equipment. These experiments brought to light a number of advantages and disadvantages of implementing SESAME at pulsed neutron sources and provided insights into the relative merits of SESAME and traditional small-angle neutron scattering.

Electrical heating for SEOP-based polarized 3He system

Development of neutron spin filters based on polarized 3He is underway at Spallation Neutron Source (SNS). We report the progress of electrical heating tests in polarized 3He based on Spin-Exchange Optical Pumping (SEOP) method. We first test the system performance based on electrical heating via non-inductance heating pads. We observe a contribution of 955 hours to the relaxation time T1 from the heating pads. We then test the electrical heating SEOP pumping system at the SNS beamline Magnetic Reflectometer. We currently obtain 73% 3He polarization in a cell with 820 cm3 in volume.

Measuring lateral magnetic structures in thin films using time-of-flight polarized neutron reflectometry

Polarized neutron reflectometry (PNR) has recently been applied to study lateral magnetic structures such as regular micron-sized magnetic arrays on a surface. To date, however, there is a lack of detailed accounts of the features observed in the scattered intensity map in the special case of time-of-flight (TOF) PNR. We present here preliminary measurement results on lithographically produced arrays of micron-sized rectangular permalloy magnetic bars. The measurements demonstrate the potential of the method to provide detailed structural information on a laterally patterned sample, as well as on its magnetic characteristics. The information can be obtained by analyzing the specular reflection along with three off-specular Bragg sheets. Most of the features seen experimentally can be interpreted by using simple heuristic arguments. In addition, we also present results of a study of lateral magnetic domains in an exchange-biased Co/CoO bilayer film to illustrate the capability of TOF PNR in the study of large lateral magnetic domains in the case when almost no off-specular scattering is detected.

Spin-Echo Resolved Grazing Incidence Scattering (SERGIS) at Pulsed and CW Neutron Sources

We have used SERGIS to probe the surface structure of a silicon diffraction grating of period 140 nm. Experiments were performed at: the Los Alamos Neutron Science Center (LANSCE) pulsed neutron source and the National Institute of Standards and Technology (NIST) continuous wave (CW) reactor neutron source. Although both sets of data show peaks of the spin echo polarization at integer multiples of the grating period, as expected, the results differ in detail. We have developed a dynamical theory, based on a Bloch wave expansion, to describe neutron diffraction from a grating. The theory explains the differences between the two sets of data without any adjustable parameters.

Neutron spin evolution through broadband current sheet spin flippers.

Controlled manipulation of neutron spin is a critical tool for many neutron scattering techniques. We have constructed current-sheet, neutron spin flippers for use in Spin Echo Scattering Angle Measurement (SESAME) that comprise pairs of open-faced solenoids which introduce an abrupt field reversal at a shared boundary. The magnetic fields generated by the coils have been mapped and compared with both an analytical approximation and a numerical boundary integral calculation. The agreement is generally good, allowing the former method to be used for rapid calculations of the Larmor phase acquired by a neutron passing through the flipper. The evolution of the neutron spin through the current sheets inside the flipper is calculated for various geometries of the current-carrying conductors, including different wire shapes, arrangements, and common imperfections. The flipping efficiency is found to be sensitive to gaps between wires and between current sheets. SESAME requires flippers with high fields and flipping planes inclined to the neutron beam. To avoid substantial neutron depolarization, such flippers require an interdigitated arrangement of wires.