High resolution neutron Larmor diffraction using superconducting magnetic Wollaston prisms

Abstract

The neutron Larmor diffraction technique has been implemented using superconducting magnetic Wollaston prisms in both single-arm and double-arm configurations. Successful measurements of the coefficient of thermal expansion of a single-crystal copper sample demonstrates that the method works as expected. The experiment involves a new method of tuning by varying the magnetic field configurations in the device and the tuning results agree well with previous measurements. The difference between single-arm and double-arm configurations has been investigated experimentally. We conclude that this measurement benchmarks the applications of magnetic Wollaston prisms in Larmor diffraction and shows in principle that the setup can be used for inelastic phonon line-width measurements. The achievable resolution for Larmor diffraction is comparable to that using Neutron Resonance Spin Echo (NRSE) coils. The use of superconducting materials in the prisms allows high neutron polarization and transmission efficiency to be achieved.The ability of conventional neutron diffraction to measure precise values of the d-spacings of crystalline materials is limited by factors such as the strength of the available neutron source and the practical length of neutron flight paths. The current limit is around Δd/d of 10 −3 . At reactor neutron sources however, high resolution measurements of Δd/d ~ 10 −6 have been achieved using the Larmor diffraction (LD) technique 1 first introduced by Rekveldt 2 . Like the neutron spin echo (NSE) technique proposed by Mezei 3 for energy encoding, the LD method makes use of Larmor precession of neutron spins in well-defined magnetic fields. The method allows the lattice spacing of the diffracting crystal to be encoded into the Larmor phase of the neutron spin by making this phase depend only on the scattering vector of the diffracting Bragg peak, a quantity that is independent of the monochromaticity and collimation of the neutron beam. This enables small changes of the lattice spacing to be measured through the change of the neutron Larmor phase instead of by measuring the change in the diffraction angle.The original Rekveldt proposal for LD involved magnetic fields before and after the sample. When the field boundaries of these two magnetic fields are aligned parallel to the crystal diffraction plane, all the diffracted neutrons will yield the same Larmor phase regardless their incident angle on the sample. Therefore, the Larmor phase of the diffracted neutrons will only depend on the geometry and intensities of the magnetic fields before and after the sample.The LD method has been used in a number of experiments and its recent applications have been summarized by Rekveldt 4 including absolute lattice spacing determination 5 and temperature induced lattice variations 6 . Up to now, LD has been implemented and routinely operated on the beamlines of TRISP (FRM II, MLZ) 1 , FLEXX (BER-II, HZB) 7 and ZETA (ILL) 8 , with a relative resolution of Δd/d ~10 −6 . Instead of using two static magnetic fields, these...

New capabilities in high-resolution neutron Larmor diffraction at ORNL

“…The details of tuning the effective tilt angle using magnetic Wollaston prisms have been discussed by Li et al (2017) and and the experimental tuning results are very close to the calculations. This provides us with confidence to tune the setup on the basis of the calculation results.…”

“…The sample used for this measurement was CuFeO 2 , which undergoes a first-order structural phase transition induced by the magnetic transition at 11 K and a second-order lowersymmetry lattice distortion at 14 K, seen using high-resolution synchrotron X-ray scattering performed on station 11-ID-C at the Advanced Photon Source, Argonne National Laboratory, Illinois, USA (Ye et al, 2006). Since lattice expansion measurements on copper have already been demonstrated by Li et al (2017), the following part will focus on lattice distortion measurements.…”

“…Recently, we also demonstrated our approach to implementing Larmor diffraction using superconducting magnetic Wollaston prisms (Li et al, 2017Li, Parnell, Wang et al, 2016), as shown in Fig. 1(b), and its principle has been explained and demonstrated Li et al, 2017).…”

“…Recently, we also demonstrated our approach to implementing Larmor diffraction using superconducting magnetic Wollaston prisms (Li et al, 2017Li, Parnell, Wang et al, 2016), as shown in Fig. 1(b), and its principle has been explained and demonstrated Li et al, 2017). Basically, by using a series of magnetic field boundaries with or without inclinations, the method provides us with the possibility of tuning the effective field boundaries by picking different combinations of the fields.…”

Neutron Larmor diffraction on powder samples

“…These instruments take advantage of the resonant neutron spin-echo technique (NRSE) to define the parallelogram-shaped field regions by radio frequency (RF) spin-flip coils (Golub & Gä hler, 1987;Gä hler & Golub, 1988). A recent development uses Wollaston prisms, triangular-shaped DC magnetic fields bounded by superconducting sheets (Li et al, 2017), as PDs. The advantages of the latter technique are the compact geometry and the increased flexibility achieved by tuning the inclination angles of the magnetic fields by electrical currents, instead of mechanical rotation as in the case of NRSE.…”