Larmor precession applications: magnetised foils as spin flippers in spin-echo SANS with varying wavelength

https://doi.org/10.1016/S0921-4526(03)00230-8Get rights and content

Abstract

A new development in SESANS instrumentation will be discussed. Magnetised foils as π-flippers for neutron polarisation were discussed in previous papers as powerful tools for constructing a strong gradient in Larmor precession in some direction. Up to now, such foils could only be used with limited flipping power and only for monochromatic neutrons. In this paper, it will be shown that it is possible to use foils also as nearly perfect flippers and moreover that such foils can be used also for varying neutron wavelength.

Introduction

The various applications of Larmor precession in spin-echo techniques with inclined front and end faces have been discussed extensively in previous papers [1], [2], [3], [4], [5], [6], [7], [8], [9] and references therein. The inclination is meant to create a gradient in Larmor precession angle in some direction with which the angle of transmission through the device can be encoded [2], [3], [4], [5], [6], [7], [8], [9]. In general, for optimal encoding it is important that a strong precession change occurs at the inclined boundary. That can happen by a strong magnetic field change or by a π-flipper that changes effectively the sense of rotation. Strong field gradients lead to inhomogeneous field line integrals over the beam cross-section. The use of π-flippers avoids most of this problem and is therefore advantageous [7]. As a π-flipper with strong inclination, a foil magnetised in plane can be used. The thickness of the foil has been chosen such that the precession angle of a polarised beam during transmission is just π and the inclination so strong that the external magnetic field is nearly perpendicular to the foil plane as illustrated in Fig. 1. Eq. (1) demonstrates that ideal flipping occurs only when the field is perpendicular to the foil. Here a foil is inclined by π/2−θ0 to a magnetic field in the z-direction and magnetised in the foil plane. The polarisation change by the foil in the direction parallel to the field Bz and initial polarisation P0 is described byP/P0=1−(1−cosϕ)(1−n2)withϕ=cλBst/sinθ0.

Here c=4.6368×1014 (T−1 m−2), n=sinθ0+Bz/Bs where Bs is the spontaneous magnetic field in the foil (μ0Ms), λ the neutron wavelength, t the thickness of the foil and θ0 the inclination angle of the foils with the x-axis. The quantity n, here the normalised z-component of the local B in the foil, describes the deviation from perpendicular orientation of the applied field with respect to magnetic induction in the foil plane. For best flipping properties, the angle ϕ will be adjusted to π and n will be minimised.

The application of this kind of flippers in a new SESANS set-up where we accepted the incomplete spin flip due to a small value of n in Eq. (1), was discussed extensively in our recent paper [7]. Recently, we were able to carry out the first SESANS experiments using this set-up [10]. Fig. 2 shows the measured correlation function of clusters of polystyrene spheres up to a spin-echo length of z=2.5 μm (z=2cλ2BzLcotθ0/2π). Here we used λ=0.2 nm wavelength neutrons, a magnetic induction Bz on the foils of maximum 30 mT and a distance L of 1 m between the foils. The very high spin-echo length achieved is mainly due to the small inclination angle of θ0∼5° of the foils with the neutron beam. Now we will discuss an essential improvement of these foil flippers, which makes them ideal flippers in the first place but makes them also applicable to variable neutron wavelengths as is required in pulsed neutron beams. Although permanently magnetised foils were used previously as spin rotators [12], they could not be used in high fields up to 0.05 T as is described in this paper, because of too low anisotropy and too low-saturation magnetisation.

Section snippets

Foils as ideal flippers for SESANS

The non-ideal flipping properties of the foils are manifested in Eq. (1) in the term containing n. This term originates from the non-perpendicular orientation of the magnetic field to the film plane, where the magnetic field direction is also the normal to the polarisation precession plane. If we introduce a field Bx by means of a coil in the x-direction in the region where the foil is located, such that the total field Bz+Bx is nearly normal to the foil plane, n will nearly vanish. Hence we

Wavelength-dependent flipping in a foil

Eq. (1) suggests that there are only two possibilities to vary the precession angle in a foil: changing the wavelength or changing the effective transmission length of the beam through the foil. The latter can be achieved by changing the angle of transmission through the foil.

Here we will discuss a less obvious way, namely by splitting the foil in two regions with opposite magnetisation, that give opposite contributions to the field line integral in the foil. By varying the lengths of those

Dynamic investigation of rotation angle ϕ

A small applied field in the x-direction of the foil has been created by a function generator delivering a periodic current of triangular shape in time, with a period of 50 ms. The neutrons are counted in time channels in phase with this current. Fig. 7 shows the precession angle as a function of time. Indeed, it has the same time dependence as the field as also expected from Eq. (3). Like in the static experiments, the absolute value of the polarisation vector shows no significant

Acknowledgements

The research described here is part of the research program of the Stichting voor fundamenteel Onderzoek der Materie (FOM), which is financially supported by the Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NOW). The authors thank R. Pynn and M. Fitzsimmons of Los Alamos center for putting at our disposal the permalloy film with which the domain wall experiments have been carried out.

References (12)

  • M.Th. Rekveldt

    Nucl. Instrum. Methods B

    (1996)
  • M. van Oossanen et al.

    Physica B

    (2000)
  • V.T. Lebedev et al.

    Physica B

    (1995)
  • F. Mezei

    Neutron spin echo

  • R. Pynn

    J. Phys. E

    (1978)
  • W.G. Bouwman et al.

    J. Appl. Cryst.

    (2000)
There are more references available in the full text version of this article.

Cited by (0)

View full text