Fe-W Supermirrors for Polarizing Neutrons
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Fe-W SUPERNIRRORS FOR POLARIZING NEUTRONS
C. F. MAJKRZAK , D. A. NEUMANN , J. R.
D. COPLEY
Institute for Materials Science and Engineering Standards (NBS), Gaithersburg, MD 20899.
MD
and R. P. (IMSE),
DINARDO
National Bureau
Department of Physics and Astronomy, University of Maryland, 20742, and IMSE, NBS, Gaithersburg, MD 20899.
Instrumentation 11973.
Division,
Brookhaven
National
of
College Park,
Laboratory,
Upton,
NY
ABSTRACT
Thin film bilayers of ferromagnetic Fe and nonmagnetic W have been deposited by sputtering on flat glass substrates according to a sequence of gradually varying bilayer thicknesses which in effect extends the critical angle for external mirror reflection for neutrons of one spin state when the Fe is magnetized in the plane of the film. The measured reflectivity of this Fe-W multilayer system is compared with that of other supermirror polarizers consisting of different materials and layer sequences.
INTRODUCTION
Polarized neutrons play an important role in scattering studies of condensed matter [1]. Consequently, an efficient means of polarizing a neutron beam is very valuable. In addition to conventional, bulk singlecrystal polarizers such as the Heusler alloy Cu MnAl, polarizing mirrors are often used for this purpose. This is possibie because for most materials the refractive index for neutrons is less than unity so that external reflection occurs from flat surfaces in air or vacuum at glancing angles up to a critical angle of the order of 0.1 degree/A neutron wavelength. Furthermore, because the neutron possesses a magnetic dipole moment, its propagation through ferromagnetic materials is determined by a birefringent refractive index. Thus for a ferromagnetic material which is magnetized parallel to its reflecting surface, the critical angle depends on the neutron spin state so that only one spin state is effectively reflected between two distinct critical angles. For many applications it is advantageous to reflect one spin state over as wide a range of angles as possible with a high efficiency. To this end polarizing supermirrors, first suggested by Mezei [2] and Turchin [3], have been developed in recent years [4,5] which in effect extend the critical angle of reflection for one spin state by constructive interference of the neutron wave in a thin-film, multi-layered structure consisting of a ferromagnetic and nonmagnetic material. Reviews of these devices and their applications are given in references 6-8. Nevertheless, it has not yet been possible to produce polarizing supermirrors with high reflectivity beyond about twice the critical angle of ordinary Ni (0.1 deg/A). The purpose of the present work has been: 1) to make a supermirror with a very high polarizing efficiency; and 2) to attempt to extend the critical angle for one spin state beyond what has been previously achieved.
Mat. Res. Soc. Symp. Proc. Vol. 103. e1918 Materials Research Society
116
SUPERMIRROR AL•ORITHMS
There are a number of algorithms for generating a sequence of layers which have been
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