Spin-Polarized Neutron Reflection from Metastable Magnetic Thin Films

Abstract

Many optical phenomena have been demonstrated using a neutron source of radiation [1]. For example, at the boundary between two media with different neutron refractive indices, a collimated neutron beam may, like light, be totally reflected when the incident angle is less than the critical glancing angle θc. Under conditions close to critical reflection, the neutron reflectivity is sensitive to the refractive index profile n(z) normal to the interface boundary [2]. The measurement of the reflected intensity for different angles of incidence θi > θc gives the depth profile of the scattering density. Variations in the neutron interaction potential, arising from inhomogeneities in the interface region, produces diffuse scattering around the specularly reflected beam which may be analyzed for interface roughness and strain effects. It is possible to extract separately the spatial dependence of the magnetisation (magnetisation profile) from the density profile at a ferromagnetic surface. The magnetic interaction potential is comparable in magnitude with the total interaction potential, which is a distinct advantage over grazing-incidence X-ray scattering where the magnetic part of the scattering cross-section is 10-5 weaker than the total interaction potential. Thus, the specular reflectivity of neutrons as a function of momentum transfer normal to the interface provides information on long-range (spin) ordering in magnetic films while the parallel momentum transfer component relates to the short-range order associated with, for example, critical fluctuations during a phase transition [3]. Similarly, the technique can be extended to magnetic super-lattices, Bragg diffraction from which provides information on the layer spacing and structure [4].