A new probe for surface magnetism: Polarized neutron reflection

Abstract

A beam of polarized neutrons of wavelength λ is reflected by a magnetized surface with an intensity R ±(λ), where R is related by optical laws to the refractive index n ± = n N ± n M of the material. While the nuclear refractive index n M is constant for a homogeneous body, n M is proportional to the magnetic induction B ( z), where z is the distance from the surface. A perturbation of B close to the surface may alter the reflectivity R ±(λ) sufficiently to allow for its detection. In the case of a superconductor in an external magnetic field (below H c1) B ( z) is non-zero only close to the surface; numerical calculations appropriate to niobium show that neutron reflection can determine the penetration length within 10%. For elemental ferromagnets the magnetism at the surface is increasingly perturbed as the temperature approaches the Curie point; in nickel the perturbation should become detectable at 0.85 T c. An effort is underway to test experimentally these ideas with a polarized neutron mirror at IPNS, using all neutron wavelengths above a cutoff of 4 Å, and the sample environments appropriate to the study of the two classes of magnetic phenomena at the surface.