Investigation of magnetism at surfaces by polarized neutron reflection (invited)

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Abstract
It is shown that the spin-dependent reflectivity of a cold neutron beam from magnetically active surfaces, measured as a function of the neutron wavelength, can provide the magnetic depth profile of the material. Following the construction of a prototype instrument at the Intense Pulsed Neutron Source at Argonne, studies were made of the magnetic disturbances at the surface of superconductors. The results obtained, and discussed here, pertain to the penetration depth in a superconductor (niobium and lead) below the critical flux entry field Hc1, and the surface superconductivity of an incipient type II superconductor (lead-bismuth alloy) below Hc3. The prospects for this technique, with particular regard to the study of the surface critical phenomena of ferromagnets, are also discussed.