Exchange interactions at the surface of a ferromagnet

Abstract

The local magnetization in magnetic surfaces obeys a Bloch T3/2 law with a prefactor kS-i that varies from one plane i to another. kS-i reflects the relative exchange interactions JS-i?/J and JS-i/J on a path parallel and perpendicular to the surface, with J the exchange constant in the bulk. The spin polarization P of the low-energy cascade electrons yields keff, which is related to kS-i by the probing depth. By measurement of P(T), keff was obtained with various surfaces of polycrystalline fcc Ni78Fe22. Deposition of submonolayers of Fe, Cr, and Ta on the clean Ni-Fe surface produced dramatic changes in JS/J. A submonolayer of Fe increased JS?/J to as much as 3, in which case at T>0 the relative magnetization is larger at the surface than in the bulk. However, deposition of additional Fe reduced JS/J again to 1. These effects are analogous to what has been observed in the fcc Fe-Ni alloys as a function of composition, but much stronger. A submonolayer of Cr did not greatly affect JS/J in contrast to a submonolayer of Ta which reduced JS/J by a factor of 10. © 1991 The American Physical Society.