Polarized Low-Energy Positrons: A New Probe of Surface Magnetism

A polarized beam of low-energy positrons has been used for the first time as a probe of surface magnetism. The polarization, P_e^-, of electrons captured at a Ni(110) surface to form positronium is measured. The temperature dependence of P_e^-, fitted by a power law, yields an exponent of β₁=0.7±0.1, in qualitative agreement with calculations of the critical exponent for the surface-layer magnetization. Rapid quenching of the ferromagnetic order is observed for submonolayer coverages of oxygen.