Ferromagnetism in ultrathin metastable films of fcc Fe, Co, and Ni (invited)

Abstract

We have applied spin-polarized neutron reflection at a grazing angle of incidence to study the ferromagnetic behavior of thin magnetic films down to the monolayer thickness level. The reflected intensities I+ and I− for spin parallel (+) or antiparallel (−) to the sample magnetization are measured near the angle θc for total reflection: deviation of the flipping ratio F=I+/I− from unity occurs only for ferromagnetic ordering. In general, deviations of F from unity are so small that an accurate determination of the magnetic moment per atom μ is not possible. The success of the present experiments relies on the enhancement (∼500×) of the magnetic signal when the film is overcoated with a nonmagnetic layer of thickness sufficient to produce constructive interference in the reflected neutron wave field. With this method, we have determined the magnetic moments in films of the metastable fcc phases of Fe and Co, grown epitaxially on Cu(001) single-crystal substrates. The fcc Fe(001) films have lowered Curie temperatures TC below 400 K for thicknesses less than 5 monolayers, similar to Ni films. In contrast, the Co(001) films exhibit a constant value of μ≊μbulk=1.8μB down to a single monolayer thickness over the same temperature range. The fcc Fe films exhibit unusual magnetic anisotropy properties which are structure and strain related, as determined by low-energy electron diffraction measurements. The results are discussed in the context of three- to two-dimensional magnetic scaling theories and predictions derived from local-spin-density functional calculations of the magnetic moments per atom as a function of volume strain.