Mössbauer-effect study of face-centered-cubic-like Fe on Cu(001) (invited) (abstract)

Abstract

More than 30 years ago, Weiss has postulated the existence of two different magnetic fcc-Fe states [high-spin (HS)/high atomic volume or low-spin (LS)/low atomic volume] in order to explain the Invar effect in fcc-Fe alloys. Such metastable states may be stabilized by epitaxial growth of ultrathin Fe films on Cu(001) under suitable conditions which depend on film thickness and growth temperature. In situ conversion-electron Mössbauer spectroscopy, combined with low-energy electron diffraction, reflection high-energy electron diffraction, and Auger electron spectroscopy, on ∼3- and ∼7-ML-thick 57Fe films grown in ultrahigh vacuum at 300 K reveals a thickness-dependent transition from a HS ferromagnetic (FM) state with an anisotropically expanded fcc (fct-like) structure (c/a≳1) to a LS antiferromagnetic (AFM) isotropic fcc state. In contrast, the stability of the HS fct-like phase is extended to at least 7 ML in films grown at low T (90 K) and annealed to 300 K. The HS-FM phase in 7 ML films is rather stable against annealing up to 500 K; annealing at 570 K leads to a HS–LS transformation which is correlated with abrupt surface segregation of Cu. By placing 2-ML-thick isotopically enriched 57Fe-probe layers into a 300 K grown natural Fe film of 7 ML total thickness we could obtain a magnetic depth profile along the film-normal direction: while the LS-AFM state (with TN∼70 K) was found at the film center and a paramagnetic Fe–Cu alloy at the Fe/Cu interface, Fe surface atoms were observed to be in a HS-FM state with a noncubic atomic environment. Only today can we begin to understand the complex behavior of fcc-Fe/Cu(001).