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Epitaxial face-centered-cubic (fcc)-like Fe films have been investigated in situ in ultrahigh vacuum by 57Fe conversion-electron Mössbauer spectroscopy (CEMS). A broad distribution of hyperfine magnetic fields, P(Bhf), with an extrapolated most-probable field of 32(2) T at magnetic saturation has been observed in ∼3 monolayers (ML) thick films grown at ∼90 or 300 K, and in ∼7 ML thick films grown at ∼90 K. Such films are in a ferromagnetic high-spin state. Their local structure is characterized by an electric quadrupole interaction, eQVZZ/4, of +0.114±0.05 mm/s from which we infer an anisotropically expanded fcc (fct-like) structure with c/a≊1.06. Approximately 5–7 ML thick films grown at 300 K are observed to be in a low-spin antiferromagnetic state below TN∼70 K, and are fcc (c/a=1) above TN. Site-selective CEM spectra taken on such films provide direct evidence for a magnetically ‘‘live’’ surface layer. The fcc→bcc transformation occurring in 35 ML thick films deposited at 300 K was observed to be incomplete.

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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).