No abstract

No abstract

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.

Low energy (16 keV) muons were used to probe the dynamic magnetic behaviour of iron nanoclusters embedded in a silver thin film matrix. The silver film was 500 nm thick and contained a volume fraction of 0.1% iron. Measurements were made in a field of 25 mT, applied normal to the plane of the film, in the temperature range 4.7 K to 300 K. At temperatures above 20 K thermal activation of the cluster moments was seen as a narrowing of the field distribution sensed by the implanted muons. An intrinsic cluster relaxation time of τ 0 = 12 ± 4 ns and an activation energy of 51 ± 9 K were deduced from fits to the data. SQUID magnetometry of thicker (1.5 µm) but otherwise identical films on graphite substrates showed the clusters to have a volume of the order of 10 −26 m 3 , from which a cubic anisotropy constant of K = 2.3 ± 0.4 × 10 5 J m −3 was calculated. Remanence measurements showed no evidence of a preferred orientation for the magnetization of the cluster assembly.

No abstract