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The production and characterization of ultra-thin iron oxide films grown on an atomically clean Ag(100) surface by molecular beam epitaxy (MBE) is presented. The goal of this work was to prepare ultra-thin FeO(100) with excellent crystallographic quality. The films were prepared with high purity 57 Fe and O 2 and afterwards analyzed in situ by means of Low Energy Electron Diffraction (LEED), X-Ray Photoelectron Spectroscopy (XPS) and Conversion Electron Mossbauer Spectroscopy (CEMS). During preparation the evaporation rate, the O 2 partial pressure, film thickness and annealing procedures were varied. The analysis of the various samples showed that in general a mixture of FeO and Fe 3 O 4 phases is obtained. We determined the best conditions to produce the desired oxide (FeO). Besides the paramagnetic phase, the antiferromagnetic phase of the FeO films was characterized by low temperature Mossbauer spectra.

We have investigated the magnetic behavior of cobalt ferrite nanoparticles with a mean diameter of 7.2 nm. AC susceptibility of colloidal cobalt ferrite nanoparticles was measured as a function of temperature T from 2 to 300 K under zero external DC field for frequencies ranging from f=10 to 10,000 Hz. A prominent peak appears in both χ′ and χ″ as a function of T. The peak temperature T2 of χ″ depends on f following the Vogel–Fulcher law. The particles show superparamagnetic behavior at room temperature, with transition to a blocked state at TBm∼94 K in ZFC and 119 K in AC susceptibility measurements, respectively, which depends on the applied field. The saturation magnetization and the coercivity measured at 4.2 K are 27.3 emu/g and 14.7 kOe, respectively. The particle size distribution was determined by fitting a magnetization curve obtained at 295 K assuming a log-normal size distribution. The interparticle interactions are found to influence the energy barriers yielding an enhancement of the estimated magnetic anisotropy, K=6×106 erg/cm3. Mossbauer spectra obtained at higher temperatures show a gradual collapse of the magnetic hyperfine splitting typical for superparamagnetic relaxation. At 4.2 K, the Mossbauer spectrum was fitted with two magnetic subspectra with internal fields Hint of 490, 470 and 515 kOe, corresponding to Fe3+ ions in A and B sites.

A low energy electron diffraction (LEED) investigation of the structure of the surface of an FeO(0 0 1) thin film grown on Ag(0 0 1) is presented. The results show that this surface has an almost bulk termination structure with a very small rumple on the first layer, which agrees with the structure found in other studies carried out on the (0 0 1) surface of oxides that have rock-salt structure. Evidences that may support a linear behaviour of the topmost layer rumple with the oxide lattice constant are also discussed.

Iron silicides were grown on Si (111) substrates by Solid Phase Epitaxy (SPE) and Reactive Deposition Epitaxy (RDE) to identify the optimum conditions to obtain the semiconduting β-FeSi2 phase. The films were produced under different growth and annealing conditions and analyzed in situ and ex situ by X-ray Photoelectron Spectroscopy, and ex situ by Conversion Electron Mossbauer Spectroscopy. The use of these techniques allowed the investigation of different depth regions of the grown layer. Films of the ɛ-FeSi and β-FeSi2 phases were obtained as well as the mixtures Fe3Si + ɛ-FeSi and ɛ-FeSi + β-FeSi2. The sequence Fe3Si→ɛ-FeSi→β-FeSi2 was found upon annealing, where the phase transformation occurred due to the migration of silicon atoms from the substrate to the surface region of the grown layer. The best conditions for the phase transformation in SPE samples were met after annealing in the range 700 – 800 °C. For the RDE samples, the transition to the beta phase occurred between 600 and 700 °C, but pure β-FeSi2 was obtained only after two hours of annealing at 700 °C.

An X-ray photoelectron spectroscopy (XPS) and reflection high-energy electron diffraction (RHEED) investigation of the growth of Cu films on a Pd(1 1 1) single crystal at room temperature is presented. Dynamically taken XPS-data as function of the deposition time show a linear variation of I Cu-3p / I Pd-3d and a periodic change of its slope indicating a nearly layer-by-layer growth process. RHEED oscillations are seen for the 3–4 first layers, also suggesting a smooth growth mode. From the evolution of the RHEED-streaks separation the in-plane Cu-atom spacing is precisely determined. Up to a coverage of ca. 2–3 monolayers (ML) Cu grows pseudomorphously on Pd(1 1 1), despite the −7.1% strain imposed by the substrate lattice parameter. Non-pseudomorphous epitaxial growth is evidenced above ca. 3–4 ML by a discontinuous change in lateral lattice spacing observed by RHEED which indicates a relaxation to the Cu(1 1 1) “natural” surface lattice parameter. In addition it is concluded that surface alloying does not take place at least at room temperature (RT)-XPS spectra taken dynamically during annealing show that alloying occurs only above RT.

On the basis of a high-precision reflection high-energy electron diffraction (RHEED) investigation details of the growth of Pd on a Cu(1 1 1) single crystal substrate at room temperature are reported. Because of the +7.6% misfit of the Pd lattice spacing as compared to Cu, perfect pseudomorphous growth would result in highly stressed ultra-thin films. RHEED analysis shows that as a function of Pd coverage initially the film starts to grow with the in-plane Cu(1 1 1) lattice parameter. With increasing coverage the lattice parameter rapidly changes to the “natural” lateral lattice parameter of Pd(1 1 1). We propose that a progressive increase of the equilibrium lateral lattice parameter by alloying (Vegards law) releases the stress in the Pd/Cu(1 1 1) system. With a coverage of n >2 ML ( n =number of monolayers, ML) pure Pd layers are formed, since the in-plane lattice parameter equals the expected value for Pd at n =2 ML. Our conclusion that Pd–Cu surface alloying acts as a relaxation mechanism in this quasi-pseudomorphous growth system, corroborates scanning tunneling microscopy [Surf. Sci. 408 (1998) 43] which also suggests formation of a random surface alloy in the earlier stages of Pd growth on Cu(1 1 1).

A Bi 2 Se 3 crystal was studied by XPS (X-Ray Photoelectron Spectroscopy) and EELS (Electron Energy Loss Spectroscopy). Al Kα radiation and an electron beam energy in the range of 0.1 to 2 keV were used, respectively, to probe a Se-terminated (0001) surface. Samples of the constituent elements (Bi and Se) have also been measured with the same setup. The core level chemical shifts obtained show that a charge transfer occurs in Bi 2 Se 3 . The spectra in the valence band region suggest that the density of states in the compound may be obtained by combining the spectra of the constituent elements, that the electronic states in the vicinity of the gap region consist of a mixture of the metal and calcogen p -orbitals and that the band arising from the valence s -orbitals occurs about 12 eV below the valence band maximum. The EELS spectra allow to identify the bulk plasmons for the three materials and the Bi5 d 3 and Bi5 d 5 interband transitions. Considerations of the energies of the Bi5 d transitions as measured by XPS and EELS indicate that the bottom of the conduction band of the compound is 1.2 eV above the Fermi level. The EELS results also shows evidence that the losses occurring at 6.4 eV in the compound and at 5.4(5.5) eV in Bi(Se) have their origins in some surface process. We suggest that they may be associated to a surface plasmon.

Fe/Ag multilayers composed of ultrathin layers were grown on Cu(1 0 0) in ultrahigh vacuum by molecular beam epitaxy. Magnetic analysis of the Fe-layers (95% enriched in 57Fe) was performed by in situ conversion electron Mossbauer spectroscopy at room temperature and 100 K. Samples with different thicknesses of the Ag-spacers (2–3 monolayers) as well as with different thicknesses of the Fe-layers were prepared in order (a) to determine whether the magnetic orientation (in-plane/out of plane) of the 57Fe-layers is changed, (b) to study its influence on the intensity of the hyperfine magnetic field and (c) to identify superparamagnetism due to small Fe islands inside the layers.

Dried water-based ionic ferrofluid powder samples containing MnFe2O4 spinel ferrite nanoparticles with average particle size of about 6 and 9 nm have been studied by Mossbauer spectroscopy and DC magnetization measurements. The Mossbauer blocking temperature of the 6 nm sample was 290 K and that of the 9 nm sample 340 K. From DC magnetization measurements ‘magnetic’ diameters of 5.2 and 8 nm and a Ms of 41 and 48 emu/g were found for the two samples, respectively.

Mossbauer spectra with superparamagnetic relaxation under the presence of an external magnetic field are calculated using diagonal hyperfine interaction and a multilevel model. The magnetization is described by a large spin S and spin–phonon interaction spin–transition probabilities are calculated from a Debye model using S =3222, Δ S z =±2 and realistic parameters of the constants involved. It is shown that the calculated spectra exhibit the typical shape of experimental spectra. Spectra calculated with a reduced S =322 or 32 demonstrate almost identical patterns when the temperature is adequately reduced.

57Fe Mossbauer effect measurements have been performed on the amorphous spin-glass alloy Fe4Ni74Si9B13. Magnetic hyperfine-split spectra are found below an average magnetic transition temperature T f ≈ 11 K , the transition being not sharp but smeared over several degrees. In the spin-glass phase the hyperfine field Hint near saturation has an average value of (230±5) kOe and shows a distribution P(H) which is zero for H ⪅ 140 kOe. The most probable hyperfine field is found to follow the relation Hint(T)=(260 kOe)·(1 − T/Tf)β over a large temperature range, with β=0.40. An external magnetic field strongly effects the spin orientation in the spin-glass phase. For T >Tf and in the presence of an external field our measurements indicate that superparamagnetic clusters exist at least up to 77 K, with an average cluster moment of about 70 μB.

FCC (γ-) Fe(100)-films prepared by epitaxial growth in UHV on Cu(100)-films were investigated by Conversion Electron Mossbauer Spectroscopy (CEMS) at 295 K using a He-CH 4 flow-gas counter. The fcc structure of the Fe films was confirmed by LEED. The Mossbauer spectra clearly demonstrate that γ-Fe(100)-films are paramagnetic at 295 K. Depending on the Fe film thickness part of the Fe atoms exists in the α-Fe phase.