Mössbauer spectroscopy and magnetic properties in thin films of FexNi100−x electroplated on silicon (1 0 0)

Fe_xNi_100−x thin films were produced by galvanostatic electrodeposition on Si (1 0 0), nominal thickness 2800 nm, and x ranging 7-20. The crystalline structure of the sample was determined by X-ray diffraction (XRD). The magnetic properties were investigated by vibration sample magnetometry (VSM) and room temperature ⁵⁷Fe Mössbauer spectroscopy. Conversion Electron Mössbauer spectroscopy (CEMS) in both film surfaces for the thick self-supported films showed that the magnetic moment direction is in the plane and conventional transmission (MS) that the directions are out of the plane films. The results were interpreted assuming a three-layer model where the external layer has in-plane magnetization and the internal one, out of plane magnetization.     

Ultrathin Fe layers on Ag (1 0 0) surface

Iron layers (0.15-10 ML thick) deposited on Ag (1 0 0) substrates were investigated by conversion electron Mössbauer spectrometry over a broad temperature range. The layers were characterized by scanning tunneling microscopy. Different forms of the layers, depending on their thickness, were observed. Minimum roughness of the layers were found at 0.15 and 10 ML thickness values. The Mössbauer spectra showed systematic thickness dependence. At low thickness values, broad doublets were observed, while above 6 ML, magnetic split spectra appeared at room temperature. At low temperatures, magnetically split spectra appeared with parameter values characteristic of Fe-Ag and Fe-Fe atomic interactions. The hyperfine split spectra indicated magnetic anisotropy and an enhanced saturation hyperfine magnetic field of ?40 T. The latter value is the highest ever measured for iron in thin layers.     

Theoretical study of oxygen adsorption at the Fe(1 1 0) and (1 0 0) surfaces

Electronic, magnetic and structural properties of atomic oxygen adsorbed in on-surface and subsurface sites at the two most densely packed iron surfaces are investigated using density functional theory combined with a thermodynamics formalism. Oxygen coverages varying from a quarter to two monolayers (MLs) are considered. At a 1/4 ML coverage, the most stable on-surface adsorption sites are the twofold long bridge sites on the (1 1 0), and the fourfold-hollow sites on the (1 0 0) surface. The presence of on-surface oxygen atoms enhances the magnetic moments of the atoms of the two topmost Fe layers. Detailed results on the surface magnetic properties, due to O incorporation, are presented as well. Subsurface adsorption is found unfavored. The most stable subsurface O, in tetrahedral positions at the (1 0 0) and octahedral ones at the (1 1 0) surface, are characterized by substantially lower binding than that in the on-surface sites. Subsurface oxygen increases the interplanar distance between the uppermost Fe layers. The preadsorbed oxygen overlayer enhances binding of subsurface O atoms, particularly for tetrahedral sites beneath the (1 1 0) surface.     

Thermal and irradiation induced interdiffusion in magnetite thin films grown on magnesium oxide (0 0 1) substrates

Epitaxial Fe₃O₄(0 0 1) thin films (with a thickness in the range of 10-20 nm) grown on MgO substrates were characterized using low-energy electron diffraction (LEED), conversion electron Mössbauer spectroscopy (CEMS) and investigated using Rutherford backscattering spectrometry (RBS), channeling (RBS-C) experiments and X-ray reflectometry (XRR). The Mg out-diffusion from the MgO substrate into the film was observed for the directly-deposited Fe₃O₄/MgO(0 0 1) films. For the Fe₃O₄/Fe/MgO(0 0 1) films, the Mg diffusion was prevented by the Fe layer and the surface layer is always a pure Fe₃O₄ layer. Annealing and ion beam mixing induced a very large interface zone having a spinel and/or wustite formula in the Fe₃O₄-on-Fe film system.     

Fourfold in-plane magnetic anisotropy in epitaxial Fe(1 1 0)/Cu(0 0 1) and Fe(1 1 0)/Ni(0 0 1) bilayers

Epitaxial Fe(1 1 0) films with thicknesses of 100-800 nm on Cu(0 0 1) and Ni(0 0 1) buffer layers grown on MgO(0 0 1) substrates have been fabricated. These films contain Fe(1 1 0) crystallites which are in the Pitsch orientation relationship. Magnetization and the fourfold in-plane magnetic anisotropy constants of these films have been determined by torque measurements. All the samples under study are characterized by a fourfold magnetic anisotropy with easy axes parallel to the [1 0 0] and [0 1 0] directions of Cu(0 0 1) and Ni(0 0 1) layers. The measured values of the constant for Fe(1 1 0)/Cu(0 0 1) are found to depend on deposition temperature; a maximum value of (2.5±0.1)×10⁵ erg/cm³ is reached after annealing at 600 °С. The in-plane torque measurements on Fe(1 1 0)/Ni(0 0 1) bilayers obtained at 300 °С, on the other hand, exhibit a constant value of (2.7±0.1)×10⁵ erg/cm³. Assuming an exchange interaction between the Fe(1 1 0) crystallites, which are in the Pitsch orientation relationship, the fourfold in-plane magnetic anisotropy has been calculated as 2.8×10⁵ erg/cm³. The deviations of the experimental values from the predicted one may be explained by the formation of a polycrystalline phase within the Fe(1 1 0) layer and a partial disorientation of the epitaxial crystallites.     

Effect of Fe substitution on the phase stability and magnetic properties of Mn-rich Ni-Mn-Ga ferromagnetic shape memory alloys

The influence of Fe additions on the martensitic transformation and magnetic properties of Mn-rich Ni-Mn-Ga alloys was investigated by substituting either 1 at% Fe for each atomic species or by substituting Ni with varying amounts of Fe. The magnetic structure of the alloys was studied using ⁵⁷Fe Mössbauer spectroscopy. Mössbauer spectra revealed typical paramagnetic features in Mn-rich Ni-Mn-Ga-Fe alloys owing to the preferential site occupancy of Fe atoms at Ni sites. The evolution of the magnetic properties and phase stability has been correlated with the chemical and atomic ordering in these alloys.     

Reconstruction and de-reconstruction of the Ir(1 0 0) surface and ultrathin Fe/Ir(1 0 0) films

The structure, energetics and magnetic properties of the quasihexagonal reconstruction of the Ir(1 0 0) surface and nanostructures formed by Fe atoms on this surface have been investigated using first-principles density functional theory with generalized gradient corrections. We find the reconstructed (1 × 5) surface to be 0.10 eV/(1 × 1) area lower in energy than the unreconstructed surface and we demonstrate that first-principles calculations can achieve quantitative agreement with experiment even for such long-period and deep-going reconstructions. For Fe coverage of 0.4 monolayers (ML) we have studied the stripe-like structure with biatomic Fe rows placed in the troughs of the (1 × 5)-reconstructed surface. Results of nonmagnetic calculations agree well with the structure inferred from STM data. Higher Fe coverages lead to a de-reconstruction of the Ir substrate. At 0.8 ML coverage a surface compound with composition Fe₄Ir is formed, which shows an appreciable buckling. In this case, a ferromagnetic calculation leads to good agreement with the low-temperature LEED data. We predict that the (1 × 5) periodicity of the mixed interface layer will persist also in thicker films with a pure Fe surface. Films with 1-4 ML Fe are predicted to be tetragonally distorted and ferromagnetic, with an axial ratio corresponding well to an elastic distortion of the Fe lattice.     

Magnetism of ferriprotoporphyrin IX monomers and dimers

The SQUID and the ⁵⁷Fe Mössbauer spectroscopy studies of the magnetic properties of monomeric and dimeric forms of iron porphyrin were performed between 2 and 305 K. The effective magnetic relaxation rate of the Fe atoms in iron porphyrin monomers exhibits complex temperature dependence, resulting from the competing spin-spin and spin-lattice relaxation processes. The dimerization of iron porphyrin dramatically speeds up the magnetic relaxation. The Fe-Fe antiferromagnetic exchange coupling constant in Fe-O-Fe dimer is J≈−110 cm⁻¹. The complementary application of SQUID and the Mössbauer spectroscopy is proposed as a new precise quantitative analytical methodology for monitoring of the aggregation process of iron porphyrin.     

Structure and magnetism of bcc(1 1 0)-like Fe films grown on Cu(0 0 1) via ion beam triangulation and spin-polarized electron capture

The structure and magnetism of thin epitaxial Fe layers grown on Cu(0 0 1) is investigated by grazing scattering of fast H and He atoms. Information on the atomic structure of the film and substrate surfaces is obtained by making use of ion beam triangulation with protons. The magnetic behavior is studied via the polarization of light emitted after capture of spin-polarized electrons into excited atomic terms during scattering of He atoms. For the formation of bcc(1 1 0)-like Fe films at higher coverages, we detect differences in structural and magnetic properties for room and low temperature growth. We suggest that the crystalline structure depends on the film morphology and that Cu impurities affect the magnetic properties.     

Fe-contacts on InAs(100) and InP(100) characterised by conversion electron Mössbauer spectroscopy

We have grown 4 nm thin films of ⁵⁷Fe on InAs(100) and InP(100) surfaces by use of MBE and studied the samples by ⁵⁷Fe conversion electron Mössbauer spectroscopy. In the case of InAs, the Mössbauer spectrum showed a sextet due to α-Fe and a further magnetically split component with slightly smaller hyperfine field, which is attributed to interface components. This result indicates that there is a relatively sharp interface between Fe and InAs. On the contrary, the spectrum of the InP sample showed a sextet with very broad lines and a smaller average hyperfine field. This suggests a strong chemical reaction between iron and the substrate, which results in the formation of a poorly crystalline phase.     

Evolution of magnetic order in mechanically alloyed Al–1 at%Fe

The evolution of ferromagnetic order in high-energy ball-milled Al–1 at% Fe before the onset of a considerable Fe–Al solid solution phase has been investigated using ⁵⁷Fe Mössbauer and bulk magnetization studies. The unmilled sample does not exhibit bulk magnetic properties and an onset of bulk magnetization is observed only after 30 min of milling, when the grain size becomes comparable to the ferromagnetic exchange length. The Curie temperatures of all the samples are less than that of pure iron. The reduction in grain size is accompanied by an increase in coercivity and reduced remanence and a decrease in T_C. The effective magnetic moment per iron atom decreases with the development of a non-magnetic, Al-rich Fe–Al solution on longer milling. The clustering of Fe at grain boundaries is responsible for the observed bulk magnetic ordering. The systematic variation of the magnetic properties has been qualitatively correlated with the evolution of microstructure, reduction in grain size and enhanced inter-granular exchange coupling.     

Effect of atomic environment on the Fe hyperfine structure and the thermal stability of magnetic order in L10 ordered Fe-Pt alloys

The ⁵⁷Fe Mössbauer effect measurements were made for the L1₀ ordered Fe-Pt alloys with 39-62 at% Pt and the effect of local atomic environment on the hyperfine structure was investigated. Furthermore, the thermal stability of magnetic order was investigated for the alloys with high Pt concentration. From the analyses of the observed Mössbauer spectra, we found that dipole-field-like anisotropic transferred hyperfine fields are mainly responsible for the large difference in hyperfine field between Fe-site and Pt-site in the Fe-rich alloys. In the Pt-rich region far from stoichiometry, the existence of many Fe-sites occupied by excess Pt atoms causes a distribution of exchange fields. Therefore, the iron atoms in different local environments may have their several hyperfine fields with different temperature dependence. The anomalous temperature dependence of the averaged hyperfine field and line broadening observed for the 61, 62 at% Pt alloys can be understood from the co-existence of various sub-spectra with different temperature dependence. As a result, the thermal stability of magnetic order is largely reduced as the Pt concentration exceeds 60 at%.     

Growth and morphology of the epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) Trilayers

Growth and surface morphology of epitaxial Fe(1 1 0)/MgO(1 1 1)/Fe(1 1 0) trilayers constituting a magnetic tunnel junction were investigated by low-energy electron diffraction (LEED) and scanning tunneling microscopy (STM). STM reveals a grain-like growth mode of MgO on Fe(1 1 0) resulting in dense MgO(1 1 1) films at room temperature as well as at 250 °C. As observed by STM, initial deposition of MgO leads to a partial oxidation of the Fe(1 1 0) surface which is confirmed by Auger electron spectroscopy. The top Fe layer deposited on MgO(1 1 1) at room temperature is relatively rough consisting of clusters which can be transformed by annealing to an atomically flat epitaxial Fe(1 1 0) film.     

Structure and electronic properties of Fe nanostructures on MgO(0 0 1)

We present and discuss X-ray absorption spectroscopy and resonant photoemission measurements on Fe nanostructures self-assembled on MgO(0 0 1). For Fe coverages below 1 ML equivalent we measured an increase of the Fe L₂₃ branching ratio and changes in the splitting, widths and relative intensities of the different final states in the L₃M₂₃M₂₃ resonant Auger peak. Scanning tunnelling microscopy indicates that the average lateral dimensions of the self-aggregated structures decrease with decreasing Fe amount, from 12 nm at 15 ML nominal Fe amount to 5 nm at 2 ML Fe. This observation allows to interpret the observed changes in the 3d band electronic properties in terms of the evolution of the Fe local atomic coordination from a bulk-like situation to a configuration where low dimensionality effects are significant.     

238U and 57Fe Mössbauer Spectroscopic Study of UFe2

We have performed ⁵⁷Fe and ²³⁸U Mössbauer measurements of UFe₂ in order to investigate its magnetic properties. From the results of ⁵⁷Fe Mössbauer spectroscopy, the ferrromagnetic ordering at 167 K is caused by the iron 3d-electrons, which hybridize strongly with the uranium 5f-conduction electrons. It is also clarified from the results of ²³⁸U Mössbauer spectroscopy that there are no magnetic moments on the uranium atoms.     

Density functional theory study of selenium adsorption on Fe(1 1 0)

The adsorption of atomic Se on a Fe(1 1 0) surface is examined using the density functional theory (DFT). Selenium is adsorbed in high-symmetry adsorption sites: the -short and long-bridge, and atop sites at 1/2, 1/4, and 1 monolayer (ML) coverages. The long bridge (LB) site is found to be the most stable, followed by the short bridge (SB) and top sites (T). The following overlayer structures were examined, p(2 × 2), c(2 × 2), and p(1 × 1), which correspond to 1/4 ML, 1/2 ML, and 1 ML respectively. Adsorption energy is −5.23 eV at 1/4 ML. Se adsorption results in surface reconstruction, being more extensive for adsorption in the long bridge site at 1/2 ML, with vertical displacements between +8.63 and −6.69% -with regard to the original Fe position-, affecting the 1st and 2nd neighbours. The largest displacement in x or y-directions was determined to be 0.011, 0.030, and 0.021 Å for atop and bridge sites. Comparisons between Se-adsorbed and pure Fe surfaces revealed reductions in the magnetic moments of surface-layer Fe atoms in the vicinity of the Se. At the long bridge site, the presence of Se causes a decrease in the surface Fe d-orbital density of states between 4 and 5 eV below Fermi level. The density of states present a contribution of Se states at −3.1 eV and −12.9 eV. stabilized after adsorption. The Fe-Fe overlap population decrease and a Fe-Se bond are formed at the expense of the metallic bond.     

Growth mode and electronic structure of Au nano-clusters on NiO(0 0 1) and TiO2(1 1 0)

The growth mode and electronic structure of Au nano-clusters grown on NiO and TiO₂ were analyzed by reflection high-energy electron diffraction, a field-emission type scanning electron microscope, medium energy ion scattering and photoelectron spectroscopy. Au was deposited on clean NiO(0 0 1)-1 × 1 and TiO₂(1 1 0)-1 × 1 surfaces at room temperature with a Knudsen cell at a rate of 0.25-0.35 ML/min (1 ML = 1.39 × 10¹⁵ atoms/cm²:Au(1 1 1)). Initially two-dimensional (2D) islands with thickness of one Au-atom layer grow epitaxially on NiO(0 0 1) and then neighboring 2D-islands link each other to form three-dimensional (3D)-islands with the c-axis oriented to the [1 1 1] direction. The critical size to form 3D-islands is estimated to be about 5 nm². The shape of the 3D-islands is well approximated by a partial sphere with a diameter d and height h ranging from 2.0 to 11.8 nm and from 0.95 to 4.2 nm, respectively for Au coverage from 0.13 to 4.6 ML. The valence band spectra show that the Au/NiO and Au/TiO₂ surfaces have metallic characters for Au coverage above 0.9 ML. We observed Au 4f spectra and found no binding energy shift for Au/NiO but significant higher binding energy shifts for Au/TiO₂ due to an electron charge transfer from Au to TiO₂. The work function of Au/NiO(0 0 1) gradually increases with increase in Au coverage from 4.4 eV (NiO(0 0 1)) to 5.36 eV (Au(1 1 1)). In contrast, a small Au deposition(0.15 to 1.5 ML) on TiO₂(1 1 0) leads to reduction of the work function, which is correlated with an electron charge transfer from Au to TiO₂ substrate.     

The interaction of carbon with Si(1 1 1):As and Si(1 1 1):H surfaces, a theoretical study

Density functional theory calculations have been performed to determine the adsorption site of carbon at the Si(1 1 1):As and Si(1 1 1):H surfaces at different coverages. The As- and H-passivated surfaces were simulated by replacing the topmost Si layer by As or by saturating the Si dangling bonds with hydrogen atoms, respectively. Different high symmetry sites were considered. Carbon was placed successively in the fourfold (T₄) or threefold coordinated (H₃), the ontop (T₁) sites or substituted for a Si atom in the S₅ position located underneath the Si adatom in the T₄ site. We found that the preferred carbon adsorption site depends on the coverage of the passivated surfaces. At low coverages i.e. at 1/16 ML and 1/3 ML, it prefers a distorted T₄ position whereas at 1 ML, it occupies an H₃ site. This contrasts with the clean surface where the most energetically favored site is the S₅ at all coverages. Carbon adsorption induces a significant change in the structural geometry of the surface atoms, leading to a charge re-arrangement in the surface layers.     

Structure analysis of stoichiometric LiNbO3(0 0 0 1) surfaces using low-energy neutral scattering spectroscopy

The atomic structure of LiNbO₃(0 0 0 1) surface was investigated by low-energy neutral scattering spectroscopy (LENS). Poled stoichiometric LiNbO₃ (SLN) samples were prepared for the measurements. The LENS was developed for surface structure and composition analysis particularly of highly insulating materials and was successfully applied to the structure analysis of the SLN(0 0 0 1) surface. The polar angle dependences of intensity of scattered He⁰ from the poled SLN surfaces indicate obvious differences between the negatively and the positively charged surfaces. It is suggested that O atoms cover the surfaces, and the first metal layers underneath the O layer consist of Li and Nb for negatively and positively charged surfaces, respectively, parallel to the applied electric field.     

Interaction of CO with atomically well-defined PtxRuy/Ru(0 0 0 1) surface alloys

The interaction of CO with structurally well-defined Pt_xRu_y surface alloys supported on Ru(0 0 0 1) was investigated by thermal desorption spectroscopy and infrared reflection-absorption spectroscopy. The surface composition and the distribution of the surface atoms were controlled by high resolution scanning tunneling microscopy. On these surfaces, which have a nearly random distribution of the two surface species, the adsorption (and desorption) of CO is strongly modified compared to the pure elemental surfaces, by strain effects and electronic ligand effects. CO adsorbs exclusively in a linear configuration on Pt and Ru atoms for all surfaces investigated. The adsorption energy of CO is lowered on the alloy surfaces with respect to both Pt(1 1 1) and Ru(0 0 0 1), similar as for pseudomorphic monolayer Pt films. For both Pt and Ru sites the adsorption strength decreases with increasing Pt concentration.