Conversion Electron Mossbauer Research Articles

57Fe conversion electron Mössbauer study on tin oxide films doped with dilute two metal ions

Tin dioxide (SnO2) films codoped with 1% mole ratio of (Co, Fe) ions, (Cu, Fe) ions, and (Ti, Fe) ions were prepared on SnO2 (1% Sb) thin film on glass by a spray pyrolysis. Fe species included in these films were characterized by 57Fe conversion electron Mossbauer (CEM) spectra measured at room temperature (RT) and low temperatures (50 K and 16 K). Three paramagnetic Fe3+ species were observed in RT CEM spectra of two metal ions codoped SnO2 films. The broad magnetic sextet was observed in 16 K CEM spectra of (Co, Fe) codoped SnO2 and (Cu, Fe) codoped SnO2 films, but not even at low temperature for (Ti, Fe) codoped SnO2 film. The relationship between isomer shift and quadrupole splitting were shown and compared with the results of ab initio calculations based on the configuration models of metal ions and oxygen defects.

Enhanced multiferroic properties of Bi0.85Nd0.15FeO3 ceramics with excess Bi2O3

Multiferroic Bi(1+x)×0.85Nd0.15FeO3 (BNF) ceramics with various Bi contents are prepared by conventional solid state reaction, and the effects of excess Bi on the structure, morphology, dielectric, ferroelectric, and magnetic properties are investigated. XRD patterns show the coexistence of rhombohedral and orthorhombic phase in all BNF ceramics due to the doping of Nd3+ ions. Bi2Fe4O9 and Bi2O3 impurity can be observed in x = 0 and x = 5%, 7.5% ceramics, respectively. Surface morphologies are closely related to the Bi content and x = 2.5% sample exhibits the largest average grain size of ∼6.72 μm. The leakage current density has been decreased by 3–4 orders of magnitudes through excess Bi. The x = 2.5% sample shows good ferroelectric property with typical hysteresis loop and a large remnant polarization of about 42 μC/cm2 at 150 kV/cm. The P-E hysteresis deteriorates in x = 5% and x = 7.5% ceramics. The PbZrO3-like superstructure related to 1/4(hh0)p diffraction spots in SAED pattern can be found in all BNF ceramics and the number of grain with superstructure observed in each sample decreases as the Bi content increases. As Bi content increases, the remnant magnetization decreases at first, and then increases. Conversion-electron Mossbauer study have ruled out the impacts of Fe2+, Fe4+, or γ-Fe2O3 on the magnetic properties, suggesting that the weak ferromagnetism comes from the destroy of spin cycloid through substitution of Bi3+ ions with Nd3+ ions. Excessive Bi3+ ions have weakened the effect of Nd3+ ions on suppression of spin cycloid and resulted in the degradation of magnetic properties. The slight improvement of magnetic properties in x = 7.5% sample is considered to attribute to Fe vacancies induced by excessive Bi.

Oxidation of iron with varying isotope content

The objective of this work is to find the features of iron oxidation with different contents of isotopes prepared in the form of foils, bulk samples, and powders. The contents of other impurity elements in pure iron samples and 57Fe iron (enrichment up to 95%) have been found to be practically identical. The oxidation products have been characterized via conversion and Auger electron Mossbauer spectroscopy, as well as via absorbed Mossbauer γ-quanta. It is noted that the natural iron foil samples with a thickness of 6.35 μm undergo air oxidation, while 57Fe isotope foil samples remain unoxidized for more than 40 years.

Spin cycloid destruction in Pr doped BiFeO3 films studied by conversion-electron Mossbauer spectroscopy

Crystalline Bi1−xPrxFeO3 films were fabricated on Si (100) substrates by sol-gel technique. X-ray diffraction revealed a decrease of lattice constants of the doped samples evidencing substitution of Bi by Pr ions. Ferromagnetism with low saturation magnetization Ms = 1–3 emu/cm3 of the films were observed by Vibrating sample magnetometry. Systematic change of Mossbauer parameters of the films with Pr doped concentration increase were obtained by Conversion-electron Mossbauer spectroscopy (CEMS). The results of both one- and two-sextet fittings of the CEMS spectra provide evidence for destruction of the spin cycloid in Pr-doped BiFeO3.

Ferromagnetic properties and conversion-electron Mossbauer spectroscopy of (La2O3)x–(Bi2O3)1−x–Fe2O3 films

(La2O3)x–(Bi2O3)1−x–Fe2O3 films were synthesized by the sol–gel technique on Si (100) substrates. The crystal structure of the samples was studied by X-ray diffraction (XRD) and high resolution transmission electron microscopy (HRTEM). The magnetic properties were studied by vibrating sample magnetometry and conversion-electron Mossbauer spectroscopy (CEMS). The samples were amorphous or nanocrystallized in structure and had ferromagnetic properties with clear enhancement of saturation magnetization when La concentration was increased. The CEMS measurement revealed two doublets in the spectra. The central shift and quadrupole splitting of the doublets were increased with increasing La concentration. This is attributed to the decreasing of s-electron density in the iron nucleus and asymmetry of the electrical field around the nucleus enhanced by La doping.

57Fe conversion electron Mössbauer study of oxidized iron-nickel alloy pins

Iron-nickel alloy pins were pretreated at 700°C and 500°C for 30 min in H2, and additionally heat-treated at 600°C for 30 min in N2 atmosphere, respectively. The oxide layers of these samples were compared with that of a non-preheated sample. Thin iron oxide layers on all samples were observed by scanning electron microscopy (SEM), X-ray diffraction (XRD) and 57Fe conversion electron Mossbauer (CEM) spectrometry. The iron products of Fe3O4 and γ-Fe2O3 were determined among various heated samples.

Laser-induced iron oxidation

Iron oxidation in air ux has been studied and the iron response to laser radiation has been evaluated. Effect of cooling rateon the oxide formation has been demonstrated. The Mossbauer conversion electron spectra have shown that a larger amountof oxides is formed in the surface layer. Any clearly expressed temperature dependence of the oxidation process has not beenobserved. It has been experimentally determined that the oxide amount depends upon the sample collection site.

Advantages of Mössbauer polarimetric methods in material studies

Mossbauer spectroscopic methods combined with the use of circularly polarized radiation offer element selective, unambiguous measurements of the orientation of hyperfine magnetic field at the nucleus. The paper presents applications of the method in investigations of antiferromagnets and distribution of magnetization inside iron-based nanoparticles. Selected examples showing development of Mossbauer conversion electron polarimetric methods are also reported.

Evolution of structural changes in nanocrystalline alloys with temperature

Structural features of the NANOPERM-type alloys Fe91 − x Mo8Cu1B x with x = 12, 15, and 17 have been investigated by Mossbauer spectroscopy. The room-temperature Mossbauer spectra of the as-quenched alloys are characteristic of disordered structural arrangement, but traces of bcc-Fe(Mo) as well as a FeMo2B2 phase have been revealed by X-ray diffraction in all the samples. These results have been confirmed by conversion-electron Mossbauer spectroscopy. The differences between the opposite sides of the ribbon-shaped samples have been shown to stem from structural distinctions. From the point of view of hyperfine interactions, the x = 12 sample exhibits paramagnetic behavior. With increasing x, a contribution from ferromagnetic regions appears gradually, thus leading to an increase in the magnetic ordering temperature in the as-quenched state. Partially crystallized samples have been prepared by controlled annealing of the original precursors for one hour at temperatures ranging from 330 to 650°C in a vacuum. The temperature of the onset of crystallization has been determined to be of 430, 450, and 470°C for x = 12, 15, and 17, respectively. During the first step of crystallization, bcc-Fe(Mo) nanosized grains are formed. Surface features of the samples investigated have also been characterized by using atomic force microscopy.

Study of low energy Ar+ ion irradiated 57Fe/Pt multilayers

This paper reports a study on the effects ofAr+ ion irradiation in57Fe/Pt multilayers havingthe structure Si (substrate)/[57Fe(19 Å)/Pt(25 Å)] × 10. The as-deposited multilayers were irradiated with 90 keVAr+ ions at afluence of 2 × 1016 ions cm−2 at different temperatures. X-ray reflectivity, x-ray diffraction and conversionelectron Mossbauer spectroscopy were used for the characterization of theirradiated samples. Using x-ray reflectivity it was observed that irradiationresults in mixing across the interfaces and deterioration of the multilayerstructure. The mixing results in the formation of fcc FePt phase and noL10 ordering is observed in the samples. The results are discussed in terms of recoildisplacements induced, energy transfers from ions supporting the observations of Bernas et al, using kinetic Monto Carlo simulations.

Hematite thin films: growth and characterization

We have grown hematite (α–Fe 2 O 3) thin films on stainless steel and (001)-silicon single-crystal substrates by RF magnetron sputtering process in argon atmosphere at substrate temperatures from 400 to 800°C. Conversion Electron Mossbauer (CEM) spectra of the sample grown on stainless steel at 400°C exhibit values for hyperfine parameter characteristic of bulk hematite phase in the weak ferromagnetic state. Also, the relative line intensity ratio suggests that the magnetization vector of the polycrystalline film is aligned preferentially parallel to the surface. The X-ray diffraction (XRD) pattern of the polycrystalline thin film grown on steel substrates also corresponds to α–Fe 2 O 3. The samples were also analyzed by Atomic Force Microscopy (AFM), those grown on stainless steel reveal a morphology consisting of columnar grains with random orientation, given the inhomogeneity of the substrate surface.

Site location of Co in β-FeSi2

In order to reveal cationic site preference in β-FeSi2, Co-substituted samples synthesized by various techniques such as molecular beam epitaxy, ion implantation, and chemical vapor transport were investigated by Fe57 conversion electron Mössbauer (CEM) as well as Co57 Mössbauer emission (ME) spectroscopy. Literature on the structure of β-FeSi2 is somewhat contradictory, especially on the point of the population of the two iron sites in the orthorhombic structure. Co57 ME and Fe57 CEM spectra both showed two quadrupole split spectral components in the crystalline phase. Hyperfine parameters indicate that Co atoms substitute Fe in both Fe positions in the orthorhombic lattice. The aim of the present study was to get reliable results on the relative population of the two iron sites and determine the substitution of the iron sites by Co atoms in the β-FeSi2 lattice. The relative intensities of the two components in the absorption and emission Mössbauer spectra were found to be very similar for the samples prepared by different techniques, indicating a homogeneous distribution of Co on both Fe sites.

Conversion Electron Mössbauer Spectroscopic Study of Subsolidus Oxidation of Olivine

Single crystal samples of natural olivine, Mg1.82Fe0.17Ni0.01SiO4, have been oxidized in air at temperatures of 600 and 800°C, respectively, and studied by conversion electron and transmission Mossbauer spectroscopy. Oxidation of olivine was found to result in ferriolivine and spinel ferrite formation; the corresponding kinetic data also were discussed.

Photon and Ion-Beam Induced Processes in Materials Studied by Hyperfine Interactions

Laser nitriding and laser cementation are investigated by Mossbauer spectroscopy and complementary methods. It is demonstrated how the backscattering versions of Conversion Electron and Conversion X-ray Mossbauer spectroscopy can contribute to the investigation of surface processes, like the laser-induced formation of nitrides and carbides. Additionally, the formation of semiconducting iron disilicide can be achieved by ion-beam mixing and pulsed laser irradiation of Fe/Si bilayers. The results of both processes are compared.

Mössbauer studies of α″-Fe16N2 and α′-Fe8N films

Conversion-electron Mossbauer spectra of epitaxial α″-Fe16N2 and α′-Fe8N films have been studied and their differences are discussed in detail. The Mossbauer spectrum of α″-Fe16N2 can be decomposed into three subspectra, which correspond to the 4d, 8h and 4c sites. The Mossbauer spectrum of α′-Fe8N can be fitted using four spectra based on a nitrogen-atom-random-distribution model. The average hyperfine field is larger (3%) for α″-Fe16N2 than for α′-Fe8N, which is approximately consistent with a 4.1% enhancement of the magnetic moments for α″-Fe16N2. The iron moments tend to locate in the film plane for α″-Fe16N2 and to arrange perpendicularly to the film plane for α′-Fe8N.

Influence of surface on the effective magnetic fields in α-Fe2O3 and FeBO3

First measurements of the effective magnetic fields as a function of the depth at which the iron ions are in the surface layer of α-Fe2O3 and FeBO3 single crystals are reported. The method used is the depth-selective conversion-electron Mossbauer spectroscopy. An analysis of experimental spectra revealed that the magnetic properties of the crystal surface vary smoothly from the bulk to surface characteristics within a layer ∼100 nm thick. The layers lying below ∼100 nm from the surface are similar in properties to the bulk of the crystal, and their spectra consist of narrow lines. The spectral linewidths increase smoothly as one approaches the crystal surface. The spectra obtained from a ∼10-nm thick surface layer consist of broad lines indicating a broad distribution of effective magnetic fields. Calculations show that the field distribution width in this layer is δ=2.1(3) T, for an average value Heff=32.2(4) T. It has been experimentally established that, at room temperature (291 K), the effective magnetic fields smoothly decrease as one approaches the crystal surface. The effective fields in a 2.4(9)-nm surface layer of α-Fe2O3 crystals are lower by 0.7(2)% than the fields at the ion nuclei in the bulk of the sample. In the case of FeBO3, the effective fields decrease by 1.2(3)% in a surface layer 4.9(9) nm thick.

Depth-selective conversion-electron Mössbauer spectroscopy of the surface of Ba-M hexaferrite single crystals

The magnetic properties of a 50-to 2-nm-thick surface layer in hexagonal ferrite BaFe12O19 single crystals are investigated for the first time. Measurements are made on a conversion-electron Mossbauer spectrometer constructed on the basis of a unique ultrahigh-vacuum magnetostatic electron analyzer of the “orange” type. An analysis of the experimental Mossbauer spectra obtained from the surface layer of BaFe12O19 50-to 2-nm-thick single crystals reveals that (1) the spectral line widths are close to the natural widths of Mossbauer lines and neither changes nor a set of the values of effective magnetic fields, as well as a paramagnetic state of iron ions, could appear due to a defect such as the “surface,” and (2) the experimental spectra are best described only under the assumption that the surface of a hexaferrite BaFe12O19 single crystal contains a 2-nm-thick layer, in which the magnetic moments of iron ions are deflected through ∼20° from the crystallographic C axis along which the magnetic moments of ions located in the bulk of the crystal are oriented.

Critical behavior of the surface of FeBO3 single crystals

The behavior of the surface and near-surface layers of macroscopic FeBO3 single crystals is studied over the temperature range from 291 K to Neel temperature (T N ) using depth-selective conversion-electron Mossbauer spectroscopy. Three different phases or states, namely, an antiferromagnetically ordered phase (similar to the crystal bulk state), a surface phase, and a transition layer between them coexist near the Neel point in a surface layer ∼500 nm thick. The critical parameters found for the bulk phase agree well with the theoretical critical index νth≅0.63 predicted by the 3D Ising model. As the crystal surface is approached, the critical parameter β increases to 0.51(2) but remains smaller than the value of β=0.8 for the surface of a semi-infinite Heisenberg model. Therefore, the effective dimensionality of the system, being equal to 3 in the bulk, decreases at the crystal surface.

Phase analysis in α-Fe after high-dose Si ion implantation by depth-selective conversion-electron Mössbauer spectroscopy (DCEMS)

Si+ ions of 50 keV in energy were implanted into α-Fe (95% 57Fe) with a nominal dose of 5 × 1017 cm−2 at 350°C. The depth distribution of the Fe-Si phases formed by ion implantation after annealing at 300 and 400°C for 1 h was studied quantitatively by depth-selective conversion-electron Mossbauer spectroscopy (DCEMS). Ordered Fe3Si and e-FeSi was observed.

Surface investigation of excimer laser irradiated phosphatized steel plates

Steel plates (St 14-05) of 1.5 mm thickness and coated with 1.5 μm of ironzinc-phosphatehydrate (ASTM 29-1429) were irradiated with an XeCl-excimer laser (Siemens XP 2020) at energy densities of 20–80 mJ/mm2 and with 2–32 pulses per spot. Depth-sensitive Mossbauer spectroscopy was carried out by means of conversion electron (CEMS) and conversion X-ray (CXMS) Mossbauer spectroscopy in order to determine the phases produced by the excimer laser treatment. Although the phosphate layer is mainly ablated during the laser treatment, there is a significant formation of Fe2P. The phosphorous phase and the wustite, with changing stoichiometries, were found in the very surface (CEMS). In deeper layers and in correlation with the energy density and the number of pulses, austenite was found in surprisingly high amounts (CEMS and CXMS). The austenite was found to be nitrogen austenite. The high Fe-N austenite content as well as the presence of some ferromagnetic Fe-N phase (e-Fe2+xN) must be ascribed to an unexpectedly high nitriding effect during the laser treatment.