Depth-selective Conversion Electron Mossbauer Spectroscopy Research Articles

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.

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.

Lowering of magnetic field intensity at the surfaces of α-Fe2O3 and FeBO3 single crystals

Depth-selective conversion electron Mossbauer spectroscopy was used to study magnetic properties of the thin surface layers of the α-Fe2O3 and FeBO3 single crystals. An analysis of the experimental spectra indicates that the magnetic properties of the layers at a depth of more than ∼100 nm from the surface are similar to the properties of crystal bulk, and the corresponding spectra consist of narrow lines. The lines gradually broaden as the crystal surface is approached. The spectra of the ∼10-nm-thick surface layers consist of broad lines, indicating a wide distribution δ=2.1 T of the effective magnetic field about its mean value of 32.2(4) T. The experimental spectra were used to determine the effective magnetic fields (Heff) for the iron ions situated in the surface layers of thickness ∼100 nm. The effective fields in these layers were found to gradually decrease at room temperature (291 K) as the crystal surface was approached. The Heff values in the 2.4(9)-nm-thick surface layer of the α-Fe2O3 crystal and 4.9(9)-nm layer of FeBO3 are 0.7(2) and 1.2(3)%, respectively, smaller than for the nuclei of the ions in the bulk of these crystals.

Reduced magnetic hyperfine field at the surface of α-Fe 2 O 3 and FeBO 3 single crystals

The surface magnetic ordering of an α-57Fe2O3 and 57FeBO3 single crystal was studied by Depth Selective Conversion Electron Mossbauer Spectroscopy (DCEMS). The depth selective Mossbauer data of 57FeBO3 up to the Neel temperature (T N = 348.35 K) reveal the coexistence of the canted antiferromagnetic bulk phase and a magnetic surface phase that shows time relaxation. Both phases are separated by a wall (interim layer) and the thickness of the magnetic surface layer increases as the crystal is heated to T N.

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.

New high-resolution electrostatic electron spectrometer for conversion electron Mössbauer spectroscopy

The construction and performance of a new electron spectrometer using an electrostatic 90°-spherical deflection analyzer (SDA) is described. The spectrometer is suited for depth-selective conversion-electron Mossbauer spectroscopy (DCEMS) on surfaces and thin films in ultra-high vacuum (UHV) at temperatures ranging between 40–1000K using K- and L-conversion electrons of57Fe. The spectrometer is designed for 2% electron-energy resolution and about 5% (of 4π) transmission for 7.3 keV K-conversion electrons of57Fe. The energy resolution has been confirmed experimentally. We demonstrate that DCEMS in general and our instrument in particular are capable of detecting a thin (∼50 Fe A) Mossbauer-inert buried oxide layer located at the surface of a stainless steel substrate and covered by an Fe/Cr bilayer.

Depth selective CEMS in the energy range 0 to 20 keV

The “depth selectivity” can be expressed by weight functions T E, Θ ( x) giving the probability that electrons, starting in depth x, reach and leave the surface with an energy E, and an emission angle Θ relative to the surface normal. T E, Θ ( x) was available up to now only for K conversion electrons of iron in the energy range 6.3–7.3 keV. To extend the available depth range for 57Fe DCEMS, we calculated T E, Θ ( x) for energies down to about 2 keV, and up to 14.4 keV, including K, L and M conversion and KLL Auger electrons. Based on these results a program has been developed to simulate for a given sample composition DCEMS and also ICEMS spectra. First results of electron transport calculations for the 19.6 keV L conversion electrons of 119Sn indicate a reduced surface sensitivity in comparison to 57Fe, but the depth resolution seems to be reasonably good for deep layers.

Mössbauer study of martensitic transformation in Xe ion-irradiated type 304 austenitic stainless steel

Foil specimens of type 304 stainless steel have been irradiated with Xe+ ions in the range of 100–400 keV and 1×1020−1×1021 ions/m2 to elucidate the dynamics of the ion-induced martensitic phase transformation in stainless steel. It has been clearly shown by depth selective conversion electron Mossbauer spectroscopy (DCEMS) that the ion-induced martensitic phase in type 304 stainless steel has grown from the surface to a depth dependent both on the ion energy and the fluence of the Xe+ ions. Especially, we observed by means of DCEMS that the extension of the martensitic phase into the interior of stainless steel has been induced with increasing ion energy. It is concluded from these results that the depth distribution of the ion-induced martensitic phase is stress-induced by the formation of the highly pressurized Xe+ inclusion in type 304 stainless steel.

Problems of electron detection in Depth-Selective Conversion Electron Mössbauer Spectroscopy

Applications of Depth-Selective Conversion Electron Mossbauer Spectroscopy (DCEMS) are limited by the long measuring times needed for collecting sufficient data statistics. To shorten the recording time, the background should be reduced and the detection efficiency for conversion electrons should be improved. For57Co/57Fe DCEMS, systematic studies were performed to investigate the origin, shape, and structure of the background components in DCEMS data distributions for various samples using channeltrons and low-noise scintillation counters as electron detectors.

Remarks on depth selective CEMS — Backscattering measurements

Depth Selective Conversion Electron Mossbauer (MB) Spectroscopy measurements usually are done in backscattering geometry under bad geometrical conditions due to low resonant count rates. Furthermore in the case of iron these investigations are often done on samples enriched with57Fe, partially polarized. Because of these conditions the spectra are modified more or less strongly by the cosine smearing effect and/or saturation effects due to high enrichment of57Fe. We have investigated these effects in the case of an α-Fe-metal sample using a high transmission orange spectrometer.

Surface sensitivity of low energy electron Mössbauer spectroscopy (LEEMS) using57Fe

Very low energy electrons (LEE) (E e ≤15 eV) are produced with high intensity directly by Mossbauerabsorption and conversion in the case of57Fe [1, 4, 5]. These electrons should be very surface sensitive due to their very low attenuation length compared to the 7.3 keV K-Conversion electrons of57Fe [5, 11]. We have examined the surface sensitivity of these resonant LEE, using nonresonant56Fe metal and56Fe stainless steel foils coated with about 20 A and 50 A57Fe, respectively. They were exposed to air after evaporation: The 20 A samples are found to be fully oxidized [5]. Depth Selective Conversion Electron Mossbauer Spectroscopy (DCEMS), performed with a high transmission orange type magnetic spectrometer [5, 6, 13] reveals a two layer structure of the 50 A samples. Low Energy Electron Mossbauer Spectroscopy (LEEMS) [5] is found to be significantly more surface sensitive than conventional DCEMS, but not as surface sensitive as Auger Electron Mossbauer Spectroscopy (AEMS) using LMM-Auger electrons of 500–600 eV, as expected due to the different mean free path. But because of the very low intensity of these Auger electrons this mode appears to be not very useful for practical application.

Effect of dynamic loading on the structure of amorphous alloy Fe66Ni12Si20B2

Transmission- and depth-selective-conversion-electron Mossbauer spectroscopy (DCEMS) were applied to study variation of structural state in the bulk and in the surface layer (500, 1000, 1500 A thick) of amorphous alloy under certain optimum conditions of the dynamic loading. The alloy structure remains amorphous. However, a certain variation of the short range order. An amorphous oxide Fe2O3 is formed on the amorphous alloy surface.

DCEMS of neon implantation in yttrium iron garnet

57Fe-enriched, epitaxial Y3Fe5O12 films have been implanted with 50 keV and 100 keV neon ions with a dose of 4·1014Ne+/cm2. Depth-selective conversion electron Mossbauer spectroscopy has been performed at 300 K and 40 K. The results show that the 50 keV-implanted sample can be interpreted as an amorphous layer on top of an almost unperturbed YIG layer. In the 100 keV-implanted film a buried amorphous layer is observed.

Structural segregation in amorphous Fe40Ni40P14B6 under heat treatment and natural ageing

Transmission and depth selective conversion electron Mossbauer spectroscopy (DCEMS) were applied usefully to study structural transformation in the bulk and in the surface layer in a≈50 and 100 nm thick amorphous alloy that was subjected to annealing and natural ageing. Analysis of the hyperfine field distributionsP(H) shows the segregation of the amorphous matrix.

Icems and dcems study of Fe layers evaporated onto Al and Si

Thin layers of57Fe (2.5 nm, 10 nm and 70 nm thickness), vacuum evaporated onto Al and Si plates, have been investigated by conversion electron Mossbauer spectroscopy (CEMS). The measurements were performed employing both a proportional counter and a channeltron for conventional and ultrahigh-vacuum integral CEMS (UHV-ICEMS) studies, respectively, and a channeltron for depth-selective CEMS (DCEMS). The phase analysis of the layers on base of experimental results has indicated the presence of metallic iron and one or two iron compounds, ascribed to iron reaction products formed with the residual gas during evaporation. These products are most likely rather stable iron nitrides, are more or less clustered and are distributed throughout the whole layer.

The effect of 40 keV helium ion irradiation on Fe-B amorphous alloys

The phase conditions of amorphous Fe100-xBx alloys (x<or=25) irradiated with 40 keV He+ ions were studied by depth-selective conversion electron Mossbauer spectroscopy (DCEMS). A gas-flow-type proportional counter was used in the DCEMS measurements and the method is shown to be useful to detect the phase changes in the near-surface regions of the alloys. It is found that the crystalline phase of alpha -iron is induced on the irradiated surface of amorphous Fe80B20 and Fe85B15 but not on Fe75B25 and that the extent of the amorphous-to-crystalline transformation depends on the depth from the surface, decreasing towards the interior of the sample. It is likely that the irradiation leads to separation of the amorphous structure into boron-depleted and boron-enriched regions. It is suggested that an amorphous structure exists which is stable against irradiation after the formation of alpha -iron.

Structural separation of amorphous Fe40Ni40P14B6 alloy during natural ageing

Both the transmission and depth selective conversion electron Mossbauer spectroscopy (DCEMS) were applied usefully to study structural transformations in the bulk and in the surface layer ∼50 and 100 nm thick during three years natural ageing of an amorphous alloy. The primary crystallization was revealed in the surface layer ∼100 nm thick on the rough side of amorphous ribbon.

Ultra high vacuum electron spectrometer for ‘DSCEMS’

Depth Selective Conversion Electron Mossbauer Spectroscopy (DSCEMS) has become a very important surface science technique during the last one decade and attracted world wide interest specially because it gives more detailed knowledge of the properties and structure of the solid surfaces. The design and details of an ultra high vacuum cylindrical mirror analyser (CMA) for DSCEMS are described and illustrated.

Surface magnetism by depth-selective cems

Some applications of energy and angular selective (“depth-selective”)57Fe conversion electron Mossbauer spectroscopy (DCEMS) for the study of magnetic surfaces, subsurface layers and interfaces are reviewed, and a brief description of the method is given. Recent attemps to determine the magnetic hyperfine field at well-characterized Fe surfaces are discussed.

Mössbauer Characterization of Iron-containing Catalysts

Recent studies on Mossbauer characterization of iron-containing catalysts are presented. Instrumental advances on Mossbauer measurement made possible insitu analysis of the catalysts under reactive atmospheres. With insitu Mossbauer analysis, the surface species produced via reactions with synthetic gas on iron catalysts were identified as various carbides (e.g. χ-Fe5C2). For iron-containing platinum-group-metal catalysts, existence of anchoring iron (III) was estimated and a schematic model for the surface structure was proposed. Good correlations between iron content and chemical states of iron in such bimetallic systems were also observed. Also introduced for their future potentiality are specific applications of Mossbauer effect, source experiment, 99Ru Mossbauer spectroscopy, and depth-selective conversion electron Mossbauer spectroscopy which is a method for surface analysis like electron spectroscopy.