Depth Selective Conversion Electron Mossbauer Research Articles

Improvements in Depth Selective Electron Mössbauer Spectroscopy

The conditions for achievement of high resolving power of depth selective conversion electron Mossbauer spectroscopy method at a combined installation ‘electron spectrometer-nuclear gamma-resonance spectrometer’ have been obtained. There has been made a considerable step in development of the method at its realization at a magnetic sector electron spectrometer with double focusing, equipped with electron source (a sample under investigation) of large-area and position-sensitive detector. The paper presents a prospective symmetrical version of a magnetic sector electron spectrometer that allows realizing more completely possibilities of the method. It is noted that the proposed method is particularly valuable for investigations of nanosystems, nanostrutures that contain Mossbauer nuclei.

Magnetic phase in the near-surface region of an FeBO3 single crystal

An FeBO3 single crystal was studied from 291 K up to the Neel temperature T(N) = 348.35 K by depth selective conversion electron Mossbauer spectroscopy in ultrahigh vacuum (10(-9) mbar). A new magnetic near-surface phase was found. Its thickness D diverges on approaching T(N) and gives a critical exponent for the correlation length of nu = 0.59(4). The phase boundary between the bulk and near-surface phase could be identified.

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.

Conversion electron Mössbauer spectroscopy using a CAMAC data acquisition system (KODAQ)

For simple depth selective conversion electron Mossbauer spectroscopy (CEMS) by using a He+CH4 proportional counter, a flexible and versatile CAMAC data acquisition system (KODAQ) which works on a popular Japanese personal computer PC9801VX (NEC) was applied. It is demonstrated that CEM spectra of the oxidized Fe thin film which is composed of Fe3O4, Fe1−xO and α-Fe metal are successfully obtained.

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.

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.

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.

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.

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.

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.

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.

Corrosion processes and their inhibition as studied by Mössbauer conversion and other electron spectroscopies

To study corrosion processes of iron and steel and measures of their inhibition, a detailed knowledge of the phase composition and of phase transformations in very thin layers close to the attacked surface of the material is necessary. The information depths of integral (ICEMS) and depth selective (DCEMS) conversion electron Mossbauer spectroscopy are well suited for such investigations, but some effort is necessary if technical samples, i.e. nonenriched in57Fe, are to be studied. In many cases of practical importance, full information on the corroded surfaces cannot be got from Mossbauer spectra only, and a combination with Auger and photoelectron spectroscopies, in-including scanning and sputter options, is found to be most informative. This is demonstrated by three examples.

A CEMS/AES study of the passivation of iron

The passivation of iron and steel (DIN 1623) was studied by integral and depth selective conversion electron Mossbauer spectroscopy and Auger electron spectroscopy. Thickness and phase composition of the passive layer formed in sulphate solution and in a phosphate buffer were determined in dependence on the anodic potential and the duration of the passivating procedure. The experimental results lead to the conclusion that not the whole oxidic layer is responsible for the passivity but only a very thin intermediate layer formed between the cubic substrate and the rhombic oxide (γ-FeOOH) cover.

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.

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.

Depth selective conversion electron mössbauer spectroscopy: comparison of experiment with Monte Carlo simulation of electron scattering

Abstract Recently reported experimental results in depth selective conversion electron Mossbauer spectroscopy have been compared with theory based on Monte Carlo simulation of the electron scattering in the absorber. Good agreement is found. The Monte Carlo simulation demonstrates clearly the importance of the angular interval selected by the electron spectrometer entrance aperture.