Conversion Electron Mossbauer Spectroscopy Research Articles

Structural Transformations in Duplex Stainless Steel CF8 Under Intensive Cold Plastic Deformation

The austenitic–martensitic transformation in austenitic–ferritic duplex stainless steel CF8 subjected to cold plastic deformation with a deformation degree ε = 10–95% is studied here using transmission Mössbauer spectroscopy (MS), conversion electron Mössbauer spectroscopy (CEMS), and X-ray diffraction (XRD) methods. It is assumed that the α′-martensite phase appeared at ε > 10%. The CEMS results showed that the formation of α′-martensite occurred most intensively in the near-surface layers of the steel, distributing in depth with the growth of the deformation degree. The volume fraction of the α′-martensite was determined based on the results of calculations carried out via the MS and XRD methods, and a good correlation was observed. A modified Olson–Cohen model was proposed to determine the dependence of the amount of α′-martensite on the deformation degree ε. The coefficients included in the Olson–Cohen expression were found.

Formation of magnetic nanoclusters in Fe implanted amorphous and crystalline SiO2

Conversion electron Mössbauer Spectroscopy (CEMS) studies have been conducted on Fe implanted amorphous and crystalline SiO2 which were annealed in air up to a temperature of 1000oC. For both samples, dramatic changes set in after the 1000oC anneal and the CEM spectra are dominated by strong ferromagnetic sextets. In the amorphous sample, the sextet is characterized by magnetic hyperfine fields of 33T, 31T and 29 T, consistent with the formation α-Fe nanoclusters. In the crystalline sample the ferromagnetic sextet has spectral parameters of δ = 0.38(3) mm/s and Bhf = 52.3T, consistent with formation of α-Fe2O3 clusters, reflecting the precipitation of the implanted Fe into such clusters. The line ratios of lines 1, 2 and 3 (and 6, 5 and 4) of the sextet are 3:4:1, reflecting alignment of the magnetic moment of the precipitates normal to the c-axis of the sample surface.

Phase Changes in the Surface Layer of Stainless Steel Annealed at a Temperature of 550 °C.

Stainless steels have the advantage of forming a protective surface layer to prevent corrosion. This layer results from phase and structural changes on the steel surface. Stainless steel samples (1.4404, 316L), whose alloying elements include Cr, Ni, Mo, and Mn, were subjected to the study of the surface layer. Prism-shaped samples (25 × 25 × 3) mm3 were made from CL20ES stainless steel powder, using selective laser melting. After sandblasting with corundum powder and annealing at 550 °C for different periods of time (2, 4, 8, 16, 32, 64, 128 h), samples were studied by conversion X-ray Mössbauer spectroscopy (CXMS), conversion electron Mössbauer spectroscopy (CEMS), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray diffraction (XRD). The main topics of the research were surface morphology and elemental and phase composition. The annealing of stainless steel samples resulted in a new surface layer comprising leaf-shaped crystals made of chromium oxide. The crystals grew, and their number increased as annealing time was extended. The amount of chromium increased in the surface layer at the expense of iron and nickel, and the longer the annealing time was set, the more chromium was observed in the surface layer. Iron compounds (BCC iron, mixed Fe-Cr oxide) were found in the surface layer, in addition to chromium oxide. BCC iron appeared only after annealing for at least 4 h, which is the initial time of austenitic-ferritic transformation. Mixed Fe-Cr oxide was observed in all annealed samples. All phase changes were observed in the surface layer at approximately 0.6 µm depth.

Study of radiation-stimulated structural phase transformations in iradiated Zr by Mössbauer spectroscopy

The article considers the study of the influence of high-energy singly charged 57Fe ions with energy of 1 MeV, a beam current on the target of ~100 nA, a dose of 5x1016 ion/cm2, on the structural-phase state of zirconium. Using the methods of Mössbauer spectroscopy in transmission geometry (MS), conversion electron Mössbauer spectroscopy (CEMS), and scanning electron microscopy (SEM), we studied the effect of high-energy singly charged 57Fe ions on the structural-phase state of zirconium. The projective ranges of 57Fe ions in zirconium were determined, the number of target atoms subjected to irradiation was calculated, and the number of displacements caused by these ions was estimated. Radiation damage to zirconium materials was simulated by irradiating samples at a charged particle accelerator with probe Mössbauer atoms to obtain nuclear physical data from the zone of influence of these atoms on the structural and phase state of irradiated samples under conditions close to reactor conditions. The electronic state of the implant in these materials was evaluated. The concentration of implanted atoms in the near-surface layer and the volume of samples was calculated. In particular, it was shown that the projective range of 57Fe ions in zirconium was 497 nm, the total number of displaced atoms was 3.4х1020, and the number of displacements per atom (dpa) was 159. The solubility of Fe in Zr is 0.03%. However, when zirconium is irradiated with 57Fe ions, as a result of thermal recombination of vacancies and interstitial atoms within the cascade of displacements caused by thermal peaks, intermetallic compounds Zr3Fe, ZrFe2 and a Zr (Fe) solid solution are formed.

Mössbauer Spectroscopy for Additive Manufacturing by Selective Laser Melting

Selective laser melting (SLM) is a technology of layer-by-layer additive manufacturing using a laser. This technology allows one to get complex-shaped, three-dimensional (3D) specimens directly from metal powder. In this technology, various metal powders are used, including different steels. Stainless steel 1.4404 (CL20ES) and maraging steel 1.2709 (CL50WS) have been investigated. The surface of samples manufactured from CL20ES and CL50WS powders by SLM (with and without combination sandblasting and annealing) was studied by conversion X-ray Mössbauer spectroscopy (CXMS) and conversion electron Mössbauer spectroscopy (CEMS). The surface morphology, elemental composition, and structure were examined by scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDS), and X-ray powder diffraction (XRD). Samples with sandblasted (corundum powder) and non-sandblasted surfaces were annealed at 540 °C (CL50WS) or 550 °C (CL20ES) for 6 h in air. Oxidation processes on surfaces of samples manufactured from both initial powders were observed after post-process annealing by CEMS and CXMS, as well as confirmed by XRD. The transformation of the austenitic to ferritic phase was observed in a sandblasted and annealed CL20ES sample by CEMS and XRD.

Spin‐Charge Conversion in Fe/Au/Sb2Te3 Heterostructures as Probed By Spin Pumping Ferromagnetic Resonance

AbstractLarge‐area antimony telluride (Sb2Te3) thin films are grown by a metal organic chemical vapor deposition technique on 4” Si(111) substrates, and their topological character probed by magnetoconductance measurements. When interfaced with Fe thin films, broadband ferromagnetic resonance spectroscopy (BFMR) shows a clear increase of the damping parameter in Fe/Sb2Te3 when compared to a reference Fe layer, which may suggest the occurrence of spin pumping (SP) into Sb2Te3. Simultaneously, X‐ray reflectivity and conversion electron Mossbauer spectroscopy evidence the development of a chemically and magnetically pure Fe/Sb2Te3 interface. However, by conducting SP‐FMR, it is shown that no spin‐to‐charge conversion (S2C) occurs in Fe/Sb2Te3, while a clear SP signal develops by introducing a 5 nm Au interlayer between Fe and Sb2Te3, with a measured inverse Edelstein effect conversion efficiency of λIEE = 0.27 nm. The results shed some light on the correlation among the chemical‐structural‐magnetic properties of the Fe/Sb2Te3 interface, the broadening of the magnetic damping parameter as detected by BFMR, and the occurrence of S2C, as probed by SP‐FMR.

Surface Micromorphology and Structure of Stainless and Maraging Steel Obtained via Selective Laser Melting: A Mössbauer Spectroscopy Study

Selective laser melting (SLM) as an additive manufacturing method makes it possible to quickly produce complexly shaped three-dimensional (3D) metal specimens from a powder. This work describes how SLM affects the surface phase composition of a 3D printed specimen, as analyzed with conversion electron Mössbauer spectroscopy (CEMS), conversion X-ray Mössbauer spectroscopy (CXMS) and X-ray diffraction (XRD). Both stainless 1.4404 (CL20ES) steel and maraging 1.2709 (CL50WS) steel have been investigated. A transformation of the phase composition from the ferritic phase into an austenitic one was proven by comparing the initial CL50WS powder and the final specimen using CXMS. This transformation takes place during the SLM process. No transformation was identified in stainless steel. The differences identified via CEMS between the surface phase composition of the final non-annealed specimens and the surface of the final annealed specimens demonstrated the oxidation of the surface layer. The oxidation occurs during the annealing of the sample in surface layers less than 1 μm thick. The quality of the surface was examined using scanning electron microscopy (SEM), which presented imperfections on the face of the final specimen. Granules of the initial powder bonded to the surface of the specimen and both irregular and spherical pores were observed.

Phase Structure Evolution of the Fe-Al Arc-Sprayed Coating Stimulated by Annealing.

The article presents the results of research on the structural evolution of the composite Fe-Al-based coating deposited by arc spray with initial low participation of in situ intermetallic phases. The arc spraying process was carried out by simultaneously melting two different electrode wires, aluminum and low alloy steel (98.6 wt.% of Fe). The aim of the research was to reach protective coatings with a composite structure consisting of a significant participation of FexAly as intermetallic phases reinforcement. Initially, synthesis of intermetallic phases took place in situ during the spraying process. In the next step, participation of FexAly fraction was increased through the annealing process, with three temperature values, 700 °C, 800 °C, and 900 °C. Phase structure evolution of the Fe-Al arc-sprayed coating, stimulated by annealing, has been described by means of SEM images taken with a QBSD backscattered electron detector and by XRD and conversion electron Mössbauer spectroscopy (CEMS) investigations. Microhardness distribution of the investigated annealed coatings has been presented.

Magnetic structure and phase transition at the surface region of Fe3O4(100)

The magnetic structure and phase transition of the near-surface region of Fe3O4(100) was investigated by 57Fe conversion electron Mössbauer spectroscopy (CEMS) and theoretical calculations. It is revealed that at 300 K the magnetization is in-plane in the surface region and cants from the in-plane to the 〈111〉 direction in a deeper region suggesting the presence of a noncollinear magnetic structure. The critical exponents for the tetrahedral and octahedral sites are estimated to be 0.24 ± 0.01 and 0.28 ± 0.01, respectively. Near the critical temperature, furthermore, the magnetization direction in the surface region was found to deviate from the in-plane direction.

PHYSICAL PRINCIPLES OF THE MESSBAUER EFFECT FOR SOLVING INNER-NUCLEAR PHENOMENA IN SOLIDS

In this work, using a number of examples, it is shown that one of the promising methods for studying the physicochemical state of matter is Mössbauer spectroscopy with all the variety of its methodological approaches. The processes occurring in the shell of atoms have a negligible effect on intranuclear phenomena, and often turn out to be inaccessible for detection by other methods. Mössbauer spectroscopy reveals these influences. Four methodological approaches have been implemented and an experimental base has been created for observing the resonant absorption of квантов-quanta by 57Fe nuclei. Techniques for absorption (MS), emission Mössbauer spectroscopy (EMS), characteristic X-ray radiation (CXR), conversion electron Mössbauer spectroscopy (CEMS) allow, practically without restrictions on the elemental composition and geometric dimensions of samples (from bulk to nanometer), to carry out studies

Probing the origins of magnetism in 2 at% Fe-implanted 4H-SiC

Here we reported an about 2 at.% Fe-implanted p-type 4H-SiC Diluted Magnetic Semiconductor (DMS). Scanning Transmission Electron Microscopy (STEM) and Atom Probe Tomography (APT) techniques were used to investigate the origins of ferromagnetic behavior. Nanoparticles (1–9 nm) were identified to be Fe3Si, Fe2Si, Fe5Si3, Fe3Si2, FeSi and FeSi2 phases, the distribution of which is related to Fe distribution. The magnetic properties were studied by Conversion Electron Mössbauer Spectroscopy (CEMS) and Superconducting Quantum Interference Device (SQUID) techniques. The contributions of these Fe-silicides to the ferromagnetism are thus discussed from a quantitative perspective.

Depth-dependent ferromagnetic spin structure and asymmetric magnetization reversal in exchange-biased Fe/FeMn bilayers

Depth-dependent spin rotation of the Fe spins upon magnetisation reversal of the ferromagnetic layer in polycrystalline Fe/FeMn exchange-biased bilayers was investigated by using 57Fe probe layers and angular hysteresis loops constructed from in-field 57Fe conversion electron Mössbauer spectroscopy (CEMS) measurements and magnetometry. Our results provide evidence for the formation of an exchange-spring-like domain wall structure inside the ferromagnetic Fe layer during magnetisation reversal along the forward branch of the hysteresis loop (H values varying from positive to negative), and coherent reversal along the recoil branch (H varying from negative to positive). This result implies asymmetric magnetisation reversal for each branch of the hysteresis loop, as indicated by depth-resolved CEMS and vibrating sample magnetometry. These results are relevant for the development of more precise quantitative models for exchange bias in polycrystalline metallic systems with Mn-based antiferromagnets such as those used in spintronic applications.

The influence of the atomic scale interface roughness on the GMR effect in Fe/Cr multilayers

In this study, we analysed Fe/Cr multilayers that had been modified by Bi, In, and Pb surfactants. Their structure, morphology and interface roughness were investigated using low-energy electron diffraction, Auger electron spectroscopy (AES), X-ray reflectivity and conversion electron Mössbauer spectroscopy (CEMS). The magnetic and magnetotransport properties of the multilayers were studied by measuring the Kerr effect and four-point magnetoresistance, respectively.We observed that the Bi surfactant caused a slight interface smoothening while the introduction of In and Pb surfactants had a negative influence on the epitaxial growth of Fe and Cr, which were associated with an increase of interface roughness. These changes did not destroy the layer continuity, which was confirmed by AES measurements. The magnetic measurements revealed an antiferromagnetic coupling of the Fe layers for the as-deposited sample as well as for the Bi and Pb-modified multilayers and its partial reduction for the In-modified sample. We observed that the increase of interface roughness that had originated from atom intermixing resulted in a reduction of the giant magnetoresistance. In these investigations in which CEMS studies were used, we were able to distinguish between the interface roughness that was caused by atom intermixing and interface corrugation.

Probing the Origins of Magnetism in 2 at.% Fe-implanted 4H-SiC

Here we reported an about 2 at.% Fe-implanted p-type 4H-SiC Diluted Magnetic Semiconductor (DMS). Scanning Transmission Electron Microscopy (STEM) and Atom Probe Tomography (APT) techniques were used to investigate the origins of ferromagnetic behavior. Nanoparticles (1-9 nm) were identified to be Fe3Si, Fe2Si, Fe5Si3, Fe3Si2, FeSi and FeSi2 phases, the distribution of which is related to Fe distribution. The magnetic properties were studied by Conversion Electron Mossbauer Spectroscopy (CEMS) and Superconducting Quantum Interference Device (SQUID) techniques. The contributions of these Fe-silicides to the ferromagnetism are thus discussed from a quantitative perspective.

Conversion electron Mössbauer spectrometer with a YAG:Ce scintillator

In this work we report the development of a new scintillation detector of conversion electrons for 57Fe conversion electron Mossbauer spectroscopy. The detector, a photomultiplier tube with a 5 μm YAG:Ce scintillator, is sensitive to both X-rays and (conversion) electrons. Electrons and X-rays have different average depths from which they come out of the sample. The detected electrons are much more likely to come from close to the surface, whereas X-rays can come from greater depths in the sample. By applying an electric field over the sample and detector, electrons were accelerated towards the detector making more electrons being detected and giving them higher energies. On the other hand, X-rays were not influenced by the electric field making the electrons distinguishable from X-rays and the background.

Influence of flexoelectricity on the spin cycloid in (110)-oriented BiFeO3 films

The influence of film orientation, strain relaxation, and flexoelectric fields on the stability of the spin cycloid in (110)-oriented $\mathrm{BiFe}{\mathrm{O}}_{3}$ epitaxial films grown on $\mathrm{LaAl}{\mathrm{O}}_{3}$ substrates is investigated. By means of advanced x-ray-diffraction techniques, we show that thinner films have very large strain gradients which give rise to high flexoelectric fields. Using low-energy Raman spectroscopy and conversion electron M\"ossbauer spectroscopy (CEMS) we show that films up to 53 nm thick possess collinear antiferromagnetic order, with no cycloidal modulation. This suppression of the cycloid is proposed to be from strain and strain-gradient-induced flexoelectric fields. On the other hand, films thicker than 90 nm show a complex spin texture consistent with two separate cycloids, likely with different propagation directions. Interestingly, CEMS analysis suggests that the two cycloids have the same spin rotation plane. The multiple cycloids are suggested to arise from different ferroelastic domains (in turn influenced by twinning in the substrate) with different strain relaxation behaviors. These results offer insight into the factors that influence cycloid stability in the less common (110) film orientation and have implications for future magnonic devices.

Influence of surface treatment on microstructure of stainless steels studied by Mössbauer spectrometry

Examination of surface microstructure of stainless steels is presented. Transmission Mossbauer spectrometry and Conversion Electron Mossbauer Spectrometry which provide information from the bulk and surface regions, respectively, were used. We concentrate on structural modifications that were caused by surface treatments including grinding, polishing, and electrolytic etching. Scanning Electron Microscopy with Energy Dispersive Spectrometry was adopted for visualization of surface differences. Formation of magnetic phases was revealed in the surface regions of X6CrNiTi1810 steel while no substantial effect of surface treatment was found in the ATABOR steel. Bulk regions in both steels are not affected by surface treatment.

Is “expanded austenite” really a solid solution? Mössbauer observation of an annealed AISI 316L nitrided sample

Expanded austenite, also known as S phase, m phase or γN phase forms during low temperature (<420 °C) nitriding of austenitic materials containing chromium. In this communication, annealing of a nitrided sample is used as a tool to understand the nature of expanded austenite. Starting from an AISI 316LN sample plasma nitrided for 4 h at 410 °C, the decomposition of expanded austenite is studied by conversion electron Mössbauer spectroscopy (CEMS) and X ray diffraction (XRD) for different annealing times at 400 °C. For the as nitrided sample, the CEMS spectra are analysed using two hyperfine field distributions (HFD). HFD A is attributed to a nitrogen enriched expanded austenite, HFD B is attributed to a bcc environment without or with very low nitrogen content. Both the local deformation of the bcc lattice and the presence of other elements as iron neighbouring atoms explain the broad range of hyperfine field in HFD B. The vanishing of HFD A, after 2 h of annealing, is explained by the formation of short-range order (SRO) between chromium and nitrogen and by the redistribution of nitrogen in excess. For the 3 h of annealing, a bcc phase is observed by XRD. We propose to describe this phase as martensite. An interplay between local internal stress and local nitrogen content is proposed to explain the unexpected formation of such a martensitic phase in a strongly gamma stabilizing environment. From the microscopic point of view expanded austenite can be considered to be constituted of at least two different environments: a gamma environment supersaturated in nitrogen and a martensitic environment without nitrogen.

Fe3C nanoparticle formation in Fe implanted HOPG and CVD diamond

We have conducted conversion electron Mossbauer spectroscopy (CEMS) studies of Fe-carbides produced in Fe implanted highly oriented pyrolytic graphite (HOPG) and CVD diamond substrates. The samples were implanted at room temperature with 57Fe ions with 40 keV and 60 keV, respectively, and corresponding fluences of 1.0·1016/cm2 and 5·1015/cm2. The formation of magnetic structures in the spectra was monitored with CEMS measurement after annealing the samples at temperatures up to 700 °C. After the high temperature annealing, the main components in the spectra were sextets whose line shapes displayed strong, asymmetric distortions, consistent with those due the formation of nanoclusters. The hyperfine magnetic fields Bhf = 21.6 T, 20.0 T and 18 (1) T were in good agreement with those observed previously for Fe3C nanoclusters 10–20 nm in size.

Thermal annealing effects on the structural, magnetic and hyperfine properties of the Fe/SnO2/Fe thin film deposited by RF sputtering method

In the present work, the effect of the post-deposition annealing process in an air atmosphere on the structural and magnetic properties of the ternary Fe/SnO2/Fe thin film, deposited by the sputtering technique was reported. X-ray diffraction (XRD) pattern confirms the formation of alpha-iron, magnetite and tin dioxide phases in the as-deposited film. Meanwhile, after the thermal annealing, the formation of hematite and tin dioxide phases was determined. Magnetization (M) measurements reveal some features after the annealing: i) a reduction of the coercive field (HC), which has been assigned to the presence of hematite, and ii) the transformation of alpha-iron/magnetite phases into the hematite phase in accordance with results obtained from XRD data analysis. Those results were also confirmed by room temperature Conversion electron Mössbauer spectroscopy (CEMS) measurements carried out using a 57Co(Rh) source. Furthermore, the doublets of the CEMS spectrum of the as-deposited film (21%) has been assigned to the signal of Fe3+ and Fe2.5+ (mixture of Fe3+ and Fe2+) ions, which diffuse into the SnO2 matrix. The spectral area of those doublets is clearly reduced to 3% after the thermal annealing. It suggests the migration of the Fe ions from the non-magnetic phase to the magnetic phase. On the other hand, CEMS spectrum of the as-deposited film carried out with using a 119 mSn source is well modeled with a doublet and a sextet. The doublet was assigned to Sn4+ ions of the SnO2 phase; meanwhile, the sextet was assigned to Sn4+ ions located at interstitial and/or substitutional sites of the alpha-iron/magnetite phases. Those tin ions sense the supertransferred hyperfine field from Fe neighbor. After the thermal annealing, the CEMS spectrum was well modeled with only a doublet related to SnO2 phase, suggesting that the tin ions have been out diffused from the magnetic phase (alpha-iron and magnetite) and incorporate into the no magnetic o-SnO2.