Mössbauer spectra and electronic structure of nonequilibrium Fe-Cu alloys produced by vapor quenching

Mössbauer spectra have been observed for nonequilibrium bcc and fcc Fe–Cu alloys sputter-deposited at several Ar gas pressures,P _Ar. These alloys are ferromagnetic at low temperatures and show sextet spectra. The fcc alloys which are paramagnetic at 290 K show asymmetric doublet spectra, indicating no serious segregation. In the alloys deposited at highP _Ar, the weak intensity ratios of the second and fifth lines of the sextet indicate a tendency of perpendicular magnetic anisotropy, while a large magnetic hyperfine field component of about 40 MA/m (500 kOe) at 4.2 K and a large quadrupole splitting component of about 0.7 mm/s at 290 K imply CuFeO₂ formation. The nonequilibrium, bcc and fcc Fe–Cu, alloys are maintained below 500 K and the phase separation is detected above 550 K. X-ray photoemission spectroscopy studies of these alloys reveals individual Fe- and Cu-d bands. The concentration dependence of peak intensities and peak positions indicate that Fe and Cu electronically intermix.     

Thermal stability of high concentration Fe-Ag and Fe-Cu alloys produced by vapor quenching

Mössbauer effect measurements were carried out for sputtered fcc Fe-Ag and Fe-Cu alloys annealed at various temperatures. At temperatures higher than 300 °C, the metastable fcc phases decompose by removing saturated Fe atoms. During the phase separation processes, the ejected Fe atoms form clusters, which initially have a fcc structure and transform to bcc particles as their sizes grow beyond a critical value.     

A systematic study of the shape of57Fe Mössbauer spectra in disordered ferromagnetic fcc Fe−Pt alloys

We investigate Fe–Pt alloys in the concentration range 24.9–33.9 at% Pt and determine the hyperfine parameters of Mössbauer spectra measured at about 20 K.     

Microstructure of supersaturated fcc Al–Fe alloys: A comparison of rapidly quenched and mechanically alloyed Al98Fe2

Alloys of the composition Al₉₈Fe₂ have been prepared by rapid quenching from the melt and mechanical alloying methods and have been studied by Xray diffraction techniques and room temperature ⁵⁷Fe Mössbauer effect methods. Results may be summarized as follows: The rapidly quenched sample is a single phase supersaturated fcc Al–Fe alloy. Mössbauer effect spectra indicate the presence of a substantially greater degree of Fe clustering than is expected for a random distribution of atoms on the lattice sites. Mechanically alloyed samples have been studied as a function of milling time and show the initial formation of a supersaturated fcc phase with microstructural properties which are quite similar to those of the rapidly quenched sample. Further milling results in the reduction of the average grain size and the formation of an amorphous phase. Mössbauer studies and previously reported phase diagrams suggest that a substantial fraction of the Fe resides in this phase.     

Mössbauer effect study of antiferromagnetic Fe–Mn–Si alloys

Antiferromagnetic Fe–30Mn–Si alloys containing 2.0–8.7 at.% Si are known to exhibit several attractive physical properties at Néel temperatures which render them candidate materials for functional alloys applications. The Néel transitions and anomalous transport phenomena have been studied extensively in a wide temperature range. In the present work, the hyperfine interactions are studied by Mössbauer spectroscopy measured at temperatures 95–623 K. It is found that the Mössbauer spectra are singlets at temperatures above the Néel temperature and doublets below the Néel temperature. The alloys have a small hyperfine field around the Fe nuclei below the Néel temperature and the hyperfine field increases linearly with increasing silicon concentration. This can be explained by the presence of a localised net magnetic moment on the Fe nuclei which is induced by the silicon atoms. A decrease in isomer shift with increasing silicon concentration is observed and this can be accounted for by the change in the occupation of the Fe 3d shell. There is a small quadrupole splitting, it increases with increasing silicon concentration, and is consistent with the lattice shrinking and magnetostriction.     

Mössbauer study of Fe-Al disordered alloys near the critical concentration

Disordered bcc Fe_1–q Al _q alloys in the composition range 0.5q0.6 were studied by Mössbauer effect measurements. The Mössbauer spectra at 300 K of all the samples consist of two paramagnetic sites, one is a singlet and the other a doublet with quadrupole splitting. The results can be interpreted by considering that the sites of this disordered system are arranged near the configurations of the Fe and Al sites of the Fe-Al ordered system.     

Magnetic Structure of FexCu100−x Magnetoresistive Alloys Produced by Mechanical Alloying

Fe _x Cu_100–x magnetoresistive alloys were produced by mechanical alloying. X-ray diffraction shows fcc structure. The room-temperature Mössbauer spectra evolves from an asymmetrical doublet below x=25%, to a broad magnetic hyperfine field distribution above this concentration. Quadrupole splitting of the doublet varies between 0.48 and 0.57 mm/s, and its isomer shift from 0.16 to 0.29 mm/s. Low-temperature Mössbauer spectroscopy displays a B _hf distribution. Magnetization measurements display different features depending on concentration, from mictomagnetism to ferromagnetism. Low-temperature magnetoresistance is measured. Samples with x20% exhibit larger magnetoresistivity ratios. Bulk and hyperfine magnetic properties are correlated in order to explain magnetoresistivity features of these samples.     

Effects of Heavy Ion Irradiation on Short-Range Order of Fe–Cr–P–C Amorphous Alloys Investigated by Mössbauer and DSC Measurements

Rapidly quenched Fe–Cr–P–C amorphous alloys irradiated with 209 MeV energy ⁸⁴Kr ions were investigated by Mössbauer spectroscopy, X-ray diffraction, and differential scanning calorimetry measurements. Significant changes were found between Mössbauer spectra of the irradiated and non-irradiated amorphous alloys. The changes were analysed by the hyperfine field distribution method, too. The crystallisation temperature as well as the phase composition at a certain stage of crystallisation were found to be different in the non-irradiated and irradiated samples. These results reflect the changes in the short-range ordering due to the irradiation with energetic heavy ions.     

Mössbauer and X-ray study of rapidly quenched and mechanically alloyed Al-Fe alloys

Mössbauer spectroscopy and X-ray diffractometry were used to study the effect of the cooling rate as well as of the milling time on the structure of rapidly quenched Al-6.8% Fe, Al-0.5% Fe and mechanically alloyed Al-8% Fe alloys. The main phase of the rapidly quenched alloys was identified as Al _m Fe besides Al₆Fe andAlFe solid solutions. In the mechanically alloyed samples (with milling time between 1.5–43 hours), we have found -Fe, AlFe solid solution and a third phase characterized by a doublet with Mössbauer parameters which are not so far from those of clusters inAlFe alloys. We have observed a continuous increase of the quantity ofAlFe solid solution, together with a significantly less increase of the third phase as a function of the milling time. Simultaneously, the quantity of alpha-iron has gradually decreased.     

Characterization and magnetic properties of Fe–Co ultrafine particles

Characterization and magnetic properties of Fe–Co ultrafine particles were investigated systematically by means of X-ray diffraction, Mössbauer spectroscopy, X-ray photoelectron spectroscopy, transmission electron microscopy, energy disperse spectroscopy analysis, chemical analysis, oxygen determination and magnetization measurement. In comparison with bulk iron–cobalt alloys, the corresponding Fe–Co ultrafine particles have significant difference in the phase structure and magnetic properties, depending on the condition of evaporating and subsequent quenching. The mechanism for the formation of the ultrafine particles as well as the origin of ferromagnetism and paramagnetism (or superparamagnetism) were discussed.     

Mössbauer Spectroscopy Study of Fe–Si Solid Solution Prepared by Mechanical Milling

A specimen of Fe–Si solid solution is prepared by ball milling of proper amounts of the pure elements (3Fe:Si) for different milling times. X-ray diffraction and Mössbauer spectroscopy have been used to characterize the solid solution. The Mössbauer spectra show four different sites corresponding to Fe atoms in a bcc structure having 0, 1, 2 and 3 Si atoms in the 1st nearest-neighbor (nn) shell. The hyperfine magnetic field decreases by 31 kOe for each Si atom in the 1st nn shell. A magnetic component with hyperfine field around (180 kOe) characterized by a broadened sextet was observed which could be due to iron sites having more than 3 Si atoms in the 1st nn. A theoretical model based on the binomial distribution was adopted to analyze the data. Good agreement between the experimental and the theoretical hyperfine field distribution in the high hyperfine field region was found, and the silicon content in the disordered A2 phase is deduced from the parameters which give the best agreement.     

57Fe Mössbauer study of Eu1−x Sr x FeO3−y

The samples of Eu_1–x Sr _x FeO_3–y (x=0.0–1.0) were prepared by the solid state reaction method. Their X-ray diffraction patterns and⁵⁷Fe Mössbauer spectra at room temperature were measured. It is found that Sr ions incorporate in the lattice of EuFeO₃, the change of crystal structure is related to the dopant.⁵⁷Fe Mössbauer spectra consist of one magnetic, one doublet and one single paramagnetic components. The Fe ions in the cubic phase are in intermediate valence state between Fe(III) and Fe(IV) and may participate in electron hopping.     

Coordination and Oxidation States of Iron Incorporated in Mesoporous MCM41

Mesoporous Fe–MCM41 samples (Si/Fe=25) were synthesized and characterized under evacuation and reducing/oxidizing treatments by in situ FTIR and Mössbauer spectroscopies. Both Fe(II) and Fe(III) located in low coordination states in top layers of pore walls exhibit Lewis acidity and may participate in Fe(III) Fe(II) processes at low temperatures (570 K). Furthermore, Fe(III) Fe(II) cycles can be achieved and repeated with participation of the full amount of iron at higher temperatures (670 K). The accompanying formation of oxygen vacancies and restoration of the structure in the reverse process does not result in extended damages; the MCM41 structure retains its stability under the conditions applied.     

Fe–Mn–Al–C Alloys: a Study of Their Corrosion Behaviour in SO2 Environments

The corrosion reaction of four Fe–Mn–Al alloys exposed to a cycling, dry–humid, SO₂ (0.001% by volume) polluted atmosphere was studied. ICEMS, XPS, AES-SAM and transmission Mössbauer spectroscopy at different temperatures were employed to characterize the corrosion products. The analytical results indicate that (i) ferrihydrite is the main component of the rust; (ii) there is an abundant presence of Mn²⁺ and SO₃ ^2–/SO₄ ^2– on the top of the corrosion layer, the concentration of SO₄ ^2– increasing with the number of cycles; and (iii) the magnetic hyperfine pattern exhibited by the series of low-temperature spectra of the rust is quite different from that observed in the rust formed under similar corrosive environments on iron and weathering steel. This latter finding is correlated with a slow rate of transformation of the Fe³⁺ species formed at the early stages of corrosion into -FeOOH, the usual final product of this type of corrosion processes. The sulphate anions, abundant inside the electrolyte during the wet periods, could be incorporated to the ferrihydrite structure being responsible for the Mössbauer spectral pattern recorded from the corrosion products at low temperatures.     

A study of Fe grown on muscovite with Mössbauer spectroscopy

Mössbauer spectroscopy has been used to study Fe grown on muscovite by thermal evaporation in 10^–6 torr; there is evidence of hcp structure as far as thickness of 7,000 Å; no evidence was found for magnetic ordering. Quadrupole splitting of –0,42 mm/s is attributed to hcp Fe; the resonance lines are broader than that of bcc resonance lines. Results are presented on the isomer shift.     

In-beam implantation of iron into germanium,silicon and diamond studied by the Mössbauer effect

The Mössbauer effect of⁵⁷Fe implanted into diamond structure semiconductors, Ge, Si and C, has been studied by in-beam implantation of⁵⁷Fe ions, which were excited to the 14 keV state by a³⁵Cl beam from a tandem Van de Graaff accelerator. The time between the stopping of the⁵⁷Fe nucleus and the emission of the 14 keV -ray is determined by the lifetime (140 ns) of the 14 keV state. In each material the Mössbauer spectrum exhibits a doublet with velocity coordinates (in mm/s) at room temperature, relative to a sodium ferrocyanide absorber, as follows: diamond (–0.99, 1.10), silicon (–0.80, 0.21), and germanium (–0.88, –0.02). In silicon and germanium crystals the spectra were observed over the temperature range between 13 K and 870 K. The relative line intensities changed dramatically and the positions of the lines shifted systematically with temperature. In addition, channelling studies were made on iron that had been implanted into silicon.     

High-Energy Ball Milling of NiAl(Fe) System

High-energy ball milling was used to promote the solubilization of iron into NiAl powder for an iron concentration range of 10–30 wt.%. The microstructural evolution induced by the intense mechanical deformations, under different milling conditions, was followed by X-ray diffraction and Mössbauer spectroscopy. The Mössbauer spectra are dominated by a magnetic sextet of about 33 T. Increasing the time and the speed of milling gives rise to a non-resolved doublet, having parameters typical of a NiAl compound with Fe atoms in solution. At the same time a reduction of lattice parameter occurs, which can be correlated to composition variations and partial disordering of the NiAl structure. Subsequent annealing modifies the Mössbauer spectra noticeably. In particular, the non-magnetic component becomes a broad singlet. Both diffraction analysis and Mössbauer spectroscopy indicate that a fcc Ni(Al,Fe) solid solution is forming in samples milled in agate. It is observed that the grain size of the milled products remains in the nanometric range even after thermal treatment, which adds interest to possible applications.     

Mössbauer spectroscopy investigation of body centered cubic Co in Co/Fe superlattices prepared with MBE

A new spectrum component is observed in Mössbauer spectra of thin body centered cubic Co layers prepared in Fe/Co superlattices doped with⁵⁷Co. It is characterized by a large magnetic hyperfine field (31.2 T) and an isomer shift nearly equal to that of -Fe. The decrease of the isomer shift in bcc Co with respect to hcp Co is consistent with smaller s to d charge transfer in bcc Co as compared to hcp Co. The cubic structure of the CoFe superlattices is evidenced with X-ray diffraction and ion-channeling measurements. The Fe/Co interface is investigated with conversion electron Mössbauer spectroscopy. It is shown that the interface alloy thickness is about six monolayers for growth temperatures up to 450 K and that increasing alloying occurs for higher growth temperatures.     

On the Phase Transformations in Cr–FeB and Fe–CrB Systems at High Temperature

The solid state interactions occurring at high temperature in the Cr–FeB and Fe–CrB systems were studied by transmission Mössbauer and X-ray diffraction techniques on samples prepared by powders carefully mixed, cold-compacted and then treated at 1000°C for times up to 16 h. In the Cr–FeB system, iron atoms liberated by the substitutional diffusion of Cr into FeB lattice preferentially destabilize the iron monoboride with formation of Cr-containing Fe₂B. In the Fe–CrB system, chromium atoms liberated by the substitutional diffusion of Fe into CrB lattice interacts with iron forming an Fe–Cr metal alloy. Moreover, zones of Cr-containing FeB and Fe₂B form at the contact between metal iron and chromium monoboride, and tend to disappear as iron is consumed by the alloying process.     

Evolution of spin clusters from the spin glass region inAuFe

Zero field Mössbauer spectra (4.2–40 K) have been obtained onAuFe alloys containing 5 and 10 at %Fe in differing metallurgical states. Graphs of the reduced magnetic hyperfine field versus reduced temperature have been compared With Brillouin curves. This comparison shows that the heat treated 5 at %Fe sample is essentially a homogeneous spin glass whereas the behaviour of the heat treated 10 at %Fe sample and alloys in an as rolled (atomically clustered) state can be better described by the additional presence of magnetic clusters of spins.