U. Gonser

Antiferromagnetism of fcc Fe thin films

Thin iron films (∼18 A and 90% enriched in Fe57) were prepared on (001) Cu single‐crystal substrates. The fcc structure was verified by electron microscopy. By Mossbauer spectroscopy it was found that antiferromagnetic ordering begins at around 80±10 K with hyperfine fields of about 16–20 kOe. Additional proof for the existence of anitferromagnetism has been obtained by measuring the films in a longitudinal (parallel to the γ‐ray direction) external magnetic field. The apparent discrepancy in the literature of ferromagnetic and antiferromagnetic ordering in fcc films might be resolved by taking into account the differences concerning the film orientations.

Modified atomic structure in a PdFeSi nanoglass: A Mössbauer study

A non-crystalline solid called a nanoglass was generated by consolidating glassy droplets with diameters of a few (typically </10) nm. The glassy droplets of chemical composition Pd70Fe3Si27 were produced by evaporating the initial substances in a cold inert gas (He) atmosphere. The atomic structure of the nanoglass was found by Mossbauer spectroscopy to differ from the structure of a melt-spun glass with a comparable chemical composition. The observed structural deviation may be understood if it is assumed that a new type of atomic structure is formed during consolidation at the interfaces between adjacent glassy droplets. This new structure is characterized by a reduced atomic density and interatomic spacings deviating from the spacings in a melt-spun glass. The technique to synthesize solids by compacting small glassy droplets may open the way to generate new types of non-crystalline solids differing structurally from melt-spun metallic glasses.

Nano-structured materials

The term “nano” in connection with crystalline or amorphous structures represents important materials from a scientific as well as from a technical point of view. In addition, it seems that, so far, Mössbauer spectroscopy is the most successful technique for obtaining information on these structures. A brief overview of the perspective which Mössbauer spectroscopy affords in the study of the structure of such materials is presented.

Mössbauer‐effect study of Co57 and Fe57 impurities in ferroelectric LiNbO3

The Mossbauer effect observed with LiNbO3:Co57 (source) and LiNbO3:Fe57 (absorber) crystals showed the existence of high‐spin Fe2+ and Fe3+ valence states. The Fe2+/Fe3+ ratio could be changed by reducing or oxidizing heat treatment. Fe3+ in sources and absorbers shows Mossbauer spectra which are typical for slow electronic relaxation between the crystal field states of the 6S5/2 state ion. For both Fe3+ and Fe2+, the principal axis of the electric field gradient is found to be parallel to the crystallographic c axis. For ferric iron Vzz is positive, while Vzz is negative and strongly temperature dependent for ferrous iron. The impurity site substitution and charge compensation mechanism are discussed.

Phase transformations of a nitrogen-implanted austenitic stainless steel (X10 CrNiTi 18-9)☆

The available results of research work performed on nitrogen-implanted austenitic CrNi steels are contradictory. The following characteristics have been observed: formation of martensite in the austenite as well as complete retransformation of martensite into austenite; solution of nitrogen up to high concentrations as well as precipitation of nitrides at low concentrations; an increase as well as a reduction of the wear resistance. The transformation of austenitic CrNi steels under nitrogen bombardment is apparently not in accordance with a fixed pattern but can go in different directions. Therefore a systematic phase analysis in conjunction with hardness and corrosion tests is indispensable for process development. On the other hand, this system is of particular interest as a model for studying phase reactions in steels with nitrogen implantation because of the variety of transformations. In this paper we first give an outline of the problems encountered in the treatment of austenitic steels with nitrogen ion beams and introduce our programme for the investigation. We then discuss conditions for γ ⇌ α transformations in either direction. In this context we differentiate between pure irradiation impact and alloying effects caused by the constitutional changes. Further investigations cover the formation of nitrides in pure austenite and the development of surfaces with different pretreatment. The analytical methods used are conversion electron Mossbauer spectroscopy, X-ray diffraction, and depth profiling by nuclear reaction analysis.

On the validity of 57Fe hyperfine field distribution calculations from Mössbauer spectra of magnetic amorphous alloys

Abstract The approximations which are made in 57Fe hyperfine field distribution calculations from magnetic amorphous alloys are discussed. A diagram which gives an immediate indication of the validity of a given hyperfine field distribution obtained using first order perturbation theory is proposed. The assumption about the distribution of the polar angles of the hyperfine field direction with respect to the EFG principal axes in transition metal-metalloid amorphous alloys is discussed. The asymmetries of the Mossbauer spectra of the latter alloys are also considered.

Antiferromagnetism of fcc iron films

Abstract In γ-Fe films of the three basic orientations (111), (110) and (100), prepared by epitaxial growth, antiferromagnetic ordering was established at low temperature.

Mössbauer study of the amorphous alloy Zr100−xFex☆

The hyperfine field distributions were determined from Mossbauer spectra of Fe91Zr9 taken for 6 K < T < 230 K. As in other alloys with invar behaviour, the distribution consists of a high field peak and a broad low field tail. Measurements in external magnetic fields show the ferromagnetic nature of the spin coupling. No evidence was found of an antiferromagnetic component at low fields. For Zr-rich alloys, the distribution of quadrupole splitting was evaluated from the spectra and compared with that found for metastable crystalline alloys. The differences between the two distributions suggest that important changes occur in the short-range order on crystallization.

Simultaneous conversion electron, conversion X-ray and transmission Mössbauer spectroscopy

Abstract Mossbauer spectroscopy can be performed in transmission (TMS) and backscattering geometry. The backscattering geometry of conversion electron Mossbauer spectroscopy (CEMS) and conversion X-ray Mossbauer spectroscopy (CXMS) enables nondestructive surface analysis to be carried out. Because of the different sampling ranges in these variations of Mossbauer spectroscopy it is of interest to measure them simultaneously. An experimental setup for simultaneous CEMS, CXMS and TMS measurements is shown.

Mössbauer spectroscopy of Fe-Ni and Fe-Pt alloys

Abstract Mossbauer measurement on Fe-Ni and on Fe-Pt in the ordered and disordered states at various temperatures and in a longitudinal external field (Hext = 50 kOe) have been analyzed. Hyperfine field distributions of Fe-Ni Invar alloys indicate that in addition to the ferromagnetic phase a second phase or state exists whose magnetic properties seem to be closely related to those of γ-Fe. No second phase or state was detected in the ordered Fe-Pt, while the situation in disordered Fe-Pt is uncertain.