F. van der Woude

Mössbauer effect in iron and dilute iron based alloys

Abstract Fifteen years of Mossbauer Effect (ME) studies have significantly widened the insight into the physical properties of iron and iron based alloys. In this review article the various contributions to the hyperfine interactions as measured with the ME technique, namely the isomer shift, the magnetic hyperfine interaction and the quadrupole interaction are summarized. Further the impurity effects as the Friedel type oscillations in the charge and the RKKY type oscillations in the spin density distribution are discussed. Special attention is paid to the peculiar magnetic properties of metallic iron and its alloys. From a comparison of the magnetic hyperfine fields and bulk magnetizations as a function of the impurity concentration and from the temperature dependence of the magnetic hyperfine fields at the various sites in iron alloys, it is concluded that the 3d magnetic moments in iron are largely localized. Further the exchange interaction is provided by the remaining small percentage of itinerant 3d electrons. Finally, from a comparison of magnetic hyperfine field and isomer shift data in alloys an overall picture of the electronic changes involved in alloying has been developed.

A study of amorphous, quasi crystalline and crystalline Al-Fe alloys by Mössbauer-effect and diffraction techniques

Abstract Amorphous Al 6 Fe, icosahedral Al-Fe and crystalline Al m Fe have been investigated by X-ray diffraction, electron diffraction and Mossbauer-effect measurements. The results show that the type and number of iron nearest neighbors is very similar in all three alloys. It is suggested that one possible explanation for the spread in Electric Field Gradient magnitudes can be the existence of holes in the aluminum network.

Magnetic properties of amorphous transition metal alloys

Abstract The chemical short-range order of metallic glasses will be discussed and it will be shown that the structural disorder has minor effects on the shape of the magnetization curve, on the magnetic moments and Curie temperatures.

Analysis of the pitch in binary cholesteric liquid crystal mixtures

The pitch p in the cholesteric liquid crystal phase of mixtures of cholesteryl chloride and p−azoxyanisole has been measured as a function of the composition c. The function p−1 (c) deviates considerably from linearity and shows an inversion of the helical twist sense. The results, as well as those of five other binary cholesteric systems published in the literature, have been analyzed in terms of the molecular theory of Goossens. In the discussion of the results of the analysis, the requirements for a consistent physical interpretation of the interaction parameters that appear in the theory have been emphasized. The results indicate that these requirements are satisfied only in simple systems that do not deviate strongly from linear behavior of p−1 (c). In more complicated systems the specific properties of the individual molecules probably play an important role in determining the cholesteric helix.

MOSSBAUER ISOMER-SHIFTS AND QUADRUPOLE SPLITTINGS IN THE AMORPHOUS IRON-BORON SYSTEM

Abstract The isomer shifts and quadrupoole splittings of amorphous Fe x B 1−x alloys (10 a/o ⪅ × ⪅ 90 a/o) were studied by 57 Fe Mossbauer spectroscopy. The isomer shift vs. composition can be described by the ▵φ ∗− and ▵n WS -terms of Miedemas cellular model to predict the heat of formation of binary alloys and an additional volume mismatch term. This term is necessary for boron-rich alloys ( x ⪅ 50 a/o). We conclude that these samples have a strained atomic structure. The values of the quadrupole splittings at the boron-rich side increase strongly, indicating that the Fe-atoms are squeezed into asymmetric atomic surroundings. Implanted iron into crystalline boron has a comparable isomer shift and quadrupole splitting as the boron-rich alloys.

Covalency effects in divalent iron compounds

In this letter we discuss the various effects of covalency on the isomer shift and the quadrupole interaction in some divalent iron compounds.

A Mössbauer study of the order-disorder transition in FePd

Abstract Using Mossbauer Effect spectroscopy we followed the disorder to order transition in Fe 50 Pd 50 . From the annealing time dependence of the ME parameters we conclude that ordering occurs within the fcc phase. Afterwards the material transforms to the tetragonal structure.

Cation valencies in cubic and tetragonal Mn1.9Fe1.1O4

Abstract Mossbauer effect studies on Mn1.9Fe1.1O4 indicate that the cubic to tetragonal phase transition is not due to a significant change in the concentration of divalent iron ions in octahedral sites.

THE SUBSTRATE PROBLEM IN X-RAY-DIFFRACTION OF BORON-RICH AMORPHOUS FE-B ALLOYS

Abstract Energy-dispersive X-ray diffraction spectra of boron-rich amorphous FeB alloys are presented. Special attention is paid to the preparation of the samples in a sputtering apparatus specially designed to give very pure materials. A solution is given to the “substrate problem”. The amorphous films are sputtered onto thin polycarbobate foils 2 μm thick fixed to a copper backing by means of SIRA adhesive wax. It is shown that the data evaluation is greatly simplified when the polycarbonate is directed towards the incident X-ray beam during the measurements.

MÖSSBAUER SPECTROSCOPY ON AMORPHOUS Fe x Zr 100-x (20<×<90) ALLOYS

The isomer shift and quadrupole splitting of amorphous Fe x Zr 100-x (20 ≤ × ≤ 90) are presented. From the concentration dependence of the isomer shift we conclude that the structure can be considered as a strainfree packing of atoms. The concentration dependence of the quadrupole splitting could not be reproduced in computer simulations, using a number of quasi-crystalline and DRPHS models.