Dietrich Schroeer

Mössbauer studies of the morin transition in bulk and microcrystalline α-Fe2O3†

Abstract Mossbauer spectra of bulk and microcrystalline hematite have been analyzed at the Morin transition for both the weak-ferromagnetic (WF) and anti-ferromagnetic (AF) phases simultaneously, increasing precision in the extracted Mossbauer parameters. The Morin transition in bulk took place in less than 0.4 K, and in the microcrystals decreased linearily with expanding lattice in agreement with its increase under external pressure. Temperature hysteresis measurements in the microcrystals gave values of the fourth-order magnetic anisotropy energy which indicated possible surface spin-pinning effects. In bulk the magnetic field decreased by 7 ± 0.5 kOe in going from the WF to the AF phase on decreasing temperature, and the isomer shift decreased by 0.014 ± 0.003 mm sec. With lattice dilation the AF quadrupolar interaction appeared to decrease, while the isomer shift change across the transition goes positive. In contrast the change in magnetic field is not simply related to this lattice dilation, indicating surface spin-pinning effects.

Mössbauer study of Fe1 + xV2−xO4 spinels for the determination of cation distributions and magnetic structure

Abstract Mossbauer experiments were carried out in the temperature range of 80–500 K on the spinel series of Fe 1 + x V 2− x O 4 . The determined cation distribution has Fe 2+ and Fe 3+ at both A and B spinel sublattice sites for t whole series except for x = 0. Charge hopping was observed at the B lattice sites. The present cation distribution reasonably predicts several experimentally observed constants such as the lattice constant, the oxygen parameter, the saturation magnetic moment and the Curie constant. The magnetic structure in best agreement with the experimental values shows the AA spins to be antiparallel for Fe, the BB spins to be parallel for identical ions and antiparallel for different ions, and shows a strongly antiparallel AB interaction between trivalent iron ions. Crystal distortions due to the Jahn-Teller effect were observed at 80 K for compositions with 0 ⩽ x ⩽ 0.75.

Mössbauer study of Fe57 and Co57 in Co1 + xV2−xO4

Abstract Mossbauer sources and absorbers, prepared by doping Fe57 and radioactive Co57 into samples of Co1−xV2−xO4 (0 ⩽ x ⩽ 1), were studied in the temperature range of 80–500 K. The source and absorber spectra are very similar. The absence of any Fe2+ at the A site can be understood by partial covalent bond formation with an anion. However, the predominance of Fe3+ at the B site (with some Fe2+ for x = 1) cannot be explained by simple crystal-field or molecular-orbital theories. The x dependence of the isomer shift and of the Fe3+B-site quadrupole interaction can be related to changes in the lattice constant and the oxygen parameter. The temperature dependence of the Fe2+B-site quadrupole interaction can be fitted in the motional-averaging model. In the range of 0 ⩽ x ⩽ 0.5 the temperature dependence of the isomer shift shows effects of chemical bonding beyond the second-order Doppler shift.

Uses of the Mössbauer Effect in Radiation-Damage Studies

A review is presented of contributions which Mossbauer spectroscopy can make to studies of radiation-damage effects. Special emphasis is placed on the unique aspects of this technique, including the following examples: in all Coulomb excitation, as well as in some radioactive decay processes, every Mossbauer gamma ray has its origin in a radiation-damage event, leading to high sensitivity; all chemical charge states of a Mossbauer ion can be simultaneously observed and identified through the isomer shift; the Mossbauer effect is particularly sensitive to the local details of the damage effects; and radiation-damage processes with recovery times as short as the nuclear lifetimes can be studied. Some of the more significant contributions of the Mossbauer effect to a variety of radiation-damage questions are illustrated by reviewing experiments from the existing literature.

Critical reading list for teachers of physics‐and‐society courses

A critical bibliography of 25 books is presented. It is to aid potential teachers of physics‐and‐society courses in acquiring competence in this subject area.

Resource Letter PNAR‐1: Physics and the nuclear arms race

This collection of article and book references was assembled to help physicists understand and teach about the nuclear arms race. Both technical and political aspects are covered. Entries are therefore marked as elementary for the layman (E), intermediate for college freshman (I), and advanced for the college senior or graduate student (A) for both the technical (t) and the social (s) aspects. Articles or books which are excerpted in a reprint volume (to be published separately by the American Association of Physics Teachers) are marked with an asterisk (*).

Brecht’s Galileo: A revisionist view

Galileo is often claimed by scientists to be the first modern physicist, and because of his conflict with the Catholic Church is seen as a heroic figure fighting for the independence of pure science. Brecht, in his play Galileo, has presented a revisionist view of Galileo. This view developed over several versions of the play, and finally used him as a symbol for all scientists who reject social responsibility for their work. Is this revisionist view of Galileo any more distorted than the portrayal of him as the patron saint of modern science?

Teaching a physics‐and‐society course

The goals, teaching methods, and requirements of a six‐year‐old physics‐and‐society course are presented and related to the course’s content and survey approach. The problems, frustrations, and rewards experienced in developing and teaching the course are shared.

Teaching about physics and the nuclear arms race

The arms race has an important scientific and technological component; but it inevitably also involves political problems. In thinking about the arms race, physicists tend to emphasize the quantitative aspects, while policy makers and society as a whole focus on the political choices. A personal experience of teaching a course on the nuclear arms race to non‐science students is described to show how teaching quantative aspects in an order‐of‐magnitude fashion can help non‐scientists decide when numbers are important, and when they may be irrelevant or misleading to policy considerations.