Lionel M. Levinson

Mössbauer Study of Some 2–17 Lanthanide‐Iron Compounds

We have studied the iron‐rich, rare‐earth‐iron intermetallic compounds R2Fe17 (R = Pr, Gd, Tm, Lu), using the Mossbauer effect in 57Fe. The measured average hyperfine field H(T/Tc) acting on the 57Fe nuclei is a unique, monotonically decreasing function of T/Tc, with no observed irregularity at temperatures corresponding to a suspected ferromagnetic‐antiferromagnetic phase transition in these compounds. The onset of paramagnetism occurs at temperatures close to the upper transition points measured by Strnat et al., namely at Tc = 282°, 472°, 271°, and 263°K for Pr2Fe17, Gd2Fe17, Tm2Fe17, and Lu2Fe17, respectively. Low‐temperature (≃90°K) spectra indicate the presence of at least four magnetically nonequivalent Fe sublattices, while for T>Tc broadened 2‐line absorption spectra typical of 57Fe at noncubic sites in paramagnetic material are observed.

Magnetic behaviour of FeF3 close to the curie temperature

Abstract FeF 3 is a weak ferromagnet with a remanence and a canting angle of the expected size [5]. The macroscopic moment has an anomalous behaviour close to the Curie temperature where a sharp drop in the canting angle is observed. The Curie point has been accurately measured by 2 methods.

Spin re-orientation in SmFeO3

Abstract The spin re-orientation phase transition in SmFeO 3 has been studied by magnetic moment measurements and the Mossbauer technique. The results tend to support a rotational process for the transition.

Theory of elastic stiffness c33 of gadolinium in the spin rotation region

Abstract We have constructed a simple, model free-energy function to interpret the anomalous decrease in the elastic stiffness c 33 of Gd in the spin rotation region. Our model qualitatively exhibits the observed drop in c 33 , and an order of magnitude estimate of the decrease is in agreement with measured values. In the presence of sufficiently large magnetic fields, spin rotation causes large changes in the shape-dependent magnetostatic self-energy, which ‘freezes out’ strain-induced spin rotation, thereby eliminating the drop in c 33 . As is borne out experimentally, we predict that the anomalous decrease in c 33 should disappear for fields of the order of 5 kG. We also evaluate the magnetic field dependence of c 33 and obtain fair qualitative agreement with the measurements.

Entropy change of spin reorientation phase transitions

Abstract We estimate the entropy change ΔS associated with spin reorientation phase transitions in TbFeO3, YbFeO3 and αFe2 O3, using magnetic anisotropy data. Satisfactory agreement with experimental measurements of ΔS is obtained.

Resolving power of coincidence Mössbauer spectroscopy

Abstract The narrowing of Mossbauer absorption lines resulting from time filtering effects is analyzed with regard to the possible utility of this technique in improving experimental line resolution. We demonstrate that in the absence of statistical noise, and provided the line shape is Lorentzian of known width and experimentally undistorted, it is always possible to solve for a completely unknown energy level scheme of a thin absorber, independent of whether or not the individual lines comprising the Mossbauer spectrum can be resolved by eye. We solve analytically a simple model to test the utility of time filtering as an aid to the determination of the parameters of Mossbauer spectra, taking into account the effects of statistical noise. Our calculation indicates that only minor improvement in accuracy can be achieved when non-zero coincidence delay times are introduced, and in view of the experimental difficulties involved in coincidence Mossbauer spectroscopy, it appears to us that time filtering methods are unlikely to compete successfully with computer techniques as a useful experimental tool for spectrum analysis. Some comments as to the relevance of the Heisenberg uncertainty principle to line narrowing are appended.