Local acoustoelasticity in poly(methyl methacrylate) by Brillouin scattering J. Appl. Phys. 54, 5456 (1983); 10.1063/1.332729 Brillouin scattering near the lowtemperature glass transitions of polymethyl methacrylate Brillouin scattering of laser light has been used to measure the frequency of hypersonic sound waves in the range of 10 10 Hz in PMMA as a function of temperature through the glass-transition region. A discontinuity in the temperature coefficient of sound velocity is observed at the glass-transition temperature; this is explained as a consequence of a corresponding discontinuity in the temperature coefficient of the specific volume (thermal expansion coefficient). The ratio of the light scattered by isothermal density fluctuations to that scattered by adiabatic density fluctuations was also measured. This ratio was large and did not change appreciably near the glass-transition temperature. The value of the Landau-Placzek ratio is approximately what one would expect from previously observed ultrasonic-velocity-dispersion data as a function of temperature well above the glass-transition temperature. Both the velocity and intensity ratio data indicate that no velocity-dispersion effects are present for the hypersonic sound waves up to temperatures 35 0 above the glass-transition temperature. These results also indicate that the glass transition is not a classical secondorder phase transition.

Internal magnetic fields in several iron—aluminum alloys ranging in composition from 19 to 28 at. % aluminum have been measured in a Mössbauer absorption experiment. In the composition range 22% to 26% aluminum three distinct internal magnetic fields were observed. At 300°K the magnitudes of these fields are in the ratio 1.00:0.88:0.72. In a 19% alloy only the two larger fields were observed, while in a 28% alloy only the largest and smallest fields were seen. The largest and smallest fields are the previously observed internal magnetic fields at iron atoms having, respectively, 8 and 4 iron nearest neighbors in an Fe3Al structure. The 0.88 field is attributed to iron atoms surrounded by 5 iron nearest neighbors in an Fe13Al3 structure. Using the known relation between the internal magnetic field and the atomic moment and the supposition that an iron atom with 3 iron nearest neighbors has no aligned moment, saturation magnetization curves of the iron—aluminum system have been calculated on a statistical basis under various known ordering conditions. In the case of annealed alloys, this calculation yields a saturation magnetization curve which agrees with measurements within experimental accuracy. The measured differences between the saturation magnetization of quenched and annealed alloys are also reproduced in these calculations. In the case of cold-worked alloys, good agreement with experiment is similarly obtained.