Mossbauer spectra of Fe 67 in a-Fe 2 0 3 have been measured as a function of particle size and temperature. Bulk a-Fe 2 03 shows a change in the sign of the quadrupole interaction in going through the Morin transition temperature, 263°K. Analyses of the spectra show that the magnetization vector is in the с plane above the transition temperature and parallel to the с axis below it. In contrast, finely divided a-FtzOz particles, sup ported on a high-area silica, do not undergo a Morin transition. The spectra show that the magnetization vector remains perpendicular to the с axis at least down to 10°K. It is evident that the spins are pinned by the surface in small particles. When the particle size is less than 135 A, the room-temperature spectrum con sists only of a quadrupole-split center line corresponding to superparamagnetic a-Fe 2 03. As the particle size is gradually increased, a 6-line hyperfine spectrum of increasing intensity appears. This transition from superparamagnetic to ferromagnetic behavior, coupled with a measurement of the average particle size by x-ray line broadening, leads to a calculated value of the crystalline anisotropy constant in the с plane of (4.7db 1.1) X Ю 4 erg per cm 3 . A sample with an average particle size just below that required for ferromagnetic behavior at room temperature, when cooled, displays an increasing fraction of ferromagnetic material as the temperature is decreased. From these data the particle size distribution of the a-FezOz is determined. In addition, these data are used to calculate an independent value of the anisotropy constant of (4.1±1.1)X10 4 erg per cm 3 . Careful measurements on the series of samples with varying particle sizes show that the quad rupole splitting increases from the bulk value of 0.42 mm sec -1 to a value of 0.98 mm sec -1 for particles with an undetermined average size of less than 100 A.

The absorption of high-frequency (60 kc. to 20 mc. per second) sound waves in fused silica shows a large peak at low temperatures (30" to 50'K.). This absorption appears to result from some property of the glassy state since the absorption does not occur in crystalline silica. Data are presented showing the variation of rigidity and internal friction with temperature and frequency. It is shown that the source of the internal friction may be a structural relaxation. Some speculations are made on the nature of the structural relaxation and its relation to the glassy state.

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The ultrasonic attenuation of the heavy-fermion superconductor UPt3 is investigated as a function of temperature, magnetic field strength, and frequency. It is shown that in the presence of a dc magnetic field at least one metastable and two stable phases exist in the superconducting state.PACS numbers: 64.70. Kb, 62.80.+f, 74.30.Gn Until recently He was known as the only existing superfluid Fermi liquid in nature where the formation of the superfluid pairs is via triplet pairing. Since the discovery of superconductivity in some of the so-called "heavy-fermion" systems, x however, whose low-temperature behavior may also be well described 2 by Landau's Fermi-liquid approach, there is an increasing number of experimental and theoretical findings which support speculations that some of these compounds-and in particular 3,4 UPt3-are promising candidates for anisotropic odd-parity-pairing superconductivity. One of the characteristic features of an odd-parity-pairing superfluid (or superconducting) state is the larger number of possible spin configurations than in the singlet case which give rise to the various phases in superfluid 3 He and to a phase diagram which is profoundly influenced by a magnetic field. In 3 He ultrasonic experiments 5 have substantially contributed to the identification of the different superfluid phases and to the understanding of the physical nature of the pairing mechanism. Therefore, the idea suggests itself that ultrasonic experiments should also provide valuable information about phase transitions and the pairing mechanism in superconducting heavy-fermion compounds.It is the main purpose of this paper to present ultrasonic attenuation experiments in the superconducting state of UPt3 and to demonstrate for the first time that-as in superfluid 3 He-the phase diagram is strongly affected by a dc magnetic field so that, in the presence of a dc magnetic field, at least one metastable and two stable superconducting phases exist in UPt3.The measurements where performed on a single crystal of UPt3 of about 7 mm length and 7 mm diameter whose c axis forms an angle of about 21° with the cylinder axis. Longitudinal sound was propagated along the c axis and generated by means of a 30-MHz overtone A'-cut quartz transducer. A conventional ultrasonic pulse spectrometer was used to measure the ultrasonic attenuation coefficient a, and the ac susceptibility (at 30 Hz) of the sample was monitored simultaneously. In all experiments the dc magnetic field Bo was oriented along the cylinder axis of the specimen, thereby giving rise to some difficulties in the calculation of the true local field because, in the sample investigated here, no common coordinate system exists in which both the magnetic susceptibility and the "demagnetization factor" are diagonal tensors. On the other hand, this orientation guarantees that Bo has a component perpendicular to the ultrasonic wave vector q. The latter is a necessary condition to make sure that zero sound may couple to the three collective modes 6 (normal flapping, cl...

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