No abstract

We have performed ultrasonic measurements on the α-pyrochlore spin compounds R 2 Ti 2 O 7 , R = Tb, Dy, and Ho. A distinct dip appears around 80 K, especially in transverse modes (C 11 − C 12 )/2 and C 44 as a function of temperature in Dy 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 compounds. This anomaly may correspond to a structural change detected by the temperature dependence of the permittivity and of the loss-angle tangent. No distinct anomaly was observed at lower temperatures in Dy 2 Ti 2 O 7 and Ho 2 Ti 2 O 7 compounds. On the other hand, a pronounced elastic softening toward low temperatures was observed in the temperature dependence of the principal elastic constants C 11 , (C 11 − C 12 )/2, and C 44 in Tb 2 Ti 2 O 7 . A detailed discussion of the highly distinct behavior among three compounds is given in terms of the magnitude of a splitting energy between the ground state and the excited one formed by the crystal-field effect. It seems that geometrically structural frustration prohibits the formation of a long-range quadrupolar ordering as well as a magnetic one, even if the coupling constant between quadrupolar moments is rather large enough in Tb 2 Ti 2 O 7 at low temperatures.

The elastic and magnetic properties of a single crystal of the bilayer manganese oxide ͑Pr 0.6 La 0.4 ͒ 1.2 Sr 1.8 Mn 2 O 7 have been investigated by means of ultrasonic and high-field magnetization measurements. Remarkable changes in the elastic constants C 11 , C 33 , C 44 , and C 66 as a function of temperature and magnetic field have been observed. In particular, a distinct elastic anomaly was observed at low temperatures and in magnetic fields when crossing the phase boundary between the paramagnetic insulating and the fieldinduced ferromagnetic metallic state. A pronounced elastic softening as a function of magnetic field ͑H͒ appears across the boundary of the low-temperature magnetic phase below around 40 K, accompanied by a distinct hysteresis. In the high-field region, however, these elastic constants exhibit a monotonic increase upon increasing the magnetic field. The high-field magnetization measurements characterizing the magnetic state point out a strong coupling between the elastic strain and the magnetic moment. The data can be described reasonably well considering a strong coupling between elastic strain and magnetic susceptibility m = ‫ץ‬M / ‫ץ‬H.

We have investigated elastic properties of the Ce-based filled skutterudite antimonides CeRu 4 Sb 12 and CeOs 4 Sb 12 by means of ultrasonic measurements. CeRu 4 Sb 12 shows a slight increase around 130 K in the temperature dependence of the elastic constants C 11 , ͑C 11 − C 12 ͒ / 2, and C 44 . No apparent lattice softening toward low temperature expected by a quadrupolar response of the 4f-electronic ground state of the Ce ion was observed at low temperatures. On the other hand, CeOs 4 Sb 12 shows a pronounced lattice softening toward low temperature in the longitudinal C 11 as a function of temperature below about 15 K, while a slight lattice softening was observed in the transverse C 44 below about 1.5 K, in contrast to no softening in CeRu 4 Sb 12 at low temperatures. Furthermore, CeOs 4 Sb 12 shows a steep drop around a phase-transition temperature of T s = 0.9 K in both C 11 and C 44 . The lattice softening observed in C 11 below about 15 K cannot be explained reasonably only by the crystalline electric-field ͑CEF͒ effect. Instead, it is most likely to be responsible for a coupling between elastic strain and a relevant renormalized quasiparticle band with a small energy gap in the vicinity of the Fermi level. The elastic properties and the 4f ground state of Ce ions in CeRu 4 Sb 12 and CeOs 4 Sb 12 are discussed in terms of the CEF effect and the band structure in the vicinity of Fermi level.

We have performed ultrasonic measurements of the filled skutterudite compound NdFe 4 P 12 to investigate the elastic properties and 4 f ground state of Nd ions split by the crystalline electric-field ͑CEF͒ effect. A pronounced elastic softening towards low temperatures was observed in the temperature dependence of the elastic constants C 11 , (C 11 ϪC 12 )/2, and C 44 . From the present results the CEF ground state of Nd ions and their low-temperature behavior are discussed in connection with the CEF ground state being degenerate with a quadrupolar moment.