P. S. Anil Kumar

The static and dynamic response of cluster glass in ${\mathrm{La}}_{0.5}$${\mathrm{Sr}}_{0.5}$${\mathrm{CoO}}_{3}$ has been experimentally investigated through linear and nonlinear ac susceptibility measurements in the frequency range 137 Hz to 1370 Hz; ac field (1--10 Oe), dc field (0--40 Oe). Field cooled and zero field cooled dc magnetization data have also been reported. An attempt has been made to compare the dynamic response of cluster glass with spin glass, reentrant spin glass, and canted spin systems. The interesting observation is that in the cluster glass system, the coercivity at low fields under the zero field cooled condition does not increase rapidly, as one approaches the so-called freezing temperature (${\mathit{T}}_{\mathit{f}}$), unlike that in typical reentrant spin glass systems. We also observed a linear relation of field cooled coercivity, ${\mathit{H}}_{\mathit{c}}^{\mathrm{FC}}$ (i.e., the coercive force to bring the thermoremanent magnetization to zero) with cooling fields (${\mathit{H}}_{\mathrm{FC}}$) at different temperatures. The variation of ${\mathit{dH}}_{\mathit{c}}^{\mathrm{FC}}$/${\mathit{dH}}_{\mathrm{FC}}$ with T(K) shows an exponential behavior for T\ensuremath{\leqslant}${\mathit{T}}_{\mathit{c}}$. We believe that this is a significant observation for cluster glass systems. \textcopyright{} 1996 The American Physical Society.

Analysis of the irreversible field-cooled (FC) and the zero-field-cooled (ZFC) magnetic susceptibilities of one ferrimagnetic and three ferromagnetic systems, measured at different applied magnetic fields, shows that the irreversibility indicated by the difference between the FC and the ZFC susceptibilities arises from magnetic anisotropy. The two susceptibilities are related to each other through the coercivity which is a measure of the anisotropy. The ZFC susceptibility can be calculated from the FC susceptibility (or vice versa) and the coercivity.

Attempts have been made to develop an artificial articular cartilage on the basis of a new viewpoint of joint biomechanics in which the lubrication and load-bearing mechanisms of natural and artificial joints are compared. Polyvinyl alcohol hydrogel (PVA-H), 'a rubber-like gel', was investigated as an artificial articular cartilage and the mechanical properties of this gel were improved through a new synthetic process. In this article the biocompatibility and various mechanical properties of the new improved PVA-H is reported from the perspective of its usefulness as an artificial articular cartilage. As regards lubrication, the changes in thickness and fluid pressure of the gap formed between a glass plate and the specimen under loading were measured and it was found that PVA-H had a thicker fluid film under higher pressures than polyethylene (PE) did. The momentary stress transmitted through the specimen revealed that PVA-H had a lower peak stress and a longer duration of sustained stress than PE, suggesting a better damping effect. The wear factor of PVA-H was approximately five times that of PE. Histological studies of the articular cartilage and synovial membranes around PVA-H implanted for 8-52 weeks showed neither inflammation nor degenerative changes. The artificial articular cartilage made from PVA-H could be attached to the underlying bone using a composite osteochondral device made from titanium fibre mesh. In the second phase of this work, the damage to the tibial articular surface after replacement of the femoral surface in dogs was studied. Pairs of implants made of alumina, titanium or PVA-H on titanium fibre mesh were inserted into the femoral condyles. The two hard materials caused marked pathological changes in the articular cartilage and menisci, but the hydrogel composite replacement caused minimal damage. The composite osteochondral device became rapidly attached to host bone by ingrowth into the supporting mesh. The clinical implications of the possible use of this material in articular resurfacing and joint replacement are discussed.

The realm of high energy, large wave vector spin waves in ultrathin films and at surfaces is unexplored because a suitable method was not available up to now. We present experimental data for an 8 ML thick Co film deposited on Cu(001) which show that spin-polarized electron energy loss spectroscopy can be used to measure spin-wave dispersion curves of ultrathin ferromagnetic films up to the surface Brillouin zone boundary.