STRONG MAGNETIC FLUCTUATIONS IN TRANSITION-METAL OXIDES

Most magnets have long-range magnetic order when the thermal energy is less than the local magnetic exchange energy (T<\Theta(CW)). Effects such as reduced dimensionality and frustration, however, can suppress the ordering transition and lead to unusual cooperative paramagnetic p hases at low temperatures. We review neutron scattering experiments ex ploring such short-range-ordered phases in insulating transition metal oxides. We discuss (V1-xCrx)(2)O-3, in which orbital fluctuations app ear to limit spin correlations to within small ''molecular'' clusters, SrCr9pGa12-9pO19, in which geometrical frustration allows local antif erromagnetic constraints to be fulfilled without long,ranged order, an d Y2BaNiO5, in which magnetic interactions occur only within chains of spins which,are unable to order because of the Haldane effect. Emphas is is placed on the common features of exchange interactions in these oxides and the important role which magnetic neutron scattering has pl ayed in understanding the unusual magnetic phenomena. (C) 1996 America n Institute of Physics.