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 phases at low temperatures. We review neutron scattering experiments exploring such short-range-ordered phases in insulating transition metal oxides. We discuss (V1-xCrx)(2)O-3, in which orbital fluctuations appear to limit spin correlations to within small ''molecular'' clusters, SrCr9pGa12-9pO19, in which geometrical frustration allows local antiferromagnetic constraints to be fulfilled without long,ranged order, and Y2BaNiO5, in which magnetic interactions occur only within chains of spins which,are unable to order because of the Haldane effect. Emphasis is placed on the common features of exchange interactions in these oxides and the important role which magnetic neutron scattering has played in understanding the unusual magnetic phenomena. (C) 1996 American Institute of Physics.