This award will support collaborative research in solid state physics between Dr. Werner P. Wolf, Yale University and Dr. J.F. Gregg, Clarendon Laboratory, Oxford University, U.K. Ultrasonic waves have long been used to study magnetoelastic properties. By measuring the transit time of a short pulse, one obtains information about changes of length resulting from magnetostrictive effects, and also about changes of elastic moduli corresponding to changes in the magnetic state of the material. Recent developments of high frequency ultrasonic transducers, using sputtered ZnO films, have made possible magnetoacoustic studies at gigahertz frequencies. A group at Oxford University led by Dr. Gregg has developed a cryogenic system for such experiments and is currently using it to study magnetic phase transitions in rare earth insulators. A group at Yale under Dr. Wolf has long experience with rare earth insulators and with phase transitions. It is proposed to enter into a collaborative arrangement in which experiments done at Oxford will be analyzed at Yale. Also, a number of collaborative experiments will be carried out jointly at Oxford. The work will concentrate on magnetic systems in which a number of basic phenomena can be studied. These include transformations between time reversed antiferromagnetic states, involving metastability, nucleation and domain growth, and other order-order and order-disorder phase transitions. Unexplained effects previously observed in magnetoelectric experiments may also be resolved. Further developments of microwave ultrasonic techniques may be expected, and these could have important applications to devices involving four-wave mixing and phase conjugation.