SELECTED STUDIES OF MAGNETISM AT HIGH-PRESSURE

Most previous studies of magnetism in various compounds under extreme conditions have been conducted over a wide pressure range at room temp erature or over a wide range of cryogenic temperatures at pressures be low 20 GPa (200 kbar). We present some of the most recent studies of m agnetism over an extended range of temperatures and pressures far beyo nd 20 GPa, i.e. in regions of pressure-temperature (P-T) space where m agnetism has been largely unexplored. Recent techniques have permitted investigations of magnetism in selected 3d transition metal compounds in regions of P-T where physical properties may be drastically modifi ed; related effects have often been seen in selected doping studies at ambient pressures. We present Fe-57 and I-129 Mossbauer isotope studi es covering the range 300-4 K to sub-megabar pressures in compounds su ch as Sr-2 FeO4, LaFeO3 and FeI2, representative of a broad class of 3 d transition metal compounds. At ambient pressure the electronic struc ture of the transition metal atom in these antiferromagnetic insulator s extends from 3d(4) to 3d(6) and has a distinct influence on the pres sure evolution of their magnetic properties. Mossbauer studies of thes e compounds ape considered in conjunction with available structural an d electrical transport data at pressure.