Studies of magnetic orders under very high pressures

Sign and values of exchange interactions, electronic band structure, and crystal field splitting are the basic parameters of any magnetic or strongly correlated electronic system. All these parameters are very sensitive to interatomic distances. Experiments under applied pressures is the only way to vary interatomic distances in a controlled way without changing chemical composition. High-pressure studies offer unique opportunities to test microscopic models and to search for new, highly exotic magnetic phenomena. We describe recent developments for magnetic neutron diffraction under very high pressures. Neutron diffraction is the most suitable tool to study spin arrangements. Progress in neutron and high pressure techniques allowed us to reach pressures up to 50 GPa, by 1-2 orders of magnitude higher than in previous neutron works. We give an overview of our recent high-pressure results. In rare-earth chalcogenides we observed a 'resonant' ferromagnetic exchange induced by pressure. In frustrated Laves phases RMn2 (R=Gd,Ho,Tb) we found new pressure induced magnetic phases and unusual 'short range order - long range order' magnetic transitions. At the end, we discuss prospects for magnetic studies in Mbar pressure range. (author)