Development of a New Method to Detect Magnetoelectric Effect of a Crystal under Bias Electric Field and its Application to Structural Phase Transition in Ordered Magnets

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1) We developed a new system of a simple construction to detect ac magnetization. It can be applied to magnetoelectric measurement under bias electric field as well as to magnetic measurements on high temperature superconductors.2) We observed linear magnetoelectric effect in PbFe_<0.5>Nb_<0.5>O_3 below 9 K and interpreted the result in terms of a structural phase transition in this crystal.3) We observed magnetoelectric effect in CoCr_2O_4 with a conical ferrimagnetic structure to determine its magnetic symmetry . The results were consistently interpreted with the theoretical model of Lyons et al..4) We made magnetoelectric studies of ferrimagnetic Y_3Fe_5O_<12> (YIG) in cooperation with the research group in Tsukuba University. The results showed a possibility of ferroelectric transition in YIG, though our Mossbauer and dialectic studies seemed to deny this possibility.5) We observed linear magnetoelectric effect in Pb_3Mn_7O_<15> and concluded that this crystal possessed both polar structure and spontaneous magnetic moment in low temperature range.6) We confirmed the ferroelectric transition of EuMn_2O_5 at 38 K reported by Sanina et al. and observed spontaneous magnetiation below this transition.The last two compounds are interesting from the view point of a new type of phase transition where both ferroelectric and magnetic orders appear at the same time. We concentrate on this problem in the following few years.