Formation and Electromagnetic Properties of Large Single Crystals of Metal Hydrides

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The present study was concerned with (1) the formation of well-defined and large single crystals of metal hydrides and (2) the investigation of electric and magnetic properties for these metal hydrides. We investigated the ternary metal hydrids based on rare-eath intermetallics, such as LaNi_5, LaCo_5, GdFe_3, Gd_6Fe_<23>, Mg_2Ni, NdCo_5, YCo_3, Y_6Fe_<23>.The single crystals of the starting intermetallics were prepared by means of triarcmelting. Laue x-ray diffraction patterns showed that they were single crystals and the maximum size of them was 8mm d*20mm l.We developed an in situ method to measure the electrical resistivity unstable hydrides (with high equilibrium hydrogen pressures). It was observed that Mg_2Ni transits from a metal to a semiconductor as a result of hydrogenation and the energy gap of Mg_2NiH_<3.1> is 0.056eV.As hydrogen is absorbed in GdFe_3, the lattice constants is increased and the compensation temperature is decreased from 280K to 161K.Under the influence of high magnetic fields, the magnetic structure of GdFe_3H_X transits from a simple ferrimagnetic one to a non-colinear one. Its critical fields decreased from 24.7T to 13.0T.The strength of the exchange interaction depends on the hydrogen composition x judging from the variation in the exchange coupling constant.