No abstract

Seedless crystallization of Ge and GaSb melts was achieved under microgravity occurring in a drop-shaft capsule. It was observed that melts levitated in Ar atmosphere, formed into a spherical shape and were solidified instantaneously when they collided with the container quartz wall. X-ray diffraction pattern of these samples showed clear Laue spots signifying a single-crystal structure, while Ge or GaSb solidified in crucibles under normal gravity did not show Laue spots. These results indicate that single crystal growth rapidly occurs without seed crystals from the spherical melt under microgravity. This can be understood by assuming that an ordered structure like a single crystal is already formed in a levitated melt, and assists rapid crystallization when the melt touches the crucible wall and latent heat is deprived.

The isotropic and anisotropicexchange energies in phosphide and arsenide compoundswith helical orferromagneticstructureshave been studiedby meansofan anisotropic twc-band model. The anisotropic term is separated into a symmetric and an antisymmetric contribution. It appears that, incrystals with non-vanishingspin-orbit interaction, acircular isotropic helm minimizes the antisymmetric contribution (AEI) to the exchange energy. The helix is stable if the AEI compensates for the increase in the symmetric anisotropic energy that the deviation from a non-collinear spin arrangement causes. The band model predicts the correct ground state. with the exception of MnP. where the helical (ground state) and the ferromagnetic (high-temperature phase) configurations correspond to the same energy. Theresuluconfirmthatthe~~land theshapeoftheFermisurface arecrucial for thestability of a helical magnetic structure.