Successive Magnetic Phase Transitions of Component Orderings in DyB sub( 4)

The successive magnetic phase transitions in DyB4 have been studied in detail by microscopic measurements using resonant X-ray diffraction and neutron scattering. It is shown that the a b-plane component of the magnetic moment is short-range-ordered in the intermediate phase where the c-axis component is long-range-ordered. It is estimated that this short-range order is dynamically fluctuating with a time scale between ~10 super( -8) to ~10 super( -11) s. Crystal field excitation has also been investigated by inelastic neutron scattering. To qualitatively understand the phase-transition phenomenon, we have studied a simple two-sublattice model with an antiferromagnetic interaction by mean-field calculation. The calculation, though without a quadrupolar interaction, successfully explains the occurrence of double phase transition, magnetic specific heat and entropy, magnetic susceptibility, and the huge elastic softening in the intermediate phase. The general success of the mean-field calculation, except for the short-range order, suggests that there is little effect of geometrical frustration on the macroscopic properties at zero magnetic field.