Structural change in DyCu2 single crystal induced by magnetic field

The metamagnetic transition in the orthorhombic antiferromagnets RCu2 (R=Ce , Pr, Dy, Tb) in high magnetic fields parallel to the c-direction is connec ted with a conversion of the magnetic Ising axis from the orthorhombic a- t o the c-direction. The new state remains stable even after switching off th e field. A giant magnetostriction (GMS) Delta L/L greater than or equal to 1% is observed at the conversion. Neutron scattering experiments on a DyCu2 single crystal reveal that this axis conversion is also connected with a s tructural transition hom the orthorhombic CeCu2-type structure to the hexag onal AlB2-type structure due to strong magnetoelastic coupling. To our know ledge this is the first observation of a change from low to high crystal la ttice symmetry by application of an external magnetic field.