Magnetic structure, magnetostriction, and magnetic transitions of the Laves-phase compound NdCO2 -: art. no. 064405

The crystal and the magnetic structures as well as magnetostriction of the Laves-phase compound NdCo2 are investigated by means of temperature-dependent high-resolution neutron-powder diffraction. The compound crystallizes in the cubic Laves phase C15 structure at high temperature, undergoes a tetragonal distortion (space group I4l/amd) around T(C)approximate to 100 K and an orthorhombic distortion (space group Fddd) at T 42 K. The temperature dependence of lattice constants, magnetostriction constant, and magnetic moment indicate that the magnetic and structural transitions are second order in the vicinity of T-C and are first-order around 42 K. Refinements of magnetic structure reveal that the Nd moment distinctly exhibits an abrupt increase at the first-order transition and the easy magnetization direction of the compound changes from [001] in the tetragonal lattice to [011] in the orthorhombic lattice, indicating a strong coupling between crystal structure and magnetic properties at zero field. Analysis of the temperature dependence of bondlength suggests a strong magnetic exchange striction in the tetragonal structure and that the abrupt increase of the Nd moment is attributed essentially to a change in crystal electric field. Field-dependent neutron diffraction reveals a decoupling of the magnetic and structural transitions under relatively modest magnetic fields.