The crystal and the magnetic structures as well as magnetostriction of the Laves-phase compound $\mathrm{Nd}{\mathrm{Co}}_2$ 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 $I{4}_1∕amd$) around $T_C\approx 100\mathrm{K}$ and an orthorhombic distortion (space group $Fddd$) at $T\approx 42K$. 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.

• Received 26 August 2004

DOI:https://doi.org/10.1103/PhysRevB.71.064405