The crystal and magnetic structures of Laves-phase compound $\left({\mathrm{Nd}}_{0.5}{\mathrm{Tb}}_{0.5}\right){\mathrm{Co}}_2$ have been investigated by high resolution neutron powder diffraction at different temperatures. Magnetization measurement and neutron diffraction reveal two magnetic transitions at $T_{\mathrm{C}}\approx 173\mathrm{K}$ and $T_{\mathrm{M}}\approx 47\mathrm{K}$, respectively. At room temperature, the compound crystallizes in the $\mathrm{Mg}{\mathrm{Cu}}_2$-type (C15) structure. Below $T_{\mathrm{C}}$, rhombohedral distortion and large anisotropic magnetostriction take place and persist down to $4\mathrm{K}$. In contrast to the binary rare earth $\left(R\right)\text{−}\mathrm{Co}$ Laves-phases $R{\mathrm{Co}}_2$, a noncollinear magnetic structure (canted) is deduced for $\left({\mathrm{Nd}}_{0.5}{\mathrm{Tb}}_{0.5}\right){\mathrm{Co}}_2$, based on the Rietveld refinement of the neutron diffraction data at $50\mathrm{K}$ and $4\mathrm{K}$. In addition, the correlation between lattice distortion and easy magnetization direction (EMD) commonly observed for binary $R{\mathrm{Co}}_2$ is violated in $\left({\mathrm{Nd}}_{0.5}{\mathrm{Tb}}_{0.5}\right){\mathrm{Co}}_2$. Though the crystal structure remains rhombohedral, the EMD of the $R$ sublattice is close to the [110] direction at $50\mathrm{K}$ of the pseudocubic lattice and along the [111] direction at $4\mathrm{K}$, respectively. The difference of the canting angle between the magnetic structures at $50\mathrm{K}$ and $4\mathrm{K}$ is small, indicating stable canted configurations. The lattice parameters exhibit a discontinuity around $T_{\mathrm{M}}$, suggesting a first-order transition between the two canted magnetic structures. The dilution of magnetic anisotropy of the $R$ sublattice and the contribution of the magnetic anisotropy of the Co sublattice are responsible for the observations.

• Received 5 October 2005

DOI:https://doi.org/10.1103/PhysRevB.73.064413