Canted magnetic structure arising from rare-earth mixing in the Laves-phase compound(Nd0.5Tb0.5)Co2

Abstract

The crystal and magnetic structures of Laves-phase compound $({\mathrm{Nd}}_{0.5}{\mathrm{Tb}}_{0.5}){\mathrm{Co}}_{2}$ have been investigated by high resolution neutron powder diffraction at different temperatures. Magnetization measurement and neutron diffraction reveal two magnetic transitions at ${T}_{\mathrm{C}}\ensuremath{\approx}173\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and ${T}_{\mathrm{M}}\ensuremath{\approx}47\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, 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 $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In contrast to the binary rare earth $(R)\text{\ensuremath{-}}\mathrm{Co}$ Laves-phases $R{\mathrm{Co}}_{2}$, a noncollinear magnetic structure (canted) is deduced for $({\mathrm{Nd}}_{0.5}{\mathrm{Tb}}_{0.5}){\mathrm{Co}}_{2}$, based on the Rietveld refinement of the neutron diffraction data at $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. In addition, the correlation between lattice distortion and easy magnetization direction (EMD) commonly observed for binary $R{\mathrm{Co}}_{2}$ is violated in $({\mathrm{Nd}}_{0.5}{\mathrm{Tb}}_{0.5}){\mathrm{Co}}_{2}$. Though the crystal structure remains rhombohedral, the EMD of the $R$ sublattice is close to the [110] direction at $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ of the pseudocubic lattice and along the [111] direction at $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$, respectively. The difference of the canting angle between the magnetic structures at $50\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ and $4\phantom{\rule{0.3em}{0ex}}\mathrm{K}$ 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.