Layered borocarbides $R{\mathrm{B}}_2\mathrm{C}$ ($R=\mathrm{D}\mathrm{y}$, Ho, and Er) have been studied by powder neutron diffraction at 2–30 K. ${\mathrm{E}\mathrm{r}\mathrm{B}}_2\mathrm{C}$ has two-sublattice antiferromagnetic order below $T_N=16.3\mathrm{K}$, but ${\mathrm{D}\mathrm{y}\mathrm{B}}_2\mathrm{C}$ and ${\mathrm{H}\mathrm{o}\mathrm{B}}_2\mathrm{C}$ show a coexistence of a conventional canted $\mathbf{k}=(000)$ ferromagnetic structure and unconventional magnetic correlations. The $\mathbf{k}=(000)$ phase orders at $T_c=8.5\mathrm{K}$ (${\mathrm{D}\mathrm{y}\mathrm{B}}_2\mathrm{C}$) and 7.1 K (${\mathrm{H}\mathrm{o}\mathrm{B}}_2\mathrm{C}$), but low-$Q$ diffraction peaks from the unconventional correlations appear above $T_c$ with different critical temperatures for different peaks: at 8, 10.5, and 15.7 K for ${\mathrm{H}\mathrm{o}\mathrm{B}}_2\mathrm{C}$. This scattering is fitted as diffraction from a Warren-type random magnetic layer lattice and may result from quadrupolar interactions between $R^{3+}$ spins.

• Received 31 May 2002

DOI:https://doi.org/10.1103/PhysRevLett.90.087201