The effect of Cu substitution on the structural and magnetic properties of $\mathrm{Tb}{\mathrm{Ni}}_{5-x}{\mathrm{Cu}}_x$ $(x=0,0.5,1.0,1.5,2.0)$ have been investigated by x-ray diffraction, magnetization measurements and neutron powder and single crystal diffraction. The electronic and the magnetic structures of $\mathrm{Tb}{\mathrm{Ni}}_5$ were studied using first principles theory. All samples were found to have the $\mathrm{Ca}{\mathrm{Cu}}_5$-type structure, space group $P6∕mmm$. The lattice parameters increase monotonically with increasing Cu concentration. The Curie temperature $T_c$ has a maximum value of $29\mathrm{K}$ at $x=1.0$. The magnetic structure of $\mathrm{Tb}{\mathrm{Ni}}_5$ at $10\mathrm{K}$ is incommensurate with a helimagnetic component [wave vector $\mathbf{q}\sim 2\pi ∕c(0,0,0.02)$] perpendicular to a ferromagnetic one. In contrast, the substituted $\mathrm{Tb}{\mathrm{Ni}}_{5-x}{\mathrm{Cu}}_x$ alloy is ferromagnetic. All magnetic moments are observed to be located on the Tb atoms. The magnetocrystalline anisotropy in the $ab$ plane is observed to be strongly increased by the Cu substitution, whereas the magnetization decreases with the Cu concentration. The observed magnetic structure of $\mathrm{Tb}{\mathrm{Ni}}_5$ is consistent with first principles calculations regarding both the magnetic moments and the helimagnetic structure. The microscopical origin of the helimagnet is analyzed and correlated to the Fermi surface topology.

• Received 13 April 2006

DOI:https://doi.org/10.1103/PhysRevB.74.094419