  Single crystals of $R$Mg$_{2}$Cu$_{9}$ ($R$=Y, Ce-Nd, Gd-Dy, Yb) were grown
using a high-temperature solution growth technique and were characterized by
measurements of room-temperature x-ray diffraction, temperature-dependent
specific heat and temperature-, field-dependent resistivity and anisotropic
magnetization. YMg$_{2}$Cu$_{9}$ is a non-local-moment-bearing metal with an
electronic specific heat coefficient, $\gamma \sim$ 15 mJ/mol K$^2$. Yb is
divalent and basically non-moment bearing in YbMg$_{2}$Cu$_{9}$. Ce is
trivalent in CeMg$_{2}$Cu$_{9}$ with two magnetic transitions being observed at
2.1 K and 1.5 K. PrMg$_{2}$Cu$_{9}$ does not exhibit any magnetic phase
transition down to 0.5 K. The other members being studied ($R$=Nd, Gd-Dy) all
exhibits antiferromagnetic transitions at low-temperatures ranging from 3.2 K
for NdMg$_{2}$Cu$_{9}$ to 11.9 K for TbMg$_{2}$Cu$_{9}$. Whereas
GdMg$_{2}$Cu$_{9}$ is isotropic in its paramagnetic state due to zero angular
momentum ($L$=0), all the other local-moment-bearing members manifest an
anisotropic, planar magnetization in their paramagnetic states. To further
study this planar anisotropy, detailed angular-dependent magnetization was
carried out on magnetically diluted (Y$_{0.99}$Tb$_{0.01}$)Mg$_{2}$Cu$_{9}$ and
(Y$_{0.99}$Dy$_{0.01}$)Mg$_{2}$Cu$_{9}$. Despite the strong, planar
magnetization anisotropy, the in-plane magnetic anisotropy is weak and
field-dependent. A set of crystal electric field parameters are proposed to
explain the observed magnetic anisotropy.

Materials Science