Physical properties of single crystallineRMg2Cu9(R=Y,Ce−Nd,Gd−Dy,Yb)and the search for in-plane magnetic anisotropy in hexagonal systems

Single crystals of $R{\mathrm{Mg}}₂{\mathrm{Cu}}₉$ ($R=\mathrm{Y}$, 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- and field-dependent resistivity and anisotropic magnetization. ${\mathrm{YMg}}₂{\mathrm{Cu}}₉$ is a non-local-moment-bearing metal with an electronic specific heat coefficient, $\ensuremath{\gamma}\ensuremath{\sim}15$ mJ/mol ${\mathrm{K}}²$. Yb is divalent and basically non-moment-bearing in ${\mathrm{YbMg}}₂{\mathrm{Cu}}₉$. Ce is trivalent in ${\mathrm{CeMg}}₂{\mathrm{Cu}}₉$ with two magnetic transitions being observed at 2.1 K and 1.5 K. ${\mathrm{PrMg}}₂{\mathrm{Cu}}₉$ does not exhibit any magnetic phase transition down to 0.5 K. The other members being studied ($R=\mathrm{Nd}$, Gd-Dy) all exhibit antiferromagnetic transitions at low temperatures ranging from 3.2 K for ${\mathrm{NdMg}}₂{\mathrm{Cu}}₉$ to 11.9 K for ${\mathrm{TbMg}}₂{\mathrm{Cu}}₉$. Whereas ${\mathrm{GdMg}}₂{\mathrm{Cu}}₉$ 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 (${\mathrm{Y}}_0.99{\mathrm{Tb}}_0.01$)${\mathrm{Mg}}₂{\mathrm{Cu}}₉$ and (${\mathrm{Y}}_0.99{\mathrm{Dy}}_0.01$)${\mathrm{Mg}}₂{\mathrm{Cu}}₉$. 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.