Physical properties of single crystalline $R$Mg${}_2$Cu${}_9$ ($R$ = Y, Ce-Nd, Gd-Dy, Yb) and the search for in-plane magnetic anisotropy in hexagonal systems

Abstract

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.

17 pages, 16 figures