Anisotropic magnetization studies of $R_2 Co Ga_8$ (R = Gd, Tb, Dy, Ho, Er, Tm, Y and Lu) single crystals

Single crystals of R$_2$CoGa$_8$ series of compounds were grown, for the first time, by high temperature solution growth (flux) method. These compounds crystallize in a tetragonal crystal structure with the space group $P4/mmm$. It has been found that R$_2$CoGa$_8$ phase forms only with the heavier rare earths, starting from Gd with a relatively large $c/a$ ratio of $\approx$ 2.6. The resultant anisotropic magnetic properties of the compounds were investigated along the two principal crystallographic directions of the crystal viz., along [100] and [001]. The nonmagnetic compounds Y$_2$CoGa$_8$ and Lu$_2$CoGa$_8$ show diamagnetic behavior down to the lowest temperature (1.8 K) pointing out the non-magnetic nature of Co in these compounds and a relatively low density of electronic states at the Fermi level. Compounds with the magnetic rare earths order antiferromagnetically at temperatures lower than 30 K. The easy axis of magnetization for R$_2$CoGa$_8$ (R = Tb, Dy and Ho) is found to be along the [001] direction and it changes to [100] direction for Er$_2$CoGa$_8$ and Tm$_2$CoGa$_8$. The magnetization behavior is analyzed on the basis of crystalline electric field (CEF) model. The estimated crystal field parameters explains the magnetocrystalline anisotropy in this series of compounds.