Structural and magnetic characterization of CeTa7O19 and YbTa7O19 with a two-dimensional pseudospin-12 triangular lattice

Triangular lattice antiferromagnets are prototypes for frustrated magnetism\nand may potentially realize novel quantum magnetic states such as a quantum\nspin liquid ground state. A recent work suggests NdTa$_7$O$_{19}$ with\nrare-earth triangular lattice is a quantum spin liquid candidate and highlights\nthe large family of rare-earth heptatantalates as a framework for quantum\nmagnetism investigation. In this paper, we report the structural and magnetic\ncharacterization of CeTa$_7$O$_{19}$ and YbTa$_7$O$_{19}$. Both compounds are\nisostructural to NdTa$_7$O$_{19}$ with no detectable structural disorder. For\nCeTa$_7$O$_{19}$, the crystal field energy levels and parameters are determined\nby inelastic neutron scattering measurements. Based on the crystal field\nresult, the magnetic susceptibility data could be well fitted and explained,\nwhich reveals that CeTa$_7$O$_{19}$ is a highly anisotropic Ising\ntriangular-lattice antiferromagnet ($g_z$/$g_{xy}$$\\sim$3) with very weak\nexchange interaction (J$\\sim$0.22~K). For YbTa$_7$O$_{19}$, millimeter sized\nsingle crystals could be grown. The anisotropic magnetization and electron spin\nresonance data show that YbTa$_7$O$_{19}$ has a contrasting in-plane magnetic\nanisotropy with $g_z$/$g_{xy}$$\\sim$0.67 similar as that of YbMgGaO$_4$. The\nabove results indicate that CeTa$_7$O$_{19}$ and YbTa$_7$O$_{19}$ with\npseudospin-1/2 ground states might either be quantum spin liquid candidate\nmaterials or find applications in adiabatic demagnetization refrigeration due\nto the weak exchange interaction.\n