High field magnetization, specific heat and NMR of the two dimensional tetramer-cuprate Na5RbCu4(AsO4)4Cl2

Complex novel 2D compound Na$_{5}$RbCu$_{4}$(AsO$_{4})$Cl$_{2}$ is a unique magnetic material, which contains layers of Cu$_{4}$O$_{4}$ tetramers [S.-J.~Hwu \textit{et al}., J. Am. Chem. Soc.,\textbf{ 124},~12404 (2002)]. The ground state following a second order low entropy phase transition at 15(1) K is antiferromagnetically ordered one [J.A. Clayhold \textit{et al}., Phys. Rev. B, \textbf{66}, 052403 (2002)]. We have measured the $^{87}$Rb and $^{35}$Cl NMR spectra and high field magnetization and specific heat at low temperatures. We discuss the field dependence of T$_{N}$, hyperfine couplings at various nuclear sites, and characteristics of the ordered state as seen from Rb and Cl nuclear sites.