(Sr$_{ 2}$,Ba$_{ 2}$)Cu$_{ 3}$O$_{ 4}$Cl$_{ 2}$ are antiferromagnetic insulators which are akin to the parent compounds of the cuprate superconductors but with two distinct magnetic ordering temperatures related to two magnetic Cu$_{ I}$ and Cu$_{ II}$ spin sublattices. Here we present a study of these materials by means of Raman spectroscopy. Following the temperature and polarization dependence of the data we readily identify two distinct features at around 3000 cm$^{ -1}$ and 300 cm$^{ -1}$ that are related to two-magnon scattering from the two sublattices. The estimated spin-exchange coupling constants for the Cu$_{ I}$ and Cu$_{ II}$ sublattices are found to be J$_{ I}\sim$139-143(132-136) meV and J$_{ II}\sim$14(11) meV for Sr(Ba) compounds. Moreover, we observe modes at around 480 and 445 cm$^{ -1}$ for the Sr and Ba containing samples respectively, that disappears at the ordering temperature of the Cu$_{ II}$. We argue that this modes may also be of magnetic origin and possibly related to interband transitions between the Cu$_{ I}$-Cu$_{ II}$ sublattices.
Categories: cond-mat.str-el cond-mat.supr-con