Incommensurate Magnetic Structure of ZnCr$_{2}$Se$_{4}$ and ZnCr$_{2}$S$_{4}$

Recent studies of chalcogenide chromium spinels have shown a coupling between ferroelectricity and magnetism. The motivation of this work is to determine the magnetic ground state, (including its symmetry properties), to comprehend the coupling of magnetic and ferroelectric order parameters in the spinels ZnCr$_{2}$Se$_{4}$ and ZnCr$_{2}$S$_{4}$. The incommensurate magnetic structures through the N\'{e}el transition in these systems have been studied by high-resolution powder neutron diffraction. Below T$_{N}$ ($\sim $22K), for both cases, the magnetic structure is described as ferromagnetic layers in the \textbf{\textit{ab}}-plane stacked in a spiral arrangement along the \textbf{c}-axis with a propagation vector \textbf{k} = (0,0,$\sim $0.46). In ZnCr$_{2}$Se$_{4}$ and ZnCr$_{2}$S$_{4, }$ the magnetic phase transition is of first order. Therefore to use the irreducible co-representation theory, for symmetry analysis, the magnetic phase is described by a linear combination of irreducible representations. In this talk we present results of Rietveld analysis on the magnetic and crystal structure through the magnetic transition.