Magnetic and Structural Properties of Sr-Doped Ba$_{2-x}$Sr$_{x}$CoO$_{4}$*

We have studied the structural and magnetic properties of a newly synthesized compound Ba$_{2-x}$Sr$_{x}$CoO$_{4}$ with different doping ($x)$ levels. Monoclinic Ba$_{2}$CoO$_{4}$ is an antiferromagnetic (AFM) insulator with N\'eel temperature $T_{N}$ = 25 K and a two-dimensional character with spins aligned in the \textit{ac} plane. The isovalent Sr doping causes changes in both crystal structure and magnetic properties. With increasing $x$, $T_{N}$ initially increases then decreases after reaching the maximum at $x$=0.5. Correspondingly, its crystal structure changes from monoclinic ($x <$ 0.5) to orthorhombic ($x\ge $0.5) at room temperature. The correlation between crystal structure and physical properties will be discussed.