Single crystal growth and study of the magnetic properties of the mixed system Ba_(3-x)Sr_xCr₂O₈

We describe the growth of Ba_(3-x)Sr_xCr₂O₈ single crystals with x=2.9 and x=2.8 in a four-mirror type optical floating-zone furnace and the influence of chemical disorder on their magnetic and structural properties studied by magnetization and heat-capacity measurements. Inelastic neutron scattering measurements on single crystalline Ba_(3-x)Sr_xCr₂O₈ with x=2.9 show three excitation modes, corresponding to the three twinned domains that confirm a monoclinic distortion. The interaction constants were determined from a random phase approximation (RPA) model, and the results are compared to the pure Sr₃Cr₂O₈ and Ba₃Cr₂O₈ systems. The intradimer interaction constant for Ba_0.1Sr_2.9Cr₂O₈ is found to be J₀ = 5.332(2) meV, about 4% smaller than that of the pure Sr₃Cr₂O₈ compound, while the interdimer exchange interaction Je is smaller by 6.9%. These results show a noticeable change in the magnetic properties by a random substitution effect.