Single-crystal growth and study of the mixed spin-dimer system $Ba_{3-x} Sr_x Cr_2 O_8$

The compounds $Sr_3 Cr_2 O_8$ and $Ba_3 Cr_2 O_8$ are insulating dimerized antiferromagnets with $Cr^{5+}$ magnetic ions. These spin$-\frac{1}{2}$ ions form hexagonal bilayers with a strong intradimer antiferromagnetic interaction that leads to a singlet ground state and gapped triplet states. We report on the effect on the magnetic properties of $Sr_3 Cr_2 O_8$ by introducing chemical disorder upon replacing Sr by Ba. Two single crystals of $Ba_{3-x} Sr_x Cr_2 O_8$ with $x = 2.9$ (3.33% of mixing) and $x = 2.8$ (6.66% of mixing) were grown in a four-mirror-type optical floating-zone furnace. The magnetic properties of these compounds were studied by magnetization measurements. Inelastic neutron-scattering measurements on $Ba_{0.1} Sr_{2.9} Cr_2 O_8$ were performed in order to determine the interaction constants and the spin gap for $x = 2.9$. The intradimer interaction constant is found to be $J_0 = 5.332 (2) meV$, about 4% smaller than that of pure $Sr_3 Cr_2 O_8$, whereas the interdimer exchange interaction $J_e$ is smaller by 7%. These results indicate a noticeable change in the magnetic properties by a random substitution effect