Magnetic order and exchange coupling in the frustrated diamond-lattice antiferromagnet MnSc2Se4

We report the magnetic properties of the $A$-site spinel compound ${\mathrm{MnSc}}₂{\mathrm{Se}}₄$. The macroscopic magnetic measurements uncover successive magnetic transitions at ${T}_{\mathrm{N}1}=2.04$ K, followed by two further transitions at ${T}_{\mathrm{N}2}=1.8$ K and ${T}_{\mathrm{N}3}=1.6$ K. Neutron powder diffraction reveals that both ${T}_{\mathrm{N}2}<T<{T}_{\mathrm{N}1}$ and $T<{T}_{\mathrm{N}3}$ orders are associated with the propagation vector $k=(3/4,3/4,0)$, while the magnetic structures are collinear amplitude modulated and helical, respectively. Using neutron powder spectroscopy we demonstrated the effect of substitution of S by Se on the magnetic exchange. The energy range of the spin-wave excitations is suppressed due to the chemical pressure of the $X$ ion in ${\mathrm{MnSc}}₂{X}₄$ ($X=\text{S}$, Se) spinels.