Magnetic order and exchange couplings in the frustrated diamond lattice antiferromagnet MnSc2Se4

We report the magnetic properties of A-site spinel compound MnSc2Se4. The macroscopic magnetic measurements uncovers successive magnetic transitions at TN1= 2.04 K, followed by two further transitions at TN2=1.8 K and TN3=1.6 K. Neutron powder diffraction reveals that both, TN2 < T < TN1 and T < TN3, 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 supressed due to the chemical pressure of the Xion in MnSc2X4 (X=S, Se) spinels.