  Magnetic topological semimetals have attracted intense attention recently
since these materials carry a great promise for potential applications in novel
spintronic devices. Here, we report an intimate interplay between lattice, Eu
magnetic order and topological semimetallic behavior in
Eu$_{1-x}$Sr$_{x}$MnSb$_{2}$ driven by nonmagnetic Sr doping on magnetic Eu
site. Different types of Eu spin reorientations are controllable by the Sr
concentration, temperature or magnetic field, and coupled to the quantum
transport properties of Dirac fermions generated by the 2D Sb layers. Our study
opens a new pathway to achieving exotic magnetic order and topological
semimetallic state via controlling spin reorientation. The effective strategy
of substituting rare-earth site by nonmagnetic element demonstrated here may be
applicable to the AMnCh$_{2}$ (A=rare-earth elements; Ch=Bi/Sb) family and a
wide variation of other layered compounds involving spatially separated
rare-earth and transition metal layers.
