Interplay between magnetism and relativistic fermions in Eu doped (Sr/Ba)MnSb$_{\mathbf{2}}$

Layered compounds AMnBi$_{2}$ (A$=$Ca, Sr, Ba, Eu, and Yb) have been established as Dirac materials with fascinating properties. In our previous work, we have demonstrated that Sr$_{1-}_{y}$Mn$_{1-}_{z}$Sb$_{2}$ (y, z \textless 0.1), isostructural to AMnBi$_{2}$, not only host relativistic fermions, but also exhibit ferromagnetic properties, with its ferromagnetism being coupled to the relativistic fermions' transport. To gain further insight into the relativistic fermion-magnetism coupling, we have synthesized a series of Eu doped (Sr/Ba)MnSb$_{2}$ single crystals and found Eu moments order antiferromagnetically. Through neutron scattering experiments, we determined the magnetic structures for Sr$_{1-x}$Eu$_{x}$MnSb$_{2}$ with x $=$ 0.2, 0.5, and 0.8. From magnetotransport measurements, we find the Eu antiferromagnetism is also coupled to relativistic fermion transport. More importantly, we observed a novel quantum phase with saturated magnetoresistivity near the quantum limit for the 10{\%} Eu doped BaMnSb$_{2}$ sample. We will discuss possible mechanisms for this novel phase.