Incorporation of Sb or Bi into GaAs enables significant engineering of the valence band offsets in GaAs and leads to a large band gap reduction.These properties make the material suitable for a variety of potential optoelectronic applications, such as infrared light-emitting diodes,photo-detector and terahertz quantum cascade lasers [1].Moreover, strong spin-orbit (SO)interactions are expected in GaAsSb and GaAsBi semiconductor alloys [2].The conduction band electron spin properties depend critically on the SO interaction.Spintronic devices could be developed and would require lower modulation electric fields based on stronger Rashba SO interactions.However, thus far, the reported spin polarization in GaAsBi is only a few percent (<~7%) [3].The limited spin polarization in GaAsBi makes it unsuitable as an efficient optical spin detector and spin light source.However, very little is known about the spin properties of GaAsSb.Understanding the spin properties in GaAsSb is an important prerequisite in realizing scalable spintronic devices based on GaAs.