In this work, employing density functional theory based electronic structure calculations, we search for an alternative to MgO as a spacer layer in a magnetic tunneling junction (MTJ), with half-metallic (HM) Co 2 MnSb as an electrode. First, we demonstrate the possibility of designing an all-Heusler alloy based MTJ with semi-conducting (SC) TiCoSb alloy as the spacer material. We probe the robustness of the HM properties of the Mn–Sb/Co interface and show that the HM property is preserved, even with various disorders and defects. The spin-dependent transport behavior indicates that these properties depend sensitively on the heterojunction interfaces and thickness of the spacer material. Further, we study the transport properties of the heterojunctions of Co 2 MnSb alloy with the well-studied insulator MgO as well as the less-explored systems like, NaCl and AlN. In these three insulating materials, the smallest complex band decay coefficient is associated with Δ 1 symmetry, unlike TiCoSb. This feature enables more desirable symmetry-based spin-filtering properties at the Γ point in the 2D Brillouin zone. We further calculate the resistance area (RA) product for all the heterojunctions, important for the realization of highly sensitive magnetic sensors and it is found that the other spacer layers yield a RA product, several orders less in magnitude compared to that of TiCoSb. Our results indicate that NaCl and AlN may be promising as an alternative to MgO as a spacer material. With the chemical compatibilities resulting into minimal interface buckling and an ultra-low RA product, TiCoSb may also be a promising new material for a MTJ with Co 2 MnSb as electrode, specifically in relation to overcoming the variability and current-injection challenges.