Effect of Nd and Gd addition on the Electrical transport and Magnetic Properties of Co2MnSi Heusler compounds ABSTRACT Heusler alloys, such as Co2MnSi and Co2MnGe, have been predicted from first– principles to be half metallic and are a remarkable class of inter metallic materials with 1:1:1 (often called Half- Heusler) or 2:1:1 composition comprising more than 1500 members. Today, more than a century after their discovery by Fritz Heusler, they are still a field of active research. and potential candidates for spintronic applications. New properties and potential fields of applications emerge constantly; the prediction of topological insulators is the most recent example. Surprisingly, the properties of many Heusler compounds can easily be predicted by the valence electron count. Their extremely flexible electronic structure offers a toolbox which allows the realization of demanded but apparently contradictory functionalities within one ternary compound. Devices based on multifunctional properties i.e. the combination of two or more functions such as superconductivity and topological edge states will revolutionize technological applications. The subgroup of more than 250 semiconductors is of high relevance for the development of novel materials for energy technologies. Their band gaps can readily be tuned from zero to 4 eV by changing the chemical composition. Thus, great interest has been attracted to the fields of thermo electrics and solar cell research. The wide range of their multifunctional properties is also reflected in extraordinary magneto-optical, magneto electronic and magneto caloric properties. The most prominent example is the combination of magnetism and exceptional transport properties in spintronic devices. This study presents the synthesis of Co2MnSi and Co2Mn(1-x)RE(x)Si, ( RE=Nd, Gd where x=0.05, 0.1) by the Arc melting technique and efficient measurement of resistivity and magnetic properties. The results show that the synthesized compounds belong to the BiF3 – Fm/3m space group which is an important crystal lattice group for spintronic device applications. The magnetic measurement and dc resistivity studies reveals that this compound belongs to the soft ferro magnetic materials. However, only 50–60 % spin polarizations were obtained experimentally for these compounds—a decrease due to the defects in the Mn and Co sublattices.