Heusler Alloys for CPP-GMR

Half-Metallic full Heusler alloys of composition X$_{2}$YZ and structure type L2$_{1}$ have aroused interest because of their potential application in CPP-GMR spin valves for readers in hard disk drives. The X and Y are typically transition metals and the Z is a non-transition metal element. The structure of these alloys can be viewed as a variant of bcc in which (100) atomic layers of X alternate with layers of YZ. The alloys Co$_{2}$MnSi and Co$_{2}$MnGe have received particular attention because of their high T$_{C}$ which exceeds 900K. We have performed first-principles calculations using the VASP code in GGA to investigate the properties of these materials. We have found them to be half-metals in bulk in agreement with previous work. We obtained minority gaps at the Fermi energy of 0.36 and 0.51eV for Co$_{2}$MnGe and Co$_{2}$MnSi, respectively. We also investigated multilayers consisting of Heusler and various possible spacer materials. Interestingly, we found that for one or two atomic layers of Cr alternating with Co2MnGe along (100), the system remained half-metallic. Cr can actually be used in this way to increase the minority gap. We found that Co$_{2}$MnGe slabs were typically not half metallic, but slabs terminated in a pure Mn layer retained a minority gap.