Theoretical study on the stability of magnetic structures in the surface and interfaces of Heusler alloys, Co2 MnAl and Co2 MnSi

We carry out the first principles calculations for bulk, surface, and bilayer system of Heusler alloys Co2 MnAl and Co2 MnSi. In the bulk system, the Mn moments feel much larger exchange (molecular) field than the Co moments. In the surface density of states of Co, the half-metallicity disappears in both systems, even though the magnetic moments are enhanced due to localization of d -states. The exchange constant of the surface Co is much reduced in Co2 MnSi. In the bilayer system with MgO, both the magnetic moments and exchange constants of Co atoms at the interface are remarkably decreased. We suppose that such instability of Co moments at the interface is responsible for the strong dependences of the magnetoresistance ratio on both the temperature and bias voltage, which was measured in the magnetic tunnel junction of Co2 MnAl (Si) MgO.