First-Principles Calculation of Electronic Structure in NiMnSb/MgO and CoMnSb/MgO Junctions

Half-metallic ferromagnets are promising candidates for electrode materials of magnetic tunnel junctions (MTJ). Extensive studies have been carried out for MTJ using Co-based full Heusler alloys, such as Co
₂
MnSi and Co
₂
FeSi, with L2
₁
structure. On the other hand, experimental attempt was limited for MTJ using Heusler alloys with C1
_b
structure such as NiMnSb and PtMnSb. First-principles calculations indicated that atomic disorder gives rise to electronic states in the minority-spin gap and easily degrades the spin polarization of NiMnSb [1]. In this study, we performed density-functional calculations of electronic structure in NiMnSb/MgO and CoMnSb/MgO junctions to assess the feasibility of these Heusler alloys as electrode materials of MTJ. We adopted the Vienna ab initio simulation package (VASP) with use of the projector augmented-wave (PAW) potential. The exchange-correlation energy is considered with a generalized gradient approximation (GGA). First, we examined stable interfacial structure of NiMnSb/MgO and CoMnSb/MgO junctions by evaluating formation energy of each structure. As a result, the MnSb-terminated interface, where Mn and Sb atoms are located on top of O atoms, is preferable energetically for both junctions. While the magnetic moment of Mn atoms at the interface increases compared to that in bulk, the Ni or Co magnetic moment on the sub-interface layer remarkably decreases for both junctions. Note that the Co magnetic moment of
_Co2
MnSi is almost unaltered even at the sub-interface layer in the MnSi-terminated Co
₂
MnSi/MgO junction [2]. The reduction of the Ni or Co magnetic moment in the interfacial region enhances thermal fluctuation of the magnetic moment, which leads to significant reduction of tunnelling magnetoresistance (TMR) ratio with increasing temperature [3]. We also found the reduction of the spin polarization in the interfacial region of the NiMnSb/MgO and CoMnSb/MgO junctions due to the presence of interfacial resonant states in the minority-spin gap. The reduction of the spin polarization and the magnetic moment in the interfacial region of CoMnSb/MgO junction can be recovered by Co or Mn atoms occupied at the vacancy sites in the interfacial region.