Origin of interfacial perpendicular magnetic anisotropy in MgO/CoFe/metallic capping layer structures.

Shouzhong Peng,Mengxing Wang,Hongxin Yang,Qianfan Zhang,Kang L Wang,Lang Zeng,Jiaqi Zhou,Ali Hallal,Mairbek Chshiev,Jiang Nan,Weisheng Zhao,Youguang Zhang

doi:10.1038/srep18173

Abstract

Spin-transfer-torque magnetic random access memory (STT-MRAM) attracts extensive attentions due to its non-volatility, high density and low power consumption. The core device in STT-MRAM is CoFeB/MgO-based magnetic tunnel junction (MTJ), which possesses a high tunnel magnetoresistance ratio as well as a large value of perpendicular magnetic anisotropy (PMA). It has been experimentally proven that a capping layer coating on CoFeB layer is essential to obtain a strong PMA. However, the physical mechanism of such effect remains unclear. In this paper, we investigate the origin of the PMA in MgO/CoFe/metallic capping layer structures by using a first-principles computation scheme. The trend of PMA variation with different capping materials agrees well with experimental results. We find that interfacial PMA in the three-layer structures comes from both the MgO/CoFe and CoFe/capping layer interfaces, which can be analyzed separately. Furthermore, the PMAs in the CoFe/capping layer interfaces are analyzed through resolving the magnetic anisotropy energy by layer and orbital. The variation of PMA with different capping materials is attributed to the different hybridizations of both d and p orbitals via spin-orbit coupling. This work can significantly benefit the research and development of nanoscale STT-MRAM.

Highlights

There is currently intense interest in magnetic tunnel junction (MTJ) with perpendicular magnetic anisotropy (PMA) for its potential to build low-power-consumption and high-density spin-transfer-torque magnetic random access memory (STT-MRAM)[1,2,3,4,5,6,7]
Further experiments revealed that a capping or seed layer adjacent to CoFeB has an essential influence on the PMA value, e.g. by replacing Ta with Hf as a capping or seed layer, the interfacial PMA increases from 1.8 erg/cm[2] to 2.3 erg/cm[2,12,13], whereas it dramatically decreases using Ru film[14]
We can find that different capping materials lead to very different PMA values

Summary

Results and Discussion

Since it has been proven that the PMA in Co(Fe)/MgO interface originates from the overlap between the interfacial O-pz orbital and the Co(Fe) 3d orbitals[9,11], here we only investigate the physical mechanism of PMA in the CoFe/X structures and compare different PMAs when the capping layer X changes. In the CoFe/Ru system, the interfacial Co atoms have little contribution to PMA, while the interfacial Ru atoms induce an in-plane anisotropy (− 0.20 erg/cm[2]) These differences at the interfaces result in a much lower PMA value for the Ru-capped system. For the interfacial Ru atoms, the primary contribution to MAE comes from the d orbitals, which lead to an in-plane anisotropy due to the negative values of the matrix elements (dxy, ) ( dx2−y2 and dxy, dxz). It can benefit the design of PMA-based MTJs with high thermal stability for advanced node STT-MRAM

Methods

Author Contributions

Additional Information