Asymmetric angular dependence of spin-transfer torques in   CoFe/Mg-B-O/CoFe magnetic tunnel junctions
  Using a first-principles noncollinear wave-function-matching method, we studied the spin-transfer torques (STTs) in CoFe/Mg-B-O/CoFe(001) magnetic tunnel junctions (MTJs), where three different types of B-doped MgO in the spacer are considered, including B atoms replacing Mg atoms (Mg$_3$BO$_4$), B atoms replacing O atoms (Mg$_4$BO$_3$), and B atoms occupying interstitial positions (Mg$_4$BO$_4$) in MgO. A strong asymmetric angular dependence of STT can be obtained both in ballistic CoFe/Mg$_3$BO$_4$ and CoFe/Mg$_4$BO$_4$ based MTJs, whereas a nearly symmetric STT curve is observed in the junctions based on CoFe/Mg$_4$BO$_3$. Furthermore, the asymmetry of the angular dependence of STT can be suppressed significantly by the disorder of B distribution. Such skewness of STTs in the CoFe/Mg-B-O/CoFe MTJs could be attributed to the interfacial resonance states induced by the B diffusion into MgO spacer. The present investigation demonstrates the feasibility of effectively enhancing microwave output power in MgO based spin torque oscillator (STO) by doping the B atoms into MgO spacer. 
Ling Tang, Zhi-Jun Xu, Xian-Jun Zuo, Ze-Jin Yang, Qing-He Gao,   Rong-Feng Linghu, Yun-Dong Guo
cond-mat.mes-hall