Spin-torque nano-oscillators in magnetic tunnel junction with double reference layers

A recent technological realization of perpendicular magnetic tunnel junctions (MTJs) with two perpendicular reference layers has opened a new route for scaling the overall size of spin-transfer torque MRAM. This is because for an antiparallel configuration of the magnetization of the reference layers (RL) the two spin-transfer torques add constructively, giving rise to more efficient switching processes of the free layer magnetization as compared to a single MTJ [1]. Here, we use a full micromagnetic code to study a double MTJ with two in-plane RL in antiparallel configuration, generating microwave signals at low drive currents and external fields. The geometry and physical parameters of the investigated device are similar to the one studied experimentally by Zeng et al. [2] with the adding of the second RL. The device has an elliptical cross section of 70x40 nm2 and thickness t=1.60 nm. Our micromagnetic simulations show that it is possible to obtain magnetization oscillations by applying low field (10 mT) and current densities of JMTJ=0.4 –1.6 MA/cm2. In addition, the power as a function of the source current describes a similar behaviour of that reported in a previous experimental study for single RL MTJ (see Fig. 2 of Ref. 2). From the comparison between the MTJ with single [2] and double RL, we see that the source current is decreased of a factor 5 (from 3.0 to 0.6 MA/cm2).