Thermal stability and magnetization switching of exchanged coupled composite free layer of perpendicular magnetic tunnel junction

Abstract

Magnetic random access memory (MRAM) devices are envisioned as a major candidate for future memory technologies. Using an exchange coupled composite free layer (ECC FL) for perpendicular magnetic tunnel junction can improve the operation as predicted by a two-spin model [1]. Here, we show that ECC FL leads to significant reduction of the switching current and increase of the efficiency defined as the ratio between the energy barrier and switching current for device sizes (20-50nm) when the magnetization dynamics can be non-uniform. The results are obtained via the micromagnetic simulations performed with the micromagnetic simulator FastMag [2]. Figure 1 shows the thermal stability factor Δ and switching current density Jc0 as a function of the surface exchange energy density Jex for different anisotropy ratios γ between the ECC sublayers (i.e. FLsoft and FLhard ). of the ECC FL. The thermal stability factor increases with Jex (Fig. 1a). The switching current density Jc0 has an optimal value of the Jex at which it is minimal (Fig. 1b). The reduction of Jc0 is greater for greater γ and it can be reduced more than half as compared to a regular FL. The corresponding efficiency exhibits the maximal improvement of over 1.65 as compared to a regular FL. The mechanism leading to these improvements is that the switching of the softer layer assists in the switching of the harder layer. It is important that the reference layer is next to the softer layer. We find that the devices of a smaller size need to have a significantly greater surface coupling strength to achieve optimal operation. Support from the Semiconductor Research Corporation is greatly acknowledged.