Enhancement of magnetic interaction and magnetic anisotropy in MTJ with multiple CoFeB/MgO interfaces for high thermal stability

Abstract

Development of CoFeB/MgO based magnetic tunneling junction with perpendicular magnetic anisotropy (PMA) (p-MTJ) [1] has greatly contributed to realize the mass production of STT-MRAM. The higher thermal stability factor (△) is required for further scaling down of MTJ. In order to achieve high △, double and quad CoFeB/MgO interfaces p- MTJs have been proposed [2-4]. Single thin nonmagnetic layer (e.g., Ta, W, MgO, etc.) is inserted in the free layer to obtain high PMA (Stack A in Fig.1) [2]. In addition, enhancement of magnetic coupling (Jcpl) between free layer 1 and free layer 2 is also important to increase △, because △ does not increase when the magnets behave magnetically independently without Jcpl. Previous study showed an enhancement of PMA in double interface p-MTJ by inserting a thin ferromagnetic layer sandwiched by two nonmagnetic layers in the free layer [5] (we call as “ferromagnetic bridge layer (FBL)”). However, Jcpl for the free layer with FBL was not considered. In this study, we investigated Kefft* and Jcpl for the free layer with FBL in the double interface p-MTJ and simulated its impact on △. We prepared two types of stack with single insertion layer (Stack A) and FBL (Stack B). All stacks were deposited by DC/RF magnetron sputtering system, and then annealed at 400oC for 1h. Figures1-2 show experimental results of total insertion layer thickness (t) dependence of Jcpl and Kefft*. Stack B with FBL showed higher Kefft* than Stack A with single insertion layer in all t range, while keeping higher Jcpl when t was up to 0.5 nm because each insertion layer thickness is much thinner than single insertion layer. This could result in higher △ in Stack B with FBL because △ is proportional to Kefft* and effective free layer volume. Detailed simulation method and results will be presented. The FBL structure is an important technology to achieve high values both in Jcpl and Kefft*, which result in high △ for double and quad CoFeB/MgO interfaces MTJ. This work is supported by CIES’s Industrial Affiliation on STT MRAM program, CIES Consortium, JST-OPERA, and CAO-SIP.