Impact of Fe80B20 insertion on the properties of dual-MgO perpendicular magnetic tunnel junctions

Abstract

Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have been studied in the recent decades as the crucial element for next generation memory applications [1]. As the data-storage layer in MTJs, the dual-MgO free layers (FLs) with a structure as MgO/CoFeB/spacer/CoFeB/MgO have been under intense development to provide high PMA and low damping after annealing, which guarantees the high thermal stability and low switching current in MTJ devices [2]. The previous efforts for the optimization of dual-MgO FLs include the engineering of non-magnetic spacer [3-5] and boron composition [6]. As a novel approach, we explore the impact of Fe80B20 inserted at both Co20Fe60B20/MgO interfaces of dual-MgO FLs under different annealing conditions in bottom-pinned MTJs. Current-in-plane tunnelling measurements are carried out to characterize magnetotransport properties of the MTJs. Conventional magnetometry measurements and ferromagnetic resonance are conducted to estimate the PMA and the Gilbert damping of dual-MgO FLs as a function of the Fe80B20 thickness and annealing temperatures. With ultrathin Fe80B20 (0.2 - 0.4 nm) inserted at the bottom interface, the tunnel magneto-resistance (TMR) can be maintained with a lower resistance-area (RA) product, while the top-FeB insertion results in a more RA drop with a similar TMR (Fig. 1(a)). The PMA of dual-MgO FLs with FeB inserted at the bottom interface shows a larger improvement than its top FeB insertion counterpart, even after 400 °C annealing (Fig. 1(b,c)). At the same time, the ultrathin FeB insertion at either interface reduces the damping constant in the FL (Fig. 1(d,e)). By optimizing the FeB insertion layer thickness and location, dual-MgO FLs with an unchanged TMR, higher effective anisotropy and lower damping can be achieved. This study demonstrates a novel approach to tune properties of dual-MgO FLs up to 400 °C annealing, which enables MTJ stacks for various applications.