Giant perpendicular magnetic anisotropy in Mo-based double-interface free layer structure for advanced magnetic tunnel junctions

Abstract

Magnetic tunnel junctions (MTJs) with perpendicular magnetic anisotropy (PMA) have advantages of high packing density due to their high thermal stability, efficient current-induced magnetic switching, and unrestricted cell aspect ratio.[1] Therefore, PMA-based MTJs (p-MTJs) have broad application prospects on high-density and non-volatile spin-transfer torque magnetic random access memories (STT-MRAMs) and other spintronics devices.[2-4] In particular, p-MTJs with CoFeB/MgO structures show high tunnel magnetoresistance ratio.[4] In the W/CoFeB/MgO stacks, the interfacial magnetic anisotropy (Ki) of 1.98 mJ/m2 is acquired after annealing at 350 °C.[5] To further improve Ki, it is an effective way to use MgO/CoFeB/X/CoFeB/MgO double-interface free layer structures, where the X represents the spacer material. [6] However, higher Ki is needed to scale down the MTJs devices and maintain high thermal stability.[7]In this work [8], we study the magnetic anisotropy of the double-interface free layer stacks with different annealing temperatures and different spacer materials. The samples of Ta(5)/MgO(2)/Co20Fe60B20(t)/X(0.4) /Co20Fe60B20(1.2)/MgO(2)/Ta(3) are deposited by sputtering system, where the numbers in parentheses indicate the thickness in nanometers, X including Ta, W, and Mo. The thickness of the bottom CoFeB layer t ranges from 0.4~3.2 nm. The films with Mo spacer layer exhibit the highest Ki of 4.06 mJ/m2 compared with the Ta- and W-based films. Furthermore, the Ki of Mo-based films remains at 2.93 mJ/m2 even after annealing at 500 °C for 1 h.Fig.1a and 1b show the structures with bottom CoFeB thickness of 0.8 nm and with spacer materials of Mo and W. As shown in the M-H (Magnetization M versus magnetic field H) curves, all the films show good PMA. Fig.1c and 1d show the corresponding high-resolution transmission electron microscopy (HR-TEM) and energy-dispersive X-ray spectroscopy (EDX) images.Saturation magnetization Ms and the total thickness of magnetic dead layer tDL can be extracted from the curves ( M/A versus tCoFeB curves), where A is the area of the sample and tCoFeB is the total thickness of CoFeB layers. The Ki can be extracted from the intercept of the Keff*teff versus teff curves, where Keff is effective magnetic anisotropy energy density and teff = tCoFeB- tDL. As shown in Fig.2a and 2b, Ki of 2.15±0.11 mJ/m2 and 2.90±0.34 mJ/m2 can be obtained for Ta- and W-based stacks, respectively. More importantly, the maximum of 4.06±0.14 mJ/m2 is acquired for the Mo-based structure after annealing at 350 °C as shown in Fig.2c. According to previous study, when the required thermal stability factor is 70, if the Ki reached 4 mJ/m2 , the device size can be reduced to 10 nm.[8] It can be shown in Fig.2d- 2f that the Ki decreases with the increase of annealing temperature. Note that the Ki reaches 2.93 mJ/m2 even after annealing at 500 °C for 1 h, which demonstrates the high thermal stability of this structure.Revealed by the HR-TEM and EDX images, the sharper interface, weaker interdiffusion, better interfacial uniformity and higher crystallinity can be seen in the Mo-based stack than the W-based stack. These results show that Mo plays an important role in improving the film quality. When the annealing temperature increases, the interfacial mixing intensifies, which leads to the decrease of Ki in Mo-based films.In summary, we study the dependence of magnetic anisotropy and film quality of the MgO/CoFeB/X/CoFeB/MgO double-interface free layer structures on the spacer material and the annealing temperature. We find that, the Mo-based structure exhibits the strongest PMA (4.06 mJ/m2). Furthermore, Ki in the Mo-based stack is still as strong as 2.93 mJ/m2 even after annealing at 500 °C. The HR-TEM and EDX images of W- and Mo-based stacks show that the high thermal stability can be attributed to the high film quality in the Mo-based films. Our work provides a practical solution to scale down MRAM device while maintaining a high thermal stability. **