Recently, dynamic random access memory (DRAM) has scaling limit because of its capacitor area. To replace DRAM, the next generation memory devices are being studied. Among them, perpendicular spin transfer torque magnetic random access memory (p-STT MRAM) is considered as a promising candidate because of easy scaling down, low power consumption, high read and write speed and non-volatility.

p-STT MRAM has several challenges for commercialization. To keep a high retention time (~10 years), STT-MRAM should have thermal stability greater than 74. Although the thickness of free layer is increased with thermal stability, the thickness of free layer is limited to maintain perpendicular magnetic anisotropy (PMA). One of studies, double MgO based p-MTJ spin-valve structure is issued for its large volume because in case of double MgO based p-MTJ spin-valve, PMA of the free layer is originated from two i-PMA between the CoFeB free layer / MgO tunneling barrier and CoFeB free layer / MgO capping layer. The i-PMA induced from the two interfaces increases the total volume of the magnetic free layer which increases the thermal stability, but it is still limited by the thickness of the individual CoFeB free layers. In our study, therefore, we propose a new structure using interfacial Co₂Fe₆B₂ and [Co/Pt]_n multi-layer as bulk PMA to achieve higher thermal stability than that of double MgO based p-MTJ spin-valve structure.

The dependence of the magnetic moment and thermal stability on the number of [Co/Pt]_n multi-layers was estimated by using vibrating sampling magnetometer (VSM) as shown in Fig.1 b. Magnetic moment t~180μemu/cm²) of i-PMA+[Co/Pt]₂ multi-layers is similar to that(~180μemu/cm²) of double MgO based p-MTJ spin-valve structure. Also, magnetic moment (t~366μemu/cm²) of i-PMA+ [Co/Pt]₅ multi-layers is about two times more than that of double MgO based p-MTJ spin-valve structure and thermal stability (~181) is two times higher than that (~74) of the double MgO based p-MTJ spin-valve structure. The thermal stability can be modulated by simply changing the number of [Co/Pt]_n multi-layers. Obviously, these results show that i-PMA + [Co/Pt]n multilayer as bulk PMA based p-MTJ spin-valve can achieve a high thermal stability.

In our presentation, we report the different characteristics of the p-MTJ spin-valve with [Co/Pt]_n multi-layers by examining the static magnetization behavior, TMR ratio and MgO crystallinity. In addition, we present how these properties influence mechanism of p-MTJ spin-valve.

Acknowledgements

This work was supported by a Basic Science Research Program grant from the National Research Foundation of Korea (NRF) funded by the Korean government (MSIP) (No. 2017R1A2A1A05001285) and the Brain Korea 21 PLUS Program in 2014.