Co2Fe6B2/MgO-Based Perpendicular Spin-Transfer-Torque Magnetic-Tunnel-Junction Spin-Valve without [Co/Pt]n lower Synthetic-Antiferromagnetic Layer
Authors: Lee Seung-Eun , Shim Tae-Hun , Park Jea-Gun 
Published: February 2020

In recently years, the research on magnetic tunnel junctions (MTJs) using CoFeB magnetic layer and MgO tunneling barrier layer has been performed for realizing perpendicular-spin-transfer-torque magnetic-random-access-memory (p-STT-MRAM). In particular, to integrate a terra-bit-level nonvolatile memory-cell, the p-MTJ needs to achieve critical device performances such as a high tunneling magnetoresistance (TMR) ratio of ~150%, a high thermal stability (°â
= E/KBT) of 74, and a low critical current density (Jo
) of 13.4 MA/cm2, where E, Ms
, kB
, and T are the energy barrier, the saturation magnetization, the Boltzmann constant, and the temperature, respectively.[1] In addition, to achieve CoFeB/MgO based p-MTJs grown on 12-inch TiN/W electrode, a design of [Co/Pt]n synthetic-anti-ferro-magnetic (SyAF) layer being operated at the ex-tu annealing above 350 oC has been intensively studied.[2] However, the [Co/Pt]n SyAF layer essentially has thicker a [Co/Pt]n SyAF layer since a [Co/Pt]n lower SyAF should couple antiferromagnetically with a [Co/Pt]n upper SyAF layer and couple ferromagnetically with a Co2Fe6B2 pinned layer at the same time.[3] Thicker SyAF layer has the problem of a high fabrication cost for p-MTJ spin-valves in the mass production of p-STT MRAM. Thus, we designed a Co2Fe6B2/MgO based p-MTJ spin-valve without a [Co/Pt]n lower SyAF layer, which can largely reduce the fabrication cost since the total multi-layer number (n) of [Co/Pt]n for the Co2Fe6B2/MgO-based p-MTJ spin-valve.
Figures 1(a) and (b) show the spin-valve structure of a [Co/Pt]n SyAF layer with lower SyAF layer, without lower SyAF layer, respectively. The [Co/Pt]n SyAF layer without a lower SyAF layer is much thinner than that with a lower SyAF layer, as you can see in figure 1. In addition, the upper SyAF layer of p-MTJ without a lower SyAF layer is much thinner than that of p-MTJ with a lower SyAF layer because of fitting for antiferromagnetically coupling between lower and upper SyAF layer. And we studied the TMR ratio dependency on bcc nanoscale capping layer thickness of two structures by using current in-plane tunneling (CIPT) technique. In result, the maximum TMR ratios of two structures were both over 150%. In case of the p-MTJ with a [Co/Pt]n lower SyAF layer, the TMR ratio rapidly increased from 19% to 159% when the bcc nanoscale capping-layer thickness (tbcc
) increased from 0.3 to 0.6 nm. Then, it abruptly decreased from 159% to 2% when tbcc
decreased from 0.6 to 0.73 nm. Also, for the p-MTJ spin-valve without a [Co/Pt]n lower SyAF layer, the TMR ratio very rapidly increased from 5% to 158% when tbcc
increased from 0.3 to 0.6 nm. Then, it abruptly decreased from 158% to 2% when tbcc
decreased from 0.6 to 0.73 nm.
In the conference, we will present the effect of Co2Fe6B2/MgO-based perpendicular spin-transfer-torque magnetic-tunnel-junction spin-valve without [Co/Pt]n lower synthetic-antiferromagnetic layer and review the advantage of p-MTJ without lower SyAF layer. In addition, we report the physical and magnetic properties of p-MTJ by using vibrating-sampling-magnetometer (VSM), transmission-electron-microscopy (TEM). Furthermore, we review the mechanism why the magnetic properties of p-MTJs are varied by the thickness of SyAF layer.
* This research was supported by the Basic Science Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education (No.2014R1A2A1A01006474) and the Brain Korea 21 PLUS Program in 2014.

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[1] K. C. Chun, H. Zhao, J. D. Harms, T. H. Kim, J. P. Wang, and C. H. Kim, IEEE J. Solid-State Circuits 48, 598, 2013.
[2] S. Ikeda, H. Sato, H. Honjo, E. C. I. Enobio, S. Ishikawa, M. Yamanouchi, S. Fukami, S. Kanai, F. Matsukura, T. Endoh and H. Ohno, in IEEE International Electron Devices Meeting (IEDM) IEEE, 2014, pp. 33.2.1.
[3] J. G. Park, T. H. Shim, K. S. Chae, D. Y. Lee, Y. Takemura, S. E. Lee, M. S. Jeon, J. U. Baek, S. O. Park, and J.P. Hong, in IEEE International Electron Devices Meeting (IEDM) IEEE, 2014, pp. 19.2.1.

Figure 1