MRAM based on spin-transfer-torque (STT) write operations has been intensively developed as a technology for saving energy. During the development of high-density STT-MRAM, issues have arisen that have placed an intrinsic physical limit on the miniaturization of STT-MRAM. Recently, we proposed and developed a voltage-control spintronics memory (VoCSM) that employs the voltage-controlled-magnetic-anisotropy (VCMA) effect for the selection of MTJs and the spin-Hall effect (SHE) for write operations [1]. We achieved high write efficiency in the VoCSM by taking the following approaches: (1) developing a self-aligned VoCSM structure; (2) improving the spin-Hall electrode to have a large spin-Hall angle; and (3) optimizing the magnetic design of the storage layer.

The VoCSM in the present study employs in-plane MTJs. The critical switching current Ic₀ in this VoCSM is expressed as follows:

Ic₀ ~ (2e/ħ)αM_St_eff{-(H_{k_eff}-H_{k_eff(V)})/2}(w_Nt_N/θ_SH)

Here, e is the charge of an electron, α is a damping constant, ħ is Planck’s constant divided by 2π, M_S is the saturation magnetization, t_eff is the effective thickness of the storage layer, H_{k_eff} is the effective perpendicular magnetic anisotropy field, H_{k_eff(V)} is the variation of H_{k_eff} by the applied voltage (V_MTJ), w_N and t_N are respectively the width and thickness of the spin-Hall electrode, and θ_SH is the spin-Hall angle.

Approach (1)

We fabricated a VoCSM consisting of a string of several MTJs on spin-Hall electrodes as shown in Fig. 1. In the self-aligned VoCSM structure, since the width of the spin-Hall electrode and that of the MTJ are same, almost all of the spin-polarized electrons can exert torque on the storage layer. This enabled a large reduction in the Ic of the self-aligned VoCSM [2]. Furthermore, when the spin-Hall electrode was made thinner by using a highly-selective patterning process, Ic was further reduced [3].

Approach (2)

We applied a novel amorphous-TaB/ β-Ta spin-Hall electrode in the VoCSM, as shown in Fig. 1. The amorphous TaB exhibits a large spin-Hall angle (θ_SH = -0.18) that is twice that of β-Ta, because extrinsic effects likely have a dominant role in the SHE of TaB [4]. We successfully prepared an extremely small magnetic dead layer (DL) of the storage layer with small interface roughness by using the TaB spin-Hall electrode. The estimated interfacial transparency of CoFeB/TaB had a relatively high value of 0.60 due to the extremely small DL.

Approach (3)

In the precessional switching mode for in-plane MTJs as represented in Fig. 1, since the spins in the storage layer precess from in-plane to out-of-plane, increasing H_{k_eff} has a big impact on lowering Ic. However, the total magnetization M_St_eff and H_{k_eff} are correlated significantly with the magnetic design of the device. In particular, although lowering the total magnetization M_St_eff has the advantage of lowering Ic, it may also have the effect of decreasing the retention Δ. In this study, we were able to maintain the retention Δ by increasing t_eff (i.e. reducing the size of DL) despite the decreased M_S. Consequently, from the perspective of lowering Ic, we successfully achieved factors such as high θ_SH, self-aligned device structure related to small w_Nt_N, high H_{k_eff}, low α, and low M_S.

Device properties of the VoCSM

By combining the self-aligned fabrication technique with a novel TaB spin-Hall electrode, a quite small value of critical switching current (Ic ~ 79 μA) was achieved despite a relatively large MTJ size (60 nm × 150 nm). We successfully reduced the write current further to 48 μA by the VCMA effect. The write efficiency of the VoCSM without the VCMA effect is about 4 times higher, and furthermore the write efficiency of the VoCSM by using the VCMA effect can reach 8 times higher compared with STT write operations. The VoCSM device also demonstrates high reliability such as a low write error rate (< 1 × 10^-8), high endurance (> 1 × 10¹² cycles), and large break-down voltage (> 2.5 V). These results indicate that VoCSM could open a path to realizing high-density nonvolatile memories with low power consumption and high-speed read and write operations.

This work was partly supported by the ImPACT Program of the Council for Science, Technology and Innovation (Cabinet Office, Government of Japan).

References

[1] H. Yoda et al., Digests of 62^nd IEDM 2016.

[2] S. Shirotori et al., IEEE Transactions on Magnetics 53, 3401104 (2017).

[3] M. Shimizu et al., Digests of IcAUMS 2018.

[4] Y. Kato et al., Digests of MMM 2017.

Fig. 1. Schematic diagram of the self-aligned VoCSM structure with TaB/Ta spin-Hall electrode and cross-sectional high-resolution TEM image of the MTJ.