The critical current density $J_{c0}$ required for switching the magnetization of the free layer (FL) in a spin-transfer torque magnetic random access memory (STT-MRAM) cell is proportional to the product of the damping parameter, saturation magnetization and thickness of the free layer, $\alpha M_S t_F$. Conventional FLs have the structure CoFeB/nonmagnetic spacer/CoFeB. By reducing the spacer thickness, W in our case, and also splitting the single W layer into two layers of sub-monolayer thickness, we have reduced $t_F$ while minimizing $\alpha$ and maximizing $M_S$, ultimately leading to lower $J_{c0}$ while maintaining high thermal stability. Bottom-pinned MRAM cells with device diameter in the range of 55-130 nm were fabricated, and $J_{c0}$ is lowest for the thinnest (1.2 nm) FLs, down to 4 MA/cm$^2$ for 65 nm devices, $\sim$30% lower than 1.7 nm FLs. The thermal stability factor $\Delta_{\mathrm{dw}}$, as high as 150 for the smallest device size, was determined using a domain wall reversal model from field switching probability measurements. With high $\Delta_{\mathrm{dw}}$ and lowest $J_{c0}$, the thinnest FLs have the highest spin-transfer torque efficiency.

中文翻译：

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用于降低 STT-MRAM 中开关电流的超薄垂直自由层

在自旋转移扭矩磁性随机存取存储器 (STT-MRAM) 单元中切换自由层 (FL) 的磁化所需的临界电流密度 $J_{c0}$ 与阻尼参数饱和磁化的乘积成正比和自由层的厚度，$\alpha M_S t_F$。传统的 FL 具有 CoFeB/非磁性垫片/CoFeB 的结构。通过减少间隔层厚度，在我们的例子中，W，并将单个 W 层分成两层亚单层厚度，我们减少了 $t_F$，同时最小化了 $\alpha$ 并最大化了 $M_S$，最终导致了更低的 $ J_{c0}$ 同时保持高热稳定性。制造了器件直径在 55-130 nm 范围内的底部钉扎 MRAM 单元，最薄 (1.2 nm) FL 的 $J_{c0}$ 最低，65 nm 低至 4 MA/cm$^2$设备，$\sim$ 比 1 低 30%。7 纳米 FL。热稳定性因子 $\Delta_{\mathrm{dw}}$ 对于最小的器件尺寸高达 150，是使用来自场切换概率测量的畴壁反转模型确定的。在高 $\Delta_{\mathrm{dw}}$ 和最低 $J_{c0}$ 的情况下，最薄的 FLs 具有最高的自旋传递扭矩效率。