Inspection of intrinsic critical currents for spin-transfer magnetization switching

We examined the relationships between critical current, I_c, and switching time, τ_p, for spin-transfer switching in two regions: (region I) τ_p≫τ₀, where thermal switching is accompanied and (region II) τ_p< several tens times τ₀, where τ₀ is the attempt time for thermal switching (≈1 ns). We estimated I_c0, defined as the intrinsic I_c at 0 K, for both regions and confirmed experimentally that those I_c0 coincided with each other at room temperature (RT). The value of I_c at τ_p=1 ns, measured with microwaves, was approximately 1.6 times the I_c0. This suggested that we use at least two times I_c0 as the writing currents of magnetic memory devices for nsec spin-transfer switching at RT. Although I_c0 for both regions were defined as I_c at 0 K (I_c^0K) in theory, they showed temperature dependence at low temperatures; |I_c0| for region I increased with decreasing temperature, and the estimated I_c^0K was approximately three times I_c0 for RT. This temperature dependence was quite different from that for region II.