  Spin transfer torque (STT) driven by a charge current plays a key role in
magnetization switching in heavy-metal/ferromagnetic-metal structures. The STT
efficiency defined by the ratio between the effective field due to STT and the
current density, is required to be improved to reduce energy compulsions in the
STT-based spintronic devices. In this work, using the harmonic Hall measurement
method, we experimentally studied the STT efficiency in platinum(Pt)/FM
structures as a function of the Pt thickness. We found that the STT efficiency
strongly depends on the Pt thickness and reaches a maximum value of 4.259
mT/($10^6$A/$cm^{2}$) for the 1.8-nm-thickness Pt sample. This result indicates
that competition between spin Hall effect (SHE) and Rashba effect as well as
spin diffusion process across the Pt layer determines the Pt thickness for the
maximum STT efficiency. We demonstrated the role played by the spin diffusion
besides the spin current generation mechanisms in improvement of the STT
efficiency, which is helpful in designing STT-based devices.
