  Magnetization switching in a fine-structured ferromagnet of nanoscale by the
spin-transfer torque excited via the spin Hall effect has attracted much
attention because it enables us to manipulate the magnetization without
directly applying current to the ferromagnet. However, the switching mechanism
is still unclear in regard to the ferromagnet having an in-plane easy axis
parallel to the current. Here, we develop an analytical theory of the
magnetization switching in this type of ferromagnet, and reveal the threshold
current formulas for a deterministic switching. It is clarified that the
current should be in between a certain range determined by two threshold
currents because the spin-transfer torque due to a large current outside the
range brings the magnetization in an energetically unstable state, and causes
magnetization precession around the hard axis.
