  Recently, it was demonstrated that field-free switching could be achieved by
combining spin-orbit torque (SOT) and Dzyaloshinskii-Moriya interaction (DMI).
However, this mechanism only occurs under certain conditions which have not
been well discussed. In this letter, it is found that the ratio of domain wall
width to diameter of nanodots could be utilized as a criteria for judging this
mechanism. Influences of different magnetic parameters are studied, including
exchange constant, DMI magnitude and field-like toque, etc. Besides, we reveal
the importance of the shrinkage of magnetic domain wall surface energy for
metastable states. Our work provides guidelines to the experiments on the
DMI-induced field-free magnetization switching, and also offers a new way for
the design of SOT-based memory or logic circuits.
