  A recent experimental demonstration of a parametric magnetization oscillation
excited by applying a microwave voltage to a ferromagnetic metal will be
applicable not only to a new magnetization switching method but also to
bio-inspired computing. It should be, however, noted that a phase of the
parametric magnetization oscillation is not uniquely locked, related to the
fact that a frequency of the microwave voltage is twice the value of the
magnetization oscillation. There are two possible phases in the parametric
oscillation state, and which of the two is realized depends on the initial
condition of the magnetization. Here, we examine two approaches to lock the
phase uniquely. One is to suppress the distribution of the initial state by
enhancing the perpendicular magnetic anisotropy before applying microwave
voltage, and the other is to use a sweeping frequency. Through numerical
simulation of the Landau-Lifshitz-Gilbert equation and quantification of locked
rate, we find that the sweeping frequency is more effective to lock the phase
of the parametric magnetization oscillation.
