  An excitation of highly nonlinear, complex magnetization dynamics in a
ferromagnet, for example chaos, is a new research target in spintronics. This
technology is applied to practical applications such as random number generator
and information processing systems. One way to induce complex dynamics is
applying feedback effect to the ferromagnet. The role of the feedback electric
current on the magnetization dynamics was studied in the past. However, there
is another way to apply feedback effect to the ferromagnet, namely feedback
magnetic field. In this paper, we developed both numerical and theoretical
analyses on the role of the feedback magnetic field causing complex
magnetization dynamics. The numerical simulation indicates the change of the
dynamical behavior from a simple oscillation with a unique frequency to complex
dynamics such as amplitude modulation and chaos. The theoretical analyses on
the equation of motion qualitatively explain several features found in the
numerical simulations, exemplified as an appearance of multipeak structure in
the Fourier spectra. The difference of the role of the feedback electric
current and magnetic field is also revealed from the theoretical analyses.
