  We excite the spin precession in rare-earth orthoferrite YFeO3 by the
magnetic field of intense terahertz pulse and probe its dynamics by transient
absorption change in the near infrared. The observed waveforms contain
quasi-ferromagnetic-mode magnon oscillation and its second harmonics with a
comparably strong amplitude. The result can be explained by dielectric function
derived from magnetorefractive Hamiltonian. We reveal that the strong second
harmonic signal microscopically originates from novel dynamics of the
quasi-ferromagnetic mode magnon at nonlinear regime, wherein spin canting angle
periodically oscillates.
