Spin canting in nonlinear terahertz magnon dynamics revealed by magnetorefractive probing in orthoferrite

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.