  The ability to drive a spin system to state far from the equilibrium is
indispensable for investigating spin structures of antiferromagnets and their
functional nonlinearities for spintronics. While optical methods have been
considered for spin excitation, terahertz (THz) pulses appear to be a more
convenient means of direct spin excitation without requiring coupling between
spins and orbitals or phonons. However, room-temperature responses are usually
limited to small deviations from the equilibrium state because of the
relatively weak THz magnetic fields in common approaches. Here, we studied the
magnetization dynamics in a HoFeO3 crystal at room temperature. A custom-made
spiral-shaped microstructure was used to locally generate a strong multicycle
THz magnetic near field perpendicular to the crystal surface; the maximum
magnetic field amplitude of about 2 T was achieved. The observed time-resolved
change in the Faraday ellipticity clearly showed second- and third-order
harmonics of the magnetization oscillation and an asymmetric oscillation
behaviour. Not only the ferromagnetic vector M but also the antiferromagnetic
vector L plays an important role in the nonlinear dynamics of spin systems far
from equilibrium.
