  Antiferromagnetic (AFM) materials possess a well-recognized potential for
ultrafast data processing thanks to their intrinsic ultrafast spin dynamics,
absence of stray fields, and large spin transport effects. The very same
properties, however, make their manipulation difficult, requiring frequencies
in THz range and magnetic fields of tens of Teslas. Switching of AFM order
implies going into the nonlinear regime, a largely unexplored territory. Here
we use THz light from a free electron laser to drive antiferromagnetic NiO into
a highly nonlinear regime and steer it out of nonlinearity with magnetic field
from a 33-Tesla Bitter magnet. This demonstration of large-amplitude dynamics
represents a crucial step towards ultrafast resonant switching of AFM order.
