  Optical pump-probe setups are commonly used for excitation and investigation
of the spin dynamics in various types of magnetic materials. However, usually
the spatially homogeneous excitation is considered. In the present study we
describe an approach for optical excitation of the nonuniform THz spin dynamics
and for probing its spatial distribution inside a magnetic crystal. We propose
to illuminate a crystal with laser pulses of properly adjusted polarization to
benefit from a strong optical birefringence inherent to the crystal. It results
in an unusual behavior of the effective magnetic field generated by the pulses
due to the inverse Faraday effect and the peculiar sign-changing dependence of
the direct Faraday effect inside the crystal. The study is performed exemplary
for yttrium orthoferrite crystal although the proposed approach is applicable
for various magnetic materials with optical anisotropy.
