  $TbMn_{6}Sn_{6}$ is a ferrimagnetic material which exhibits a highly unusual
phase transition near room temperature where spins remain collinear while the
total magnetic moment rotates from out-of-plane to in-plane. The mechanisms
underlying this phenomenon have been studied in the quasi-static limit and the
reorientation has been attributed to the competing anisotropies of Tb and Mn,
whose magnetic moments have very different temperature dependencies. In this
work, we present the first measurement of the spin-reorientation transition in
$TbMn_{6}Sn_{6}$. By probing very small signals with the transverse
magneto-optical Kerr effect (TMOKE) at the Mn M-edge, we show that the
re-orientation timescale spans from 12 ps to 24 ps, depending on the laser
excitation fluence. We then verify these data with a simple model of spin
precession with a temperature-dependent magnetocrystalline anisotropy field to
show that the spin reorientation timescale is consistent with the reorientation
being driven by very large anisotropies energies on approximately $\approx$ meV
scales. Promisingly, the model predicts a possibility of 180o reorientation of
the out-of-plane moment over a range of excitation fluences. This could
facilitate optically controlled magnetization switching between very stable
ground states, which could have useful applications in spintronics or data
storage.
