Uncovering the Timescales of Spin Reorientation in $TbMn_{6}Sn_{6}$

Kagome ferrimagnets are of fundamental interest because of their tunable magnetic, electronic and topological properties, with potential applications for quantum-enabled technologies and spintronics. Here we present the first direct measurement of the intrinsic timescale of the spin reorientation transition in the Kagome ferrimagnet $TbMn_{6}Sn_{6}$. This material exhibits a magnetic phase transition near room temperature, where spins remain collinear while the total magnetic moment rotates from out-of-plane to in-plane. This reorientation has been attributed to the competing anisotropies of Tb and Mn, whose magnetic moments have very different temperature dependencies. By probing at the Mn M-edge using high harmonic extreme ultraviolet pulses, we measure reorientation timescales between 6 and 18 ps, depending on the laser excitation fluence. Our model of the magnetization dynamics shows that this timescale is consistent with a spin reorientation driven by very large anisotropy energies, on meV energy scales. Our model also predicts a possibility of a 180$^{\circ}$ reorientation of the out-of-plane moment, that could facilitate optically controlled magnetization switching between very stable ground states, for applications in spintronics or data storage.