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 $\mathrm{TbM}{\mathrm{n}}_6\mathrm{S}{\mathrm{n}}_6$. 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 5 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 the 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.

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• Laser-induced demagnetization
• Magnetism
• Magnetization dynamics
• Spin dynamics
• Ultrafast magnetization dynamics