Strongly extended diffusion length for the nonequilibrium magnons in Y 3 F e 5 O 12 by photoexcitation

${\mathrm{Y}}_{3}\mathrm{F}{\mathrm{e}}_{5}{\mathrm{O}}_{12}$ (YIG) is known for its long magnon diffusion length. Although it has the known lowest damping rate, an even longer diffusion distance is still highly desired since it may lead to a much more efficient information transmission and processing. While most of previous works focused on the generation and detection of magnons in YIG, here we demonstrate how to depress the damping rate during the diffusion of magnon. By selectively exciting the spin state transition of the Fe ions in YIG, we successfully increase magnon diffusion length by one order of magnitude, i.e., from the previous reported \ensuremath{\sim}10 \ensuremath{\mu}m up to \ensuremath{\sim}156 \ensuremath{\mu}m (for the sample prepared by liquid phase epitaxy) and \ensuremath{\sim}180 \ensuremath{\mu}m (for the sample prepared by pulsed laser deposition) at room temperature. The diffusion length, determined by nonlocal geometry, is \ensuremath{\sim}30 \ensuremath{\mu}m for the magnons induced by visible light and above 150 \ensuremath{\mu}m for the laser of 980 nm. In addition to thermal gradient, light excitation affects the electron configuration of the $\mathrm{F}{\mathrm{e}}^{3+}$ ion in YIG. Long-wavelength laser is more effective since it causes a transition of the $\mathrm{F}{\mathrm{e}}^{3+}$ ions in $\mathrm{Fe}{\mathrm{O}}_{6}$ octahedron from a high spin to a low spin state and thus causes a magnon softening which favors a long-distance diffusion. The present work paves the way toward an efficient tuning of magnon transport which is crucially important for magnon spintronics.