One pillar of quantum magnonics is the exploration of the utilization of the mediation role of magnons in different platforms to develop quantum technologies. The efficient coupling between magnons and various quantum entities is a prerequisite. Here, we propose a scheme to enhance the spin-magnon coupling by the magnonic Kerr nonlinearity in a YIG sphere. We find that the Kerr-enhanced spin-magnon coupling invalidates the widely used single-Kittel-mode approximation to magnons. It is revealed that the spin decoherence induced by the multimode magnons in the strong-coupling regime becomes not severe, but suppressed, manifesting as either population trapping or persistent Rabi-like oscillation. This anomalous effect is because the spin changes to be so hybridized with the magnons that one or two bound states are formed between them. Enriching the spin-magnon coupling physics, the result supplies a guideline to control the spin-magnon interface.

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• Magnons
• Open quantum systems & decoherence
• Spin dynamics
• Surface magneto-optical Kerr effect
• Non-Markovian processes