Developing telecom-compatible spin-photon interfaces is essential towards scalable quantum networks. Erbium ions (${\mathrm{Er}}^{3+}$) exhibit a unique combination of a telecom ($1.5\mathrm{\mu }\mathrm{m}$) optical transition and an effective spin-$1/2$ ground state, but identifying a host that enables heterogeneous device integration while preserving long optical and spin coherence remains an open challenge. We explore the potential of ${\mathrm{Er}}^{3+}:{\mathrm{CeO}}_2$ films on silicon and study the ${\mathrm{Er}}^{3+}$ spin coherence, offering low nuclear spin density and the potential for on-chip integration. We demonstrate a $38.8\mathrm{\mu }\mathrm{s}$ spin coherence, which can be extended to $176.4\mathrm{\mu }\mathrm{s}$ with dynamical decoupling. Pairing experiments with cluster correlation expansion calculations, we identify spectral diffusion induced by bath ${\mathrm{Er}}^{3+}$ spin flip-flops as the dominant decoherence mechanism and provide pathways to millisecond-scale coherence.

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• Spin coherence
• Spin relaxation
• Cluster expansion
• Electron spin resonance