We report on the temperature dependence of the spatially modulated spin-polarization amplitude $\mathrm{\Delta }{K}_{\text{spin}}$, which is a hallmark of the superconducting Fulde-Ferrell-Larkin-Ovchinnikov (FFLO) state. For that, we use ${}^{13}\mathrm{C}$ nuclear magnetic resonance (NMR) spectroscopy performed on the organic conductor ${\beta }^{\prime \prime }\text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$. From a comparison of our experimental results to a comprehensive modeling of the ${}^{13}\mathrm{C}$ NMR spectra, we determine the evolution of $\mathrm{\Delta }{K}_{\text{spin}}$ upon condensation of the FFLO state. Further, the modeling of the spectra in the superconducting phase allows to quantify the decrease of the average spin susceptibility, stemming from the spin-singlet coupling of the superconducting electron pairs in the FFLO state of ${\beta }^{\prime \prime }\text{−}{\left(\mathrm{ET}\right)}_2{\mathrm{SF}}_5{\mathrm{CH}}_2{\mathrm{CF}}_2{\mathrm{SO}}_3$.

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• FFLO
• Spin-singlet pairing
• Superconductivity
• Superconductors
• Nuclear magnetic resonance