Resumen: Waveguide quantum electrodynamics studies interactions of matter with photons traveling via a transmission guide and how these can be exploited to control quantum emitters and to establish quantum correlations between them. Here, we explore the competition between such light-mediated interactions with intrinsic matter-matter interactions. For this, we couple a superconducting line to a magnetic material made of organic free radical molecules. We find that molecules belonging to one of the two crystal sublattices form one-dimensional spin chains. Temperature then controls spin correlations along these chains in a continuous and monotonic way. In the paramagnetic region (T > 0.7 K), the microwave transmission evidences a collective coupling of quasi-identical spins to the propagating photons, with coupling strengths that reach values close to the dissipation rates. As T decreases, the growth of spin correlations, combined with the anisotropy in the spin-spin exchange constants, tend to suppress the collective spin-photon coupling. In this regime, the spin visibility in transmission also reflects a gradual change in the nature of the dominant spin excitations, from single-spin flips to bosonic magnons. Idioma: Inglés DOI: 10.1038/s43246-025-00898-w Año: 2025 Publicado en: Communications materials 6 (2025), 172 [8 pp.] ISSN: 2662-4443 Financiación: info:eu-repo/grantAgreement/ES/AEI/AEI PID2022-140923NB-C21 Financiación: info:eu-repo/grantAgreement/ES/DGA/E09-23R-QMAD Financiación: info:eu-repo/grantAgreement/EC/H2020/862893/EU/Molecular spin qudits offering new hope for quantum computing/FATMOLS Financiación: info:eu-repo/grantAgreement/ES/MICINN-AEI/PRTR-C17.I1 Financiación: info:eu-repo/grantAgreement/ES/MICINN/CEX2023-001286-S Financiación: info:eu-repo/grantAgreement/EUR/MICINN/TED2021-131447B-C21 Tipo y forma: Artículo (Versión definitiva)