López Martín, RaúlLepesant, MathieuLacroix, Lise-MarieToro, José A. deLópez Ortega, Alberto2025-07-022025-07-022025-05-15López-Martín, R., Lepesant, M., Lacroix, L. M., Toro, J. A. de, López-Ortega, A. (2025). High-temperature superspin glass and low-temperature glassy exchange bias in passivated FeCo nanoparticles. Surfaces and Interfaces, 65, 1-7. https://doi.org/10.1016/j.surfin.2025.106514.2468-023010.1016/j.surfin.2025.106514https://academica-e.unavarra.es/handle/2454/54368Conventional powders, dense systems of magnetic nanoparticles, often combine intra- and inter-particle magnetically glassy properties, which may complicate their interpretation. To shed light on this matter, we have studied 9 nm FeCo particles synthesized by thermal co-decomposition of metal amides after a passivation layer around 2 nm thick has formed in ambient conditions. The saturation magnetization, 117 emu/g, is consistent with the above metallic core/ferrite shell picture. The high magnetic moment and concentration of the particles yield, via strong interparticle interactions, a remarkable room temperature superspin glass-like phase (with freezing temperature above 350 K) for such small particles, as confirmed by the de Almeida-Thouless analysis. Additionally, we detect a spin glass-like freezing at the atomic scale (within the particles). Its corresponding feature, a small hump under small fields in the temperature dependence of the magnetization, closely agrees with the onset of the exchange bias effect (∼ 60 K) measured, unlike it is customary, with repeated field-coolings. The spin-disordered nature of the core/shell interface is further proved by a strong training effect of the exchange bias field, among others. This magnetic behavior offers an indirect proof of structural interface disorder even in fully passivated metallic particles.application/pdfeng© 2025 The Authors. Published by Elsevier B.V. This is an open access article under the CC BY-NC-ND license.Core-shellDisordered materialsMagnetic interfaceNanoparticlesSpin-glassinfo:eu-repo/semantics/article2025-07-02info:eu-repo/semantics/openAccess