High-temperature superspin glass and low-temperature glassy exchange bias in passivated FeCo nanoparticles
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- AEI/Plan Estatal de Investigación Científica y Técnica y de Innovación 2021-2023/PID2021-122613OB-I00/ES/
- AEI/Plan Estatal de Investigación Científica y Técnica y de Innovación 2021-2023/CNS2022-135787/
- AEI/Plan Estatal de Investigación Científica y Técnica y de Innovación 2021-2023/PID2022-142267NB-I00/ES/
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Conventional 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.
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