Low-temperature anomalies in muon spin relaxation of solid and hollow γ−Fe2O3 nanoparticles: A pathway to detect unusual local spin dynamics

By means of muon spin relaxation measurements we unraveled the temperature spin dynamics in monodis-perse maghemite spherical nanoparticles with different surface to volume ratio, in two samples with a full core(diameterD∼4andD∼5 nm) and one with a hollow core (external diameterD∼7.4 nm). The behaviorof the muon longitudinal relaxation rates as a function of temperature allowed us to identify two distinct spindynamics. The first is well witnessed by the presence of a characteristic peak for all the samples around theso-called muon blocking temperatureTμ+B. A Bloembergen-Purcell-Pound (BPP)-like model reproduces theexperimental data around the peak and at higher temperatures (20<100 K) by assuming the Néel reversaltime of the magnetization as the dominating correlation time. An additional dynamic emerges in the sampleswith higher surface to volume ratio, namely, full 4 nm and hollow samples. This is witnessed by a shoulderof the main peak forT<20 K at low longitudinal field (μ0H≈15 mT), followed by an abrupt increase ofthe relaxation rate atT<10 K, which is more evident for the hollow sample. These unusual anomalies ofthe longitudinal relaxation rate forT