Low-temperature anomalies in muon spin relaxation of solid and hollow γ- F e2 O3 nanoparticles: A pathway to detect unusual local spin dynamics

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