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- Basini, M., et al.
(författare)
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Low-temperature anomalies in muon spin relaxation of solid and hollow gamma-Fe2O3 nanoparticles : A pathway to detect unusual local spin dynamics
- 2020
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Ingår i: Physical Review B. - : American Physical Society (APS). - 2469-9950 .- 2469-9969. ; 102:19
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Tidskriftsartikel (refereegranskat)abstract
- 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 similar to 4 and D similar to 5 nm) and one with a hollow core (external diameter D similar to 7.4 nm). 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 T-B(mu+). A Bloembergen-Purcell-Pound (BPP)-like model reproduces the experimental data around the peak and at higher temperatures (20 < T < 100 K) by assuming the Neel 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 < 20 K at low longitudinal field (mu H-0 approximate to 15 mT), followed by an abrupt increase of the relaxation rate at T < 10 K, which is more evident for the hollow sample. These unusual anomalies of the longitudinal relaxation rate for T < T-B(mu+) are suggested to be due to the surface spins' dynamical behavior. Furthermore, for weak applied longitudinal magnetic field (mu H-0 approximate to 15 mT) and T < T-B(mu+) we observed damped coherent oscillations of the muon asymmetry, which are a signature of a quasistatic local field at the muon site as probed by muons implanted in the inner magnetic core of the nanoparticles. The muon spin relaxation technique turns out to be very successful to study the magnetic behavior of maghemite nanoparticles and to detect their unusual local spin dynamics in low magnetic field conditions.
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| 2. |
- Bordiga, Manuela, et al.
(författare)
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Coccolithophore carbonate during the last 450 ka in the NW Pacific Ocean (ODP site 1209B, Shatsky Rise)
- 2014
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Ingår i: Journal of Quaternary Science. - : Wiley. - 0267-8179 .- 1099-1417. ; 29:1, s. 57-69
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Tidskriftsartikel (refereegranskat)abstract
- This study focuses on Pleistocene-Holocene sediments from the Shatsky Rise (Ocean Drilling Program Site 1209B, NW Pacific Ocean). We quantify the contribution of calcite made by coccoliths in oceanic sediments, investigating the role of calcareous nannoplankton during the last 450 ka. Coccolith carbonate constitutes 60-90% of bulk carbonate. Coccolith carbonate accumulation rates (CARs) and CaCO3 fine fraction confirm that coccolithophores are major contributors to the carbonate export and accumulation. Primary productivity shows highest values from marine isotope stage (MIS) 12 to 8. Thereafter, although coccolith calcite content remains high, other sediment components, such as as foraminifera and biogenic opal, seem to be favored, perhaps related to an increase in fertilization by eolian dust. Our results demonstrate the important role of coccolithophore production and sedimentation on the regulation of ocean carbonate chemistry on time scales >1000-100 000 years. On glacial-interglacial scales, coccolithophore productivity could have affected deep-water saturation by buffering deep-sea CO2 through increased carbonate dissolution episodes. Spectral and wavelet analyses are consistent with CARs primarily driven by glacial-interglacial variability and obliquity-controlled changes. Coccolith-based paleoceanographic reconstructions allow us to establish that during the last 450 ka the mid-latitudes of the NW Pacific are controlled by the dynamics of the El Nino Southern Oscillation perturbations and Boreal Monsoon system.
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