Nanosized antiferromagnets show anomalously large magnetization and superparamagnetism, having complicated magnetic properties due to a competition between core and surface spins in addition to the surface-induced effect, the finite size effect, and the magnetic moment distribution. A significant distribution of magnetic moments, which are responsible for the superparamagnetism, can also exist in real ensembles of nanoparticles, making the analysis of magnetization difficult. It has been a key issue for the understanding of the complicated magnetic property to correctly separate a superparamagnetic component from an antiferromagnetic component that linearly depends on magnetic fields in the observed magnetization. Here, we report the separation of these two components observed in the magnetization of antiferromagnetic NiO nanoparticles by applying the scaling law in the magnetization of superparamagnets. The distribution of the magnetic moment was taken into account without a priori assumption of the distribution function. The magnetic moments causing superparamagnetism had a bimodal distribution with different mean values. The contribution of the two different superparamagnetic components could thus be newly clarified and was ascribed to the uncompensated spins in the core and on the surface. The magnetic structure that leads to the bimodal distribution is discussed in light of the inhomogeneity of the surface structure.

Fluorescent carbon dots are nontoxic nanoparticles composed of carbon, exhibiting advantageous properties for applications in bioimaging and functional materials. We present a methodology for synthesizing fluorescent nitrogen-doped carbon dots (N-CDs) using starch, a biopolymer, and urea as the sources of nitrogen, via the microwave-assisted hydrothermal method. Furthermore, the dependence of the fluorescence spectra and fluorescence quantum yield of N-CDs on the initial concentration of urea in the reactant solution was examined, thereby providing a comprehensive understanding of the influence of nitrogen doping on the CDs. The fluorescence of N-CDs was tunable by varying the excitation wavelength. Stronger fluorescence intensity was observed for a moist phosphate salt/N-CD composite, in contrast to the weaker fluorescence exhibited by a dried one. Fluorescence lifetime measurements revealed that the change in fluorescence intensity can be attributed to the suppression of the non-radiative deactivation process. This observation highlights the critical importance of the interaction between water molecules and surface functional groups in controlling the photophysics of the excited state of N-CDs.