Size dependence of the blocking temperatures and electron magnetic resonance spectra in NiO nanoparticles

Size dependence of the blocking temperature TBTB (determined by SQUID magnetometry) and electron magnetic resonance (EMR) spectra are reported for NiO nanoparticles (NPs) with diameter D=5D=5–20 nm. Comparison of the TBTB vs. DD variations in the oleic acid (OA) coated and uncoated NiO NPs is made. The higher blocking temperatures observed for the uncoated NPs are suggested to result from the effects of the interparticles interactions (IPI). For the coated NPs with negligible IPI, the expected linear variation of TBTB with volume VV of the NPs is valid if effective anisotropy constant Keff increases with decreasing VV. The decreasing intensity of the EMR signal with increasing DD is suggested to result from the surface Ni2+ spins whose density decreases with increasing D.