Core-shell silica (SiO2)-coated Fe3O4 nanoparticles of shell thickness between 2 and 52 nm have been synthesized and characterized. The saturation magnetization scales with SiO2 wt % and these samples exhibit negligible hysteresis at room temperature, as compared to the bulk Fe3O4. The peak in the temperature-dependent ac susceptibility Tm shifts toward higher temperature with increasing frequency. Data of pristine to 40 wt % SiO2-coated samples fit to critical slowing-down behavior model yielding relaxation time of ∼10-10 s. For particles of higher shell thickness, smaller relaxation time of ∼10-13 s is obtained. Temperature-dependent electron spin resonance measurements suggest overall weakening of magnetic interactions in Fe3O4 nanoparticles of thicker SiO2 shell. Variation of shell thickness in these core-shell nanoparticles could lead to diverse ground states ranging from interacting to noninteracting systems.