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Hexane-based magnetic fluids with uniform magnetite nanoparticles of 4, 6, and 8 nm have been prepared by thermal decomposition methods and seed-mediated growth techniques. Their monodispersities were characterized by dynamic light scattering and transmission electron microscopy. We further dispersed them in paraffin wax matrices to study their magnetic properties. The size-dependent magnetic behavior of the magnetite-wax nanocomposites was investigated by the VSM in the temperature ranging from 77 to 300 K. The results show that the thermomagnetization curves follow Bloch's law σ(T)=σ(0)(1-BTb), and the Bloch exponent differs from b=1.5 for the bulk phase. Based on the Belov-Arrott plots samples with 6 and 8 nm Fe3O4 nanoparticles have the magnetic ordering temperatures (Tm) above 300 K, while the Tm for a sample containing 4-nm Fe3O4 nanoparticles is approximately 150 K.