It is well known that the coercivity of magnetic nanomaterials increases up to a maximum and then decreases to zero with decreasing particle size. However, until now, no single synthesis method has been able to produce magnetic nanoparticles with a wide range of sizes, i.e., from 10 to 500 nm, in order to uncover the coercivity evolution. Here we report the characterization of magnetite (Fe3O4) multi-granule nanoclusters (MGNCs) to demonstrate the transitional behaviour of coercivity. The M–H curves indicate that our samples had a relatively high saturation magnetization (MS) value of ~70 emu/g and that the coercivity (Hc) increased to the maximum value of ~48 Oe until the nanoclusters reached a size of ~120 nm; the coercivity then gradually decreased to zero.

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We report the synthesis and characterization of size-tunable MnxFe3−xO4 ferrite nanoclusters of different sizes ranging from ∼30 to ∼120 nm. The nanoclusters synthesized via a hydrothermal polyol process show high crystallinity and a narrow size distribution. The magnetic properties of the nanoclusters demonstrate well-behaved magnetization and low-coercivity characteristics, ferrimagnetically at a large size, and superparamagnetically at a small size.

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