Magnetic multi-granule nanoclusters: A model system that exhibits universal size effect of magnetic coercivity

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 (Fe 3 O 4 ) 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 ( M S ) value of ~70 emu/g and that the coercivity ( H c ) increased to the maximum value of ~48 Oe until the nanoclusters reached a size of ~120 nm; the coercivity then gradually decreased to zero.