Structural and magnetic characteristics of monodispersed Fe and oxide-coated Fe cluster assemblies

We systematically studied structural and magnetic characteristics of size- monodispersed Fe and oxide-coated Fe cluster assemblies with the mean cluster sizes of 7-16 nm. Transmission electron microscopy and scanning electron microscopy (SEM) observations show that the Fe clusters in the assemblies maintain their original size at room temperature. In the SEM images, a random stacking of the Fe clusters and a porous structure with a low cluster packing fraction of about 25% are observed. For the Fe cluster assemblies, magnetic coercivity (H-c) at room temperature increases from 4x10(1) to 4x10(2) Oe by increasing the mean cluster size from 7.3 to 16.3 nm. Using the experimental values of the coercivity at Tgreater than or equal to100 K and the fitting values of blocking temperature T-B from H-c=H-c0[1-(T/T-B)(1/2)], we estimated the values of magnetic anisotropy constant K of the order of 10(6) erg/cm(3) from T-B=KV/25k(B), which is larger by an order of magnitude than the bulk Fe value (5x10(5) erg/cm(3)). Such a large effective anisotropy at Tgreater than or equal to100 K is ascribed to the large surface anisotropy effects of the small clusters and the low cluster-packing fraction of the Fe cluster assemblies. For the oxide-coated Fe cluster samples, the coercivity strongly depends on the oxygen gas flow rate during deposition, cluster size, and temperature. In the case of a high oxygen gas flow rate (namely high surface-oxidized clusters), the ferrimagnetic oxide shell crystallites also affect the coercivity at T>50 K: The hysteresis loop shift disappears, leading to a complex change in the coercivity and an enhancement of the effective anisotropy constant. (C) 2002 American Institute of Physics.