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

Abstract

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 (Hc) at room temperature increases from 4×101 to 4×102 Oe by increasing the mean cluster size from 7.3 to 16.3 nm. Using the experimental values of the coercivity at T⩾100 K and the fitting values of blocking temperature TB from Hc=Hc0[1−(T/TB)1/2], we estimated the values of magnetic anisotropy constant K of the order of 106 erg/cm3 from TB=KV/25kB, which is larger by an order of magnitude than the bulk Fe value (5×105 erg/cm3). Such a large effective anisotropy at T⩾100 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.