Building high-performance magnetic materials out of gas-phase nanoclusters

Abstract

It is well known that free magnetic nanoclusters display novel properties including enhanced magnetic moments in ferromagnetic metals, ferrimagnetism in antiferromagnetic metals and the appearance of magnetism in non-magnetic metals. A contemporary challenge is to build some of the intrinsic novel behaviour into macroscopic materials. XMCD measurements of exposed mass-selected Fe clusters in the size range 180–700 atoms deposited in situ onto HOPG substrates reveal that adsorbed clusters show an enhancement in both orbital and spin moments relative to the bulk. Coating the exposed Fe clusters with Co increases the spin moment still further without significantly changing the orbital moment. Increasing the surface density of Fe clusters to induce a significant interaction reduces the orbital moment to its small bulk value but an enhancement in the spin moment remains even in thick cluster-assembled films. The anisotropy in isolated Fe and Co clusters embedded in Ag matrices is shown to be uniaxial with the anisotropy axes randomly oriented in three dimensions. Thick cluster films adopt a domainless correlated super-spin glass magnetic configuration as a result of the frustration between the inter-cluster exchange coupling and random anisotropy. This state is magnetically soft, which is a valuable technological attribute in high-moment films.