High performance magnetic materials produced by assembling gas-phase magnetic nanoclusters

Abstract

The magnetic behaviour of supported Fe nanoclusters containing a few hundred atoms and coated with Co or embedded in Ag matrices has been studied using XMCD and magnetometry. Isolated mass-selected clusters deposited in situ on HOPG substrates in UHV have significantly enhanced spin moments, relative to the bulk, that increase with decreasing cluster size. Coating the exposed clusters with Co enhances the spin moment across the size range studied (180–680 atoms) by a further 10% and, after accounting for systematic errors in the XMCD measurement, compares well with a recent calculation. It is concluded that 180-atom Fe clusters coated with Co have a total (orbital+spin) moment of ≈2.7 μB per atom. Isolated unfiltered Fe clusters with a median size of 3 nm embedded at low volume fraction in Ag have a uniaxial anisotropy with an anisotropy constant of 2.63×105 J/m3. With increasing density in the matrix, a rapid increase in the intial susceptibility at 300 K is observed. This is attributed to the clusters forming exchange-coupled aggregates that behave as larger particles. At the highest densities including pure cluster films the assemblies form correlated super-spin glasses. Thus cluster-assembled films are naturally magnetically soft due to the anisotropy being randomised on the nanometre scale.