Magnetism in Fe Nanoclusters ? From Isolated Particles to Nanostructured Materials

Abstract

We have studied the evolution of the magnetic behaviour of Fe nanoclusters from isolated particles adsorbed on a surface and exposed in UHV to dense interacting assemblies. The clusters were produced by a UHV-compatible gas aggregation source and were studied in situ by synchrotron radiation techniques based on dichroism and ex situ after transfer into a UHV-capable vibrating sample magnetometer. Isolated and exposed 250-atom Fe clusters deposited on HOPG substrates exhibit enhanced orbital and spin magnetic moments that decay towards the bulk value with increasing size or with increasing particle density on the surface. The individual particles appear to be magnetically isotropic but a strong in-plane anisotropy develops as the coverage is increased to a complete cluster monolayer. At low temperature, thick cluster films have an asperomagnetic configuration that develops into a two-dimensional correlated super spin glass with increasing temperature.

Magnetic Nanoparticles: Surface Effects and Properties Related to Biomedicine Applications

“…Computational studies also reported different anisotropy and magnetic moment at the surface of magnetic clusters embedded in matrices [ 26 ]. Synchrotron radiation studies revealed that both spin and orbital moments at the surface are different from those of the bulk counterparts [ 27 ]. Thermal measurements also reported that the structure of nanoparticles and the strength of their surface anisotropy control their magnetic properties [ 28 ].…”

Theoretical FeL2,3- andK-edge x-ray magnetic circular dichroism spectra of free iron clusters

“…Magnetic properties of transition metal clusters, in particular, have attracted a lot of attention recently-both due to fundamental reasons and due to potential applications in magnetic recording technology. X-ray magnetic circular dichroism ͑XMCD͒, defined as the difference between the absorption rate for left-and right-circularly polarized x-rays in magnetic targets, ⌬ = ͑+͒ − ͑−͒ , is a frequently used tool for studying magnetism in clusters, [1][2][3][4][5][6] because it has several capabilities not supplied by other magnetic techniques ͑as, for example, chemical and angular momentum specificity͒.…”

“…11,12 However, several XMCD measurements were performed on clusters supported by a substrate or embedded in a matrix. Studies of supported Fe clusters of a few hundreds to thousands of atoms [1][2][3][4] suggested a substantial enhancement of orb as well as of the ratio orb / spin with respect to the bulk. Similarly, XMCD experiments on small supported Fe clusters of just several atoms 5,6 suggested a strongly sizedependent orb .…”

Nanosized iron clusters investigated with in situ transmission electron microscopy

“…Because a large fraction of atoms in nanoclusters ͑sizes ϳ5 nm) are surface atoms, the thermal and magnetic properties [7][8][9] are quite different from the bulk counterparts. Although many investigations of low-temperature physical properties of magnetic nanoclusters have been performed, [10][11][12][13][14][15][16] only a few studies have been conducted on nanocluster properties at higher temperatures. 7,17 The latter are necessary because device operation might lead to heating and alteration of structural components that may affect the functional performance.…”