Introduction to Nanotechnology

a
42. METAL NANOCLUSTERS 87
Figure 4.f5. A series of electron microscope pictures of gold nanoparlicles containing approxi-
mately 460 atoms taken at various times showing fluctuation-induced changes in the structure.
(With perrnisslcln from S. Sugano and H. Koizumi, in MfcroCruster Physm, Spinger, Berlin.
1998, p. IS.)
rnorncnt of the e!ectron. The total moment of the atom mill be the vector slim of thc
momcnts ofeach elcctron ofthe atom. In energy levels filled with an cvcn number of
elecmons the electron magnetic morncnts are paired oppositely, and thc net magnetic
niomcnt is zero. Thus most atoms in solids do not have a net magnetic moment.
Therc are, however, some transition ion atoms such as iron, rnangmcsc, and cobalt,
which have partially filled inner dorbital levels, :rnd hence they posess a net
magnetic moment. Crystals of thesc atonis can bccome ferrornagnclic when thc
rnagnctic moments of all the atoms arc aligned in thc same direction. In this section
we diwuss the magnetic properties 06 nanoclusters of metal atoms that have a net
magnct moment. In a cluster the magnetic moment or each atom will interact with
the moments ef the other atoms, and can force all the moments to align in one
dircction with respect lo some symmetry axis of the clusler. The clwstcr will have a
net moment, and is said to be mapnctized. The magnetic moment of snch clusters
can bc measured by rncans of the Stcm-Gedach expcriment ihstratcd in Fig. 4.16.
The cluster particles are sent into R region whcrc there is an inhomogeneous
magnetic field, which scpurates the particles according to whethcr their magnetic
moment is up or down. From the bcam separation, and a knowledge OS the strength
and gradient of the magnetic field, the magnetic moment can bc determined.
Howcvcr, Tor magnetic nanoparticles the measured magnetic moment is Found to
be len than the value Tor ii perfect paraitel alignment of the moments in the cluster.
The atoins of the cluster vibrate, and this vibratimonal energy increases with
temperature. These vibrations cause some misalignment of the magnetic moments
of thc individual atoms of the cluster sc) that the net magnetic moment nt the cluster
is less than what it would be if all the atom had their moments ahgmd in the same
direction. The component of the niagnctic moment of an individual cluster will
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