Magnetic Moment of Co
Clusters are small aggregates of atoms with dimensions on the order of
1 - 100 nanometers. When bulk matter is scaled down to these dimensions its
properties begin to change in a variety of interesting ways. Small particles of
metals and semiconductors have electric, magnetic, and optical properties which
are different from the bulk and depend on size. A familiar example is the
coloration of stained glass which is due to small metal particles annealed into
the glass from metal salts. In recent years, chemists have discovered reactions
to grow metal and semiconductor nanoparticles in organic solutions which allow
them to be segregated by size. Like the particles preciptated in stained glass,
these nanoparticles have brilliant colors that differ from those of the bulk and
depend on the particle size. There has also been much interest
in the magnetic properties of small metal clusters - which have technological
applications in magnetic data storage, and medical imaging.
The projects in our group study the properties of free clusters in a
molecular beam. Because of their small size, a typical cluster will have a
large fraction of its atoms on the surface. This makes it very difficult to
distinguish between effects which are due to their small size and effects
which are due to surface interactions with a solvent or matrix. Free
clusters, which condense in the gas phase and can be collimated into a molecular
beam, are an experimental platform where the properties of small clusters can be
studied without the complication of surface interations.
Our experiments focus on how the electric and magnetic properties of clusters
vary with size, to try and understand the emergence of these properties in the
Click this link to see a diagram of our apparatus
Clusters of all sizes are produced in a molecular beam by ablating a sample rod
with a pulsed laser. The beam is then analyzed by a Time of Flight
Mass Spectrometer (TOFMS) which disperses the clusters into a spectrum by their mass.
In the TOF spectrometer, the cluster beam (which contains clusters of all sizes) is
ionized by a UV laser and accelerated toward a detector by an electric field.
The time of flight depends on the mass of the cluster, with the lightest
clusters arriving first and the heaviest clusters arriving last. An actual
spectrum is shown below, taken from a beam of cobalt clusters.
Each peak in the above spectrum represents a cluster with a specific number of
In many of our experiments we direct the cluster beam through an electric or
magnetic field and we want to measure the deflection of cluster beam by the
field. The TOF spectrometer also gives detailed information about the
deflection of the beam, because a cluster that has been deflected by an electric
or magnetic field will have a slight lead or lag compared to the time of flight
of an undeflected cluster. The deflection is manifested in the mass spectrum
by a shifting of the peak of the deflected cluster. The magnitude of the
deflection can be calculated from the magnitude of the peak's shift.
Our recent work has focused on corellated electron effects in clusters and
alloys such as ferromagnetism and ferroelectricity.
We are studying the details of how these propeties vary with cluster size and
temperature. Another interesting series of experiments involves how the
properties of clusters change when magnetic impurities are added to make
See our publications page for details.