Magnetism of Alloy Clusters in a Molecular Beam
Shuangye Yin, Xiaoshan Xu, Anthony Liang, and Walt A. de Heer
Stern-Gerlach deflections of free clusters in a molecular beam has
provided many surprising facts about cluster magnetism. At the
reduced size of clusters (2 - 1000 atoms) magnetic enhancement is
observed in Iron, Cobalt, and Nickel. Rhodium and Manganese clusters
have been found to be ferromagnetic while their bulk crystals are
paramagnetic, and antiferromagnetic.
Many insights into the magnetic properties of materials have
been gained by considering the properties of alloys. The effects
of mixing in a crystal atoms of different elements has greatly
increased the range of magnetic properties attainable by materials.
We are curious about the effects of impurities on the magnetic
properties of small metal clusters.
Alloy samples are perpared in a furnace, which allows the composition
to be controlled. The sample is vaporized in a chamber by a pulse
laser which causes it to exit through a skimmer nozzle as a molecular
beam. The molecular beam is then collimated and directed into a
Stern-Gerlach magnet. After that the time-of-flight spectrum is
taken which allows the beam to be sorted by cluster size, composition,
and deflection. The molecular beam experiment is described in
detail here. A sample spectrum taken
from a Cobalt-Manganese experiment is shown in figures 1 and 2.
Sample spectrum from a CoMn experiment. Replacing
one Cobalt atom with a Manganese atom reduces the mass of a cluster
by 4 amu. They are thus clearly separated from each other in the
time of flight spectrum. The bottom graph shows the spectrum with
the magnetic field turned on in red. The shifting is due to the
magnetism of the clusters.
Integrated intensity under each CoNMnM
cluster peak. The intensities are close to a binomial
distribution with maximum intensity at the same Co:Mn ratio
as the sample rod. This behavior contrasts with the abnormal
intensity distribution of AuNCoM clusters.
see here for a comparison
Gold-Cobalt alloy clusters
The electronic shell structure of pure Gold clusters (fig. 3) favors
the stability of clusters with a magic number of atoms. These
magic numbers correspond to a completely filled electronic shell.
(for Gold the magic numbers are 8, 20, 34, ...). In addition to
being favored by the cluster formation process, the magic number
clusters also have a much higher photoionization potential.
Because their ionization potential is higher than the 6.5eV of the
eximer laser used to ionize them in our spectrometer, the intensity
of the peaks corresponding to the magic number clusters is smaller
than the other clusters which are easily ionized by the laser.
Fig.3 - Mass spectrum of pure Gold clusters
When Cobalt atoms are added to the alloy, the magic numbers
change. For every Cobalt atom added to a Gold cluster, the number
of gold atoms required to form a closed electronic shell is
shifted downward by two. This trend in the shifting
of the magic numbers persists to impurity levels of about
5 Cobalt atoms per cluster. Beyond this point the clusters revert
to the binomial mass spectrum expected for a binary alloy.
Interestingly this transition is accompanied by a change in the
clusters' magnetic properties. (fig. 5)
Total magnetic moments of AuNCoM clusters
Each cobalt atom contributes about 2 μB to the
total magnetic moment. Additional Gold atoms generally reduce
the total moment. Note the anomalous behavior of the
To put the differences between alloy clusters and bulk alloys
in perspective, a plot comparing magnetic moments of AuCo
clusters with AuCo bulk alloy is shown in fig. 6.
Generalized Slater-Pauling plot of
Co1-xAux alloy as compared with alloy
clusters. The clusters have larger average moments due to
the narrower bands. The similarity of the reducing trend with
Au doping suggests that the mechanism is similar. There are
deviations from the trend for small clusters.
Bismuth - Manganese alloy clusters
New! - To appear in PRB:
Measurement of magnetic moments of free BiN
S. Yin, X. Xu, R. Moro, and W. A. de Heer
Phys. Rev. B 72, 174410 (2005)
Pure Bismuth clusters have a very small magnetic moment of
less than 3 μB. The addition of Manganese
atoms has a very large effect on the total moment. There
are also some specific combinations of
BiMMnN that have peculiarly large
Fig.7 - Magnetic moments of
BiNMnM. Note the particularly
large moments of Bi5Mn3,
Fig.8 - Magnetic moments of
BiNMnM as a function of
M and N. The diameter of each data
point is proportional to its magnitude.
The enhancements are clear compared to pure Bi
While it is hard to discern any trends from the data above, a
histogram of the moments show that there are two states of
magnetic ordering for the BiMn clusters. We propose that the
peak of centered around 3μB per
Mn atom corresponds to a ferromagnetic coupling of the local
moments of the Manganese atoms, and the peak around
1.3μB corresponds to a ferrimagnetic coupling
(where the moments are anti-aligned).
Histogram of the magnetic moments per Mn atom of the BiMn
clusters. Note that the distribution is bimodal. The
peak at 3μB, is identified with ferromagneic
coupling between Mn moments. The peak at 1.3 μB
is identified with ferrimagnetic coupling.
Alloy clusters show a variety of interesting magnetic
properties. Our future work will example the 3d and 4d
transition metal alloy clusters and their electric, magnetic
and optical properties.