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Magnetism of Alloy Clusters in a Molecular Beam

Introduction

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.

Experimental Methods

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.

Fig.1 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.

Fig.2 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)

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 AuNCo4 clusters.

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.

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 Bi_N Mn_M clusters 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 μ_B. The addition of Manganese atoms has a very large effect on the total moment. There are also some specific combinations of Bi_MMn_N that have peculiarly large moments.

Fig.7 - Magnetic moments of BiNMnM. Note the particularly large moments of Bi5Mn3, Bi9Mn4, Bi19Mn5, and Bi12Mn6. 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 clusters.

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 3μ_B per Mn atom corresponds to a ferromagnetic coupling of the local moments of the Manganese atoms, and the peak around 1.3μ_B corresponds to a ferrimagnetic coupling (where the moments are anti-aligned).

Fig.9 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.

Summary

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.