Magnetic spin and orbital moments of size-selected free iron cluster ions Fe{n}{+} (n=3-20) have been determined via x-ray magnetic circular dichroism spectroscopy. Iron atoms within the clusters exhibit ferromagnetic coupling except for Fe{13}{+}, where the central atom is coupled antiferromagnetically to the atoms in the surrounding shell. Even in very small clusters, the orbital magnetic moment is strongly quenched and reduced to 5%-25% of its atomic value while the spin magnetic moment remains at 60%-90%. This demonstrates that the formation of bonds quenches orbital angular momenta in homonuclear iron clusters already for coordination numbers much smaller than those of the bulk.

Spin and orbital magnetic moments of cationic iron, cobalt, and nickel clusters have been determined from x-ray magnetic circular dichroism spectroscopy. In the size regime of n = 10 − 15 atoms, these clusters show strong ferromagnetism with maximized spin magnetic moments of 1 µB per empty 3d state because of completely filled 3d majority spin bands. The only exception is Fewhere an unusually low average spin magnetic moment of 0.73 ± 0.12 µB per unoccupied 3d state is detected; an effect, which is neither observed for Co + 13 nor Ni + 13 . This distinct behavior can be linked to the existence and accessibility of antiferromagnetic, paramagnetic, or nonmagnetic phases in the respective bulk phase diagrams of iron, cobalt, and nickel. Compared to the experimental data, available density functional theory calculations generally seem to underestimate the spin magnetic moments significantly. In all clusters investigated, the orbital magnetic moment is quenched to 5 − 25 % of the atomic value by the reduced symmetry of the crystal field. The magnetic anisotropy energy is well below 65 µeV per atom.

A new femtosecond time-resolved interferometer was developed that utilizes interference fringes in the frequency domain to obtain simultaneously difference phase spectra (DPS) and difference transmission spectra with a multichannel spectrometer. For the first time to our knowledge, transient oscillations were observed in DPS and the spectral shift of a probe pulse was time resolved together with the rise in DPS, which is clear evidence for induced phase modulation in absorptive materials.

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Dichromium oxide cluster anions, Cr2On− (n=1–3), were found to possess highly spin-polarized electronic structures, which were revealed by the measurements of photoelectron spectra and the analyses by the density-functional calculations. Their spin magnetic moments were as large as 9, 9, and 7 μB for n=1, 2, and 3, respectively, due to a ferromagnetic coupling between local spins on the chromium atoms. The ferromagnetic spin couplings were caused predominantly by a superexchange-type Cr–Cr interaction through an oxygen atom at the bridge site, where a significant mixing of Cr 3d with O 2p orbitals stabilized the ferromagnetic states. The high-spin characters of Cr2On− are in striking contrast to that of a pure chromium dimer, which is known to exhibit an antiferromagnetic spin coupling due to the strong Cr–Cr covalent bond. The present ferromagnetic spin couplings should, therefore, be induced by oxidation. These findings support a concept that a chemical reaction controls magnetic properties of molecules and clusters.

The photoelectron spectra of Co−n (3≤n≤70) were measured at the photon energy of 4.025 eV by use of a XeCl excimer laser. For Co−n with n=3, 4, and 6, the geometric and electronic structures were obtained from the spectra in comparison with the calculated spectra by the spin-polarized DV-Xα method. The spectra observed are reproduced reasonably well by the calculation with postulating the most probable geometrical structures. It is revealed that the 3d band with the majority spin is separated by 1.0–2.8 eV from that with the minority spin; the former is completely filled while the latter is partly filled and extends above Fermi level. The magnetic moments and the average exchange energies of these cluster anions were estimated. For Co−n with n≥7, the observed electron affinity depends linearly on the reciprocal of the cluster radius and approach the work function of a cobalt metal, as n increases. Below n=6, the electron affinity deviates from the linear dependence. This finding indicates that a size-dependent transition in the electronic structure occurs at n≂7. The spherical conducting drop model suggests the presence of mobile electrons in Co−n with n≥7.

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The photoelectron spectrum of the dichromium oxide cluster anion, Cr2O-, and the analysis by the density-functional theory revealed that the spins of the two Cr atoms in Cr2O- are ferromagnetically coupled, and that its total spin magnetic moment is as large as 9 mu(B). This ferromagnetic spin coupling is induced by oxidation; the mixing of Cr 3d with O 2p orbitals plays an important role in a spin coupling between the localized electrons at the two Cr sites bridged by the O atom. The present finding is in marked contrast to the pure chromium dimer, which is known to be antiferromagnetic due to the strong sextuple Cr-Cr bond.