Ferrimagnetic cagelikeFe4O6cluster: Structure determination from infrared dissociation spectroscopy
Physical Review B
Cationic iron-oxide clusters of several sizes and stoichiometries have been synthesized and studied isolated in the gas phase. Vibrational spectra of the clusters have been measured using resonant IR-induced dissociation of Fe n O m+2 + → Fe n O m + + O 2 in the 250-1250 cm −1 range. Density-functional theory was used to investigate the geometry and spin configuration of the representative Fe 4 O 6 0/+ cluster. Its lowest-energy state was found to be an almost tetrahedral cagelike structure
... a ferrimagnetic arrangement of spins, resulting in total cluster spin of S = 5 for the neutral cluster. These results were confirmed for Fe 4 O 6 + by the comparison of the calculated infrared spectrum to the experimentally obtained one. With the continuous trend of increasing the density of electronic devices, novel concepts are needed to answer the increasing technical problems. Many of these concepts require the creation of dedicated nanosized building blocks with well understood and a priori designed properties. For application in quantum computing and storage, for example, chemically synthesized magnetic molecules were suggested as possible units. 1 In such a "bottom-up" approach, magnetic clusters represent the smallest chunks of matter where condensed matter properties start to appear, magnetic order, in particular. However, these properties are still drastically different from those of the bulk matter, making the clusters a new object of physical research. Such atomic clusters may be as small as containing only tens of atoms. 2 They allow a large freedom in manipulation through varying their composition and size, adding one atom at a time. 3 From various materials, transition-metal oxides represent the widest variety of phenomena, from ferroelectricity and magnetism to superconductivity, often combined. In cluster form, many unusual phenomena have been predicted. 4-8 The knowledge of their detailed magnetic, electronic, and ionic structure is essential for various applications, such as the formation of novel materials or design of next generation devices, to fundamental issues as the functioning of quantum and thermodynamics laws in ͑sub-͒ nanoscale systems. To explore the intrinsic properties of nanoparticles free of external perturbations, experiments are best performed in the gas phase. However, the experimental data on the properties of gas-phase transition-metal oxide clusters are still scarce. A preferred method to obtain information on structure and bonding is vibrational spectroscopy. Unfortunately, in most cases, isolated clusters can only be studied in molecular beams or ion traps 9 and the low particle density rules out direct absorption measurements. Furthermore, clusters are often produced in broad size distributions, making size selectivity essential. To overcome such problems, mass selected clusters can be embedded and accumulated in rare gas matrices. 10, 11 In those experiments, interactions with the rare gas matrix may induce shifts of the absorption lines, and additionally, it cannot be excluded that structural changes during the deposition occur. Intense tunable infrared sources such as free-electron lasers allow different approaches. Thus, titanium 12 and zirconium 13 oxides were investigated with the help of infrared resonance enhanced multiple-photon ionization spectroscopy. Infrared photodissociation spectroscopy helped to determine the structure of oxide clusters of vanadium, 14 niobium and tantalum 15,16 as well as binary vanadium-titanium oxide clusters. 17 However, the possible existence of magnetic order has not been investigated for either of these systems. Here we report an experimental investigation of cationic iron-oxide clusters in the gas phase. Their vibrational spectra are measured via resonant dissociation of the target clusters ͑Fe n O m + ͒ tagged with molecular oxygen. Quantum chemical calculations allow us to derive the cluster structure as well as its magnetic state by comparison of the calculated vibrational spectra for low-energy isomers to the experimental data. A ferrimagnetic structure slightly distorted from C 3v symmetry is found for a "bulk-stoichiometric" Fe 4 O 6 + cation cluster while the lowest-energy neutral Fe 4 O 6 isomer is of exact C 3v symmetry. The calculations also demonstrate a correlation between the cluster's geometrical and magnetic configurations. The cluster cations are produced by ablating the metal with the second harmonic output ͑532 nm, 10 mJ͒ of a pulsed Nd-yttrium aluminum garnet laser and quenching the plasma with a short pulse of a gas mixture containing 0.5% oxygen in helium. After expansion into vacuum the cluster distribution in the molecular beam is analyzed using a reflectron time-of-flight mass spectrometer. The cluster beam is overlapped with a counterpropagating intense infrared laser beam, delivered by the free electron laser for infrared experiments ͑FELIX͒. This laser can produce intense several s long pulses of tunable IR radiation in the 40-2500 cm −1 range containing up to 100 mJ per pulse. Each pulse consists of a train of 0.3-3 ps long micropulses of typically 10 J, spaced by 1 ns. For more details of the measurement procedure see Ref. 16.