Thin films grown by pulsed-laser deposition from targets of Sn 0.95 Fe 0.05 O 2 are transparent ferromagnets with Curie temperature and spontaneous magnetization of 610 K and 2.2 A m 2 kg Ϫ1 , respectively. The 57 Fe Mössbauer spectra show the iron is all high-spin Fe 3ϩ but the films are magnetically inhomogeneous on an atomic scale, with only 23% of the iron ordering magnetically. The net ferromagnetic moment per ordered iron ion, 1.8 B , is greater than for any simple iron oxide with superexchange interactions. Ferromagnetic coupling of ferric ions via an electron trapped in a bridging oxygen vacancy (F center͒ is proposed to explain the high Curie temperature. © 2004 American Institute of Physics. ͓DOI: 10.1063/1.1650041͔ First generation spin electronics 1 was based on magnetoresistive sensors and memory elements using electrodes made from alloys of the ferromagnetic 3d metals Fe, Co and Ni. There is an ongoing quest for ferromagnetic semiconductors with a Curie temperature well above room temperature, which could be used for a second generation of spin electronics, as well as a search for transparent ferromagnets which can add an optoelectronic dimension. Much recent interest has been generated by high temperature ferromagnetism in oxide and nitride materials such as ZnO with Co or Mn doping, 2-4 TiO 2 ͑anatase͒ with Co, 5 GaN with Mn, 6 AlN with Cr, 7 and SnO 2 with Mn ͑Ref. 8͒ or Co. 9 Doubts linger as to whether these are homogeneous, single-phase materials, particularly since the well-accepted mechanisms for ferromagnetic coupling, via spin-polarized p-band holes like those in Ga 1Ϫx Mn x As, 10 or via double exchange as in mixed valence manganites, 11 do not seem to apply in these oxides and nitrides.Following a recent report by Ogale et al. 9 of high temperature ferromagnetism with a giant cobalt moment in Codoped SnO 2 , we undertook an investigation of the magnetism of Fe-doped SnO 2 . We find atomic-scale inhomogeneity and remarkably strong ferromagnetism, for which a novel ferromagnetic exchange mechanism is suggested.Ceramic targets of Sn 0.95 Fe 0.05 O 2 were first prepared by solid-state reaction of SnO 2 and FeO or 57 Fe 2 O 3 at 1150°C. Rietveld analysis of the x-ray diffraction patterns of the targets showed SnO 2 with a trace of ␣-Fe 2 O 3 ͑Fig. 1͒. Elemental maps of the targets obtained by energy-dispersive x-ray ͑EDAX͒ diffraction indicated a nonuniform iron distribution, with some tendency to accumulate iron in regions 2-4 m in size which were identified as Sn-doped hematite. The ceramics were ferromagnetic, with magnetization at 5 K of 2.3 A m 2 kg Ϫ1 (Ϸ1.2 B /Fe), and a Curie temperature of 360 K. Mössbauer spectra showed that all the iron was highspin Fe 3ϩ , and 88% of it was magnetically ordered with a hyperfine field of 53.3 T at 19 K. The ferromagnetism cannot be attributed to the Sn-doped hematite, which is a canted antiferromagnet with a weak net moment. 12 The thin films were deposited on R-cut sapphire substrates using a KrF excimer laser operating at 248 nm and 10 Hz. Laser ...

A nitrogenase-inspired biomimetic chalcogel system comprising double-cubane [Mo 2 Fe 6 S 8 (SPh) 3 ] and single-cubane (Fe 4 S 4 ) biomimetic clusters demonstrates photocatalytic N 2 fixation and conversion to NH 3 in ambient temperature and pressure conditions. Replacing the Fe 4 S 4 clusters in this system with other inert ions such as Sb 3+ , Sn 4+ , Zn 2+ also gave chalcogels that were photocatalytically active. Finally, molybdenum-free chalcogels containing only Fe 4 S 4 clusters are also capable of accomplishing the N 2 fixation reaction with even higher efficiency than their Mo 2 Fe 6 S 8 (SPh) 3 -containing counterparts. Our results suggest that redox-active iron-sulfide-containing materials can activate the N 2 molecule upon visible light excitation, which can be reduced all of the way to NH 3 using protons and sacrificial electrons in aqueous solution. Evidently, whereas the Mo 2 Fe 6 S 8 (SPh) 3 is capable of N 2 fixation, Mo itself is not necessary to carry out this process. The initial binding of N 2 with chalcogels under illumination was observed with in situ diffuse-reflectance Fourier transform infrared spectroscopy (DRIFTS). 15 N 2 isotope experiments confirm that the generated NH 3 derives from N 2 . Density functional theory (DFT) electronic structure calculations suggest that the N 2 binding is thermodynamically favorable only with the highly reduced active clusters. The results reported herein contribute to ongoing efforts of mimicking nitrogenase in fixing nitrogen and point to a promising path in developing catalysts for the reduction of N 2 under ambient conditions. nitrogenase mimics | chalcogel | N 2 fixation | ammonia synthesis | photocatalytic T he reduction of atmospheric nitrogen to ammonia is one of the most essential processes for sustaining life. Currently, roughly half of the fixed nitrogen is supplied biologically by nitrogenase, while nearly the other half is from the industrial Haber-Bosch process, which operates under high temperature (400-500°C) and high pressure (200-250 bar) in the presence of a metallic iron catalyst (1). Nitrogenase, a two-component protein system comprising a MoFe protein and an associated Fe protein, carries out this "fixation" in nature under ambient temperature and pressure (2-4). N 2 substrate binding and activation take place at the ironmolybdenum-sulfur cofactor (FeMoco), and in some cases, Mofree iron-sulfur cofactor FeFeco and iron-vanadium-sulfur cofactor FeVco cofactors. Electron transfer during this catalytic process is believed to proceed from a [4Fe:4S] cluster located in the Fe protein to another Fe/S cluster (the P cluster) buried in the MoFe protein and finally to the FeMoco (Fig. 1A) (2, 5, 6). Whereas the role of Mo in the reactivity of nitrogenase has been the subject of long debate, iron is now well recognized as the only transition metal essential to all nitrogenases, and recent biochemical and spectroscopic data point to iron as the site of N 2 binding in the FeMoco (7-9). Naturally, understanding and mimicking how the nitrogenas...

There are recent reports of weak ferromagnetism in graphite and synthetic carbon materials such as rhombohedral C(60) (ref. 4), as well as a theoretical prediction of a ferromagnetic instability in graphene sheets. With very small ferromagnetic signals, it is difficult to be certain that the origin is intrinsic, rather than due to minute concentrations of iron-rich impurities. Here we take a different experimental approach to study ferromagnetism in graphitic materials, by making use of meteoritic graphite, which is strongly ferromagnetic at room temperature. We examined ten samples of extraterrestrial graphite from a nodule in the Canyon Diablo meteorite. Graphite is the major phase in every sample, but there are minor amounts of magnetite, kamacite, akaganéite, and other phases. By analysing the phase composition of a series of samples, we find that these iron-rich minerals can only account for about two-thirds of the observed magnetization. The remainder is somehow associated with graphite, corresponding to an average magnetization of 0.05 Bohr magnetons per carbon atom. The magnetic ordering temperature is near 570 K. We suggest that the ferromagnetism is a magnetic proximity effect induced at the interface with magnetite or kamacite inclusions.

Controlled assembly of single-crystal, colloidal maghemite nanoparticles is facilitated via a high-temperature polyol-based pathway. Structural characterization shows that size-tunable nanoclusters of 50 and 86 nm diameters (D), with high dispersibility in aqueous media, are composed of ~13 nm (d) crystallographically oriented nanoparticles. The interaction effects are examined against the increasing volume fraction, φ, of the inorganic magnetic phase that goes from individual colloidal nanoparticles (φ= 0.47) to clusters (φ= 0.72). The frozen-liquid dispersions of the latter exhibit weak ferrimagnetic behavior at 300 K. Comparative Mössbauer spectroscopic studies imply that intra-cluster interactions come into play. A new insight emerges from the clusters' temperature-dependent ac susceptibility that displays two maxima in χ''(T), with strong frequency dispersion. Scaling-law analysis, together with the observed memory effects suggest that a superspin glass state settles-in at T B~ 160-200 K, while at lowertemperatures, surface spin-glass freezing is established at T f~ 40-70 K. In such nanoparticleassembled systems, with increased φ, Monte Carlo simulations corroborate the role of the inter-particle dipolar interactions and that of the constituent nanoparticles' surface spin disorder in the emerging spin-glass dynamics.

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