U52B-01 INVITED
Antiferromagnetic Nanoparticles of Iron Oxides: Their Magnetic Properties, and the Effects of Aggregation and Interparticle Interactions
Antiferromagnetic materials such as a-Fe2O3 (hematite), a-FeOOH (goethite) and ferrihydrite, are commonly found in geological environments, but they have typically attracted limited attention in magnetic studies, because their sublattice magnetizations are aligned antiparallel and hence particles of these materials is often thought to have negligible magnetic moments. A number of studies have, however, revealed that nanoparticles of antiferromagnetic materials show a wealth of fascinating magnetic properties. For instance, nanoparticles may have a net magnetic moment due to uncompensated spins, with implications for the rock magnetic signature [1]. Recently, it has also been proposed that the thermal energy may excite the magnetic structure of antiferromagnetic nanoparticles and a thermoinduced magnetic moment may occur [2]. Thermoinduced magnetism is a novel concept of (nano)magnetism where the magnetization increases with temperature. At room temperature the thermoinduced magnetic moment can be similar in magnitude to that originating from uncompensated spins. Magnetic dipole interactions between antiferromagnetic nanoparticles can be considered negligible [3-5] despite magnetic moments from e.g. uncompensated spins, but magnetic exchange interactions can be established between surface atoms of neighboring particles e.g. by drying aqueous suspensions of particles [3-5]. Interparticle exchange interactions significantly influence the properties of individual particles, e.g. superparamagnetic relaxation may be suppressed [3], and the direction of the sublattice magnetizations may deviate from the easy axis as defined by magnetic anisotropy [5]. The effect of interactions on the magnetic properties appears associated with the attachment of the particles, either as a result of oriented attachment or of a more random aggregation [4,5]. Macroscopic handling like grinding and ultrasonic treatment may diminish the nanoscopic coupling [6]. The results stress that the properties of nanoparticles, in addition to differing from bulk properties, have to be described in terms of aggregation state and interactions. [1] Robinson P. et al. (2002), Nature 438, 517. [2] Morup S., and C. Frandsen (2004), Phys. Rev. Lett. 92, 217201. [3] Frandsen C., and S. Morup (2003), J. Magn. Magn. Mater. 266, 36. [4] Frandsen C,. et al. (2005), Physical Review B 72, 214406. [5] Frandsen C., and S. Morup (2005), Phys. Rev. Lett. 94, 027202. [6] Frandsen, C., and S. Morup (2006), J. Phys. Cond. Matter 18, 7079.
U52B-02
Testing for Discrete vs. Agglomerated Nanoparticles of 'Iron Oxides'
Nanoparticles of 'iron oxides' such as ferrihydrite, goethite, hematite and magnetite are found in nature as discrete or agglomerated ('clumped') particles, and with or without dopants such as Al, as and Si. The presence of clumping has important repercussions on both physical and chemical properties of these compounds. We present evidence from a number of nanoparticle systems in which dc and ac susceptibility measurements at low temperatures provide a non-destructive way to assess the presence of agglomeration and the roles of the dopants in modifying such behavior. In addition, we use quantitative kinetics for Fe (III) reduction or Fe(II) oxidation by quinones as our main chemical tool for sample aliquots.
U52B-03
The Fate of
Natural nanoparticles are attracting a great deal of attention due to their unique role as agents of elemental
transport and their increased reactivity in geologic systems. Although significant progress has been made in
understanding their behavior in the Earth's critical zone (i.e. near-surface environments), there is a severe lack of
information on their stability for a wider range of geologically relevant temperatures. Here, we describe the first
direct observations of the dynamic behavior of natural nanoparticles at near atomic scale, revealing that their
thermal stability is not only dependent on particle size, but also on the surrounding host mineral. Native Au
nanoparticles (mean diameter ~4 nm) incorporated in an As-rich pyrite from "invisible" Au ores were observed
during in-situ heating up to 650°C. While isolated Au nanoparticles melt, with their melting point being a
function of size, we show that when incorporated in a sulfide host, Au nanoparticles react to increased
temperature by dissolving into the pyrite matrix and forming larger particles in an Ostwald-type ripening process.
The dissolution temperatures are much lower than melting temperatures of isolated nanoparticles and are as
well strongly size-dependent. These findings provide new insights into the fate of nanoparticulate Au and other
metals during geological processes and throughout their metallurgical recovery from refractory ores. The size
distribution of the particles may be an indicator of the geologic history of the ore because, as we show, the mean
particle diameter sets un upper limit to the maximum temperature of the host rock. Furthermore, results suggest
that nanoparticulate minerals, usually documented in low-temperature (T<100°C) aqueous
environments, can also occur and survive at higher temperatures when incorporated into refractory host phases.
http:www.geo.lsa.umich.edu/compmin/
U52B-04
Magnetite and Native Gold Nanoparticles: Relation with Hydrothermal-Sedimentary Exhalative Environments in Iron-Ore Deposit
We report for the first time on the finding of magnetite and native gold nanoparticles in an iron-ore deposit. Initial results contribute to understanding the peculiar behaviour and distinctive physical, chemical and magnetic characteristics of the nanoparticles in the iron ore. Study of the natural magnetite nanoparticles provides information on the grain-size effects in their properties and in their nanomineral phases of transformation from temperature effects. This is used to develop a model of the genetic and environmental conditions of nanoparticles formation. Crystallographic studies in magnetite nanoparticles and berthierine were performed by high resolution TEM, and by X-ray diffraction, differential and gravimetric thermal analysis and Mössbauer spectroscopy. We observe magnetite nanoparticles semi-spherical shapes from 2 to 14 nm included in berthierine; continuous formation magnetite nanoparticles for nucleation in the hydrothermal-diagenetic processes provoks aggregates of nanoparticles associated to berthierine, both minerals are genetic indicators of hydrothermal-sedimentary exhalative environments. In our study, we found one locality of berthierine and chamosite in the Peña Colorada deposit. Results permit to observe in the magnetite nanoparticles a high heating resistance, high Curie temperature, Tc = 690 oC d high frequency dependent factor. Mössbauer spectroscophy shows a doublet of distinctive nanophases of Fe2 and Fe3 resulting from magnetite with superpragmatic behaviour.