HR: 10:20h
AN: U52B-01 INVITED     [Abstracts]
TI: Antiferromagnetic Nanoparticles of Iron Oxides: Their Magnetic Properties, and the Effects of Aggregation and Interparticle Interactions
AU: * Frandsen, C
EM: fraca@fysik.dtu.dk
AF: Technical University of Denmark, Department of Physics, Building 307 , Kgs. Lyngby, DK-2800, Denmark
AB: 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.
DE: 1540 Rock and mineral magnetism
DE: 3929 NMR, Mossbauer spectroscopy, and other magnetic techniques
DE: 5112 Microstructure
SC: Union [U]
MN: 2007 Joint Assembly