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Abstract: We use dynamic susceptometry measurements to extract semiempirical temperature-dependent, 255–400K, magnetic parameters that determine the behavior of single-core nanoparticles useful for SQUID relaxometry in biomedical applications. Volume susceptibility measurements were made in 5K degree steps at nine frequencies in the 0.1–1000Hz range, with a 0.2mT amplitude probe field. The saturation magnetization (M s ) and anisotropy energy density (K) derived from the fitting of theoretical susceptibility to the measurements both increase with decreasing temperature; good agreement between the parameter values derived separately from the real and imaginary components is obtained. Characterization of the Néel relaxation time indicates that the conventional prefactor, 0.1ns, is an upper limit, strongly correlated with the anisotropy energy density. This prefactor decreases substantially for lower temperatures as K increases. We find, using the values of the parameters determined from the real part of the susceptibility measurements at 300K, that SQUID relaxometry measurements of relaxation and excitation curves on the same sample are well described. [ABSTRACT FROM AUTHOR]

Abstract: Magnetic characterization of spherical, oxide-free, bcc iron nanoparticles synthesized with β-diketone surfactants has been performed. The results of this characterization, which included particles with diameters ranging between 2 and 5nm show that the nanoparticles have an average anisotropy of 1.9×106±0.3×106 J/m3, which is more than an order of magnitude greater than the magnetocrystalline anisotropy of bulk iron. Despite their unusually large anisotropy, these particles can have saturation magnetizations of up to 210Am2/kg (slightly lower than bulk iron). High-energy X-ray diffraction data indicates that the Fe particles have a distorted bcc lattice, which could, at least in part, explain the magnetic behavior of these nanoparticles. Dipolar coupling between particles, while present, is weak and cannot account for the high anisotropy of these nanoparticles. [Copyright &y& Elsevier]