title: Magnetic resonance in nanoparticles: between ferro- and paramagnetism.
abstract: Magnetic nanoparticles of γ-Fe(2)O(3) coated with organic molecules and suspended in liquid and solid matrices, as well as non-diluted magnetic fluid, have been studied by electron magnetic resonance (EMR) at 77-380 K. Slightly asymmetric spectra observed at room temperature become much broader and symmetric, and shift to lower fields upon cooling. An additional narrow spectral component (with a line-width of 30 G) is found in diluted samples; its magnitude obeys the Arrhenius law with an activation temperature of about 850 K. The longitudinal spin-relaxation time, T(1)≈10 ns, is determined by a specially developed modulation method. The angular dependence of the EMR signal position in field-freezing samples points to substantial alignment, suggesting the formation of dipolar-coupled aggregates. The shift and broadening of the spectrum upon cooling are assigned to the effect of the surface-related anisotropy. To describe the overall spectral shape, the 'quantization' model is used which includes summation of resonance transitions over the whole energy spectrum of a nanoparticle considered as a giant exchange cluster. This approach, supplemented with some phenomenological assumptions, provides satisfactory agreement with the experimental data.
authors.affiliation: Norfolk State University Center for Materials Research, Norfolk, VA, USA.
authors.forename: N
authors.initials: N
authors.lastname: Noginova
authors.forename: F
authors.initials: F
authors.lastname: Chen
authors.forename: T
authors.initials: T
authors.lastname: Weaver
authors.forename: E P
authors.initials: EP
authors.lastname: Giannelis
authors.forename: A B
authors.initials: AB
authors.lastname: Bourlinos
authors.forename: V A
authors.initials: VA
authors.lastname: Atsarkin