Magnetic resonance in nanoparticles: between ferro- and paramagnetism

Abstract

Magnetic nanoparticles of γ-Fe2O3 coated with organic molecules and suspended in liquid and solid matrices, aswell as non-diluted magnetic fluid, have been studied by electron magneticresonance (EMR) at 77–380 K. Slightly asymmetric spectra observed at roomtemperature become much broader and symmetric, and shift to lower fields uponcooling. An additional narrow spectral component (with a line-width of 30 G) isfound in diluted samples; its magnitude obeys the Arrhenius law with anactivation temperature of about 850 K. The longitudinal spin-relaxation time,T1≈10 ns, is determined by a specially developed modulation method. The angular dependence ofthe 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 thesurface-related anisotropy. To describe the overall spectral shape, the ‘quantization’ modelis used which includes summation of resonance transitions over the whole energy spectrumof a nanoparticle considered as a giant exchange cluster. This approach, supplemented withsome phenomenological assumptions, provides satisfactory agreement with theexperimental data.