  Magnetic nanoparticles of gamma-Fe2O3 coated by organic molecules and
suspended in liquid and solid matrices, as well as a 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, symmetric,
and shift to lower fields upon cooling. An additional narrow spectral component
(with the line-width of 30 G) is found in the diluted samples, its magnitude
obeying the Arrhenius law with the activation temperature of about 850 K. The
longitudinal spin-relaxation time, T1 >> 10 ns, was determined by the specially
developed modulation method. Angular dependence of the EMR signal position in
field-freezing samples unambiguously points to the domination of the uniaxial
magnetic anisotropy. Substantial alignment is achieved in moderate freezing
fields of 4-5 kG, suggesting formation of dipolar-coupled chains consisting
from several particles separated by organic nanolayers. The shift and
broadening of the spectrum upon cooling are ascribed to the role of the surface
layer, which is considered with taking into acount the strong surface-related
anisotropy. To describe the overall spectrum shape, a quantization model is
used which includes summation of the resonances corresponding to varios
orientations of the particle magnetic moment. This approach, supplemented with
some phenomenological assumptions, provides satisfactory agreement with the
experimental data.
