Electron paramagnetic resonance is used to investigate ionic cobalt fe
rrite-based magnetic fluids. Two magnetic fluid samples having particl
es with different average radius of 6.8 nm and 7.8 nm were doped with
Cu2+ at different concentrations. Changes on the resonance line shape
of the Cu2+-ion in the aqueous phase was analyzed taking into account
the particle polydispersity, the contribution of the dipolar held due
to the surrounding magnetic nanoparticles, and the electrostatic inter
action between the surface charged magnetic nanoparticles and the Cu2-probe. The particle polydispersity parameters we found from the reson
ance line shape analysis are in excellent agreement with the parameter
s obtained from electron microscopy. The Cu2+ spatial distribution fun
ction around the magnetic nanoparticles gives very reasonable values f
or both, the particle-particle distance and the ionic layer thickness
at the particle surface. (C) 1997 American Institute of Physics.