Superparamagnetic and ferrimagnetic resonance of ultrafine Fe3O4 particles in ferrofluids

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Abstract
Coated ultrafine magnetite particles (average diameter around 100 Å) in five nonmagnetic liquids (commercial ’’ferrofluids’’) have been investigated by electron magnetic resonance at room temperature. A narrow signal can be attributed to the superparamagnetic fraction of the particles. A simple ’’superparamagnetic narrowing’’ model is consistent with the measurements. The ferromagnetic fraction gives a broad line. Both lines narrow upon dilution because the interparticle dipolar interaction is reduced. Variations of the viscosity have only minor effects on the signals. Dilution in trifluoroacetic acid seems to destroy the coating. The resulting agglomeration and sedimentation changes the magnetic resonance drastically. The reasons for the high value of the cubic anisotropy constant K 1? (−5.8×105 erg/cm3) found for the ultrafine particles are discussed.

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