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Magnetic interactions in water based ferrofluids studied by Mössbauer spectroscopy

Various samples of ferrofluids consisting of colloidal suspensions of surfacted cobalt ferrite or magnetite nanoparticles in water were studied by x-ray diffraction and temperature dependent Mössbauer spectroscopy. Information about the particle mean size, the size dispersion and the effective magnetic anisotropy energy was obtained for each sample. The results are consistent with the formation of a magnetic dead layer at the particle surface, whose thickness depends on the surfactant–ferrite combination. The magnetic relaxation processes are faster in the colloidal suspensions of magnetite particles as compared with the suspensions of cobalt ferrite particles. The type of the surfactant also influences the magnetic relaxation behaviour, and hence the macroscopic properties of the ferrofluid at ambient temperature.