Nuclear magnetic resonance and magnetization study of surfactant-coated epsilon-Co nanoparticles

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Abstract

A combination of 59Co spin-echo nuclear magnetic resonance (NMR) and dc magnetization measurements have been carried out on two samples of surfactant-coated Co nanoparticles in disordered assemblies; 6.5 nm diameter ε-Co and 12 nm diameter hcp-Co. The ε-Co nanoparticles exhibit superparamagnetic behavior with a blocking temperature TB = 70 K, while the hcp-Co nanoparticles remain ferromagnetic up to room temperature. In addition, the initial susceptibility for the ε-Co nanoparticles can be described by the Curie–Weiss law with a negative Θ = −158 K. The NMR signal from the hcp-Co sample is strong at 77 K with no applied magnetic field; the spectra are straightforward and similar to that for bulk hcp Co. However, the NMR signal from the ε-Co sample is not detectable at 77 K, even with fields up to 7.5 kOe. A NMR signal appears at 4.2 K; the echo amplitude increases dramatically with applied field. The spectra, which must be corrected for T2 effects, are quite broad and characteristic of the small particle size. Due to the broadening, there were no observable spectral features which could be assigned to the two Co sites in crystalline ε-Co. The results are discussed in the light of interparticle interactions which reduce the initial susceptibility and lead to negative Θ-values. Estimates are given for the magnetic dipolar and magnetocrystalline anisotropy energies of the particles in both samples.

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