Ferromagnetic Nuclear Resonance of Single-Domain Cobalt Particles

Nuclear magnetic resonance in single-domain fcc cobalt particles 100 to 150 {} in diameter has been observed at 216.85 Mc/sec at 300^∘{}K. The negative shift in resonance frequency with applied field establishes that the hyperfine field at the cobalt nucleus is negative. A super-regenerative oscillator spectrometer is described that has sufficient sensitivity to observe the weak resonance. The shift in nuclear resonance frequency in single-domain cobalt particles is calculated in a self-consistent way for low and high applied-field limits. The effects of particle size and superparamagnetic behavior have been observed and explained for cobalt particles less than 90 {} in diameter. A theoretical analysis of the rf excitation in single-domain and multidomain particles is included.