Nuclear Resonance in Ferromagnetic Cobalt

The observation of nuclear magnetic resonance in ferromagnetic cobalt is reported. The resonance frequency for finely divided face-centered-cubic material has been measured in the intermediate temperature range. The frequency extrapolated to 0°K is 217.2 Mc/sec, and the temperature dependence in this range is in general agreement with that of the magnetization. This frequency implies a hyperfine field of 217 500 oe, which is in good agreement with the field deduced from specific heat measurements on hexagonal cobalt. The agreement in the two structures indicates that there is no dipolar contribution to the hyperfine field. The theoretical implications of this observation are discussed. The resonance line is inhomogeneously broadened with a half width of 400 kc/sec. A pattern of beats is observed at high passage rates which makes it possible to determine a spin-spin time of 25 μsec. By varying the modulation frequency under conditions of intermediate saturation the spin lattice relaxation time is measured to be 280 μsec. The resonance signal is remarkably intense, being ${\text{5×10}}^5$ stronger than calculated for a dipole transition in the driving radio frequency field. It is shown from the saturation measurements that the rf field at the cobalt nucleus is ${10}^3$ times stronger than the external driving field. These intensity and saturation measurements, as well as the observed line shape, establish that the resonance is driven by domain wall motion. Only those spins within the domain walls are affected. An external field reduces the intensity of the resonance but produces no shift in the resonance frequency. Both these effects are consistent with domain wall excitation. Spin-spin relaxation is interpreted as a spin wave coupling reduced in intensity by the broadening of the resonance spectrum. The spin-lattice relaxation is by spin diffusion away from the domain walls and ultimately to the lattice by coupling with the conduction electrons as suggested by the temperature dependence of the relaxation time.