Nuclear Relaxation in Ferromagnetic Cobalt

Abstract

Nuclear-magnetic spin-spin relaxation processes of ${\mathrm{Co}}^{59}$ have been studied in magnetically saturated particles of fcc cobalt. The transverse relaxation has been studied experimentally by observing the two-rf-pulse spin-echo envelope decay. This decay is initially oscillatory, with the oscillations damping in a time of order of the transverse-relaxation time ${T}_{2}$ into exponential decay. The oscillatory behavior is dependent on the pulse widths, and the relaxation time ${T}_{2}$, determined from the exponential decay rate, varies as the square root of the local field. Theory is presented which shows that the observed oscillatory behavior can be explained qualitatively by assuming that the dominant spin-spin interaction is of the Suhl-Nakamura type, with the resonance line broadened by microscopic inhomogeneities. The period of the oscillations gives the rms microscopic inhomogeneous linewidth 100 kHz. Using the correlation-function technique, the relaxation function has been derived for this system. This theory is similar to that of Kubo and Tomita for the exchange-narrowing problem, and agrees well with the observed nonoscillatory transverse relaxation. From the derived relaxation function and observed transverse-relaxation results, a model of the homogeneous line is deduced. The line is assumed to be Lorentzian, with a cutoff of the order of the second moment of the Suhl-Nakamura interaction. Using this model and the theory of quantum statistics of irreversible processes, a theory of spectral diffusion is derived. Spectral diffusion was studied experimentally by monitoring the decay of the three-rf-pulse-stimulated echo. The new theory presented here agrees well with these experimental results.