Spin-spin relaxation and multiple echoes in manganese ferrites

The Suhl-Nakamura or indirect spin-spin interaction is found to be the predominant relaxation mechanism for manganese nuclei on the A sites in manganese ferrites at temperatures below 4.2 ^∘{}K. The relaxation time of the first spin echo has been measured at T=1.5 and 4.2 ^∘{}K as a function of frequency and applied field. The results are compared to the predictions of the calculated Suhl-Nakamara relaxation rate assuming an inhomogeneously broadened resonance line. The amplitude of the first echo is calculated using the density-matrix approach and it is shown that the first echo, via the Suhl-Nakamura interaction, acts as an effective third rf pulse which causes the refocusing of the spin system resulting in the observed second and third echoes. The relaxation times of these additional echoes are calculated and compared with the observed values.