Effect of spatial spin modulation on the relaxation and NMR frequencies of 57Fe nuclei in a ferroelectric antiferromagnet BiFeO3

The 57Fe NMR spectra of the 57Fe-enriched (95.43%) ceramics of a ferroelectric antiferromagnet BiFeO3 with a spatially modulated magnetic structure have been studied. It is established that a cycloidal spin modulation in BiFeO3 causes a spatial modulation of the spin-spin relaxation rate along the magnetic cycloid period and results in inhomogeneous broadening of the local NMR lineshape. It is also found that the local magnetic moments of Fe ions in various parts of the cycloid depend differently on the temperature, which is indicative of a difference in the spin wave excitation. The observed phenomena can be explained in terms of the Shul-Nakamura indirect nuclear interaction which becomes effective at high concentrations of the magnetoactive nuclei and low temperatures. Similarity of the obtained experimental results to the regularities of NMR in the Bloch walls is discussed.