Observation of spin modulated magnetic structure at Bi- and Fe-sites in BiFeO3 by nuclear magnetic resonance

NMR spectra were measured in Fe-57 enriched ferroelectric-antiferromagnet BiFeO3 with space modulated magnetic structure of cycloidal type, which has large period lambda = 620 +/- 20 Angstrom incommensurate with the lattice parameter. This structure leads to space modulation of local field on Fe-57 nuclei as well as space periodic spin-spin relaxation and specific NMR line shape with frequency variable local width. Space modulated magnetic structure induces also an appearence of hyperfine magnetic field H-hf of 11-12 T at Bi position which has been observed by zero-field Bi-209 NMR. Complicated Bi-209 spectrum consists of various lines with asymmetric line shape. It was simulated assuming that Hhf rotates in space following cycloidal magnetic structure with the same period as at Fe-site and anharmonic parameter 0.95.