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.

The evolution of the electronic structure and magnetic properties with Co substitution for Fe in the solid solution Fe${}_{1-x}$Co${}_x$Ga${}_3$ was studied by means of ab initio band-structure calculations and nuclear spin-lattice relaxation $1/T_1$ of the ${}^{69,71}$Ga nuclei. The ${}^{69,71}(1/T_1)$ was studied as a function of temperature in a wide temperature range of 2–300 K for the concentrations $x=0.0$, 0.5, and 1.0. In the parent semiconducting compound FeGa${}_3$, the temperature dependence of the ${}^{69}(1/T_1)$ exhibits a huge maximum at about $T\sim 6$ K indicating the existence of in-gap states. The opposite binary compound, CoGa${}_3$, demonstrates a metallic Korringa behavior with $1/T_1\propto T$. In Fe${}_{0.5}$Co${}_{0.5}$Ga${}_3$, the relaxation is strongly enhanced due to spin fluctuations and follows $1/T_1\propto T^{1/2}$, which is a unique feature of weakly and nearly antiferromagnetic metals. This itinerant antiferromagnetic behavior contrasts with ab initio band-structure calculations, where a ferromagnetic state with an ordered moment of 0.5 ${\mu }_B$/f.u. is predicted. The results are discussed in terms of the interplay between the localized and itinerant magnetism including in-gap states and spin fluctuations.