Domain structure dynamics in ferromagnetic CrBr3 studied by nuclear magnetic resonance

Nuclear magnetic resonance of Cr-53 in ferromagnetic CrBr3 in external magnetic fields was studied at 4.2 K. In zero field, a well-resolved NMR triplet (at 54.434 +/- 0.297 MHz) was observed from domains, a single resonance (54.86 MHz) from domain walls, and a weak signal from Bloch lines (54.0 MHz). The NMR broadening, which is observed when the saturation field is achieved (3.08 kOe for the field parallel to the c(6)-axis), is explained by the inhomogeneous penetration of the field into the sheet-formed crystal. The signal intensities at different RF polarizations indicate that the NMR enhancement coefficient is determined by the rotation of domain magnetization in the case of domains, and by the oscillatory motion of Bloch lines in the case of Bloch lines and domain walls. The ferromagnetic system does not shield the nuclei from the external magnetic field applied perpendicular to the c(6)-axis. Such a field removes the NMR degeneracy in domain walls, thus forming a broad absorption band with distinct maxima. This band cannot be explained only in terms of the external field and the induced dipolar fields. The Cr+3 magnetization is assumed to depend, besides on orientation, on static gradients, which may be due to the spin-wave excitation and zero oscillations of domain walls and Bloch lines.