Nuclear magnetic resonance in noncollinear antiferromagnet Mn$_3$Al$_2$Ge$_3$O$_{12}$

The $^{55}$Mn nuclear magnetic resonance spectrum of noncollinear 12-sublattice antiferromagnet Mn$_3$Al$_2$Ge$_3$O$_{12}$ has been studied in the frequency range of 200–640 MHz in the external magnetic field $\mathbf H\parallel[001]$ at $T = 1.2$ K. Three absorption lines have been observed in fields less than the field of the reorientation transition $H_c$ at the polarization $\mathbf h\parallel\mathbf H$ of the rf field. Two lines have been observed at $H>H_c$ and $\mathbf h\perp\mathbf H$. The spectral parameters indicate that the magnetic structure of manganese garnet differs slightly from the exchange triangular 120-degree structure. The anisotropy of the spin reduction and (or) weak antiferromagnetism that are allowed by the crystal symmetry lead to the difference of $\approx3\%$ in the magnetization of sublattices in the field $H<H_c$. When the spin plane rotates from the orientation perpendicular to the $C_{3}$ axis to the orientation perpendicular to the $C_4$ axis, all magnetic moments of the electronic subsystem decrease by $\approx2\%$ from the average value in the zero field.