  In the current work, we report the investigation of RbLn$_2$Fe$_4$As$_4$O$_2$
(Ln = Tb, Dy and Ho) and RbFe$_2$As$_2$ by $^{57}$Fe M\"ossbauer spectroscopy.
Singlet pattern has been observed for all the samples indicating the absence of
static magnetic ordering down to 5.9\,K on the Fe sublattice. The observed
intermediate value of the isomer shift confirms the effective self charge
transfer effect for the studied superconductors. Debye temperatures of these
samples have been determined by the temperature dependence of the isomer shift.
Most importantly, we observe different spectral line width broadening behaviors
for samples with different rare earth elements, and no line width broadenings
for samples Ln=Tb and Ho. These results suggest that the observed magnetic
fluctuations at the Fe site might be sensitive to the magnetic behaviors of the
rare earth magnetic moments.
