$^{57}$Fe Mössbauer spectroscopy study of SmFe$_{3-x}$Al$_x$(BO$_3)_4$ ($x = 0$–$0.28$) multiferroics

SmFe$_{3-x}$Al$_x$(BO$_3)_4$ multiferroic single crystals with ($x = 0$–$0.28$) have been studied in the temperature range $T= 3.8 {-}298\,$К using $^{57}$Fe Mössbauer spectroscopy and X-ray diffraction analysis. An increase in the Mössbauer hyperfine quadrupole splitting with the content of aluminum $x$ impurities has been found. For all studied samples, the Mössbauer Debye temperatures $\Theta_M$ of iron ions have been determined in good agreement with the values for iron ions calculated from X-ray diffraction data. It has been shown that the low-temperature Mössbauer spectral lines of single crystals doped with Al in a magnetically ordered state are broadened compared to the corresponding lines of the undoped SmFe$_3$(BO$_3)_4$ ferroborate; this broadening is best approximated within the multilevel spin relaxation model. The Néel temperatures $T_N$ of the magnetic phase transition have been determined for all studied SmFe$_{3-x}$Al$_x$(BO$_3)_4$ samples. It has been (found that the Néel temperature $T_N$ decreases nonlinearly with an increase in the content $x$ of aluminum, and the type of three-dimensional magnetic ordering changes from planar to Ising.