Specific features of magnetic properties of rare-earth ferroborates Sm$_{1-x}$La$_x$Fe$_3$(BO$_3$)$_4$

Single crystals of rare-earth ferroborates Sm$_{1-x}$La$_x$Fe$_3$(BO$_3$)$_4$ ($x$ = 0, 0.5, 0.75) have been grown by the flux method. Their magnetic properties have been investigated in the temperature range from 2 to 300 K in magnetic fields up to 9 T. It has been found that the substitution of nonmagnetic ions La$^{3+}$ for magnetic ions Sm$^{3+}$ in Sm$_{1-x}$La$_x$Fe$_3$(BO$_3$)$_4$ ferroborates leads to an increase in the magnetic susceptibility of the compound. The behavior of the magnetic properties of the grown single crystals has been explained qualitatively in the framework of the phenomenological model. The parameters of $d$–$d$ and $d$–$f$ exchange interactions have been estimated.