Study of the magnetic properties of neodymium and samarium iron borates by the method of erbium spectroscopic probe

Iron borates NdFe$_3$(BO$_3$)$_4$ and SmFe$_3$(BO$_3$)$_4$ activated with 1% erbium, with a huntite structure (space symmetry group $R$32) were investigated by the method of erbium spectroscopic probe. From an analysis of the temperature dependence of the transmission spectra in the region of the $^4I_{15/2}$ $\to$ $^4I_{13/2}$ transition in the Er$^{3+}$ ion, it was found that both studied compounds order antiferromagnetically at $T_{\mathrm{N}}\approx$ 33 K into an easy-plane magnetic structure. No other phase transitions were found.