Spatially modulated magnetic structure of AgFeO$_2$: Mцssbauer study on $^{57}$Fe nuclei

The results of the Mцssbauer study of ferrite AgFeO$_2$ manifesting multiferroic properties (at $T\leq T_{\text{N}2} $) have been presented. The hyperfine interaction parameters of $^{57}$Fe nuclei have been analyzed in a wide temperature range including the points of two magnetic phase transitions ($T_{\text{N}2}\approx (7$–$9)$ K and $T_{\text{N1}}\approx (15$–$16)$ K). It has been shown that the Mussbauer spectra of the $^{57}$Fe nuclei are sensitive to the variations of the character of the magnetic ordering of Fe$^{3+}$ ions in the studied ferrite. The results of the model identification of a series of spectra ($4.7$ K ${}\leq T\leq T_{\text{N2}}$) under the assumption of the cycloid magnetic structure of ferrite AgFeO$_2$ have been presented. The analysis of the results has been performed in comparison with the literature data for other oxide multiferroics.