~(57)Fe M?ssbauer study of new multiferroic AgFeO_2

摘要

The present work reports results of the ~(57)Fe M?ssbauer measurements on AgFeO_2 powder sample recorded at various temperatures including the points of both magnetic phase transitions. The ~(57)Fe M?ssbauer spectra of AgFeO_2 measured in the paramagnetic range (T > T_(N1)) consist of one quadrupole doublet with rather high quadrupole splitting of ?_(300K) = 0.66 ± 0.01 mm/s for Fe~(3+) ions. In order to predict the sign of electric field gradient (EFG) at ~(57)Fe nuclei, we calculated the lattice contribution to the electric field gradient (EFG) at ~(57)Fe nuclei, which emphasized the importance of the dipolar contributions, with resultant oxygen polarizabilities in the range of α_O = 0.83 ?~3, in agreement with the results obtained previously for other delafossite-like oxides. In the temperature range of T_(N2) < T < T_(N1), M?ssbauer spectra gave clear evidence for the existence of a distribution of the hyperfine magnetic fields H_(hf) at ~(57)Fe nuclei. We present the results of a model fitting of the spectra based on an assumption of the cycloid magnetic structure of AgFeO_2 atT < T_(N2). The obtained data were analysed in comparison with published data on M?ssbauer studies of oxide multiferroics.