Structural and magnetic properties of RSr₂Fe₃O ₉ (R=La, Y, Pr, and Gd)

We have investigated the RSr₂Fe₃O₉ (R = La, Y, Pr, and Gd) materials by several experimental techniques, including x-ray and neutron diffraction, magnetic susceptibility, and Mossbauer spectroscopy measurements. All materials studied are single phase and crystallize in the hexagonal perovskite structure. Magnetic susceptibility studies reveal that, for R = La and Pr, the Fe ions order antiferromagnetically at about T_N = 190 K. Short range magnetic correlations are observed up to 250 K. For R = La and Pr, ⁵⁷Fe Mossbauer studies reveal two inequivalent magnetic sextets below 190 K, with the area ratio 2:1. Above T_N, one singlet is observed with an isomer shift value typical to Fe⁴⁺. For R = Y and Gd, the magnetization curves do not lend themselves to easy determination of the magnetic transition due to an extra magnetic phase which exists up to 350 K. This phase is a result of deficiency of oxygen. The ⁵⁷Mossbauer spectra at 300 K indicate two superimposed singlet lines, and contain a small magnetic sextet. There is no indication that the R sublattice in RSr ₂Fe₃O₉ (R = Pr and Gd) orders magnetically down to 1.5 K.