Magnetic Response and Hyperfine Magnetic Fields at Fe Sites of Sr_3Fe_2MO_9 (M = Mo, Te, W, U) Double-Perovskites

摘要

We have studied the isostructural series of double-perovskites Sr_3Fe_2MO_9 (M = Mo, Te, W, U) by Moessbauer spectrometry and AC susceptibility measurements. The hyperfine structure of Moessbauer spectra at room temperature is attributed to the presence of high spin state Fe~(3+) ions sensing both static and fluctuating magnetic hyperfine fields with different relative areas that depend on M. The magnetically split signal - indistinguishable from the background in the Mo compound spectrum - increases with the Fe-site disorder in the sequence Mo < U < Te < W. The spectra at 77 K demonstrate that the W-perovskite sample is already magnetically fully ordered, while the other three cations suggest ordering temperatures that increase from Te to U to Mo. At 4.2 K all the spectra are completely magnetically split and display hyperfine fields that range from 49 up to 53 T. Coincident with the X-ray and neutron diffraction results, the hyperfine parameters are consistent with Fe atoms centered in oxygen octahedral units, coordinated to different numbers of M-centered octahedra. The AC susceptibility response is χ′_(max) ≈ 3.5 x 10~(-5) emu/g.Oe for the Mo compound and increases for the W, Te and U compounds with values of χ′_(max)1.6 x 10~(-4), 3.0 x 10~(-4), and 9.4 x 10~(-3) emu/g.Oe, respectively. The out-of-phase component, χ″, could only be detected for the U compound. Its frequency dependence displays a shift that denotes a spin-glass-like state arising from the chemical disorder.