8. "Magnetic Response and Hyperfine Magnetic Fields at Fe Sites of Sr3Fe2MO9 (M = Mo, Te, W, U) Double-Perovskites"

L. A. BAUM; S. J. STEWART; ROBERTO CARLOS MERCADER; J. M. GRENÉCHE

Hyperfine Interactions

Año: 2004 vol. 156 p. 157 - 163

0304-3843

We have studied the isostructural series of double-perovskites Sr3Fe2MO9 (M = Mo, Te, W, U) by Mössbauer spectrometry and AC susceptibility measurements. The Mössbauer spectra at room temperature display Fe3+ signals belonging to atoms sensing static and relaxing magnetic 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 split and display hyperfine fields 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 c’max»3.5x10-5emu/g.Oe for the Mo compound and increases for the W, Te and U compounds with values of c’max1.6x10-4, 3.0x10-4, and 9.4x10-3emu/g.Oe, respectively. The out-of-phase component, c", 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.