Mössbauer spectroscopy in high T_c superconductors

Mossbauer spectroscopy has recently been applied to study the new high T_c superconducting compounds RBa₂Cu₃O_z, using isotopes of rare earths mainly¹⁵⁵Gd and¹⁵¹Eu, and⁵⁷Fe, with different amounts of Fe ions replacing Cu. It was shown that magnetic moments on the rare earth site do not interfere with superconductivity. Fe at low concentrations (<1%) was found to replace Cu mainly in the Cu(1) site, and the Mossbauer spectra reveal different quadrupole doublets-fol lowing the different oxygen coordination around the Fe ion. The change of the relative intensities of the different doublets with z can easily be followed. For higher iron concentrations, it seems that increasing amounts of iron replace Cu in the Cu(2) site. For z<6.5, the iron reflects the magnetic ordering of Cu in this site, and the ordering temperature as function of z can be obtained. The agreement between neutron diffraction and Mossbauer measurements prove that Fe is a good probe for the magnetic behaviour of the Cu(2) ions. At low temperatures, Fe Mossbauer spectra of Fe in the Cu(1) site are also magnetically broadened, for all z. Superconducting-magnetic phase diagrams are also obtained in Y_1-x Pr_x Ba₂ Cu₃O_z as function of x and z. For z=7.1, T_N changes sharply with x. T_N=300, 230 and 35 K for x=0.8, 0.6 and 0.4 respectively, whereas for z=6.1 T_N changes very little with x. Mossbauer measurements performed on 5 at %⁵⁷Fe doped in CalaBaCu₃O_z show that most of the la occupy the Ba site. For z=7 about half the iron in the Cu(2) sites are magnetically ordered, with H_eff=520 kOe and T_N=400 K, even though the sample is superconducting with T_c=35 K. The possibility of coexistence between superconductivity and magnetic order in these systems will be discussed.