Spin glass freezing and superconductivity in YBa2(Cu0.94Fe0.06)3O7

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Abstract
We have studied the dynamics of the iron spins in a superconducting YBa2(Cu0.94Fe0.06)3O7 sample by neutron time‐of‐flight measurements. The range of scattering vector q was 0.1<q−1, and the energy resolution 80 μeV. The temperature was varied in the range 1.8 K<Tq dependence of the magnetic intensity mainly reflects the magnetic form factor of iron which shows that the iron spins are almost uncorrelated. The elastic and quasielastic intensities strongly vary with temperature. A spin‐glass‐like freezing is revealed at low temperature by a sharp decrease of the quasielastic intensity, an increase of the elastic, or resolution limited intensity and a minimum in the quasielastic width. The freezing temperature (Tf≂18 K) corresponds to that already determined by a magnetic splitting in Mössbauer experiments. Above Tf, the relaxation of the iron spins in the paramagnetic state is modified by the occurrence of superconductivity. We observe a slight increase of the quasielastic intensity and of the quasielastic width at the superconducting transition (Tc≂74 K), possibly related to the electron‐spin coupling.