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要旨:
The magnetic and superconducting properties of 14% Ir-doped EuFe2As2 are
studied by means of dc and ac magnetic susceptibilities, electrical
resistivity, specific heat and Eu-151 and Fe-57 M ssbauer spectroscopy
(MS) measurements. Doping of Ir in EuFe2As2 suppresses the Fe spin
density wave transition and in turn gives rise to high temperature
superconductivity below 22.5 K with a reentrant feature at lower
temperature. Magnetization and Eu-151 M ssbauer data indicate that the
Eu2+ spins order magnetically below 18 K. Fe-57 MS studies show a line
broadening in the absorption spectra below 18 K due to transferred
hyperfine field from the magnetically ordered Eu sublattices. A
pronounced lambda-shape peak in the specific heat supports a
second-order phase transition of Eu2+ magnetic ordering with a strong
ferromagnetic component, as confirmed by the magnetic field dependences
of the transition. For a single crystal, the in-plane resistivity
(p(ab)) ab and out-of-plane susceptibility (chi(c)) show superconducting
transitions with zero resistance and diamagnetism, respectively. But the
in-plane susceptibility (chi(ab)) does not show any diamagnetic
shielding against external fields. The observed non-zero resistance in
the temperature range 10-17.5 K, below the superconducting transition
temperature, suggests the possible existence of a spontaneous vortex
state in this superconductor.
