Specific heat, magnetic susceptibility and superconductivity of YBa2Cu3O7−δ doped with iron

Abstract

Abstract YBa 2 (Cu 1- x Fe x ) 3 O 7- δ with x =0, 1%, 2% and 4% was prepared by the citrate pyrolysis method. The samples were characterized by X-ray diffraction, micrographs, electron microprobe, a.c. susceptibility near the critical temperature T c , field cooling Meissner effect, d.c. susceptibility from 100 to 250 K and specific heat from 1 to 300 K. These measurements confirm that iron does not segregate. The localized moment is close to 4.0 μ B /Fe-atom and the constant component of the susceptibility increases from 283 to 530 x 10 -6 /mol-f.u. Calorimetric data indicate that for x ⩾2% the superconducting phase is no longer homogeneous. The low-temperature linear term in the specific heat reflects a growing non-superconducting fraction. The apparent contradiction between a high Meissner effect at 4 K and the absence of any definite specific heat jump at T c in the sample with x =4% is understood as an effect of the short coherence length in the presence of repulsive defects. Using the latter tetragonal sample as a normal reference compound, the shape of the specific heat difference C s - C n near T c for the undoped sample ( calorimetric transition width: 1.3 K, δC / T c =49 mJ /( K 2 mol )) is discussed.