MOSSBAUER STUDIES OF SUPERCONDUCTING LA2-XCA1+XCU2O6 DOPED WITH FE-57

Mossbauer spectroscopy and magnetization measurements were employed to study the superconducting oxide (La2-xCa1+x(Cu1-yFey)2O6)-Fe-57 (x=0.18,y=0.01) prepared under oxygen pressures of 2 atm (non-superconducting), 50 atm (T(c)=9 K) and 400 atm (T(c)=43 K). In addition to powder samples, samples with aligned crystal direction were used in a Mossbauer study for evaluating the direction and the sign of the electric field gradient. From the paramagnetic spectra at room temperature, it was concluded that, in the superconducting samples, a larger part of the Fe atoms in Cu sites have an excess oxygen neighbor between two CuO2 planes compared to the non-superconducting sample. The excess oxygen possibly induces superconductivity by increasing holes doped in the CuO2 plane. The measurements at low temperatures showed that the samples prepared in 2 and 50 atm O2 are magnetically ordered with a transition temperature of about 15 K, consistent with the results of magnetization measurements. In contrast, the magnetic ordering was largely suppressed even at 4.2 K in the superconducting sample prepared in 400 atm O2.