COEXISTENCE OF SUPERCONDUCTIVITY AND MAGNETIC ORDER IN YBA2CU4O8

Fe-57 Mossbauer spectroscopy, magnetic susceptibility and powder x-ray-diffraction measurements were used to study superconductivity and magnetic order in YBa2(Cu1-xFex)4O8+delta. T(c) is decreasing with x, disappearing for x > x(c) almost-equal-to 0.04. For x < x(c) iron substitutes Cu, predominantly in the Cu(1) site exhibiting a single quadrupole Mossbauer spectrum at 90 K. For x > x(c) magnetic order is observed in the Cu(2) site, T(N) = 380(5) K for x = 0.1 and H(eff) (Cu(2), 4.2 K)=510(2) koe. However, the most surprising discovery is that for x-0.025, for which T(c) = 27(2) K, the Fe in the Cu(1) site orders magnetically at T(N) = 30(2) K and H(eff) (Cu(1), 4.2 K)=461(2) kOe. The coexistence and competition between superconductivity and magnetic order in the Cu(l) and Cu(2) sites in YBa2Cu4O8 are discussed in terms of the previously observed phase diagrams for Y1-xPrxBa2(Cu1-yFey)3Oz.