Coexistence and competition between superconductivity and magnetic order in YBa₂Cu₄O₈

Magnetic susceptibility and ⁵⁷Fe Mossbauer spectroscopy were used to study superconductivity and magnetic order in YBa₂(Cu_1-xFe_x)₄O_8+δ. T_c is decreasing with x, disappearing for x > x_c 0.04. For x < x_c the crystal structure is orthorhombic and Mossbauer spectra show that iron substitutes Cu, predominantly in the Cu(1) site. Moreover, for the superconducting compound with x=0.025 and T_c=27 (2) K, the Cu(1) site orders magnetically at T_N=30(2) and H_eff (Cu(1), 4.2 K)=461(2) kOe, thus superconductivity and magnetism coexist. For x>x_c the crystal structure changes to tetragonal, and 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. The coexistence of superconductivity and magnetic order in the Cu(1) site and the competition between superconductivity and magnetic order in the Cu(2) sites in YBa₂Cu₄O₈ are discussed in terms of the previously observed phase diagrams for Y_1-xPr_xBa₂(Cu_1-yFe_y)₃O_7+δ.