Soft Magnetic Switching in a FeSr₂YCu₂O_7.85 Superconductor with Unusually High Iron Valence

Ozone oxidation has allowed the stabilization of a very high iron oxidation state in the FeSr₂YCu₂O_7.85 cuprate, in which a long-range magnetic ordering of the high valent iron cations coexists with the superconducting interactions (magnetic ordering temperature T_N = 110 K > superconducting critical temperature T_c = 70 K). The somewhat unexpected A-type AFM structure, with a μ(Fe) ∼ 2 μ_B magnetic saturation moment associated with the hypervalent iron sublattice, suggests an unusual low spin state for the iron cations, while the low dimensionality of the magnetic structure results in a soft switching toward ferromagnetism under small external magnetic fields. The role of the crystal structure and of the high charge concentration in the stabilization of this unusual electronic configuration for the iron cations is discussed.