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

Abstract

Ozone oxidation has allowed the stabilization of a very high iron oxidation state in the FeSr 2 YCu 2 O 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.

Metal-to-Insulating Transition in the Perovskite System YSr2Cu2FeO8−δ(0 < δ < 1) Modeled by DFT Methods

“…Neutron diffraction results and Mossbauer spectra of YSr 2 Cu 2 FeO 7.85 yielded a magnetic moment μ 0 (Fe) = 1.7−2 μ B per Fe atom that the authors attributed to Fe 4+ ion in low-spin configuration (t 2g 4 e g 0 ). 24 No magnetic moment was detected for Cu 3+/2+ ions. In this regard, although quantitative comparison of magnetic moments with experiments is complicated due to the observed Cu/Fe antisite mixing 24,50 and differences in the oxygen content, the GGA, and SCAN methodologies offer a more appropriate description than the GGA + U.…”

“…The crystal structure and superconducting behavior of the compound with the highest oxygen content, YSr 2 Cu 2 FeO 7.86 , has previously been reported. 24 In this work, the crystal structure of YSr 2 Cu 2 FeO 7.08 is refined by NPD. Table 1 lists the results of the Rietveld refinement of the NPD pattern of YSr 2 Cu 2 FeO 7.08 (Figure 2).…”

“…24 No magnetic moment was detected for Cu 3+/2+ ions. In this regard, although quantitative comparison of magnetic moments with experiments is complicated due to the observed Cu/Fe antisite mixing 24,50 and differences in the oxygen content, the GGA, and SCAN methodologies offer a more appropriate description than the GGA + U.…”

“…45 In the present work, we have performed calculations using the idealized stoichiometric compositions YSr 2 Cu 2 FeO 7 and YSr 2 Cu 2 FeO 8 and perfect Cu/Fe ordering within the crystal structures. The crystallographic model of YSr 2 Cu 2 FeO 8 oxide has been constructed taking the initial positions from YSr 2 Cu 2 FeO 7.86 (ICSD file 11514), 24 considering a ferromagnetic 2 × 2 × 1 superstructure of the tetragonal a p × a p × 3a p unit cell shown in Figure 1a (that is, Y 4 Sr 8 Cu 8 Fe 4 O 32 composition). For YSr 2 Cu 2 FeO 7 oxide, the 6a p × √2a p × √2a p (S.G. Ima2) unit cell has been used for the calculations (Y 4 Sr 8 Cu 8 Fe 4 O 28 ), corresponding to an ideal arrangement of the tetrahedral chains (Figure 1b).…”

“…The highly oxidized term YSr 2 Cu 2 FeO 7.85 obtained experimentally in ozone flow, with Fe in the unusual 4+ formal oxidation state and mixed Cu 3+ and Cu 2+ valences, is a metallic oxide with a superconducting transition at T c = 70 K. 24 The compound presents magnetic ordering arising from the ferromagnetic (FR) interactions below T N = 110 K between Fe 4+ cations with parallel in-plane spins in the FeO 2−δ layers coupled with a soft character of the antiferromagnetic (AFM) interactions between layers. 24 The low oxygen content terms YSr 2 Cu 2 FeO 7.08 , 27 YSr 2 Cu 2 FeO 7.11 , 28 and YSr 2 Cu 2 FeO 7.04 , 26 prepared under reducing conditions, are insulators with mainly Fe 3+ and Cu 2+ . In between those extremes, air-synthesized YSr 2 Cu 2 FeO 7.56 , mixed valence Fe 3+ /Fe 4+ and Cu 2+ /Cu 3+ oxide, exhibits an interesting electrochemical behavior associated with catalytic activity in the oxygen reduction reaction (ORR), being a potential air electrode for solid oxide fuel cells (SOFCs).…”