Low-temperature ferromagnetism in La₄Ba₂Cu₂O₁₀

Abstract

THERE have been a few reports of ferromagnetic behaviour in the high-transition-temperature (high-T_c) copper oxide superconductors. For example, Na-doped La₂CuO₄ annealed in an inert gas, which is superconducting below 30 K, exhibits a ferromagnetic-like transition near 10 K (ref. 1), although the origin of this behaviour is not clear. In early studies we found that impure samples of LaBa₂Cu₃O₇ prepared under a reduced oxygen atmosphere showed evidence of ferromagnetic-like behaviour near 5 K. We have now identified the origin of the ferromagnetic behaviour as being due to the presence of an impurity phase, La₄Ba₂Cu₂O₁₀ (the 422 phase). This phase is an insulator and is homologous to the 'green' phase (Y₂BaCuO₅) found as an impurity in the Y–Ba–Cu–O system. The 422 phase contains square-planar CuO₄ units as do the high-T_c copper oxide superconductors, but in this compound each of these units lies perpendicular to those around it. Magnetization measurements indicate an effective magnetic moment of 1.66µ_B per copper ion, with a saturation magnetic moment of 0.95 µ_B per Cu. These results suggest that the compound is a localized ferromagnet in which the copper ions are magnetically ordered. We suggest that the ferromagnetic coupling may originate in the orthogonality of the CuO₄ units. There may be analogies with K₂CuF₄, which is ferromagnetic below 6.5 K (ref. 2) as a result of two-dimensional ordering of the d hole in Cu²⁺ arising from the Jahn–Teller interaction³. The 422 phase could provide flux-pinning centres in bulk superconducting LaBa₂Cu₃O_7–y.