LOW-TEMPERATURE FERROMAGNETISM IN LA4BA2CU2O10

THERE have been a few reports of ferromagnetic behaviour in the high-transition-temperature (high-Tc) 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-Tc 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 1.66μB per copper ion, with a saturation magnetic moment of 0.95 μB 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 LaBa2Cu3O7-y. © 1990 Nature Publishing Group.