Local magnetic order in superconducting YBa₂(Cu_1-xM_x)₃O₇ (M=Co,Fe)

We have examined the local magnetic order which is introduced into YBa₂Cu₃O₇ by substituting low levels (1–4 %) of Co or Fe. The order is evidenced through the appearance of an internal field on the spin of ¹⁷⁰Yb³⁺ Mössbauer probes diluted at the Y³⁺ sites. Low substitution levels only marginally influence the superconducting properties, yet introduce local magnetic order over the whole sample volume. The local order concerns the Cu sublattices and fluctuates with rates that depend on the substituted ion, its concentration, and the temperature.