MUON-SPIN ROTATION STUDY OF ANTIFERROMAGNETIC ORDER IN HYDROGENATED YBA2CU4O8 - EVIDENCE FOR A LOCAL STRUCTURAL-CHANGE IN THE VICINITY OF T-C

Zero-field muon-spin rotation (ZF-mu SR) experiments were performed on hydrogen-doped YBa2Cu4O8. The observation of well defined muon-spin p recession frequencies unambiguously demonstrates the presence of local magnetic order. From the magnitude and the temperature dependence of the mu SR frequencies we conclude that YBa2Cu4O8 has an antiferromagne tically ordered counterpart similar to that observed in YBa2Cu3O6. A j ump of one of the observed mu SR frequencies in the temperature around 80 K is discussed in terms of a local structural change of the apical oxygen to which the muon is bound.