Magnetic phase transitions in 90-K superconductors Ho1Ba2Cu3O(7-delta) and Gd1Ba2Cu3O(7-delta) observed by ZF-muon(+)SR

Freezing of holmium magnetic moments was observed below 3 K in a 90-K superconductor Ho1Ba2Cu3(sub 7-delta) by zero-field positive muon spin relaxation (ZF-mu+ SR) measurements. The ZF-mu+SR function revealed a drastic change of mu+ local field from a critically fluctuating regime (4 to 7 K) to a quasi-static regime below 3 K (of nearly Gaussian distribution). The observation indicates that holmium magnetic moments are short-range ordered around 2 K. In contrast, a 90-K superconductor Gd1Ba2Cu3O(sub 7-delta) showed a zero-field spin precession at 2 K, indicating a long-range ordering of gadolinium magnetic moments.