Magnetic Phase Transitions in 90 K Superconductors HoBa2Cu3O7-δ and GdBa2Cu3O7-δ Observed by ZF- µ+SR

Abstract
Freezing of holmium magnetic moments was observed below 3 K in a 90 K superconductor HoBa2Cu3O7-δ by zero-field positive muon spin relaxation (ZF-µ+SR) measurements. The ZF-µ+SR function revealed a marked change of µ+ local field from a critically fluctuating regime (4–7 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 GdBa2Cu3O7-δ showed a zero-field spin precession at 2 K, indicating long-range ordering of gadolinium magnetic moments.

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