Abstract:

A comparative study of powder and bulk specimens of MgB2 by electron spin resonance (ESR) verifies the presence of intense conduction electron spin resonance (CESR) in the normal state. A low concentration paramagnetic center stemmimg from the initial amoprhous boron powder along with traces of Fe3+ impurities are identified in the ESR spectra. Intense microwave absorption, that distorts CESR below Tc, is observed in fine powders implying enhanced microwave dissipation due to the viscous flux motion.

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