Observation of an extended region of magnetic reversibility in Nb and NbSe2.

Vibrating-reed measurements of the flexural resonances of thin cantilevered samples of polycrystalline Nb foils and quasi-two-dimensional NbSe{sub 2} single crystals reveal extended regions of temperature {ital T} and magnetic field {ital H} for reversible flux-line (FL) motion when the long reed dimension is perpendicular to the applied field. An irreversibility line,'' defined by the peak temperature of the reciprocal quality factor, is found to be frequency (mode)-independent for resonant frequencies 5{times}10{sup 2}{lt}{ital f}{lt}5{times}10{sup 4} Hz, implying that the time scale of FL pinning is extremely {ital T} and {ital H} dependent. We conclude that observation of an extended region of magnetic reversibility is not restricted to high-{ital T}{sub {ital c}} or extremely anisotropic materials, and depends upon the geometric aspects of samples with respect to the applied field direction.