Zero-Field Nuclear Spin Relaxation and Resonance Absorption in Superconducting Aluminum

Several field-cycling spin-relaxation experiments have been carried out on ${\mathrm{Al}}^{27}$ nuclei in normal and superconducting aluminum metal. Information has been obtained on magnetic-field motion during transition from the normal to the superconducting state. Audio-frequency nuclear resonance absorption at zero external dc field was observed in the superconducting state from 0.56 to 1.2°K. The dependence of this absorption upon the amplitude and duration of the audio-frequency field indicates that the major absorption occurs in the London penetration region. The resonance absorption signal decreases in proportion to $\ln \alpha$ in the superconducting state, compared to $e^{-\alpha }$ in the normal state, where the absorption parameter $\alpha$ is a function of audio-frequency field intensity, frequency, and time of application. The magnitude of the absorption indicates that the nuclear spin order is concentrated in a sheath of thickness $a_0\approx {10}^{-5\mathrm{}} \mathrm{to} {10}^{-4\mathrm{}}$ cm near the surface of the sample, and is created during the normal-to-superconducting phase change initiated by reducing the external magnetic field from above the critical value to zero. Nuclear order in the bulk of the sample is lost because of the rapid Meissner-effect flux expulsion during the demagnetization. The ordered sheath is a consequence of nearly adiabatic field variation during surface nucleation of the superconducting phase. The temperature variation of $a_0$ was measured. An apparent decrease in $a_0$ near the transition temperature is tentatively attributed to additional audio-frequency nuclear absorption in regions of trapped flux.