Pattern of Current Flow in Nonideal Type-II Superconductors in Longitudinal Magnetic Fields. I

The longitudinal magnetization of a wire of a nonideal type-II superconductor in the presence of a stationary longitudinal magnetic field $H_a$ is seen to oscillate between paramagnetic and diamagnetic as a conduction current is impressed and removed. When a conduction current I near the critical value $I_c$ is present, the longitudinal magnetization is paramagnetic and attains a maximum. With $I=0\mathrm{}$ after passage of a conduction current $I\lesssim I_c$, the longitudinal magnetization is diamagnetic. This diamagnetic magnetization appreciably exceeds the Meissner-effect diamagnetism exhibited by the sample at corresponding fields. This paramagnetism and diamagnetism have been investigated throughout the range $0<\mathrm{}{H}_a<H_{c2\mathrm{}}$. The variation of the longitudinal magnetization through paramagnetic and diamagnetic states as the conduction current is isothermally varied through a full cycle starting at $I\lesssim I_c$ has also been measured. All of the observations are satisfactorily described by a simple model which requires that macroscopic conduction and induced currents flow in helical paths which seek to preserve the existing magnetic-flux configuration and maintain a constant inward-directed Lorentz-force density throughout the specimen in the presence of a stationary longitudinal magnetic field.