Rotational magnetic measurements of vortex flux pinning in Nb at 4.2 K

Magnetization vector (M) measurements were made on a thin disk of compact Nb powder at 4.2 K. Our results reveal that the vortex-flux density B consists of two components: B_R a rotational vortex-flux density which rotates rigidly with the sample, and B_F a frictional vortex-flux density which rotates frictionally relative to the sample and stays at a fixed angle (θ_F) relative to the applied field H. This indicates the existence of a broad distribution in the strength of vortex pinning centers. Moreover, the average pinning torque per vortex (τ_p/μ) which is represented by the experimental value Hsin(θ_F) decreases as H increases (τ_p/μ = Hsin(θ_F), where μ is the magnetic moment per vortex). This is consistent with the collective-pinning process of vortex bundling.