Hysteretic losses of a type‐II superconductor in parallel ac and dc magnetic fields of comparable magnitude

Measurements of 50‐Hz hysteretic losses in Nb3Ge material have been performed in the presence of a dc‐bias magnetic field. The measurements were made by an electrical technique with both the ac and dc magnetic fields parallel to the axis of long Nb3Ge‐coated cylinders. With a fixed ac amplitude h0, the losses are observed to decrease as the dc field H0 is raised from zero, to pass through a minimum, and then to increase monotonically. The position of the minimum is observed to occur when H0=h0−Hs, where Hs is the field at which initial entry of flux into the sample is observed. Minimum depths which reach 30% of the zero‐bias loss are found for samples in which Hs is large (?900 Oe). This ’’minimum’’ effect and other aspects of ac losses in the presence of a dc‐bias field are explained qualitatively by a critical‐state model that takes account of the magnetic field dependences of the critical current density Jc(B) and the surface barrier ΔH (B). The experimental results are also compared with the results o...