Hysteresis losses and magnetic phenomena in ribbons of type II superconductors in the noncollinear regime

Abstract

We have measured hysteresis losses, W(h0, hb, Hb0), in ribbons of VTi and Nb subjected to oscillating magnetic fields of various amplitudes h0 directed transverse to a static bias magnetic field Hb0, hence the name, noncollinear regime. A second bias field hb along h0 may also be introduced. Hb0 and h0 are directed along the length and width of the ribbon respectively or vice versa (90° rotation). The families of data curves are compared with the predictions of double critical state models where (i) dB/dx=±FP(B)/B governs the critical gradients of the magnetic induction and (ii) dθ/dx=±FP(B)F(θ)/B2, the spatial variation of the orientation of the sheets of flux lines, with θ=0 along Hb0. The effect on W(h0,hb,Hb0) of choosing F(θ)=1/tan θ, k and k/(1+‖θ‖)2 is illustrated and the latter is seen to yield the best agreement. The locus of the magnetization along the length and width of the samples, 〈M∥〉 and 〈M⊥〉, have been monitored separately and simultaneously during the cycles of h0(t) for various h0 and Hb0 with hb=0 or h0. These observations, in particular the curves of the magnetization along Hb0, constitute severe tests of any model. The corresponding curves generated by the model using F(θ)=k/(1+‖θ‖)2 are presented. The physical origin of F(θ) is explored.