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

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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.