check this note on page 15,
**broken link removed**
When you apply a H field (generated by electric current loop) to the ferrite coil, the magnetic flux B will be different when H applied at different time. Say at t=0, You applied H of 1 A/m, B is 0.1 T for example, when at t=0.1s, you keep the H to be 1 A/m, B will change from 0.1 T as time goes on. Since the coil remembers its history. This make things quite complicated at least to me when model it in full-wave solver.