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We describe how the ferromagnetic spin echo technique may be used to make a direct and microscopic measurement of the motion of flux lines in a superconductor under the influence of an applied current. Our experimental results agree well with theory and indicate that both the value and distribution of velocities of flux lines may be measured.

The magnetic properties of the bilayer ruthenate Sr3Ru2O7 as investigated by neutron scattering are reviewed. No long range magnetic order is observed down to 40 mK, but instead a metamagnetic transition which is induced by a moderate magnetic field at low temperatures. In zero field, the spectrum of the spin fluctuations reveals that at low temperature the magnetic correlations in this material are antiferromagnetic in nature. The wavevectors at which the signal is strongest are incommensurate and appear to be determined by Fermi surface nesting. The low energy scale of the fluctuations suggests that they are intimately related to the metamagnetic transition.

We describe briefly the mixed state of type II superconductors in a magnetic field and the relationships between the structure of this state and important properties of the superconductor. We show how the “field contrast” in the mixed state may be measured by neutron diffraction and describe the first observation by this technique of flux lines in the high-Tc superconductor YBCO. We compare the information obtainable from neutron and muon spin rotation measurements of mixed state properties and show that these two techniques are complementary.