A image of the flux line lattice in the magnetic superconductor TmNi2B2C The hexagonal arrangement of magnetic flux lines in pure Nb imaged using neutrons.

Figure 2. Example of a square FLL diraction pattern showing several higher order reflections. This diraction pattern was obtained on TmNi2B2C at T = 1.9 K and H = 3 kOe. The orientation with respect to the crystalline directions is shown by the arrows.

Figure 25. Example of FLL diraction pattern for ErNi2B2C at T = 3.2 K and H = 750 Oe. The arrows show the directions for radial intensity sampling, labelled by the FLL Bragg reflections that they cut.

Figure 27. FLL diraction patterns for ErNi2B2C at 1000 Oe (top), 750 Oe (middle) and 500 Oe (bottom) and T = 3.2 K. The orientation with respect to the crystalline axis is shown in the top and is identical for all three elds. The diraction patterns were obtained using a single crystal orientation, centered with respect to the rst order FLL reflections. 44 Ris{R{1084(EN)

The superconducting and magnetic H-T phase diagram of HoNi2B2C (see ref.18 for more details).

Observation of a square ¯ux-line lattice in the unconventional superconductor Sr2RuO4 Realignment of the flux-line lattice by a change in the symmetry of superconductivity in UPt3

Neutron Scattering Study of the Flux Lattice in YNi2B2C

Final Comments High quality crystals have permitted extensive studies of the magnetic ordering and the form of the vortex lattice in the mixed state for the magnetic superconductor ErNi2B2C. Neutron scattering has been an essential part of this work. The data forces rather strong constraints on any microscopic model of the coexistence of magnetism and superconductivity in this material and as such should prompt further theoretical studies of these effects.

Studies of the Vortex Lattice RNi2B2C can be produced as single crystals of high purity and with minimal pinning of the magnetic flux in the superconducting state. Early studies of the non-magnetic members of this series showed that the high field structure of the vortex lattice was square rather than hexagonal. High resolution experiments presented here demonstrated that there were, in fact, two transitions a “square” to “distorted hex” transition at high field followed by a 45° reorientation transition at lower field. These effects have also been predicted by a “non-local” London model of the superconducting state. Good qualitative agrrement is found with this model. It is essential to understand the mixed state of the non-magnetic compounds before attempting to extract information on the influence of the magnetic order.

Studies of the Vortex Lattice in ErNi2B2C In ErNi2B2C all three possible morphologies of the vortex lattice are observed in different field ranges. There is a considerable temperature dependence of the distortion with the square configuration moving to higher magnetic field as the temperature is increased. The distortion shows a very well- defined dip at the antiferromagnetic ordering temperature. This would suggest a marked decrease in the “square” to “distorted hex” field at TN. A decrease in integrated intensity of the Bragg reflections also occurs in this region, this effect mirrors what happens to HC2. As yet we have not been able to examine the 45° reorientation transition in the same region.

The “Ferromagnetic” Transition in ErNi2B2C Below 2.5 K magnetisation studies suggest that there is a ferromagnetic component associated with the magnetic order. In our small angle neutron scattering we observe “rods” of scattering which seem to coexist with scattering from the vortex lattice. These rods can be detected below 2.1 K. These rods also exist in a zero field cooled sample. In the same temperature range even harmonics of the magnetic modulation appear and increase in intensity while the odd harmonic intensity decreases. Currently, we interpret these facts as indicating that the “squared” magnetic order develops “ferromagnetic” domain walls. The rod scattering corresponds to spin slips at these features.