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, labelledby 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 crystallineaxis is shown in the top and is identical for all three elds. The diractionpatterns were obtained using a single crystal orientation, centered with respect tothe 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 theunconventionalsuperconductor Sr2RuO4

Realignment of the flux-line latticeby a change in the symmetryof superconductivity in UPt3

Neutron Scattering Study of the Flux Lattice in YNi2B2C

Final CommentsHigh quality crystals have permitted extensive studies ofthe magnetic ordering and the form of the vortex latticein the mixed state for the magnetic superconductorErNi2B2C. Neutron scattering has been an essential partof this work.The data forces rather strong constraints on any microscopicmodel of the coexistence of magnetism and superconductivityin this material and as such should promptfurther theoretical studies of these effects.

Studies of the Vortex LatticeRNi2B2C can be produced as single crystals of high purity and withminimal pinning of the magnetic flux in the superconducting state.Early studies of the non-magnetic members of this series showed thatthe high field structure of the vortex lattice was square rather thanhexagonal. High resolution experiments presented here demonstratedthat there were, in fact, two transitions a “square” to “distorted hex”transition at high field followed by a 45° reorientation transition at lowerfield. These effects have also been predicted by a “non-local” Londonmodel of the superconducting state. Good qualitative agrrement is foundwith this model.It is essential to understand the mixed state of the non-magnetic compoundsbefore attempting to extract information on the influence of themagnetic order.

Studies of the Vortex Lattice in ErNi2B2CIn ErNi2B2C all three possible morphologies of the vortex lattice areobserved in different field ranges.There is a considerable temperature dependence of the distortion withthe square configuration moving to higher magnetic field as the temperatureis increased.The distortion shows a very well-defined dip at the antiferromagneticordering temperature. This would suggest a marked decrease in the“square” to “distorted hex” field at TN. A decrease in integrated intensityof the Bragg reflections also occurs in this region, this effect mirrorswhat happens to HC2.As yet we have not been able to examine the 45° reorientation transitionin the same region.

The “Ferromagnetic” Transition in ErNi2B2CBelow 2.5 K magnetisation studies suggest that there is a ferromagneticcomponent associated with the magnetic order.In our small angle neutron scattering we observe “rods” of scatteringwhich seem to coexist with scattering from the vortex lattice. These rodscan be detected below 2.1 K. These rods also exist in a zero field cooledsample.In the same temperature range even harmonics of the magnetic modulationappear and increase in intensity while the odd harmonic intensitydecreases.Currently, we interpret these facts as indicating that the “squared”magnetic order develops “ferromagnetic” domain walls. The rod scatteringcorresponds to spin slips at these features.