Flux pinning in (Re)Ni2B2C superconductors

Global and local magnetisation and transport resistivity measurements have been made on single crystals of the magnetic superconductor HoNi2B2C. Re-entrant or suppressed superconductivity is observed due to the onset of an a-axis incommensurate ordering of the Ho moments resulting in a strong suppression of B-c2 between temperatures of about 5 K and 6 K. Hysteresis in the global magnetisation is rather weak and asymmetric at all temperatures below T-c (similar to 8.9 K) suggesting that bulk pinning is weak and that surface and/or geometrical barrier effects are important. The form of the hysteresis curves at different temperatures is examined using a scaling analysis allowing regimes of similar pinning or surface barrier determined behaviour to be identified. Increased symmetry of the hysteresis is observed between 5 K and 6 K, coinciding with a sharp decrease in the flux flow resistivity. This suggests an increase in bulk pinning between these temperatures. We support our results with recent local magnetisation measurements made using a miniature linear Hall probe array. Measured flux profiles confirm the dominance of surface barriers over most of the phase diagram below T-c, except in the narrow temperature range between similar to 5 K and 6 K where bulk pinning becomes important. The data presented show that the competing effects of superconducting and magnetic order also have a profound influence on the vortex pinning properties of the superconducting state