Effects of magnetic and non-magnetic doping on the vortex lattice in MgB${}_2$

Abstract

Using small-angle neutron scattering we have studied the vortex lattice in superconducting MgB${}_2$ with the magnetic field applied along the $c$-axis, doped with either manganese or carbon to achieve a similar suppression of the critical temperature. For Mn-doping, the vortex lattice phase diagram remains qualitatively similar to that of pure MgB${}_2$, undergoing a field-and temperature-driven $30^{\circ }$ rotation transition, indicating only a modest effect on the vortex-vortex interaction. In contrast, the vortex lattice rotation transition is completely suppressed in the C-doped case, likely due to a change in the electronic structure which affects the two-band/two-gap nature of superconductivity in MgB2. The vortex lattice longitudinal correlation length shows the opposite behavior, remaining roughly unchanged between pure and C-doped MgB${}_2$ while it is significantly reduced in the Mn-doped case. However, the extensive vortex lattice metastability and related activated behavior, observed in conjunction with the vortex lattice transition in pure MgB${}_2$, is also seen in the Mn doped sample. This shows that the vortex lattice disordering is not associated with a substantially increased vortex pinning.

To appear in the Journal of Applied Crystallography for the special issue "Magnetic Small-Angle Neutron Scattering - from nanoscale magnetism to long-range magnetic structures"

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