Metastable Vortex Lattice Dynamics in MgB₂ Studied by Small Angle Neutron Scattering

Using the technique Small Angle Neutron Scattering (SANS), I studied the vortex lattice (VL) in MgB₂ for a number of field-temperature histories mapping out the ground state VL phase diagram and fully exploring the second order rotation transition behavior of the lattice which differed from theoretical predictions. Additionally, long lived metastable VL phases were discovered, the existence of which was unexpected in such a clean sample.To explore the origin of the metastability in MgB₂, I performed a detailed study of the metastable VL in MgB₂ as it was gradually driven to the ground state by small decreases in the applied magnetic field using SANS. Our measurements showed that metastable VL domains persist in the presence of substantial vortex motion and thus provide definitive evidence that the metastability cannot be attributed to the pinning of vortices to defects in the crystal.Using SANS in combination with a small AC magnet, I studied the VL in MgB₂ as it was driven from a metastable to the ground state by application of a few mT AC magnetic field oriented either parallel or perpendicular to the vortices in the sample. The transition to the ground state showed two distinct power-law behaviors which indicates a novel kind of vortex motion.