Measuring Energetics of Magnetic Vortices Using Small-Angle Neutron Scattering

Superconducting vortices and magnetic skyrmions are two types of mesoscale magnetic vortices that arrange themselves into periodic lattices in condensed matter systems. One method to study these structures is small angle neutron scattering (SANS), which is uniquely capable of resolving magnetic order throughout the bulk of a crystal in reciprocal space. Through careful modelling, the information that SANS provides can be used to extract more than just the magnetic structure, including the fundamental energetics and symmetries which are at play. This dissertation discusses three projects of this nature.

Skyrmions are of direct interest for future spintronic memory applications due to their intrinsic topological protection. Measurements of the energy barrier associated with this protection have often relied on generating metastable configurations of skyrmions which decay away over an observable period of time. We have developed a method of measuring the energy barrier in equilibrium (not metastable) skyrmion lattices (SkLs) by exploiting a hysteresis in the SANS signal of the prototypical skyrmion material MnSi. By modelling this hysteresis with a simple Preisach free energy and comparing it with atomistic spin simulations, the activation barrier is found to be several eV/ skyrmion. Additionally, it is confirmed that the SkL forms progressively in domains several hundred skyrmions in size.

The most promising applications of skyrmions involve using them as bits in a racetrack memory device. Such devices will likely drive the skyrmions into motion by using electric currents, and while the current density required to move skyrmions is substantially less than what is required to move domain walls, it still results in significant power consumption. We have sought to develop a new skyrmion device architecture to more efficiently produce SANS-visible SkL motion. By exploiting the Magnus force that current exerts on the SkL, we believe our new device will drive the SkL into a rotation at lower current densities than previously reported. Since rotations are visible in reciprocal space, this motion will be observable with SANS. Additionally, by measuring the rotation as a function of current density, we will be able to directly map the SkL-crystal lattice interaction potential and measure elastic properties of the SkL.

Superconducting vortex lattices (VLs) are highly sensitive to anisotropies in the underlying superconductivity. In the two-band superconductor MgB_2, orientation of the triangular VL is dictated by competing 6 and 12 fold anisotropies in the ab crystal plane. When these two contributions are comparable in energy, the VL fragments into counter rotated domains in the so called L phase. We have observed that when the VL is rotated out of the ab plane, the L phase rapidly shrinks in size. Above a critical rotation angle Omega_0, the phase disappears entirely, which can be explained by the 12 fold term in the superconducting anisotropy reversing sign.