# Structural studies of metastable and equilibrium vortex lattice domains in MgB2

## TL;DR

This study investigates the structural transition of vortex lattice domains in MgB2 from metastable to equilibrium states using small-angle neutron scattering, revealing domain sizes and the nature of vortex arrangements under varying conditions.

## Contribution

It provides the first detailed structural analysis of vortex lattice domain evolution in MgB2 during transition from metastable to equilibrium states.

## Key findings

- Longitudinal correlation length remains constant during transition.
- Vortex lattice behaves as a system of straight rods.
- Estimated maximum domain size is approximately 50 microns.

## Abstract

The vortex lattice in MgB2 is characterized by the presence of long-lived metastable states, which arise from cooling or heating across the equilibrium phase boundaries. A return to the equilibrium configuration can be achieved by inducing vortex motion. Here we report on small-angle neutron scattering studies of MgB2, focusing on the structural properties of the vortex lattice as it is gradually driven from metastable to equilibrium states by an AC magnetic field. Measurements were performed using initial metastable states obtained either by cooling or heating across the equilibrium phase transition. In all cases, the longitudinal correlation length remains constant and comparable to the sample thickness. Correspondingly, the vortex lattice may be considered as a system of straight rods, where the formation and growth of equilibrium state domains only occurs in the two-dimensional plane perpendicular to the applied field direction. Spatially resolved raster scans of the sample were performed with apertures as small as 80 microns, corresponding to only 1.2*10^6 vortices for an applied field of 0.5 T. These revealed spatial variations in the metastable and equilibrium vortex lattice populations, but individual domains were not directly resolved. A statistical analysis of the data indicates an upper limit on the average domain size of approximately 50 microns.