Observation of a Field-Driven Structural Phase Transition in the Flux Line Lattice in ${\mathrm{ErNi}}_2{B}_2\mathrm{C}$

Small-angle neutron scattering and magnetic decoration both demonstrate a topological transition in the flux line lattice (FLL) in ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$. The high-field square lattice slowly transforms into a hexagonal lattice via an area preserving [100] rhombohedral distortion below roughly 500 Oe. The square FLL is aligned with the [110] direction of the tetragonal crystal, while the two domains of the hexagonal FLL are aligned with [100] and [010]. The differences in pinning for the two FLL topologies are reflected in the rf kinetic inductance.