The flux-line lattice in the compound ${\mathrm{ErNi}}_2{\mathrm{B}}_2\mathrm{C}$, which has a tendency to ferromagnetic order in the $a$- $b$ plane, is studied with the external magnetic field direction close to the $c$ axis. We show the existence of an instability, where the direction of flux lines spontaneously tilts away from that of the applied field as a prelude to the onset of ferromagnetic order and the predicted spontaneous vortex phase. This tilting instability is accompanied by a decreasing longitudinal correlation length of the flux lattice, as observed.

• Received 22 November 1996

DOI:https://doi.org/10.1103/PhysRevLett.78.3745