Observation of the spontaneous vortex phase in the weakly ferromagnetic superconductor ErNi2B2C: A penetration depth study

The coexistence of weak ferromagnetism and superconductivity in ErNi2B2C suggests the possibility of a spontaneous vortex phase (SVP) in which vortices appear in the absence of an external field. We report evidence for the long-sought SVP from the in-plane magnetic penetration depth Δλ(T) of high-quality single crystals of ErNi2B2C. In addition to expected features at the Neel temperature TN = 6.0 K and weak ferromagnetic onset at TWFM = 2.3 K, Δλ(T) rises to a maximum at Tm = 0.45 K before dropping sharply down to ~ 0.1 K. We assign the 0.45 K maximum to the proliferation and freezing of spontaneous vortices. A model proposed by Koshelev and Vinokur explains the increasing Δλ(T) as a consequence of increasing vortex density, and its subsequent decrease below Tm as defect pinning suppresses vortex hopping.