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

Abstract

The coexistence of weak ferromagnetism and superconductivity inErNi2B2C suggests the possibility of a spontaneous vortexphase (SVP) in which vortices appear in the absence of an externalfield. We report evidence for the long-sought SVP from the in-planemagnetic penetration depth Δλ(T) of high-qualitysingle crystals of ErNi2B2C. In addition to expectedfeatures at the Néel temperature TN = 6.0 K and weakferromagnetic onset at TWFM = 2.3 K, Δλ(T) risesto 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 andfreezing of spontaneous vortices. A model proposed by Koshelev andVinokur explains the increasing Δλ(T) as aconsequence of increasing vortex density, and its subsequentdecrease below Tm as defect pinning suppresses vortex hopping.