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 N\'{e}el temperature TN = 6.0 K and weak
ferromagnetic onset at TWFM=2.3K, 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.Comment: 5 pages including figures; added inset to Figure 2; significant
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