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

¹ Department of Physics, University of Illinois at Urbana-Champaign 1110 W. Green Street, Urbana IL 61801, USA
² Los Alamos National Laboratory, MST-10 - Los Alamos, NM 87545, USA
³ National Creative Research Initiative Center for Superconductivity and Department of Physics, Pohang University of Science and Technology Pohang 790-784, Republic of Korea
⁴ National Institute for Materials Science - 1-2-1 Sengen Tsukuba, Ibaraki 305-0047, Japan

Received: 3 November 2005
Accepted: 13 January 2006

Abstract

The coexistence of weak ferromagnetism and superconductivity in 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 of high-quality single crystals of . In addition to expected features at the Néel temperature and weak ferromagnetic onset at , rises to a maximum at before dropping sharply down to . We assign the maximum to the proliferation and freezing of spontaneous vortices. A model proposed by Koshelev and Vinokur explains the increasing as a consequence of increasing vortex density, and its subsequent decrease below T_m as defect pinning suppresses vortex hopping.