Europhys. Lett., 73 (5), pp. 772-778 (2006)
DOI: 10.1209/epl/i2005-10465-7

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

E. E. M. Chia¹, M. B. Salamon¹, T. Park², H.-J. Kim³, S.-I. Lee³ and H. Takeya<sup>4

1</sup>  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 in final form 13 January 2006
published online 27 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.

PACS
74.25.Nf - Response to electromagnetic fields (nuclear magnetic resonance, surface impedance, etc.).
74.25.Ha - Magnetic properties.
74.70.Dd - Ternary, quaternary and multinary compounds (including Chevrel phases, borocarbides, etc.).

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