Europhys. Lett.
Volume 73, Number 5, March 2006
Page(s) 772 - 778
Section Condensed matter: electronic structure, electrical, magnetic, and optical properties
Published online 27 January 2006
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 $\chem{ErNi_{2}B_{2}C}$: A penetration depth study

E. E. M. Chia1, M. B. Salamon1, T. Park2, H.-J. Kim3, S.-I. Lee3 and H. Takeya4

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 in final form 13 January 2006
published online 27 January 2006

The coexistence of weak ferromagnetism and superconductivity in $\chem{ErNi_{2}B_{2}C}$ 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 $\Delta \lambda (T)$ of high-quality single crystals of $\chem{ErNi_{2}B_{2}C}$. In addition to expected features at the Néel temperature $T_{N} = 6.0\un{K}$ and weak ferromagnetic onset at $T_{WFM}=2.3\un{K}$, $\Delta \lambda (T)$ rises to a maximum at $T_{m}=0.45\un{K}$ before dropping sharply down to $\sim
0.1\un{K}$. We assign the $0.45\un{K}$ maximum to the proliferation and freezing of spontaneous vortices. A model proposed by Koshelev and Vinokur explains the increasing $\Delta \lambda (T)$ as a consequence of increasing vortex density, and its subsequent decrease below Tm as defect pinning suppresses vortex hopping.

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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