Ferromagnetism and Spontaneous Vortex Formation in Superconducting ErNi2B2C
S.-M. Choi, NCNR/CHRNS and University of Maryland
J.W. Lynn, NCNR
D. Lopez, P.L. Gammel and C.M. Varma, Bell Laboratories
P.D. Canfield and S.L. Budíco, Iowa State University
The competition between magnetic order and superconductivity has stimulated much interest, especially due to the role that magnetic fluctuations are thought to play in high-Tc materials. Recently, superconducting ErNi2B2C (TC = 11 K) was reported to develop a net magnetization below 2.3 K, well below the onset of long range spin-density-wave order at TN = 6 K. We have carried out a comprehensive study  of the magnetic order and vortex structure in this material, and directly demonstrated that this transition does indeed correspond to the development of a net atomic magnetization that coexists with superconductivity and these results in the spontaneous formation of vortices in the system.
Superconducting ErNi2B2C orders magnetically at 6 K into a transversely polarized spin density wave structure [2,3]. At 2.4 K additional higher-order peaks become observable as the magnetic structure becomes more square-wave-like. Below 2.3 K a new series of peaks has develops that polarized neutron measurements unambiguously establish as due to the development of the new (weak) ferromagnetism at TWFM. These results suggest the intriguing possibility that in an applied field vortices will form spontaneously when the net atomic magnetization is present. The small angle scattering data of the vortex structure (Fig. 1 top) show that the lattice has the expected spacing at lower applied fields. At higher fields, as the field begins to penetrate, vortices spontaneously form in addition to those expected from the applied field alone, increasing the vortex density and shifting the vortex peak. The T dependence of this shift (Fig. 3 bottom) makes it clear that this behavior is directly related to onset of the net magnetization in the system. It is also interesting to note that the vortex pinning is enhanced in both magnetic phases, which may prove useful in high current applications of superconductors.
Fig. 1. Intensity of the vortex scattering vs. wave vector Q at 850 Oe above and below the weak ferromagnetic transition. The shift in the peak position demonstrates that additional vortices spontaneously form at low temperatures. The temperature dependence (bottom) shows that this spontaneous vortex formation is directly related to the weak ferromagnetic transition.
 J. W. Lynn and S. Skanthakumar, Handbook on the Physics and Chemistry of Rare Earths, Chap. 199, Vol. 31, ed. by K. A. Gschneidner, Jr., L. Eyring, and M. B. Maple, North Holland, Amsterdam (2001), p. 313.
 J. W. Lynn, S. Skanthakumar, Q. Huang, S. K. Sinha, Z. Hossain, L. C. Gupta, R. Nagarajan, and C. Godart, Phys. Rev. B55, 6584 (1997).
 S.-M. Choi, J. W. Lynn, D. Lopez, P. L. Gammel, P. C. Canfield and S. L. Bud'ko, Phys. Rev. Lett. 87, 107001 (2001).
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