N I S T Center for Neutron Research
Accomplishments and Opportunities 2001
Ferromagnetism and Spontaneous Vortex Formation in Superconducting Er Ni2 B2 C
The competition between magnetic order and superconductivity has had an interesting history. The first examples of true long range magnetic order coexisting with superconductivity were provided by the ternary Chevrel-phase superconductors and related compounds. The magnetic ordering temperatures were all low (≈ 1 K), indicating that dipolar interactions dominate the energetics of the magnetic system. Similar behavior is found for the rare earth ordering in the hole-doped cuprates (Refer to Reference 1). The first systems where exchange clearly dominates the magnetic energetics were provided by the electron-doped cuprates (Refer to Reference 1), followed by the borocarbides of central interest here (Refer to Reference 2).
Among these systems, the rare and interesting situation where ferromagnetic interactions are present has attracted special attention because of the competitive nature between the superconducting screening and the internally generated magnetic field. In the Chevrel phase materials this competition gives rise to long wavelength oscillatory magnetic states and/or reentrant superconductivity. Recently, superconducting Er Ni2 B2 C ( Tc = 11 K ) appeared 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 this results in the spontaneous formation of vortices in the system (Refer to Reference 3).
Superconducting Er Ni2 B2 C orders magnetically at 6 K into a transversely polarized spin density wave structure (Refer to References 2 and 3), with the modulation wave vector δ along the a axis and the spins along the b direction. A portion of a scan along the a axis is shown in Figure 1 at three temperatures. The initial ordering is a simple spin density wave as shown in the schematic, with an incommensurate modulation of δ approximately equal to 0.55 a * (a* = 2 π / a ). At 2.4 K additional higher-order peaks become observable as the magnetic structure squares up as indicated schematically at the bottom of Figure 1. These peaks are all odd-order harmonics as expected for a square-wave magnetic structure.
Below the weak ferromagnetic transition (TWFM) at 2.3 K, the data (Refer to Figure 1) indicate that a new series of peaks has developed, which are even harmonics of the fundamental wave vector. Polarized neutron measurements unambiguously establish that both the odd-order peaks and the even-order peaks are magnetic in origin.
The integrated intensities of the odd- and even-order harmonics are shown in Figure 2 as a function of T. The even-order peaks abruptly develop below 2.3 K, concomitant with the development of the new (weak) ferromagnetism at TWFM. There is a substantial thermal hysteresis associated with the weak ferromagnetic transition, suggesting that this transition is first order in nature.
Graphics Caption FIGURE 1. Scan along the [h 00] direction at T = 1.3 K, 2.4 K, and 4.58 K. The data have been offset for clarity. Above the weak ferromagnetic transition at 2.3 K we observe the fundamental peak at ≈ 0.55 along with odd-order harmonics, while below a new set of even harmonics develops (arrows). Also shown is a schematic of the spin density wave order for T just below TN, and the (undistorted) wave when it squares up.
Graphics Caption FIGURE 2. Integrated intensities of the odd (blue circles) and even (red circles) order harmonics as a function of T.
Graphics Caption FIGURE 3. Intensity of the vortex scattering versus wave vector Q at 850 O e 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.
The present neutron results demonstrate that a net magnetization develops in Er Ni2 B2 C in the magnetically ordered state at low temperatures, making this the first such “ferromagnetic-superconductor” since Ho Mo6 S8, Ho Mo6 Se8, and Er Rh4 B4. For Er Ni2 B2 C the net magnetization is much smaller than for these earlier systems, which allows coexistence with superconductivity over an extended T range. This presents the intriguing possibility that in an applied field vortices will form spontaneously when this net atomic magnetization is present. The small angle scattering data of the vortex structure (Refer to Figure 3 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 (Refer to Figure 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.
 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).
S.-M. Choi and J. W. Lynn
N I S T Center for Neutron Research
National Institute of Standards and Technology
Gaithersburg, MD 20899-8562
University of Maryland
College Park, MD 20742
D. Lopez, P. L. Gammel, and C. M. Varma
Murray Hill, NJ 07974
P. C. Canfield and S. L. Bud’ko
Ames Laboratory and Department of Physics and Astronomy
Iowa State University
Ames, IA 50011
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