Anomalous paramagnetic effects in the mixed state ofLuNi2B2C

Abstract

Anomalous paramagnetic effects in dc magnetization were observed in the mixed state of LuNi2B2C, unlike any reported previously. It appears as a kink-like feature for H ≥ 30 kOe and becomes more prominent with increasing field. A specific heat anomaly at the corresponding temperature suggests that the magnetization anomaly is due to a true bulk transition. A magnetic flux transition from a square to an hexagonal lattice is consistent with the anomaly.In cuprate (high-T c ) superconductors, high-transition temperatures (T c ) and short coherence lengths (ξ) lead to large thermal fluctuation effects, opening a possibility for melting of the flux line lattice (FLL) at temperatures well below the superconducting transition temperature. A discontinuous step in dc magnetization and a sudden, kink-like drop in resistivity signified the first order nature of the melting transition from the vortex lattice into a liquid. 1,2,3 In conventional type II superconductors, with modest transition temperatures and large coherence lengths, vortex melting is also expected to occur in a very limited part of the phase diagram, 4 but it has yet to be observed experimentally. In the rare-earth nickel borocarbides RNi 2 B 2 C (R = Y, Dy, Ho, Er, Tm, Lu), the coherence lengths (ξ ∼ = 10 2Å ) and superconducting transition temperatures (16.1 K for R = Lu) lie between these extremes, suggesting that the vortex melting will be observable and may provide further information on vortex dynamics. Indeed, Mun et al. 5 reported the observation of vortex melting in YNi 2 B 2 C, based on a sharp, kink-like drop in electrical resistivity.Recently, a magnetic field-driven FLL transition has been observed in the tetragonal borocarbides. 6,7,8,9 The transition from square to hexagonal vortex lattice occurs due to the competition between sources of anisotropy and vortex-vortex interactions. The repulsive nature of the vortex interaction favors the hexagonal Abrikosov lattice, whose vortex spacing is larger than that of a square lattice. The competing anisotropy, which favors a square lattice, can be due to lattice effects (fourfold Fermi surface anisotropy), 10 unconventional superconducting order parameter, 11 or an interplay of the two. 12,13 In combination with non-negligible fluctuation effects, the competition leads to unique vortex dynamics right below the H c2 line in the borocarbides, namely a reentrant vortex lattice transition. 9 Fluctuation effects near the upper critical field line wash out the anisotropy effect, stabilizing the Abrikosov hexagonal lattice. 14,15 Here, we report the first observation of paramagnetic effects in the dc magnetization M of the mixed state of LuNi 2 B 2 C. The kink-like feature in M and the corresponding specific heat feature for H ≥ 30 kOe signify the reentrant FLL transition, which is consistent with the low-field FLL transition line inferred from small angle neutron scattering (SANS). 9 Single crystals of LuNi 2 B 2 C were grown in a Ni 2 B flux as described elsewhere 16 and were post-growth annealed at T = 10...

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