Magnon specific heat of single-crystal borocarbidesRNi2B2C(R=Tm,Er, Ho, Dy, Tb, Gd)

Zero-field specific heats of the single crystals RNi 2 B 2 C (R = Er, Ho, Dy, Tb, Gd) were measured within the temperature range 0.1 K < T< 25 K. Linearized spin-wave analysis was successfully applied to account for and to rationalize the thermal evolution of the low-temperature magnetic specific heats of all the studied compounds (as well as the one reported for TmNi 2 B 2 C) in terms of only two parameters, namely, an energy gap A and a characteristic temperature θ. The evolution of θ and A across the studied compounds correlates very well with the known magnetic properties. θ, as a measure of the effective Ruderman-Kittel-Kasuya-Yosida exchange couplings, scales reasonably well with the de Gennes factor. A, on the other hand, reflects predominately the anisotropic properties: ∼2 K for GdNi 2 B 2 C, ∼6 K for ErNi 2 B 2 C, ∼7 K for TbNi 2 B 2 C, and ∼8 K for each of HoNi 2 B 2 C and DyNi 2 B 2 C. The equality in A of HoNi 2 B 2 C and DyNi 2 B 2 C, coupled with the similarity in their magnetic configurations, indicates that a variation of x in the solid solution Ho x Dy 1 - x Ni 2 B 2 C (x<0.8 and T c <T N ) would not lead to any softening of Δ. This supports the hypothesis of Cho et al. (Ref. 35) concerning the influence of the collective magnetic excitations on the superconducting state. This work underlines the importance of spin-wave excitations for a valid description of low-temperature thermodynamics of borocarbides.