Magnon specific heat of single-crystal borocarbides RNi2B2C (R=Tm, Er, Ho, Dy, Tb, Gd) -: art. no. 224407

Zero-field specific heats of the single crystals RNi2B2C (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 TmNi2B2C) in terms of only two parameters, namely, an energy gap Delta and a characteristic temperature theta. The evolution of theta and Delta across the studied compounds correlates very well with the known magnetic properties. theta, as a measure of the effective Ruderman-Kittel-Kasuya-Yosida exchange couplings, scales reasonably well with the de Gennes factor. Delta, on the other hand, reflects predominately the anisotropic properties: similar to2 K for GdNi2B2C, similar to6 K for ErNi2B2C, similar to7 K for TbNi2B2C, and similar to8 K for each of HoNi2B2C and DyNi2B2C. The equality in Delta of HoNi2B2C and DyNi2B2C, coupled with the similarity in their magnetic configurations, indicates that a variation of x in the solid solution HoxDy1-xNi2B2C (x<0.8 and T-c<T-N) would not lead to any softening of Delta. This supports the hypothesis of Cho (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.