Spin-wave contribution to specific heat of superconducting antiferromagnets RNi2B2C (R = Er, Dy)

RNi2B2C (R = Tm, Er, Ho, Dy) superconducts at, respectively, Tc = 11, 11, 8, 6 K. Below, respectively, TN ~ 1.5, 6, 5, 10.6 K, each (For Dy below Tc) shows a coexistence of superconductivity and antiferromagnetism. Since their magnetic structures consist of weakly coupled magnetic RC that are separated by superconducting Ni2B2, it is of interest to investigate the character and dimensionality of the involved spin-wave SW excitations. For that purpose, magnetic specific heat CM(T) of well characterized RNi2B2C (R = Er, Dy) were measured in the coexistence regime. For R = Ho, SW contribution was reported as 0.29T3 exp(-5.3/T) J/mol K, indicative of magnon excitation in a 3d antiferromagnetic (AF) state with an energy gap A kBTN. For R = Tm, a reported linear-in-T contribution implies a gapless magnon excitation in a 2d FM sheet. The SW contribution of R = Er below 2 K is 0.27(Δ2 + 4TA + 6T2) exp(-Δ/T) J/mol K where Δ = 5.9±0.1 K: a gapped dispersion relation of a 2d FM sheet. The contribution of R = Dy below 2.5 K is 0.05T J/mol K: a 2d FM gapless dispersion relation. Assuming an anisotropic Neel AF ground structure and using standard SW theory we are able to obtain an explicit expression for the energy gap in terms of anisotropy field and interplanar coupling. Such an expression is used qualitatively to rationalize the observed differences in the magnon contributions of these superconducting AFs.