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

RNi2B2C (R = Tm, Er, Ho, Dy) superconducts at, respectively, T-c = 11, 11, 8, 6 K. Below, respectively, T-N similar or equal to 1.5, 6, 5, 10.6 K, eac h (For D-Y below T-c) shows a coexistence of superconductivity and antiferr umagnetism. Since their magnetic structures consist of weakly coupled magne tic RC that are separated by superconducting Ni2B2, it is of interest to in vestigate the character and dimensionality of the involved spin-wave SW exc itations. For that purpose, magnetic specific heat CM(T) of well characteri zed RNi2B2C (R = Er, Dy) were measured in the coexistence regime. For R = H o, SW contribution was reported as 0.29T(3) exp(-5.3/T) J/mol K, indicative of magnon excitation in a 3d antiferromagnetic (AF) state with an energy g ay Delta approximate to k(B)T(N). For R = Tm, a reported linear-in-T contri bution implies a gapless magnon excitation in a 2d FM sheet. The SW contrib ution of R = Er below 2 K is 0.27(Delta (2) + 4T Delta + 6T(2)) exp(-Delta /T) J/mol K where Delta = 5.9 +/-0.1 K: a gapped dispersion relation of a 2 d FM sheet. The contribution of R = Dy below 2.5 K is 0.05T J/molK: a 2d FM gapless dispersion relation. Assuming an anisotropic Neel AF ground struct ure and using standard SW theory we are able to obtain an explicit expressi on for the energy gap in terms of anisotropy field and interplanar coupling . Such an expression is used qualitatively to rationalize the observed diff erences in the magnon contributions of these superconducting AFs. (C) 2001 Elsevier Science B.V. All rights reserved.