Magnetic Specific Heat within the Superconducting Phase of Single Crystal ErNi₂B₂C

抄録

Using magnetic specific heat [C_M(T)], we investigated the character and dimensionality of the spin wave excitations within the coexistence phase of single crystal ErNi₂B₂C. As known, the magnetic structure of ErNi₂B₂C consists of a transversely polarized sinusoidally modulated state that is squared-up, presumably, below the onset point of the weak ferromagnetic state T_WF ≈ 2.2 K. We observed that the thermal evolution of C_M(T) within the sinusoidally modulated state (T_WF<T<T_N) is distinctly different from the one below T_WF: for T_WF<T<T_N, C_M(T) evolves as 0.45 T² J/mol K suggesting a 2d excitations with a gapless linear dispersion relation. On the other hand, for T< T_WF, C_M(T) is dominated by an exponential factor, indicative of gapped AF excitations. The gap (∼6 K) is approximately equal to T_N and to the separation between the lowest lying Kramer doublets. Finally, based on entropy arguments and on features of the exchange striction, we comment on the influence of the magnetoelastic coupling on the low-temperature magnetic properties of this compound.