Specific heat of (Ca_1-xSr_x)₃Ru₂O₇ single crystals

We have measured the specific heat of crystals of (Ca_1-xSr_x)₃Ru₂O₇ using ac- and relaxation-time calorimetry. Special emphasis was placed on the characterization of the Néel (T_N = 56 K) and structural (T_c = 48 K) phase transitions in the pure, x = 0 material. While the latter is believed to be first order, detailed measurements under different experimental conditions suggest that all the latent heat (with L ∼ 0.3 R) is being captured in a broadened peak in the effective heat capacity. The specific heat has a mean-field-like step at T_N, but its magnitude (Δ c_P ∼ R) is too large to be associated with a conventional itinerant electron (e.g. spin-density-wave) antiferromagnetic transition, while its entropy is too small to be associated with the full ordering of localized spins. The T_N transition broadens with Sr substitution while its magnitude decreases slowly. On the other hand, the entropy change associated with the T_c transition decreases rapidly with Sr substitution, and is not observable for our x = 0.58 sample.