Antiferromagnetic Transition in Sr_{14- X}Ca_xCu₂₄O₄₁ Investigated by Specific Heat Measurements

Results for low temperature specific heat for single crystals of ladder/chain materials Sr_14-xCa_xCu₂₄O₄₁ (x= 0, 8, 10, 11.5) in magnetic field up to ∼ 15 T are reported. A sharp peak of the specific heat of a single crystal Sr_2.5Ca_11.5Cu₂₄O₄₁, was found at ≃ 2.3 K, clearly indicating the onset of a long range order. In measurements of the specific heat in magnetic fields the transition temperature, T_N, was found to shift to lower temperatures with increasing magnetic field, indicating that the ordering is antiferromagnetic, which has been confirmed by neutron scattering measurements. With decreasing x T_N rapidly shifts to lower temperatures. For x=10 T_N decreases to ≃ 1 K and for x= 0 no anomaly indicating the transition was found down to ∼ 0.6K.