Antiferromagnetic transition in Sr14-xCaxCu24O41 investigated by specific heat measurements

Results for low temperature specific heat for single crystals of ladder/cha in materials Sr14-xCaxCu24O41 (x = 0, 8, 10, 11.5) in magnetic field up to similar to 15 T are reported, A sharp peak of the specific heat of a single crystal Sr2.5Ca11.5Cu24O41, was found at similar to 2.3 K, clearly indicat ing the onset of a long range order, In measurements of the specific heat i n magnetic fields the transition temperature, T-N, was found to shift to lo wer temperatures with increasing magnetic field, indicating that the orderi ng is antiferromagnetic, which has been confirmed by neutron scattering mea surements, With decreasing x T-N rapidly shifts to lower temperatures. For x = 10 T-N decreases to similar to 1 K and for x = 0 no anomaly indicating the transition was found down to similar to 0.6 K.