Low-temperature specific heat and ferromagnetism in La4Ba2Cu2O10and related compounds

The authors report the low-temperature specific heat and the magnetization measurements of the Cu-O ferromagnet La4Ba2Cu2O10 and the related compounds. The overall feature of the specific heat in La4Ba2Cu2O10 agrees to that of the mean field approximation with 1/2 spin system and is more 3-dimensional than that of 2-dimensional Cu ferromagnet K2CuF4. The measured magnetizations on the single crystals and the estimated anisotropy suggest the Heisenberg system with Ising-like anisotropy due to the comparably large interlayer coupling. In the system of Nd substitution for La, the ferromagnetic transition is not observed because Nd ions having the f-spins may disturb the exchange interaction between interlayer Cu2+ ions.