Comparative study of specific heat measurements in LaMnO3, La1.35Sr1.65Mn2O7, La1.5Sr0.5NiO4 and La1.5Sr0.5CoO4

We present the temperature dependence of the specific heat, without external magnetic field and with H= 9 T, for LaMnO3, La1.35Sr1.65Mn2O7, La1.5Sr0.5NiO4 and La1.5Sr0.5CoO4 single crystals. We found that spin-wave excitations in the ferromagnetic and bilayer-structure La1.35Sr1.65Mn2O7 were suppressed by the 9 T magnetic field. On the other hand, the external magnetic field had no effect in the specific heat of the other three antiferromagnetic samples. Also, the electronic part of the interactions were removed at very low temperatures in the La1.5Sr0.5NiO4 single crystal, even with a zero applied magnetic field. Below 4 K, we found that the specific heat data for La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4 crystals could be fitted to an exponential decay law. Detailed magnetization measurements in this low temperature interval showed the existence of a peak close to 2 K. Both results, magnetizations and specific heat suggest the existence of an anisotropy gap in the energy spectrum of La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4 compounds.