Possible anisotropy gap in La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4 detected through specific heat and magnetization measurements

Magnetization and specific heat measurements, as a function of temperature, were performed on single crystals of La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4, under different applied magnetic fields ( H ) . The specific heat in La1.35Sr1.65Mn2O7 was decreased for H = 9 T parallel to the crystal c axis, compared with H = 0 , possibly due to a suppression of spin-wave excitations (magnons) in that ferromagnetic bilayer structure. On the other hand, the applied magnetic field had no effect in the specific heat of the antiferromagnetic La1.5Sr0.5NiO4. For H = 9 T and below the temperature of 4 K the specific heat data, for each crystal, was well fitted by an exponential decay law. This allowed the calculation of energy gaps around 1 meV for both compounds, in close agreement with Δ = 2 μ B H for an expected energy gap in the magnon spectrum. Detailed magnetization measurements showed monotonic variations below 4 K and a steep increase close to 2 K. Both magnetization and specific heat measurements suggest the existence of an anisotropy gap in the energy spectrum of La1.35Sr1.65Mn2O7 and La1.5Sr0.5NiO4.