Possible anisotropy gap in La 1.35Sr 1.65Mn 2O 7 and La 1.5Sr 0.5NiO 4 detected through specific heat and magnetization measurements
Magnetization and specific heat measurements, as a function of temperature, were performed on single crystals of La 1.35Sr 1.65Mn 2O 7 and La 1.5Sr 0.5NiO 4, under different applied magnetic fields (H). The specific heat in La 1.35Sr 1.65Mn 2O 7 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 La 1.5Sr 0.5NiO 4. 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 ∆=2μBH 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 La 1.35Sr 1.65Mn 2O 7 and La 1.5Sr 0.5NiO 4.