Low Temperature Specific Heat of (La1-xSrx)2NiO4+δ

We studied the low temperature specific heat C of nonsuperconducting oxides (La1_xSrx)2NiO4+δ in a wide range of x up to 0.65, along with the lattice parameters at room temperature, electrical resistivity ρ and magnetic susceptibility χ. We observed the qualitative change in both ρ and χ at x=0.20 ∼ 0.25 corresponding to that in c/a, which is ascribed to the transition from the high-spin to low-spin state of Ni ions. The anomalous hump in C below 10 K was observed. It disappears gradually above x∼0.20 and coincides with the transition of the spin states. However, this hump is well described by a modified Schottky-type contribution and possibly originates from the presence of the two-level system due to a glassy magnetic state.