LOW-TEMPERATURE SPECIFIC-HEAT OF (LA1-XSRX)2NIO4+DELTA

We studied the low temperature specific heat C of nonsuperconducting oxides (La1-xSr(x))2NiO4+delta in a wide range of x up to 0.65, along with the lattice parameters at room temperature, electrical resistivity rho and magnetic susceptibility chi. We observed the qualitative change in both rho and chi at x = 0.20 approximately 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 approximately 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.