Impurity-Induced Spin-State Crossover in La0.8Sr0.2Co1-xAlxO3

We have prepared a set of polycrystalline samples of La0.8Sr0.2Co1-xAlxO3 (0 <= x <= 0.2), and have measured the magnetization as functions of temperature and magnetic field. We find that the average spin number per Co ion (S co ) evaluated from the room-temperature susceptibility is around 1.2-1.3 and independent of x. However, we further find that S-Co evaluated from the saturation magnetization at 2 K is around 0.3-0.7, and decreases dramatically with x. This naturally indicates that a significant fraction of the Co3+ ions experience a spin-state crossover from the intermediate-to low-spin state with decreasing temperature in the Al-substituted samples. This spin-state crossover also explains the resistivity and the thermopower consistently. In particular, we find that the thermopower is anomalously enhanced by the Al substitution, which can be consistently explained in terms of an extended Heikes formula.