Metal-insulator transition in Sr2-xLaxCoO4 driven by spin-state   transition
  We sought the origin of the metal-insulator transition in Sr2-xLaxCoO4, using electron-correlation corrected density functional calculations. Our results show that Sr2CoO4 is in an intermediate-spin (IS) state and a strong Co4+ 3d-O 2p hybridization is responsible for its ferromagnetic metallicity. Upon La doping, however, a spin-state transition occurs in Sr1.5La0.5CoO4: IS-Co4+ x 2 + 1e --> LS-Co4+ + HS-Co3+ (LS: low spin; HS: high spin). Then the spin-state transition suppresses an electron hopping via a spin-blockade and gives rise to the insulating behavior of Sr1.5La0.5CoO4. A corresponding superexchange accounts for its ferromagnetism. Thus, spin state could provide a way to tune materials properties. 
Hua Wu
cond-mat.str-el