Spin state and phase competition in TbBaCo2O5.5 and the lanthanide series LBaCo2O5+δ (0≤δ≤1) -: art. no. 092413

A clear physics picture of TbBaCo2O5.5 is revealed on the basis of density-functional theory calculations. An anti ferromagnetic (AFM) superexchange coupling between the almost high-spin Co3+ ions competes with a ferromagnetic (FM) interaction mediated by both p-d exchange and double exchange, being responsible for the observed AFM-FM transition. And the metal-insulator transition is accompanied by an xy/xz orbital-ordering transition. Moreover. this picture can be generalized to the whole lanthanide series, and it is predicted that a few room-temperature magnetoresistance materials could be found in LBa(1-x)A(x)Co(2)O(5+delta) (L=Ho, Er, Tm, Yb, Lu; A =Sr, Ca. Mg).