  A clear physics picture of TbBaCo$_{2}$O$_{5.5}$ is revealed on the basis of
density functional theory calculations. An antiferromagnetic (AFM)
superexchange coupling between the almost high-spin Co$^{3+}$ 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 LnBa$_{1-x}$A$_{x}$Co$_{2}$O$_{5+\delta}$
(Ln=Ho,Er,Tm,Yb,Lu; A=Sr,Ca,Mg).
