  The complicated electronic, magnetic, and colossal magnetoresistant (CMR)
properties of Sr and Ca doped lanthanum manganites can be understood by
spin-polarized first-principles calculations. The electronic properties can be
attributed to a detailed balancing between Sr and Ca induced metal-like O 2p
and majority-spin (majority-spin) Mn eg delocalized states and the
insulator-like minority-spin (minority-spin) Mn t2g band near the Fermi level
(EF). The magnetic properties can be attributed to a detailed balancing between
O mediated antiferromagnetic superexchange and delocalized majority-spin Mn
eg-state mediated ferromagnetic spin-spin couplings. While CMR can be
attributed to the lining up of magnetic domains trigged by the applied magnetic
field, which suppresses the trapping ability of the empty Mn t2g states that
resists the motion of conducting Mn majority-spin eg electrons.
