Phase separation in degenerate magnetic oxide semiconductors

A theory of mixed electronic-impurity phase separation in degenerate magnet ic oxide semiconductors, including high-T-c superconductors and materials w ith colossal magnetoresistance (CMR), is developed. Such a separation can o ccur in materials with excess oxygen, if they are simultaneously doped with an acceptor impurity whose atoms are frozen in position. Oxgyen acts as an acceptor, which can diffuse through the crystal. Then, for example, mangan ites can break up into ferromagnetic and antiferromagnetic regions with all holes and oxygen ions concentrated in the former and with no holes or oxyg en ions in the latter. Such two-phase systems can possess CMR and anomalous thermoelectric power, and they can make a transition from an insulating in to a highly conducting state as temperature increases. The reverse insulato r-metal transition is also possible. c 1998 American Institute of Physics. [S1063-7834(98)02311-9].