s~gnificalltly h~gher than that of the corresponding thin-film sample in L1 (30°/6). The x-ray analysis of this sample confirmed tbe cubic perovskite structure with a = 3.82 4 .It has been suggested (3) that the CMR effect in the Ca-doped LaMnO? system originates from "double exchange" electronhopping processes between Mn3+ and Mn4+ ne~ghboring sites through the 0'anion between them (10). In these processes, the e, itinerant electron conduction or hopping rate t, = tocos(0,J/2), where 8, is the angle between local s~i n directions of electrons on neighboring sites i and j, and to is the hopping rate In a perfect ferro~nagnetic state. The application of an external field may align the disordered local spins during a ferromagnetic transition and thereby increase electron conduction or even ~nduce an insulator-metal transition through strong interaction between itinerant electron and local spins. In this model, the MR effect may be induced by an external field w11en the system is in tknsition toward the ferromagnetic state. In this aspect, our data are consistent with this model. that is. MR effects appear only below the C u r~e temperat~lre (T -150 to 200 K) and decreaae below the sacration field (-30 to 40 K) (Fig. 3).However, the role of alkaline earth ion size in the MR effect remains unclear. In doned LaMnO, systems, it appears that smaller d o~a n t size favors the MR effect (5), whereas In doped LaCo03 systems, the opposlte effect 1s observed.
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