EFFECT OF THE MAGNETIC-FIELD ON THE SPIN, CHARGE AND ORBITAL ORDERED STATES IN PEROVSKITE-TYPE MANGANESE OXIDES

In the case of La1-xSrxMnO3, the ferromagnetic and metallic states hav e been understood in terms of the double exchange model, while in the compounds with relatively narrow one-electron bandwidth, such as Nd1-x SrxMnO3 or Pr1-xCaxMnO3, the double exchange is weakened and the charg e-ordered (Mn3+/Mn4+ ordered) state emerges at x similar to 1/2. In th e charge-ordered state, the e(g)-orbitals also order simultaneously an d the antiferromagnetic non metallic state is realized at low temperat ures. Recent investigations, however, have shown that the antiferromag netic charge-ordered state can be altered to the ferromagnetic metalli c state by applying an external magnetic field. The competition betwee n the double exchange and the charge/orbital-ordering instability play s an important role in the observed field-induced transition, and also in other versatile field-induced phenomena in the perovskite-type man ganese oxides. The robustness of charge ordering which depends on both of the (R, A) cations and the carrier concentration is a key factor f or those unconventional features. (C) 1998 Elsevier Science B.V. All r ights reserved.