Electronic structure of CMR manganites (invited)

A ''colossal'' negative magnetoresistance (CMR) occurs in manganites at a first-order ferromagnetic transition. The Mn4+ and high-spin Mn3+ ions each contain localized t(3) configurations; the t(3)-p(pi)-t(3) superexchange interactions are antiferromagnetic. The orbital degeneracy of localized Mn3+:t(3)e(1), E-5(g) configurations is lifted by cooperative static or dynamic Jahn-Teller deformations. Strong e-electron coupling to oxygen displacements, static or dynamic, introduces ferromagnetic e(1)-p(sigma)-e(0) interactions either via superexchange or, for fast Mn3+ to Mn4+ electron transfer relative to the spin-relaxation time (tau(h) < tau(s)), via a stronger double exchange. At the first-order ferromagnetic transition, a change from tau(h) approximate to <(h)over bar /W-sigma> > omega(R)(-1) to tau(h) < omega(R)(-1) occurs within mobile molecular units, where W-sigma is the bandwidth for states of e-orbital parentage and omega(R)(-1) is the period of the optical-mode lattice vibration that traps a mobile hole as a small-polaron Mn4+ ion. T-C increases with the fraction of double-exchange couplings, and this fraction increases with W-sigma and omega(R) at the transition from polaronic to itinerant-electron behavior below T-C. The bandwidth W sigma similar to epsilon(sigma)lambda(sigma)(2) cos phi[cos(theta(ij)/2)] depends on the covalent mixing parameter lambda(sigma), which increases with pressure, as well as on the Mn-O-Mn bond angle (180 degrees-phi), which increases with the tolerance factor t that measures the equilibrium bond-length mismatch, and on the angle theta(ij) between neighboring spins so that W-sigma increases with the spontaneous magnetization on cooling below T-C. In the compositions Ln(0.7)A(0.3)MnO(3) with A=Ca or Sr, T-C increases with t over the range 0.96 less than or equal to t less than or equal to 0.98 where the transition at T-C is first order. The CMR is greatest near t approximate to 0.96; it reflects a trapping out of mobile holes with decreasing temperature in the paramagnetic phase and their progressive release with decreasing temperature in the ferromagnetic phase where spin entropy is exchanged for configurational entropy. (C) 1997 American Institute of Physics.