Charge ordering via electron-electron interactions in the colossal-magnetoresistive manganites

Abstract

The coupled magnetic and charge-order transition observed in the manganites of the type R 1Ϫx M x MnO 3 near half filling (xӍ1/2) is shown to be the result of the interplay between the doubleexchange, superexchange, and the Coulomb interaction terms in an electronic Hamiltonian. At half filling and temperature Tϭ0 we find, as we increase the strength of the extended-Hubbard repulsion, a first-order transition from a charge-nonordered ferromagnetic metal ͑FN͒ to a charge-ordered antiferromagnetic and insulating ͑AFO͒ ground state. The AFO-FN transition is also obtained by increasing T; however, a small degree of charge order remains in the ferromagnetic phase. The charge-ordered state also ''melts,'' as observed, on the application of a magnetic field, which causes a rapid drop in the transition temperature. Qualitative differences in behavior between members of the manganite series can be understood in terms of small variations in the interaction parameters. ͓S0163-1829͑97͒00930-2͔The observation 1 of colossal magnetoresistance ͑CMR͒ in manganites, which provide the classic examples of the double-exchange magnetic coupling, 2-4 has generated new interest in their intriguing properties. These are compounds of the form R 1Ϫx M x MnO 3 (R ϭ La, Nd, or Pr and M ϭ Sr, Ca, Ba, or Pb͒, which were studied extensively in the 1950s. [5][6][7] In addition to the CMR, other noteworthy properties include ͑1͒ a close competition between ferromagnetic phases, favored by the double-exchange mechanism, antiferromagnetic phases, favored by superexchange and correlation effects, and a paramagnetic phase, which appears at high temperatures T, 7-13 ͑2͒ at half filling, xϭ0.5, this charge ordering can be thought of nominally as Mn 3ϩ ions on one sublattice and Mn 4ϩ ions on the other, superimposed on any magnetic ordering that might exist. For instance, Nd 0.5 Sr 0.5 MnO 3 and La 0.5 Ca 0.5 MnO 3 are charge-ordered antiferromagnets 7,11 ͑AFO's͒ with the ''CE'' crystal structure ͑Fig. 1͒ at low T; on increasing T they go to a ferromagnetic phase via a first-order transition. The AFO phase is insulating but the ferromagnetic phase is metallic. On further increasing T, the ferromagnetic phase goes to an insulating paramagnetic phase via a continuous transition. ͑3͒ The AFO-FN transition temperature T AF→F falls rapidly with increasing magnetic field H, and ͑4͒ at fixed T, the AFO phase can be ''melted'' by the application of the magnetic field.Theoretical studies of these manganites have concentrated on the double-exchange mechanism, 2-4 the effects of electron-phonon interactions, 14 spin-polaron and offdiagonal localization effects, 15 and on spiral, 16 canted, 4,17 or spin and orbital 18 orderings. The relative importance of the various interactions on the physical properties of the manganites is currently the subject of a lively debate.In this paper, we show that the coupled magnetic and charge-order transition observed in these materials, at or near half filling, is described well by an electronic Hamiltonian containing the Hubbard...

Electronic structure and exchange interactions in the manganese-based pyrochlore oxides

“…Being of itinerant character, these electrons are expected to both conduct well and mediate double exchange. Notice that the Coulomb interaction terms in the Hamiltonian that led to the very interesting charge-ordering phenomena in the perovskites 14,15 are unimportant for the pyrochlores because of the small number of Zener carriers and they have been omitted from the Hamiltonian, Eq. ͑1͒.…”

Doping and Field-Induced Insulator-Metal Transitions in Half-Doped Manganites

“…The competition between the CE and A phases appears even in a simple one-orbital model [6] because of the interplay of ferromagnetic double-exchange and AF superexchange between the core t 2g spins of Mn (see also Fig. 1).…”

“…The lowest temperature phase seems to be either the CE phase, consisting of ferromagnetic zigzag chains with relative antiferromagnetic (AF) order (as in La 1/2 Ca 1/2 MnO 3 , where it was first proposed [2,3], and in Nd 1/2 Sr 1/2 MnO 3 [4] or Nd 1/2 Ca 1/2 MnO 3 [5]) or an A-type phase, i.e, ferromagnetic planes with relative AF alignment (as in Pr 1/2 Sr 1/2 MnO 3 [4]). The competition between the CE and A phases appears even in a simple one-orbital model [6] because of the interplay of ferromagnetic double-exchange and AF superexchange between the core t 2g spins of Mn (see also Fig. 1).…”

“…This can be viewed as another manifestation of the colossal magneto-resistance (CMR) in doped manganites [1]. An explanation is that this arises from the proximity of the CE phase to a ferromagnetic phase [6,10,14]; but it is difficult to understand why the parameters in so many systems should all be so finely tuned as to be near the phase boundary.Recently, starting from a large JT coupling picture, a two-fluid e g electron model, one polaronic and localized, and the other band-like and mobile, was proposed and shown to explain, in particular, the CMR in the orbital liquid regime [15]. In this letter, we show how the two types of electrons can emerge from a realistic microscopic model, even at intermediate JT couplings, in the half-doped case where orbital and charge order have to be explicitly included.…”

“…This can be viewed as another manifestation of the colossal magneto-resistance (CMR) in doped manganites [1]. An explanation is that this arises from the proximity of the CE phase to a ferromagnetic phase [6,10,14]; but it is difficult to understand why the parameters in so many systems should all be so finely tuned as to be near the phase boundary.…”