Magnetic and orbital ordering in cuprates and manganites

Oleś, Andrzej M.; Cuoco, Mario; Perkins, Natalia B.
July 2000
AIP Conference Proceedings;2000, Vol. 527 Issue 1, p226
Academic Journal
We address the role played by orbital degeneracy in strongly correlated transition metal compounds. The mechanisms of magnetic and orbital interactions due to double exchange (DE) and superexchange (SE) are presented. Specifically, we study the effective spin-orbital models derived for the d[sup9] ions as in KCuF[sub3], and for the d[sup4] ions as in LaMnO[sub3], for spins S = 1/2 and S = 2, respectively. The magnetic and orbital ordering in the undoped compounds is determined by the SE interactions that are inherently frustrated, carrying both antiferromagnetic (AF) and ferromagnetic (FM) channels due to low-spin and high-spin excited states, respectively. As a result, the classical phase diagrams consist of several magnetic phases which all have different orbital ordering: either the same orbitals (x[sup2] - y[sup2] or 3z[sup2] - r[sup2]) are occupied, or two different linear combinations of e[subg] orbitals stagger, leading either to G-AF or to A-AF order. These phases become unstable near orbital degeneracy, leading to a new mechanism of spin liquid. The model for d[sup4] Mn[sup3+] ions in collosal magnetoresistance compounds provides and explanation of the observed A-AF phase, with the orbital order stabilized additionally by the Jahn-Teller effect. Possible extensions of the model to the doped compounds are discussed both for the insulating polaronic regime and for the metallic phase. It is shown that the spin waves are well described by SE in the insulating regime, while they are explained by DE for degenerate e[subg] orbitals in the metallic FM regime. Orbital excitations contribute to the hole dynamics in FM planes of LaMnO[sub3], characterized by new quasiparticles reminiscent of the t-J model, and a large redistribution of spectral weight with respect to mean-field treatments. Finally, we point out some open problems in the present understanding of doped manganites.


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