Origin of the Antiferroorbital Ordering in LaMnO₃and Other Related Compounds

Abstract

In order to get a realistic view of the origin of the antiferroorbital (AFO) order in LaMnO 3 , MnF 3 and KCuF 3 , both the cooperative Jahn-Teller effect (CJTE) and the orbital-dependent exchange interaction (OEI) are taken into account for the d electron-lattice system in compounds with the perovskite-type structure. The phonon-mediated orbital interaction in the CJTE and the d electron-mediated one in the OEI are treated by use of the mean-field approximation with preserving the symmetry of the crystal, and are joined to give an effective Hamiltonian. It is stressed that the CJTE favors any of the FO and AFO orders, while the OEI does only the AFO order. The present theory is consistent with the observed fact that the AFO order is characterized by an arrangement of alternate mixed orbitals of d x 2 Ày 2 and d 3z 2 Àr 2 , the AFO phase survives even at sufficiently high temperatures in the paramagnetic phase, and the AFO order is always accompanied by the ferrodistortion and the antiferrodistortion.

Polarized SSXANES study of spin ordering in ferromagnetic and paramagnetic phases of La1.2Sr1.65Ca0.15Mn2O7

“…Existence of orbital ordering and spin ordering in AF phase in LaMnO 3 has been extensively studied experimentally [2,14,[17][18][19] as well as theoretically [2,14,[20][21][22][23][24][25][26]. The persistence of orbital ordering in the PM phase has been suggested by, for example, neutron diffraction [27] and X-ray absorption [32,33] and since the orbital ordering is coupled to the spin ordering [20], this should imply existence of local spin ordering in the PM phase as well.…”

High-pressure structural and spectroscopic studies on transition metal compounds

“…Two possibilities of orbital ordering exist: (i) if all of the Jahn-Teller active polyhedra deform in the same manner, one speaks of a ferrodistortion or ferro-orbital ordering, like in Mn 3 O 4 [88]. (ii) In some other cases, however, like in LaMnO 3 [89], the distortions may order in such a way so that the orbitals are aligned perpendicular to each other. This configuration is called antiferro-orbital ordering.…”