Charge, orbital, and magnetic ordering inLa0.5Ca0.5

Abstract

The unusual magnetic properties of La 0.5 Ca 0.5 MnO 3 were found to be associated with structural and magnetic ordering phenomena, resulting from the close interplay between charge, orbital, and magnetic ordering. Analysis of synchrotron x-ray and neutron powder diffraction data indicates that the anomalous and hysteretic behavior of the lattice parameters occurring between T C ϳ225 K and T N ϳ155 K is due to the development of a Jahn-Teller ͑J-T͒ distortion of the MnO 6 octahedra, the d z 2 orbitals being oriented perpendicular to the orthorhombic b axis. We observed an unusual broadening of the x-ray Bragg reflections throughout this temperature region, suggesting that this process occurs in stages. Below T N , the development of well-defined satellite peaks in the x-ray patterns, associated with a transverse modulation with qϭ[1/2Ϫ,0,0], indicates that quasicommensurate ͑ϳ0͒ orbital ordering occurs within the a-c plane as well. The basic structural features of the charge-ordered low-temperature phase were determined from these satellite peaks. The lowtemperature magnetic structure is characterized by systematic broadening of the magnetic peaks associated with the ''Mn ϩ3 '' magnetic sublattice. This phenomenon can be explained by the presence of magnetic domain boundaries, which break the coherence of the spin ordering on the Mn ϩ3 sites while preserving the coherence of the spin ordering on the Mn ϩ4 sublattice as well as the identity of the two sublattices. The striking resemblance between these structures and the structural ''charge ordering'' and ''discommensuration'' domain boundaries, which were recently observed by electron diffraction and real-space imaging, strongly suggests that these two types of structures are the same and implies that, in this system, commensurate long-range charge ordering coexists with quasicommensurate orbital ordering.

Charge and lattice dynamics of ordered state in La1/2Ca1/2MnO3: infrared reflection spectroscopy study

“…5 (c,d)). This could be due to the variation of relevant bond distances, documented in the neutron diffraction studies [5,7] and/or the effect of magnetic order on corresponding force constants, similar to those reported for other magnetic materials [21,22].…”

“…5(a)). These new lines in the spectra could be a consequence of either the formation of a novel orthorhombic phase below T C =234 K, which has the same symmetry (space group Pnma), but slightly different lattice parameters compared to the room temperature phase of La 1/2 Ca 1/2 MnO 3 [7], or the appearance of a superstructure with doubled a lattice parameter and the space group P2 1 /m [5]. The observed intensity increase over rather wide temperature interval below T C indicates that the volume fraction of the novel phase increases upon sample cooling, in agreement with [7].…”

“…Well defined anomalies of physical properties at commensurate carrier concentrations of x=N/8 (where N=1, 3, 4, 5, and 7) and x = L/3 (where L=1,2) were unambiguously established. Of particular interest is the phenomenon of charge and orbital ordering, most clearly pronounced for x=1/2 [4,5,6,7,8,9,10].…”

“…Well defined anomalies of physical properties at commensurate carrier concentrations of x=N/8 (where N=1, 3, 4, 5, and 7) and x = L/3 (where L=1,2) were unambiguously established. Of particular interest is the phenomenon of charge and orbital ordering, most clearly pronounced for x=1/2 [4,5,6,7,8,9,10].In this communication we report the results of an infrared reflection study of La 1/2 Ca 1/2 MnO 3 over broad frequency range and at temperatures, covering the transitions from the high temperature paramagnetic to ferromagnetic and, upon further cooling, to antiferromagnetic phase. By making use of Kramers-Kronig analysis we obtained the spectral dependence of conductivity.…”

“…5 (c,d)). This could be due to the variation of relevant bond distances, documented in the neutron diffraction studies [5,7] and/or the effect of magnetic order on corresponding force constants, similar to those reported for other magnetic materials [21,22].It is important to note that the dielectric response of La 1/2 Ca 1/2 MnO 3 , as obtained from the reflection spectroscopy data, is consistent with the phase separation sce-3 nario for manganese oxides (see review [23] and references cites therein). Indeed, even below T N the dielectric function is not typical of an insulator, but contains the Drudelike component, a signature for presence in the sample of a conducting phase.…”