Quantum magnons of the intermediate phase of the half-doped magnanite oxides

MANUEL, LUIS O.; BUITRAGO, I. R.; VENTURA, CECILIA

Buenos Aires

Workshop; X Latin American Workshop on Magnetism, Magnetic Materials and their Applications; 2013

Comisión Nacional de Energía Atómica

At half doping, the ground state of manganite perovskite oxides like \comp{R}{Ca}, where R is a trivalent ion such as La, Pr, etc, is still unclear. In three-dimensional compounds, experiments have found obstacles to determine unambiguously the combined magnetic, charge and orbital order present. Many experimental findings agree better with the intermediate phase, introduced by Efremov, van den Brink and Khomskii in 2004 [Nature Mats. 3, 853]. This consists of spin dimers ( thus incorporating aspects of the Zener polaron phase (ZP) proposed in 2002 by Daoud-Aladine et al. [Phys. Rev. Lett. 89, 097205]), though formed by a pair of parallel Mn spins of different magnitude, in principle (thereby allowing for a degree of Mn charge disproportionation $\delta$: not necessarily as large as that of \Mn{3}-\Mn{4} in Goodenough's original CE phase [Phys. Rev. 100, 564 (1955)]). In the intermediate phase, consecutive spin dimers localed along the planar zig-zag chains are oriented at a constant relative angle $\phi$ between them. Varying $\delta$ and $\phi$, the intermediate phase should allow to continuously interpolate between the two limiting cases of: the CE phase and the dimer phase denoted as "orthogonal intermediate $\pi/2-$phase". It is not easy to find a microscopic model able to describe the phenomenological intermediate phase adequately for the spin, charge, and orbital degrees of freedom simultaneously. Here, we focus on the study of the quantum spin excitations of a planar model of interacting localized spins, which we found can stabilize the intermediate phase classically. The model includes ferromagnetic nearest-neighbour (NN) couplings along the zig-zag chains, antiferromagnetic (AF) inter-chain coupling, as well as other couplings along the chains such as: diagonal AF second-neighbour and biquadratic NN inter-dimer couplings. We discuss the comparison of the quantum magnons of the intermediate phase with those of the CE and ZP phases, in the context of recent experimental results.