Spin excitations of the intermediate phase of half-doped manganites

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

Parma

Simposio; JEMS2012 Joint European Magnetic Symposia; 2012

Dipartimento de Fisica, Universita degli Studi di Parma

Near half-doping, perovskite Mn oxides exhibit rich phase diagrams with a strong competition of different phases, due to the interplay of the magnetic, charge and orbital degrees of freedom. At half-doping, two main phases were introduced in order to describe most of the experimental findings. One is the joint spin, charge and orbital ordering known as CE phase, proposed by J. Goodenough in 1955. The other is the Zener polaron (or spin dimer) phase proposed in 2002 by Daoud-Aladine et al. In some half-doped manganites, though, experiments are better described by an intermediate phase between them, introduced in 2004 by Khomskii et al. In fact, the intermediate phase proposed consists of spin dimers, though formed by a pair of parallel spins of different magnitude in principle (thus allowing for a charge disproportionation p, though not necessarily as large as that of Mn3+-Mn4+ in the CE phase). Consecutive spin dimers located along the planar zig-zag chains are oriented at a constant relative angle θ between them. Varying p and θ the intermediate phase allows a continuous interpolation between the CE and the Zener polaron phases. In this work, we propose a microscopic model based on localized spins which has the intermediate phase as its classical ground state. We then use this model to predict the quantum spin excitations of the intermediate phase of half-doped manganites. New inelastic neutron scattering experiments able to measure the spin excitations of 3-D manganites, could be compared with our predictions and thus allow to settle the open question about the nature of their ground state at half-doping.