Spin Polaron Effective Magnetic Model for La_{0.5}Ca_{0.5}MnO_3

TL;DR

This paper proposes a magnetic model for La_{0.5}Ca_{0.5}MnO_3 based on a Zener polaron picture, revealing complex interactions that interpolate between different magnetic structures and suggest a non-bipartite polaron order.

Contribution

It introduces a cubic Heisenberg model with specific interactions derived from ab initio calculations, challenging the conventional charge order paradigm.

Findings

Model interpolates between A and CE-type magnetic structures.

Ab initio calculations suggest a non-bipartite polaron ordering.

Behavior influenced by tuning frustrating intraplanar interactions.

Abstract

The conventional paradigm of charge order for La_{1-x}Ca_xMnO_3 for x=0.5 has been challenged recently by a Zener polaron picture emerging from experiments and theoretical calculations. The effective low energy Hamiltonian for the magnetic degrees of freedom has been found to be a cubic Heisenberg model, with ferromagnetic nearest neighbor and frustrating antiferromagnetic next nearest neighbor interactions in the planes, and antiferromagnetic interaction between planes. With linear spin wave theory and diagonalization of small clusters up to 27 sites we find that the behavior of the model interpolates between the A and CE-type magnetic structures when a frustrating intraplanar interaction is tuned. The values of the interactions calculated by ab initio methods indicate a possible non-bipartite picture of polaron ordering differing from the conventional one.