Effect of Jahn-Teller coupling on Curie temperature in the Double Exchange Model

TL;DR

This paper investigates how Jahn-Teller coupling influences the magnetic properties of manganites, showing that increased JT energy suppresses ferromagnetism and lowers the Curie temperature through an effective model derived from the double exchange framework.

Contribution

The study introduces a new effective Heisenberg model incorporating Jahn-Teller effects and analyzes its impact on magnetic phase transitions and Curie temperature in manganites.

Findings

Jahn-Teller coupling suppresses ferromagnetism in the model.

Increasing JT energy lowers the critical electron density for phase transition.

Jahn-Teller coupling significantly reduces the Curie temperature.

Abstract

We consider the two-band double exchange model for manganites with Jahn-Teller (JT) coupling and explore the suppression of the ferromagnetism because of the JT distortion. The localized spins of the $\emph{t}_{2g}$ electrons are represented in terms of the Schwinger bosons, and two spin-singlet Fermion operators are introduced instead of the $e_{g}$ electrons' operators. In terms of the new Fermi fields the on-site Hund's interaction is in a diagonal form and one accounts for it exactly. Integrating out the spin-singlet fermions, we derive an effective Heisenberg model for a vector which describes the local orientations of the total magnetization. The exchange constants are different for different space directions and depend on the density $n$ of $\emph{e}_{g}$ electrons and JT energy. At zero temperature, with increasing the density of the $\emph{e}_{g}$ electrons the system undergoes phase transition from ferromagnetic phase $(0<n<n_c)$ to A-type antiferromagnetic phase $(n_c<n)$. The critical value $n_c$ decreases as JT energy is increased. At finite temperature we calculate the Curie temperature as a function of electron density for different JT energy. The results show that JT coupling strongly suppresses the spin fluctuations and decreases the Curie temperature.