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

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.Comment: 4 pages, 3 figure

Canted Ferromagnetism in Double Exchange Model with on-site Coulomb Repulsion

The double exchange model with on-site Coulomb repulsion is considered. Schwinger-bosons representation of the localized spins is used and two spin-singlet Fermion operators are introduced. In terms of the new Fermi fields the on-site Hund's interaction is in a diagonal form and the true magnons of the system are identified. The singlet fermions can be understood as electrons dressed by a cloud of repeatedly emitted and reabsorbed magnons. Rewritten in terms of Schwinger-bosons and spin-singlet fermions the theory is U(1) gauge invariant. We show that spontaneous breakdown of the gauge symmetry leads to \emph{\textbf{canted ferromagnetism with on-site spins of localized and delocalized electrons misaligned}}. On-site canted phase emerges in double exchange model when Coulomb repulsion is large enough. The quantum phase transition between ferromagnetism and canted phase is studied varying the Coulomb repulsion for different values of parameters in the theory such as Hund's coupling and chemical potential.Comment: 8 pages, 6 figure