Magnetic Instabilities and Phase Diagram of the Double-Exchange Model in Infinite Dimensions

Dynamical mean-field theory is used to study the magnetic instabilities and phase diagram of the double-exchange (DE) model with Hund's coupling J_H >0 in infinite dimensions. In addition to ferromagnetic (FM) and antiferromagnetic (AF) phases, the DE model supports a broad class of short-range ordered (SRO) states with extensive entropy and short-range magnetic order. For any site on the Bethe lattice, the correlation parameter q of a SRO state is given by the average q=<sin^2(theta_i/2)>, where theta_i is the angle between any spin and its neighbors. Unlike the FM (q=0) and AF (q=1) transitions, the transition temperature of a SRO state (T_{SRO}) with 0<q<1 cannot be obtained from the magnetic susceptibility. But a solution of the coupled Green's functions in the weak-coupling limit indicates that a SRO state always has a higher transition temperature than the AF for all fillings p<1 and even than the FM for 0.26\le p \le 0.39. For 0.39<p<0.73, where both the FM and AF phases are unstable for small J_H, a SRO phase has a non-zero T_{SRO} except close to p=0.5. As J_H increases, T_{SRO} eventually vanishes and the FM dominates. For small J_H, the T=0 phase diagram is greatly simplified by the presence of the SRO phase. A SRO phase is found to have lower energy than either the FM or AF phases for 0.26\le p<1. Phase separation (PS) disappears as J_H-->0 but appears for J_H\neq 0. For p near 1, PS occurs between an AF with p=1 and either a SRO or a FM phase. The stability of a SRO state at T=0 can be understood by examining the interacting DOS,which is gapped for any nonzero J_H in an AF but only when J_H exceeds a critical value in a SRO state.