Three--body Correlation Effects on the Spin Dynamics of Double--Exchange Ferromagnets

Abstract

We present a variational calculation of the spin wave excitation spectrum of double--exchange ferromagnets in different dimensions. Our theory recovers the Random Phase approximation and 1/S expansion results as limiting cases and can be used to study the intermediate exchange coupling and electron concentration regime relevant to the manganites. In particular, we treat exactly the long range three--body correlations between a Fermi sea electron--hole pair and a magnon excitation and show that they strongly affect the spin dynamics in the parameter range relevant to experiments in the manganites. The manifestations of these correlations are many-fold. We demonstrate that they significantly change the ferromagnetic phase boundary. In addition to a decrease in the magnon stiffness, we obtain an instability of the ferromagnetic state against spin wave excitations close to the Brillouin zone boundary.Within a range of intermediate concentrations, we find a strong softening of the spin wave dispersion as compared to the Heisenberg ferromagnet with the same stiffness, which changes into hardening for other concentrations. We discuss the relevance of these results to experiments in colossal magnetoresistance ferromagnets.

arXiv comment14 pages, 11 figures, published in Phys. Rev. B (1 figure added, references added)

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• Magnons in Ferromagnetic Metallic Manganites
• Spin dynamics in (III,Mn)V ferromagnetic semiconductors: the role of correlations
• Femtosecond Control of the Magnetization in Ferromagnetic Semiconductors
• Manipulating Femtosecond Spin--Orbit Torques with Laser Pulse Sequences to Control Magnetic Memory States and Ringing
• Non--Heisenberg Spin Dynamics of Double-Exchange Ferromagnets with Coulomb Repulsion
• Spin-charge coupling in a band ferromagnet: magnon-energy reduction, anomalous softening, and damping
• Magnetization Relaxation and Collective Spin Excitations in Correlated Double--Exchange Ferromagnets
• Correlating Quasi-Electron Relaxation with Quantum Femtosecond Magnetism in the Order Parameter Dynamics of Insulating Manganites
• Fermionic representation for the ferromagnetic Kondo lattice model -- diagrammatic study of spin-charge coupling effects on magnon excitations
• Nonlinear Optical studies of the Transient Coherence in the Quantum Hall System
• Strongly anharmonic collective modes in a coupled electron-phonon-spin problem
• Spin-charge and spin-orbital coupling effects on spin dynamics in ferromagnetic manganites
• Magnon self energy in the correlated ferromagnetic Kondo lattice model: spin-charge coupling effects on magnon excitations in manganites