Effects of the on-site Coulomb repulsion in double-exchange magnets

Abstract

We investigate the zero-temperature phase diagram and spin-wave properties of a double exchange magnet with on-site Hubbard repulsion. It is shown that even within a simple Hartree -Fock approach this interaction (which is often omitted in theoretical treatments) leads to qualitatively important effects which are highly relevant in the context of experimental data for the colossal magnetoresistance compounds. These include the asymmetry of the doping dependence of spin stiffness, and the zone-boundary "softening" of spin wave dispersion. Effects of Hubbard repulsion on phase separation are analyzed as well. We also show that in the ferromagnetic phase, an unusual temperature-dependent effective electron-electron interaction arises at finite T . The mean-field scheme, however, does not yield the experimentally observed density of states depletion near the Fermi level. We speculate that proper treatment of electron-electron interactions may be necessary for understanding both this important feature and more generally the physics of colossal magnetoresistance phenomenon.

Evolution of Spin-Wave Excitations in Ferromagnetic Metallic Manganites

“…This is in sharp contrast to the expectation of all DE based models, where D increases with x [solid line in Fig. 5(a) [3]. The (green) solid, (blue) dash-dotted, and (red) dashed lines in (c) are calculations from different models in [15].…”

“…Three classes of models have been proposed to explain the origin of such deviations. The first is based on the DE mechanism, considering the effect of the on-site Coulomb repulsion [3] or the conducting electron band (e g ) filling dependence of the DE and superexchange interactions [11]. The second suggests that magnon-phonon coupling [8,12] or the effects of disorder on the spin excitations of DE systems [13] is the origin for the zone boundary magnon softening.…”

“…In the strong Hundcoupling limit, spin-wave excitations of a DE ferromagnet below the Curie temperature T C can be described by a Heisenberg Hamiltonian with only the nearest neighbor exchange coupling [2]. At the long wavelength (small wavevector q), spin-wave stiffness D measures the average kinetic energy of charge carriers and therefore should increase with increasing x [2,3]. While spin dynamics of some manganites initially studied appeared to follow these predictions [4,5], later measurements revealed anomalous zone boundary magnon softening deviating from the nearest neighbor Heisenberg Hamiltonian for other materials with x ∼ 0.3 [6,7,8,9,10].…”

“…The possibility of on-site disorder-induced zone boundary softening is ruled out, as such a theory expects an enhanced softening with either increasing disorder or decreasing x [13], both contrary to the observation. Similarly, it is unclear how onsite Coulomb repulsion in a DE mechanism can explain the doping independent behavior of the spin-wave stiffness [3]. The magnon and A 1−x A ′ x -site optical phonon coupling (crossing) scenario postulated for LCMO30 [8] also has difficulty in explaining the evolution of J 4 /J 1 , as the average A 1−x A ′ x -site mass and frequencies of associated optical phonon modes do not vary dramatically from LCMO25 to SSMO45.…”

Renormalization Group Approach for the Double Exchange Ferromagnets

“…So far, some magnetic properties has been studied by investigating the magnon spectrum at non-RudermanKittelKasuyaYosida (non-RKKY) indirect exchange in conducting ferromagnets [3], in spin wave theory of double exchange ferromagnets [4], by application of continuous canonical transformation [5] and slave fermion approach to the quantum double-exchange (DEX) model [6], by analysis of on-site Hubbard repulsion eects [7] and interacting spin waves in the ferromagnetic Kondo lattice model [8], etc. Also Monte Carlo scheme of enhanced ferromagnetism from electron electron interactions in double exchange models [9] has been considered.…”