Ferromagnetic polarons in manganites

Abstract

Using the Lanczos method in linear chains we study the double exchange model in the low concentration limit, including an antiferromagnetic super-exchange K. In the strong coupling limit we find that the ground state contains ferromagnetic polarons whose length is very sensitive to the value of K/t. We investigate the dispersion relation, the trapping by impurities, and the interaction between these polarons. As the overlap between polarons increases, by decreasing K/t, the effective interaction between them changes from antiferromagnetic to ferromagnetic. The scaling to the thermodynamic limit suggests an attractive interaction in the strong coupling regime (J h > t) and no binding in the weak limit (J h ≃ t).

Origin of the multiferroicity in BiMn2O5 from first-principles calculations

“…1, the Mn 4 + cations are linked by O2 and O3 oxygens along the c direction, and the two Mn 4 + neighbors are also linked by Mn 3 + cations via O3 and O4 atoms. The magnetic indirect (Mn 4 + -O-Mn 3 + -O-Mn 4 + ) coupling mechanism should be super-exchange or double-exchange [32,33], because the lattice distortion in this compound leads to the degeneration within Mn1 3d (t 2g and e g ) and Mn2 3d (d 3z2 À r2 , d xy , d xz , and d yz ) orbitals. The double-exchange magnetic mechanism requires Mn 3 + (t 2g 3 , e g 1 ) and Mn 4 + (t 2g 3 , e g 0 ) as magnetically equivalent sites for e g electrons hopping and for maintaining the e g hole carrier concentration, however, the different spin arrangement and orbital occupation prevent the double-exchange magnetic interaction and in favor of the super-exchange magnetic interaction.…”

Self-trapped magnetic polaron: Exact solution of a continuum model in one dimension

“…[3][4][5]8,19 The three terms in the Hamiltonian describe, respectively, the kinetic energy of the itinerant electron, the Hund's-rule coupling between the electron and the localized spin on each site on the lattice, and the antiferromagnetic exchange between the localized spins. The symbols a i † and a i denote the creation and the annihilation operators for the electron state with spin at site i, ͗i j͘ denotes summation over nearest-neighbor sites, s i is the spin of the itinerant electron, and S i denotes the spin of the localized electron, the latter treated as classical spins.…”

“…[9][10][11][12][13] Interest on the magnetic polaron problem has recently resurfaced in light of its relevance to a number of new systems, e.g., the high-T c materials and the colossal magnetoresistive manganites. [14][15][16][17][18][19] In this paper, we formulate a continuum model and solve it exactly using the Euler-Lagrange method and study the energetics and the spin structure of the magnetic polaron. The Euler-Lagrange equations are solved numerically using an iterative procedure that yields ''exact'' results in the sense that the solution can be obtained to any arbitrary accuracy.…”

New recommended values of the fundamental physical constants (CODATA 2006)

“…Other authors also found ferromagnetic polarons for the almost empty lower Kondo band (i.e. very few electrons) for S = 1/2 core spins [28,29], for the 1D AF Kondo model with few electrons [30], and for the 1D paramagnet at higher temperatures [31]. Small ferromagnetic droplets were predicted from energy considerations [32].…”