Onset of metallic ferromagnetism in a doped spin–orbital chain

Abstract

Starting from a spin-orbital model for doped manganites, we investigate a competition between ferromagnetic and antiferromagnetic order in a one-dimensional model at finite temperature. The magnetic and orbital order at half filling support each other and depend on a small antiferromagnetic superexchange between t2g spins and on an alternating Jahn-Teller potential. The crossover to a metallic ferromagnetic phase found at finite doping is partly suppressed by the Jahn-Teller potential which may localize eg electrons.[published in phys. stat. sol. 242, 311 (2005)] Copyright line will be provided by the publisher 1 Introduction The transition from an insulating to metallic behavior in doped manganites, observed as a function of temperature or filling, is one of the outstanding problems in the field of strongly correlated systems. The theoretical challenge is to understand a rich variety of metallic, insulating, magnetically ordered and orbital ordered phases of doped manganites in terms of the dynamics of correlated e g electrons which involves their orbital degrees of freedom [1]. Magnetic interactions: spin superexchange (SE) and double exchange (DE), as well as orbital SE, frustrate each other and their competition leads to several magnetic phases, including phase separation [2].While orbital order supports A-type antiferromagnetic (AF) phase in LaMnO 3 [3], one expects that e g orbitals play an important role in the insulating ferromagnetic (FM) phase at finite doping, below the crossover to a metallic FM phase [1]. Here we investigate a one-dimensional (1D) model derived from the realistic spin-orbital model for manganites [3], showing an interplay between spin and orbital order.

Temperature effects in a spin‐orbital model for manganites

“…In reality it depends on the total number of d electrons occupying two interacting Mn ions [5]. However, in the undoped systems one finds 4 d configuration at each site and one may replace this part of the superexchange by a Heisenberg Hamiltonian with a properly chosen exchange constant 0 J > ¢ favoring AF order:…”

“…depending on the angle ij θ between the spins and on the phase ij χ , which cancels at 0 x = . The superexchange term J H describes g e orbital interactions, as derived recently for a 1D chain [4] from the full coupled spin-orbital dynamics by replacing the spin scalar products by the actual averages…”

“…Ref. [6]) combined with exact diagonalization for the electron degrees of freedom [4,7]. In this method, the many-particle Hamiltonian for each spin configuration S determined by { } calculated from the lowest eigenstates and we found that at most one or two percent are missed for…”

“…For the same reason, spin order is easily destroyed with rising temperature, while alternating orbital correlations can persist to temperatures where FM order has already melted. A scenario for the AF phase observed in LaSrMnO 4 is presented.2 Effective spin-orbital model at finite temperatureWe study an effective spin-orbital superexchange model derived for the one-dimensional (1D) chain [4], generalized here to two-dimensional (2D) planes of layered manganites. The model depends on spin configuration S and takes the form, ( ) J J z…”

“…We study an effective spin-orbital superexchange model derived for the one-dimensional (1D) chain [4], generalized here to two-dimensional (2D) planes of layered manganites. The model depends on spin configuration S and takes the form, ( ) J J z…”