Ferromagnetic Kondo model for manganites: Phase diagram, charge segregation, and influence of quantum localized spins

Abstract

The phase diagram of the ferromagnetic Kondo model for manganites is investigated using computational techniques. In clusters of dimensions 1 and 2, Monte Carlo simulations in the limit where the localized spins are classical show a rich low temperature phase diagram with three dominant regions: (i) a ferromagnetic phase, (ii) phase separation between hole-poor antiferromagnetic and hole-rich ferromagnetic domains, and (iii) a phase with incommensurate spin correlations. Possible experimental consequences of the regime of phase separation are discussed. Studies using the Lanczos algorithm and the Density Matrix Renormalization Group method applied to chains with localized spin 1/2 (with and without Coulombic repulsion for the mobile electrons) and spin 3/2 degrees of freedom give results in excellent agreement with those in the spin localized classical limit. The Dynamical Mean Field (D = ∞) approximation was also applied to the same model. At large Hund coupling phase separation and ferromagnetism were identified, again in good agreement with results in low dimensions. In addition, a Monte Carlo study of spin correlations allowed us to estimate the critical temperature for ferromagnetism T F M c in 3 dimensional clusters. It is concluded that T F M c is compatible with current experimental results.Actually, the phase diagram of La 1−x Ca x MnO 3 is very rich with not only ferromagnetic phases, but also regions with charge-ordering and antiferromagnetic correlations at x > 0.5 [6], and a poorly understood "normal" state above the critical temperature for ferromagnetism, T F M c , which has insulating characteristics at x ∼ 0.33. Finding an insulator above T F M c is a surprising result since it would have been more natural to have a standard metallic phase in that regime which could smoothly become a ferromagnetic metal as the temperature is reduced. Some theories for manganites propose that the insulating regime above T F M c is caused by a strong correlation between electronic and phononic degrees of freedom [7]. Other proposals include the presence of Berry phases in the DE model that may lead to electronic localization [8]. On the other hand, the regime of charge ordering has received little theoretical attention and its features remain mostly unexplored. To complicate matters further, recent experiments testing the dynamical response of manganites have reported anomalous results in the ferromagnetic phase using neutron scattering [9], while in photoemission experiments [10] the possible existence of a pseudogap above the critical temperature was reported.

Inhomogeneous magnetism in La-dopedCaMnO3. II. Nanometric-scale spin clusters and long-range spin canting

“…1). From a theoretical point of view, the formation of an inhomogeneous G-AFM+FM spin-canted state in electron-doped manganites, evidenced in this work, might be the result of a balance between the well-known electronic instability of the homogeneous spin-canted G-AFM state 14,15,16,17,18 and the large Coulomb energy cost of a radical phase segregation scenario where purely FM droplets are formed into a pure G-AFM background.…”

Charge-orbital ordering and phase separation in the two-orbital model for manganites: Roles of Jahn-Teller phononic and Coulombic interactions

“…For this purpose it is necessary to compare the energies of the various ground states and construct a plot of the density n vs following steps already described in detail in previous publications, 8 and in the previous section. Figure 9͑a͒ shows that all the densities studied here are actually FIG.…”

Spin and lattice effects in the Kondo lattice model

“…As most of the recent approaches to CMR are based on Monte Carlo simulations of one dimensional models, 15 it has been argued that for finite temperatures a classical spin represents a reasonable approximation. However, this may not be the best approach, as the quantum fluctuations are the strongest in one dimension.…”