Ferromagnetic Polarons Research Articles

Ferromagnetic polarons inLa0.5Ca0.5MnO3andLa0.33Ca0.67MnO3

Unrestricted Hartree-Fock calculations on La0.5Ca0.5MnO3 and La0.33Ca0.67MnO3 in the full magnetic unit cell show that the magnetic ground states of these compounds consist of 'ferromagnetic molecules' or polarons ordered in herring-bone patterns. Each polaron consists of either three or five Mn ions separated by O- ions with a magnetic moment opposed to those of the Mn ions. Ferromagnetic coupling within the polarons is strong while coupling between them is relatively weak. Magnetic moments on the Mn ions range between 3.8 and 3.9 Bohr magnetons in La0.5Ca0.5MnO3 and moments on the O- ions are -0.7 Bohr magnetons. Each polaron has a net magnetic moment of 7.0 Bohr magnetons, in good agreement with recently reported magnetisation measurements from electron microscopy. The polaronic nature of the electronic structure reported here is obviously related to the Zener polaron model recently proposed for Pr0.6Ca0.4MnO3 on the basis of neutron scattering data.

Magnetic polarons in the one-dimensional ferromagnetic Kondo model

The ferromagnetic Kondo model with classical corespins is studied via unbiased Monte Carlo simulations. We show that with realistic parameters for the manganites and at low temperatures, the double-exchange mechanism does not lead to phase separation in one-dimensional chains but rather stabilizes individual ferromagnetic polarons. Within the ferromagnetic polaron picture, the pseudogap in the one-particle spectral function ${A}_{k}(\ensuremath{\omega})$ can easily be explained. Ferromagnetic polarons also clear up a seeming failure of the double-exchange mechanism in explaining the comparable bandwidths in the ferromagnetic and paramagnetic phase. For our analysis, we extend a simplified model, the finite-temperature uniform hopping approach (UHA), to include polarons. It can easily be evaluated numerically and provides a simple quantitative understanding of the physical features of the ferromagnetic Kondo model.

Magnetic polarons, clusters, and their effect on the electric properties of weakly doped lanthanum manganites

The resistivity, magnetoresistance, thermopower, and magnetic susceptibility of La1−xAxMnO3(A≡Ca,Sr;x=0.07–0.1) single crystals are investigated in the temperature range from 77 to 400 K. Sharp changes in the properties (the resistivity activation energy ΔEρ, its temperature coefficient γ, the thermopower activation energy ΔES, the magnetoresistance, and the appearance of spontaneous magnetization) of these crystals occur near a temperature of 275±25 K, which is approximately twice as high as their Curie point TC and approximately half of the structural transition temperature. The results are explained by the phase separation: the formation of ferromagnetic clusters. The phase separation occurs through the coalescence of small-radius unsaturated magnetic polarons, in which only two or three magnetic moments of Mn are polarized, into a large-radius ferromagnetic polaron (a cluster about 10–12 A in size) with several charge carriers. As a result, the short-range order occurs in the cluster at a temperature of about 275 K, which is close to TC of conducting doped manganites. The results of the experimental studies of the resistivity and the magnetoresistance as functions of temperature and magnetic field and the estimates agree well with the cluster model.

Magnetic polarons in Ca1−xYxMnO3

Experimental evidence show that in the magnetoresistive manganite Ca_(1-x) Y_xMnO_3, ferromagnetic (FM) polarons arises in an antiferromagnetic (AF) background, as a result of the doping with Yttrium. This hypothesis is supported in this work by classical Monte Carlo (MC) calculations performed on a model where FM Double Exchange (DE) and AF Superexhange (SE) compite.

Quantum decoherence in the spectral functions of undoped lamno3.

We study the one hole spectral function in a model for LaMnO3 including both the effects of orbital ordering and the quantum decoherence due to the antiferromagnetic coupling between the ferromagnetic layers. We find that the classical picture of a ferromagnetic polaron does not apply and free dispersion is replaced by rigid quasiparticles on the scale of magnetic excitations, while the spectra are dominated by the incoherent spectral weight at high energies. These results have important implications on the in-plane transport and angular resolved photoemission in the manganites.

Ferromagnetic polarons in dilute La-doped Ca 2RuO 4

Ca 2RuO 4 is an antiferromagnetic insulator with a Neel temperature at T N =110 K and a Mott transition at T MI =357 K . Slight substitution of trivalent La for divalent Ca in Ca 2RuO 4 not only drastically reduces T MI but also simultaneously precipitate robust ferromagnetism in the antiferromagnetic host. It is believed that the ferromagnetic behavior is closely associated with the kinetic energy of the electron added from the La ion that modifies the magnetic structure, and as a result, forming ferromagnetic polarons in the antiferromagnetic background. These dramatic changes underline the subtlety of the competition between the antiferromagnetic and ferromagnetic coupling in Ca 2RuO 4.

Coexistence of large and small polarons in manganites

The interplay of the electron-phonon interaction and of the double- and superexchange magnetic effects is analyzed in the ${\mathrm{La}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}$ perovskites with $0<x<0.5.$ By using an analytical variational scheme that allows us to treat the electron-phonon interaction in a fully quantum manner it is shown that in the intermediate electron-phonon coupling regime a charge-density instability occurs near the metal-insulator transition, induced by either temperature or hole doping. The system segregates in antiferromagnetic or paramagnetic small polarons and ferromagnetic large polaron domains characterized by different values of the lattice distortions that are satisfactory compared with those experimentally observed.

Ferromagnetic polarons in manganites

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 ${(J}_{h}\ensuremath{\simeq}t).$

Magnetic polarons in materials with colossal magnetoresistance

We construct a phase diagram for the materials with colossal magnetoresistance. The basic model which describes these materials is a double-exchange model. We rigorously prove that a ground state of this model corresponds to a phase-separated state with small ferromagnetic polarons inside an antiferromagnetic matrix. The general picture of a percolative state, which emerges from our calculations, is in good agreement with recent experiments on doped manganites.

Bound ferromagnetic and paramagnetic polarons as an origin of the resistivity peak in ferromagnetic semiconductors and manganites

A theory of resistivity is developed for ferromagnetic semiconductors, possibly, including manganites. The theory is based on analysis of the interaction of the free and bound charge carriers with the magnetization of the crystal. The temperature dependence of free energy for nonionized donors and free electrons is calculated for the spin-wave and paramagnetic regions. In addition to the trapping by the ferromagnetic fluctuations (the ferromagnetic polarons), the electron trapping by the random magnetization fluctuations as T → is taken into account (the paramagnetic polarons). For the nondegenerate semiconductors, the theory makes it possible to explain a nonmonotonic temperature dependence of the activation energy, with the value for T = 0 being lower than that for T → ∞. For degenerate semiconductors, the theory explains a metal-insulator transition that occurs with increasing temperature in samples with relatively low charge carrier density. If the density is larger, a reentrant metal-insulator transition should take place, so that the crystal is highly conductive as T → ∞.

Double-exchange model: phase separation versus canted spins

We study the competition between different possible ground states of the double-exchange model with strong ferromagnetic exchange interaction between itinerant electrons and local spins. Both for classical and quantum treatment of the local spins the homogeneous canted state is shown to be unstable against a phase separation. The conditions for the phase separation into the mixture of the antiferromagnetic and ferromagnetic/canted states are given. We also discuss another possible realization of the phase-separated state: ferromagnetic polarons embedded into an antiferromagnetic surrounding. The general picture of a percolated state, which emerges from these considerations, is discussed and compared with results of recent experiments on doped manganaties.

Magnetic polarons in layered antiferromagnetic systems

The energy and stability conditions are found for free ferromagnetic polarons (ferrons) in the antiferromagnetic semiconductors with a layered magnetic structure. Unlike a similar problem for the staggered antiferromagnets, solved long ago, this geometrically different problem was not considered yet. In particular, the difference between these two problems consists in the fact that the ferromagnetic region, which traps the charge carrier, is well defined for the staggered structure, and is not for the layered structure. The latter is due to ferromagnetic planes inherent in this structure. It is established that, nevertheless, the ferromagnetic region is almost spherical for large ferrons. The De Gennes single-spin magnetic polaron is unstable at realistic values of parameters. The energy and magnetic moment of the ferron bound to a donor or acceptor are calculated for the case of the double exchange.

Double-exchange magnets: Spin dynamics in the paramagnetic phase

The electronic structure of perovskite manganese oxides is investigated in terms of a Kondo lattice model with ferromagnetic Hund coupling and antiferromagnetic exchange between ${t}_{2g}$ spins using a finite temperature diagonalization technique. Results for the dynamic structure factor are consistent with recent neutron-scattering experiments for the bilayer manganite ${\mathrm{La}}_{1.2}{\mathrm{Sr}}_{1.8}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}.$ The susceptibility shows Curie-Weiss behavior and is used to derive a phase diagram. In the paramagnetic phase with pronounced antiferromagnetic fluctuations charge carriers form ferromagnetic polarons.

Ferromagnetic Polarons in the Kondo Lattice Model

Using a bosonization technique we describe the ferromagnetic polaron formation in one-dimensional Kondo lattice models with partial conduction band filling. We show that at low electron concentration large ferromagnetic polarons are formed. The width of these polarons will gradually decrease as the electron concentration increases and finally local Kondo singlets will dominate. Bipolarons or multipolarons will not form in the lattice.

Electron-doped manganese perovskites: The magnetic polaron state

Using the Lanczos method in linear chains we study the ground state of the double exchange model including an antiferromagnetic super-exchange in the low concentration limit. We find that this ground state is always inhomogeneous, containig ferromagnetic polarons. The extention of the polaron spin distortion, the dispersion relation and their trapping by impurities, are studied for diferent values of the super exchange interaction and magnetic field. We also find repulsive polaron polaron interaction.

Ferromagnetic Clusters and Magnetic Polarons as Evidenced by ESR in La1.35Sr1.65Mn2O7

${\mathrm{La}}_{1.35}{\mathrm{Sr}}_{1.65}{\mathrm{Mn}}_{2}{\mathrm{O}}_{7}$ is investigated in the regime where 3D magnetic order is lost but 2D correlations are strong. ESR shows that the localized ${t}_{2g}$ spins are coupled into intralayer ferromagnetic clusters of fixed size. The ${e}_{g}$ carriers form polarons with an effective spin growing from $1/2$ to 7. ESR and transport properties suggest that the 3D order originates from the onset of coherence of the ferromagnetic clusters mediated by the propagation of the polarons.

Giant magnetoresistance behaviour in in-situ La0.60Sr0.40MnO3 films grown on Si substrates by pulsed laser deposition

We report the synthesis and giant magnetoresistance characteristics of as-deposited La0.60Sr0.40MnO3 films on Si(100) substrates by pulsed laser deposition (PLD). The deposited films exhibit a pervoskite-cubic structure with (110) orientation. Giant magnetoresistance behaviour, with more than 37% change in resistivity (Δρ/ρ0=ρ(0)−ρ(H)/ρ(0)), was obtained at room temperature under a magnetic field of 12 T. The giant magnetoresistance observed in our in-situ La0.60Sr0.40MnO3 films can be explained by the ferromagnetic phase transition and magnetic polaron behaviour.

Low-energy and low-temperature properties of untwinned YBa2Cu4O8: Gaps or condensation.

We report on experimental evidence of different electronic phases in the superconductor ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$: simple metallic CuO chains and highly correlated ${\mathrm{CuO}}_{2}$ planes. ${\mathrm{YBa}}_{2}$${\mathrm{Cu}}_{4}$${\mathrm{O}}_{8}$ is a genuine untwinned compound; hence, we were able to determine the anisotropy of the resistivity along the a and b directions. Along the b direction (chain and plane conduction channels), a normal metallic temperature behavior of the chain dominates. For the a direction (only plane conduction channel), the resistivity reveals an unconventional behavior with a kink at 160 K, becoming linear at higher temperatures. Further, we present results of the dynamical (optical) conductivity of the ${\mathrm{CuO}}_{2}$ plane as a function of frequency and temperature. The frequency dependence of the optical conductivity is consistent with a model of ferromagnetic polarons in an antiferromagnetic matrix. This is further confirmed by a depolarization experiment which is sensitive to crystal regions with different spin polarizations. The temperature behavior of the thermal occupation of the ground state is in agreement with a real-space condensation.

Doping states in the two-dimensional three-band Peierls-Hubbard model.

Doping states in a two-dimensional three-band Peierls-Hubbard model for the copper oxides are investigated with inhomogeneous Hartree-Fock (HF) and random-phase approximations. The doping states are sensitive to small changes of interaction parameters because they easily change local energy balance between different interactions around added holes. For the parameter values derived from constrained-density-functional methods for the copper oxides, added holes form isolated small ferromagnetic polar- ons. When the parameters are varied around these values, different types of doping states are obtained: For stronger on-site repulsion at Cu sites, larger ferromagnetic polarons are formed, which are qualitatively different from the small polarons; for stronger nearest-neighbor Cu-O repulsion, polarons are clumped or there occurs phase separation into Cu- and O-hole-rich regions; Intersite electron-lattice coupling rapidly changes the small polarons by quenching a Cu magnetic moment and locally distorting the lattice in an otherwise undistorted antiferromagnetic background. This is regarded as a rapid crossover from a Zhang-Rice singlet to a covalent molecular singlet, and occurs substantially below a critical strength for destruction of the stoichiometric antiferromagnetic state. However intrasite electron-lattice coupling, in contrast to the intersite coupling, does not dramatically affect the hole-doping states. Doping induces modes in magnetic, optical, and vibronic response functions. Local infrared-active phonon modes are induced in infrared absorption spectra for finite electron-lattice coupling. They are correlated with doping-induced particle-hole excitations observed in optical absorption spectra and in magnetic excitation spectra. These doping-induced particle-hole excitations are associated with the local HF eigenstates in the charge-transfer gap. Each doping state has distinctive excitation spectra in the magnetic, optical, and vibronic channels. In particular, the hole-doping states with small polarons have doping-induced, infrared absorption peaks on the low-frequency side of the stoichiometric peak, while the electron-doping states have them on the high-frequency side.

The t-J z model: two holes pairing by means of one string

The string mechanism of two holes pairing in the t-J z model for the weak-coupling limit ( J⪡ t) is considered within the continuous approximation. Pairing is realized by means of an antiferromagnetic string connecting two holes. The stability of the paired state is determined by competition between this state and the ferromagnetic polaron states. For two holes on a square lattice pairing takes place if J/t1.02×10 -2 the bound energy is 0.91 ( tJ 2) 1 3 . For a small but finite hole concentration x⪅( J/t) 1 3 pairing takes place if J/t2.96×10 -2.