Double exchange

Dilute ferromagnetic oxides having Curie temperatures far in excess of 300 K and exceptionally large ordered moments per transition-metal cation challenge our understanding of magnetism in solids. These materials are high-k dielectrics with degenerate or thermally activated n-...

Double-exchange mechanisms in RE$_{1-x}$AE$_{x}$MnO$_{3}$ manganites (where RE is a trivalent rare-earth ion and AE is a divalent alkali-earth ion) relies on the strong exchange interaction between two Mn$^{3+}$ and Mn$^{4+}$ ions through interfiling oxygen 2p states. Nevertheless, the role of RE and AE ions has ever been considered "silent" with respect to the DE conducting mechanisms. Here we show that a new path for DE-mechanism is indeed possible by partially replacing the RE-AE elements by Mn$^{2+}$-ions, in La-deficient La$_{x}$MnO$_{3-\delta}$ thin films. X-ray absorption spectroscopy demonstrated the relevant presence of Mn$^{2+}$ ions, which is unambiguously proved to be substituted at La-site by Resonant Inelastic X-ray Scattering. Mn$^{2+}$ is proved to be directly correlated to the enhanced magneto-transport properties because of an additional hopping mechanism trough interfiling Mn$^{2+}$-ions, theoretically confirmed by calculations within the effective single band model. The very idea to use Mn$^{2+}$ both as a doping element and an ions electronically involved in the conduction mechanism, has never been foreseen, revealing a new phenomena in transport properties of manganites. More important, such a strategy might be also pursed in other strongly correlated materials.

The generic changes of the electronic compressibility in systems which show magnetic instabilities is studied. It is shown that, when going into the ordered phase, the compressibility is reduced by an amount comparable to the its original value, making charge instabilities also possible. We discuss, within this framework, the tendency towards phase separation of the double exchange systems, the pyrochlores, and other magnetic materials.

The effect of elemental substitutions on the properties of the ferromagnetic, conducting, highly magnetoresistive compound La2/3Ca1/3MnO3 has been studied. The results of Co doping and Ni doping on the Mn site and Gd doping on the La site are reported. These compounds were investigated by x-ray diffraction, magnetization measurements, resistivity measurements, thermopower measurements, and by paramagnetic resonance. The result of

The structure of bound magnetic polarons in an antiferromagnetic matrix is studied in the framework of two-dimensional (2D) and three-dimensional (3D) Kondo-lattice models in the double exchange limit (J_H >> t). The conduction electron is bound by a nonmagnetic donor impurity and forms a ferromagnetic core of the size about the electron localization length (bound magnetic polaron). We find that the magnetic polaron produces rather long-range extended spin distortions of the antiferromagnetic background around the core. In a wide range of distances, these distortions decay as 1/r^2 and 1/r^4 in 2D and 3D cases, respectively. In addition, the magnetization of the core is smaller than its saturation value. Such a magnetic polaron state is favorable in energy in comparison to usually considered one (saturated core without extended distortions).

Epitaxially grown thin films of La1-xPbxMn0.8Ru0.2O3 (0.2<=x<=0.4) on LAO (001) substrate using pulsed electron deposition technique shows a systematic decrease in metal to insulator transition from 300 to 250 K when hole concentration varies from 40% to 20%. However, an increase in magnetoresistance ratio by ~5%-15% is observed for Ru substituted films at the respective Curie temperatures when compared to the parent La0.6Pb0.4MnO3 film. Transport and magnetoresistive properties show that Ru substitution maintains a considerable hole carrier density even for La0.8Pb0.2Mn0.8Ru0.2O3 (8282) composition to stabilize the double exchange interactions.

The reactivity of the Zn–Mn–O system, prepared by conventional ceramic routes using ZnO and MnO 2 as starting materials are described and correlated with the magnetic response. X-ray diffraction, X-ray photoelectron spectroscopy and Raman spectroscopy techniques have ...

J. Gutiérrez Departamento de Electricidad y Electrónica, Universidad del País Vasco/EHU, Box 644, E-48080 Bilbao, Spain. ... JM Barandiarán Departamento de Electricidad y Electrónica, Universidad del País Vasco/EHU, Box 644, E-48080 Bilbao, Spain. ...

The effect of Cr-doping on the Mn site in Nd1−ySryMnO3 (y=0.5 and 0.55) single crystals grown by the floating zone method has been studied. The y=0.5 mother material exhibits a transition at TC=260 K to be a ferromagnetic metal (FMM) and at TN=160 K to be an antiferromagnetic insulator (AFMI). The doping of 5 at% Cr3+ ions lowers TC to be 250 K and completely suppresses the AFMI. The y=0.55 mother material exhibits a transition at TN=230 K to be an antiferromagnetic metal . The doping of 5 at% Cr3+ ions changes the state to be similar to y=0.5 mother material having the FMM and AFMI. The doping of 10 at% Cr3+ ions completely suppresses the AFMI and keeps the FMM. In both cases, small amount of Cr-doping stabilizes the FMM, which is probably caused by provoking the double exchange interaction between Mn3+/Mn4+ pairs.