Browsing by Author "Norpoth, Jonas"

The electronic structure of Pr1-xCaxMnO3 has been investigated using a combination of first-principles calculations, x-ray photoelectron spectroscopy (XPS), x-ray absorption spectroscopy (XAS), electron-energy loss spectroscopy (EELS), and optical absorption. The full range of compositions, x = 0,1/2,1, and a variety of magnetic orders have been covered. Jahn-Teller as well as Zener polaron orders are considered. The free parameters of the local hybrid density functionals used in this study have been determined by comparison with measured XPS spectra. A model Hamiltonian, valid for the entire doping range, has been extracted. A simple local-orbital picture of the electronic structure for the interpretation of experimental spectra is provided. The comparison of theoretical calculations and different experimental spectra provide a detailed and consistent picture of the electronic structure. The large variations of measured optical absorption spectra are traced back to the coexistence of magnetic orders (respectively, to the occupation of local orbitals). A consistent treatment of the Coulomb interaction indicates a partial cancellation of Coulomb parameters and supports the dominance of the electron-phonon coupling.

Although the magnetic phase diagrams of bulk and thin film samples of Pr1-xCaxMnO3 (x <= 0.5) are widely explored, few works have been published on the magnetic properties of nanoparticles, especially in the lightly, doped regime. In this paper, microwave irradiation was used to synthesize the Pr0.977Ca0.023MnO3 and Pr0.964Ca0.036MnO3 Manganite phases with Pnma space group in the form of anisotropic nanoparticles. The phase identification, structural characteristics, and formation of the nanostructures were analyzed by Rietveld analysis of the X-ray diffraction patterns, electron microscopy, and combining thermal analysis with infrared spectroscopy, respectively. Transport measurements on the annealed samples revealed the insulating nature, and electrical conduction occurs through thermally activated hopping of small polarons. The Mn-O-c-Mn tilt angles in the MnO6 octahedra show considerable flattening (160.9 degrees and 167.6 degrees for x = 0.023 and x = 0.036, respectively), possibly enhancing the electronic double exchange and promoting ferromagnetism. Ferromagnetic ordering of Mn spins was indeed observed below 109 K, and interestingly, the magnetic moment for x = 0.036 was 3.92 mu(B)/f.u. at 5 K, which is higher than the saturated Mn magnetic moment (3.8 mu(B)). The enhanced magnetization is attributed to ordering of the Pr spins.

The mechanism of the electric-pulse induced resistance change effect in Au/Pr0.65Ca0.35MnO3/SrTi0.99Nb0.01O3 thin-film samples is studied by means of in situ electrical stimulation inside a transmission electron microscope. A detailed equivalent-circuit model analysis of the measured current voltage characteristics provides crucial information for the proper interpretation of the microscopy results. The electrical transport data of the electron-transparent samples used for the in situ investigations is verified by comparison to measurements of unpatterned thin-film samples. We find comprehensive evidence for electrochemical oxygen vacancy migration affecting the potential barrier of the pn junction between Pr0.65Ca0.35MnO3 and SrTi0.99Nb0.01O3 as well as the resistance of the manganite bulk. The high-resistance state formation in the Pr0.65Ca0.35MnO3 bulk is frequently accompanied by structural transformations, namely detwinning and superstructure formation, most likely as the result of the joint impact of dynamic charge inhomogenities by oxygen vacancy migration and injection of high carrier densities at the electrodes.

The magnetization structure and the mechanisms of magnetic interaction in an artificial two-phase magnet are analyzed with magnetic force microscopy and magnetooptics. A model system built up of arrays of hard magnetic CoPt squares (5 mu m edge length) embedded in a soft magnetic permalloy matrix is investigated. Special emphasis is put on the characterization of the matrix magnetization and on the interaction between the matrix and the CoPt squares. Different effects on different length scales are observed. At large distances to the hard magnetic dots, the magnetization of the matrix relaxes and is dominated by the characteristic pattern of an antidot array. The stability of this pattern with respect to magnetic fields and its dependence on the magnetic history of the system is analyzed. Nearby the dots, the magnetostatic coupling between the matrix and the CoPt induces a fine scale modulation of the magnetization of the matrix. At inter-dot distances up to 1 mu m, the dot matrix interaction is prevalent and the matrix contributes essentially to the dot-dot interaction. We have studied the characteristics of this modulation, particularly considering their decay length and the interaction transfer mediated by the modulation. (C) 2011 American Institute of Physics. [doi:10.1063/1.3626059]

Fabrication of thin film nanocomposites via decomposition and self-assembly from the vapor phase is a promising path for cost-effective fabrication of multifunctional materials. In particular, oxides as a new class of energy materials allow for rich functionalities, e. g., by combining p-and n-doped systems in catalytic or light harvesting units. Combining A-site doped perovskites ABO(3) with CoFe2O4 spinel, we have investigated thin film phase composition and nanocomposite morphology in the pseudobinary system La0.6Sr0.4BO3-CoFe2O4 for B = Fe, Co, and Mn. We observe formation of an epitaxial two-phase nanocomposite for B = Fe, i.e., the coexistence of La0.6Sr0.4FeO3 and CoFe2O4. In contrast, for B = Co or Mn nanocomposites are formed, where perovskite La0.6Sr0.4BO3 solid solutions coexists with Co-rich spinel and periclase phases. We derive conclusions for the preparation of perovskite-spinel nanocomposites with well-designed doping levels.

The relation among structure, electric transport, and photovoltaic effect is investigated for a pn heterojunction with strong correlation interactions. A perovskite interface is chosen as a model system consisting of the p-doped strongly correlated manganite Pr0.64Ca0.36MnO3 (PCMO) and the n-doped titanate SrTi1-yNbyO3 (y = 0.002 and 0.01). High-resolution electron microscopy and spectroscopy reveal a nearly dislocation-free, epitaxial interface and give insight into the local atomic and electronic structure. The presence of a photovoltaic effect under visible light at room temperature suggests the existence of mobile excited polarons within the band-gap-free PCMO absorber. The temperature-dependent rectifying current-voltage characteristics prove to be mainly determined by the presence of an interfacial energy spike in the conduction band and are affected by the colossal electroresistance effect. From the comparison of photocurrents and spatiotemporal distributions of photogenerated carriers (deduced from optical absorption spectroscopy), we discuss the range of the excited polaron diffusion length.