Jonas Norpoth

Effects of interaction and disorder on polarons in colossal resistance manganite Pr0.68Ca0.32MnO3 thin films

The colossal magnetoresistance effect (CMR), the drop of the electric resistance by orders of magnitude in a strong magnetic field, is a fascinating property of strongly correlated electrons in doped manganites. Here, we present a detailed analysis of the magnetotransport properties of small polarons in thin films of the low bandwidth manganite Pr0.68Ca0.32MnO3 with different degrees of preparation-induced octahedral disorder. The crystal and defect structure is investigated by means of high-resolution transmission electron microscopy. We apply the small polaron theory developed by Firsov and Lang in order to study the hopping mobility in the paramagnetic phase and its changes due to the formation of the antiferromagnetic charge ordered (CO) and the ferromagnetic metallic phases. Although it represents a single particle theory, reasonable estimates of small polaron properties such as formation energy, activation energy and transfer integral are possible, if the effects of interactions and disorder are taken into account. Beyond the well-known effect of the magnetic double exchange on the transfer integral, we show that the emergence of band transport of small polarons in the CMR transition sensibly depends on the degree of octahedral disorder, the polaron–polaron interactions and the resulting long range order leading to a structural phase transition in the CO phase.

Quantitative imaging of stray fields and magnetization distributions in hard magnetic element arrays

In order to determine magnetic stray field and magnetization distributions of thin magnetic patterns and arrays, we developed a new quantitative imaging technique based on magneto-optical indicator films (MOIF) combined with inverse magnetostatic methods. The method is applied to hard magnetic FePt and PrCo5 films which exhibit out-of-plane and in-plane easy magnetization axes, respectively. The films are patterned with standard electron beam lithography into square shaped elements with sizes between 10 μm and 500 nm. The magnetization values obtained from the MOIF method are compared to those of integral magnetometer measurements and show a good agreement, if the sensor properties are taken into account, properly. As an outlook, a concept for combining MOIF imaging with magnetic force microscopy is demonstrated which allows for quantitative magnetization imaging with a resolution down to 10 nm.

Manipulating the dipolar magnetic interactions in FePt square arrays: The role of edge roughness

The high magnetocrystalline anisotropy energy in hard magnetic materials may sustain magnetization distributions in the remanent state, which exhibit a high number of magnetic surface charges and thus significant magnetic stray fields. Here, we focus onto the stray field distribution in the exterior and the demagnetization field distribution in the interior of hard magnetic FePt elements without and with artificial saw tooth edge roughness. Our experiments and calculations reveal that external stray fields and internal demagnetization fields are considerably modified by the artificial edge roughness.

Electronic structure of Pr1−xCaxMnO3

The electronic structure of

Using Magneto-Optical Measurements for the Evaluation of a Hybrid Magnetic Shape Memory (MSM)-Based Microactuator

{\mathrm{Pr}}_{1\ensuremath{-}x}{\mathrm{Ca}}_{x}{\mathrm{MnO}}_{3}

Magnetostatic interaction mechanisms in a two-dimensional composite magnet

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,

Interfacial reconstruction and superconductivity in cuprate–manganite multilayers of YBa2Cu3O7−δ and Pr0.68Ca0.32MnO3

x=0,1/2,1

CHARACTERIZATION OF A MAGNETIC MICROACTUATOR FOR MANIPULATING NANOPARTICLES IN GENE DELIVERY APPLICATIONS

, 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 co-existence 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.

In situ TEM analysis of resistive switching in manganite based thin-film heterostructures

Magnetic shape memory (MSM) materials present a new class of alloys, which can be used for creating electromagnetic microactuators facilitating a linear motion. The concept of the hybrid MSM microactuator pursued is using a pair of discrete MSM strips sandwiched between microstators. For the microstator fabrication, MEMS technology is applied. The two MSM strips are elongated alternatively, with one strip showing strain when exposed to a magnetizing field, while the other one is mechanically collapsed by its elongating counterpart, and vice versa. For inducing the elongation in the MSM strips, a critical field strength H crit has to be exceeded in vertical direction within the MSM strip. For analyzing what magnetic field strength is reached by the microstators and if it is greater than the critical field strength H crit required to inducing a reorientation of the twin variants in the MSM material, magneto-optical measurements were carried out.

Polaron Absorption for Photovoltaic Energy Conversion in a Manganite-titanate pn Heterojunction

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 μm 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 μm, the dot matrix interaction is prevalent and...