Microwave Slot Transmission Line Filled with Ferroelectric and Ferromagnetic Layers

Now when the technology is fast developing it is very important to investigate new hybrid structures. One way is to use ferrite ferroelectric layered structures. Theoretical and experimental investigation of such structures was made. These structures have advantages of both layers and it is possible to tune the behavior of this structure by external electric and magnetic field. But these structures have some disadvantages connected with presence of thick ferroelectric layer. One way to overcome this problem is to use slotline. So this is another new way to create hybrid ferrite ferroelectric structures, but it is needed to create new theory and find experimental proof that the behavior of these structures can be tuned with external magnetic and electric fields.

Microwave Processes in Ferrite-ferroelectric Layered Structure

This work is devoted to investigation of wave processes in new hybrid ferrite/ferroelectric structures. Spin wave devices based on ferrite films have disadvanteges. And their applications are limited. Investigated structures allow to overcome disadvantages. This investigation helps to create new class of devices. Electromagnetic analysis of hybrid spin-electromagnetic waves in ferrite/ferroelectric structures were done. As a result dispersion relation was found. Numerical solution of this dispersion relation gave us follow results. These structures can be effectively tuned by external electric and magnetic field. Methods to increase tuning range were suggested. It was found that such structures have one basic disadvantage which is connected with presence of thick ferroelectric layer. To solve this problem is to use thin ferroelectric films. But this decreases tuning range. It was confirmed by experiment that this structures can be effectively tuned by electric and magnetic fields. Resonance characteristics of ferrite/ferroelectric resonator were succesfully tuned by magnetic and electric field.

First-principles electronic theory of non-collinear magnetic order in transition-metal nanowires

The structural, electronic and magnetic properties of one-dimensional 3d transition-metal (TM) monoatomic chains having linear, zigzag and ladder geometries are investigated in the frame-work of first-principles density-functional theory. The stability of long-range magnetic order along the nanowires is determined by computing the corresponding frozen-magnon dispersion relations as a function of the 'spin-wave' vector q. First, we show that the ground-state magnetic orders of V, Mn and Fe linear chains at the equilibrium interatomic distances are non-collinear (NC) spin-density waves (SDWs) with characteristic equilibrium wave vectors q that depend on the composition and interatomic distance. The electronic and magnetic properties of these novel spin-spiral structures are discussed from a local perspective by analyzing the spin-polarized electronic densities of states, the local magnetic moments and the spin-density distributions for representative values q. Second, we investigate the stability of NC spin arrangements in Fe zigzag chains and ladders. We find that the non-collinear SDWs are remarkably stable in the biatomic chains (square ladder), whereas ferromagnetic order (q =0) dominates in zigzag chains (triangular ladders). The different magnetic structures are interpreted in terms of the corresponding effective exchange interactions J(ij) between the local magnetic moments μ(i) and μ(j) at atoms i and j. The effective couplings are derived by fitting a classical Heisenberg model to the ab initio magnon dispersion relations. In addition they are analyzed in the framework of general magnetic phase diagrams having arbitrary first, second, and third nearest-neighbor (NN) interactions J(ij). The effect of external electric fields (EFs) on the stability of NC magnetic order has been quantified for representative monoatomic free-standing and deposited chains. We find that an external EF, which is applied perpendicular to the chains, favors non-collinear order in V chains, whereas it stabilizes the ferromagnetic (FM) order in Fe chains. Moreover, our calculations reveal a change in the magnetic order of V chains deposited on the Cu(110) surface in the presence of external EFs. In this case the NC spiral order, which was unstable in the absence of EF, becomes the most favorable one when perpendicular fields of the order of 0.1 V/Å are applied. As a final application of the theory we study the magnetic interactions within monoatomic TM chains deposited on graphene sheets. One observes that even weak chain substrate hybridizations can modify the magnetic order. Mn and Fe chains show incommensurable NC spin configurations. Remarkably, V chains show a transition from a spiral magnetic order in the freestanding geometry to FM order when they are deposited on a graphene sheet. Some TM-terminated zigzag graphene-nanoribbons, for example V and Fe terminated nanoribbons, also show NC spin configurations. Finally, the magnetic anisotropy energies (MAEs) of TM chains on graphene are investigated. It is shown that Co and Fe chains exhibit significant MAEs and orbital magnetic moments with in-plane easy magnetization axis. The remarkable changes in the magnetic properties of chains on graphene are correlated to charge transfers from the TMs to NN carbon atoms. Goals and limitations of this study and the resulting perspectives of future investigations are discussed.

The paramagnetic phase boundary of EuTe at low temperatures

Taking into account the presence of long-range dipolar interactions, we propose a model hamiltonian to calculate the canted-paramagnetic phase boundary of EuTe at low temperatures. By using spin-wave techniques we show that the critical field depends on T2 asymptotically. Our calculations are in good agreement with the experimental data. © 1981.

Entwicklung der zeitaufgelösten Photoemissions-Elektronenmikroskopie für die Untersuchung der Magnetisierungsdynamik von mikrostrukturierten magnetischen Schichten

Im Rahmen dieser Arbeit wurde die zeitaufgelöste Photoemissions Elektronenmikroskopie (TR-PEEM) für die in-situ Untersuchung ultraschneller dynamischer Prozesse in dünnen mikrostrukturierten magnetischen Schichten während eines rasch verändernden externen Magnetfelds entwickelt. Das Experiment basiert auf der Nutzung des XMCD-Kontrasts (X-ray magnetic circular dichroism) mit Hilfe des zirkularpolarisierten Lichts von Synchrotronstrahlungsquellen (Elektronenspeicherringen BESSY II (Berlin) und ESRF (Grenoble)) für die dynamische Darstellung der magnetischen Domänen während ultraschneller Magnetisierungsvorgänge. Die hier entwickelte Methode wurde als erfolgreiche Kombination aus einer hohen Orts- und Zeitauflösung (weniger als 55 nm bzw. 15 ps) realisiert. Mit der hier beschriebenen Methode konnte nachgewiesen werden, dass die Magnetisierungsdynamik in großen Permalloy-Mikrostrukturen (40 µm x 80 µm und 20 µm x 80 µm, 40 nm dick) durch inkohärente Drehung der Magnetisierung und mit der Bildung von zeitlich abhängigen Übergangsdomänen einher geht, die den Ummagnetisierungsvorgang blockieren. Es wurden neue markante Differenzen zwischen der magnetischen Response einer vorgegebenen Dünnfilm-Mikrostruktur auf ein gepulstes externes Magnetfeld im Vergleich zu dem quasi-statischen Fall gefunden. Dies betrifft die Erscheinung von transienten raumzeitlichen Domänenmustern und besonderen Detailstrukturen in diesen Mustern, welche im quasi-statischen Fall nicht auftreten. Es wurden Beispiele solcher Domänenmuster in Permalloy-Mikrostrukturen verschiedener Formen und Größen untersucht und diskutiert. Insbesondere wurde die schnelle Verbreiterung von Domänenwänden infolge des präzessionalen Magnetisierungsvorgangs, die Ausbildung von transienten Domänenwänden und transienten Vortizes sowie die Erscheinung einer gestreiften Domänenphase aufgrund der inkohärenten Drehung der Magnetisierung diskutiert. Ferner wurde die Methode für die Untersuchung von stehenden Spinwellen auf ultradünnen (16 µm x 32 µm groß und 10 nm dick) Permalloy-Mikrostrukturen herangezogen. In einer zum periodischen Anregungsfeld senkrecht orientierten rechteckigen Mikrostruktur wurde ein induziertes magnetisches Moment gefunden. Dieses Phänomen wurde als „selbstfangende“ Spinwellenmode interpretiert. Es wurde gezeigt, dass sich eine erzwungene Normalmode durch Verschiebung einer 180°-Néelwand stabilisiert. Wird das System knapp unterhalb seiner Resonanzfrequenz angeregt, passt sich die Magnetisierungsverteilung derart an, dass ein möglichst großer Teil der durch das Anregungsfeld eingebrachten Energie im System verbleibt. Über einem bestimmten Grenzwert verursacht die Spinwellenmode nahe der Resonanzfrequenz eine effektive Kraft senkrecht zur 180°-Néel-Wand. Diese entsteht im Zentrum der Mikrostruktur und wird durch die streufeldinduzierte Kraft kompensiert. Als zusätzliche Möglichkeit wurden die Streufelder von magnetischen Mikrostrukturen während der dynamischen Prozesse quantitativ bestimmt und das genaue zeitliche Profil des Streufelds untersucht. Es wurde gezeigt, dass das zeitaufgelöste Photoemissions Elektronenmikroskop als ultraschnelles oberflächensensitives Magnetometer eingesetzt werden kann.

Quantum fluctuations stabilize skyrmion textures

We study the quantum spin waves associated to skyrmion textures. We show that the zero-point energy associated to the quantum spin fluctuations of a noncollinear spin texture produce Casimir-like magnetic fields. We study the effect of these Casimir fields on the topologically protected noncollinear spin textures known as skyrmions. In a Heisenberg model with Dzyalonshinkii-Moriya interactions, chosen so the classical ground state displays skyrmion textures, we calculate the spin-wave spectrum, using the Holstein-Primakoff approximation, and the associated zero-point energy, to the lowest order in the spin-wave expansion. Our calculations are done both for the single-skyrmion case, for which we obtain a discrete set of skyrmion bound states, as well as for the skyrmion crystal, for which the resulting spectrum gives the spin-wave bands. In both cases, our calculations show that the Casimir magnetic field contributes up to 10% of the total Zeeman energy necessary to delete the skyrmion texture with an applied field.

Anomalous Excitation Spectra of Frustrated Quantum Antiferromagnets

We use series expansions to study the excitation spectra of spin-1/2 antiferromagnets on anisotropic triangular lattices. For the isotropic triangular lattice model (TLM), the high-energy spectra show several anomalous features that differ strongly from linear spin-wave theory (LSWT). Even in the Neel phase, the deviations from LSWT increase sharply with frustration, leading to rotonlike minima at special wave vectors. We argue that these results can be interpreted naturally in a spinon language and provide an explanation for the previously observed anomalous finite-temperature properties of the TLM. In the coupled-chains limit, quantum renormalizations strongly enhance the one-dimensionality of the spectra, in agreement with experiments on Cs2CuCl4.

Competing anisotropies in exchange biased nano-structured thin films

The magnetic anisotropies of a patterned, exchange biased Fe50Mn50/Ni80Fe20 system are studied using ferromagnetic resonance, supplemented by Brillouin light scattering experiments and Kerr magnetometry. The exchange biased bi-layer is partially etched into an antidot geometry so that the system approximates a Ni80 Fe20 layer in contact with antidot structured Fe50 Mn50 . Brillouin light scattering measurements of the spin wave frequency dependence on the wave vector reveal a magnonic band gap as expected for a periodic modulation of the magnetic properties. Analysis of the ferromagnetic resonance spectra reveals 8-fold and 4-fold contributions to the magnetic anisotropy. Additionally, the antidot patterning decreases the magnitude of the exchange bias and modifies strongly its angular dependence. Softening of all resonance modes is most pronounced for the applied magnetic field aligned within 10◦ of the antidot axis, in the direction of the bias. Given the degree to which one can tailor the ground state, the resulting asymmetry at low frequencies could make this an interesting candidate for applications such as selective/directional microwave filtering and multi-state magnetic logic.

Neutron Scattering Studies on Complex Magnetic Structures in Magnetoelectric Materials

In this thesis, the magnetic properties of four transition-metal oxides are presented. Their multiferroic and magnetoelectric phases have been investigated by means of different neutron scattering techniques. The materials TbMnO3 and MnWO4 belong to the group of spin-induced multiferroics. Their ferroelectric polarization can be explained by the inverse DzyaloshinskiiMoriya interaction. Another common feature of both materials is the presence of subsequent magnetic transitions from a spin-density wave to a spin spiral. The features of the phase transitions have been studied in both materials and it could be shown that diffuse magnetic scattering from the spin spiral is present even in the ordered spin-density wave phase. The excitation spectrum in the multiferroic phase of TbMnO3 was investigated in detail and a comprehensive dataset was obtained using time-of-flight spectroscopy. A spin-wave model could be obtained which can quantitatively describe the full dispersion. Furthermore, the polarization of the zone-center excitations could be derived which fit well to data from inelastic neutron spectroscopy and infrared spectroscopy. With the combination of spherical neutron polarimetry and a poling of the sample by an electric field, it was possible to observe the chiral magnetic component of the magnetic excitations in TbMnO3 and MnWO4. The spin-wave model for TbMnO3 obtained in this thesis is able to correctly describe the dispersion of this component. The double tungstate NaFe(WO4)2 is isostructural to the multiferroic MnWO4 and develops a complex magnetic phase diagram. By the use of neutron diffraction techniques, the zero-field structure and high-field structures in magnetic field applied along the b-axis could be determined. The data reveal a direct transition into an incommensurate spin-spiral structure. The value of the incommensurability is driven by anharmonic modulations and shows strong hysteresis effects. The static and dynamic properties in the magnetoelectric spin-glass phase of Ni0.42Mn0.58TiO3 were studied in detail. The spin-glass phase is composed of short-ranged MnTiO3 and NiTiO3-type order. The antiferromagnetic domains could be controlled by crossed magnetic and electric fields, which was visualized using spherical neutron polarimetry. A comprehensive dataset of the magnetic excitations in the spin-glass phase was collected. The dataset revealed correlations in the hexagonal plane which are only weakly coupled along the c-axis. The excitation spectra could be simulated by taking into account the MnTiO3-type order.

Badanie sprzężeń magnetosprężystych w multiferroikach heterostrukturalnych

Wydział Fizyki

Quantum oscillations in quasi-one-dimensional metals with spin-density-wave ground states

We consider the magnetoresistance oscillation phenomena in the Bechgaard salts (TMTSF)(2)X, where X = ClO4, PF6, and AsF6 in pulsed magnetic fields to 51 T. Of particular importance is the observation of a new magnetoresistance oscillation for X = ClO4 in its quenched state. In the absence of any Fermi-surface reconstruction due to anion order at low temperatures, all three materials exhibit nonmonotonic temperature dependence of the oscillation amplitude in the spin-density-wave (SDW) state. We discuss a model where, below a characteristic temperature T* within the SDW state, a magnetic breakdown gap opens. [S0163-1829(99)00904-2].

Wave-vector dependence of spin and density multipole excitations in quantum dots

We have employed time-dependent local-spin density-functional theory to analyze the multipole spin and charge density excitations in GaAs-AlxGa1-xAs quantum dots. The on-plane transferred momentum degree of freedom has been taken into account, and the wave-vector dependence of the excitations is discussed. In agreement with previous experiments, we have found that the energies of these modes do not depend on the transferred wave vector, although their intensities do. Comparison with a recent resonant Raman scattering experiment [C. Schüller et al., Phys. Rev. Lett. 80, 2673 (1998)] is made. This allows us to identify the angular momentum of several of the observed modes as well as to reproduce their energies

Optical evidence for the proximity to a spin-density-wave metallic state in Na0.7CoO2

We present the optical properties of Na0.7CoO2 single crystals, measured over a broad spectral range as a function of temperature (T). The capability to cover the energy range from the far-infrared up to the ultraviolet allows us to perform reliable Kramers-Kronig transformation, in order to obtain the absorption spectrum (i.e., the complex optical conductivity). To the complex optical conductivity we apply the generalized Drude model, extracting the frequency dependence of the scattering rate (Gamma) and effective mass (m*) of the itinerant charge carriers. We find that Gamma(omega) at low temperatures and for similar to omega. This suggests that Na0.7CoO2 is at the verge of a spin-density-wave metallic phase.

Wave-vector dependence of spin and density multipole excitations in quantum dots

We have employed time-dependent local-spin density-functional theory to analyze the multipole spin and charge density excitations in GaAs-AlxGa1-xAs quantum dots. The on-plane transferred momentum degree of freedom has been taken into account, and the wave-vector dependence of the excitations is discussed. In agreement with previous experiments, we have found that the energies of these modes do not depend on the transferred wave vector, although their intensities do. Comparison with a recent resonant Raman scattering experiment [C. Schüller et al., Phys. Rev. Lett. 80, 2673 (1998)] is made. This allows us to identify the angular momentum of several of the observed modes as well as to reproduce their energies

Incommensurate spin-density-wave and metal-insulator transition in the one-dimensional periodic Anderson model

Fundação de Amparo à Pesquisa do Estado de São Paulo (FAPESP)