Strongly coupled charge, orbital, and spin order in TbTe3

We report a ground state with strongly coupled magnetic and charge density wave orders mediated via orbital ordering in the layered compound TbTe3. In addition to the commensurate antiferromagnetic (AFM) and charge density wave (CDW) orders, new magnetic peaks are observed whose propagation vector equals the sum of the AFM and CDW propagation vectors, revealing an intricate and highly entwined relationship. This is especially interesting given that the magnetic and charge orders lie in different layers of the crystal structure where the highly localized magnetic moments of the Tb3+ ions are netted in the Tb-Te stacks, while the charge order is formed by the conduction electrons of the adjacent Te-Te layers. Our results, based on neutron diffraction and resonant x-ray scattering, reveal that the charge and magnetic subsystems mutually influence each other via the orbital ordering of Tb3+ ions. © 2020 American Physical Society.S. C. thanks K. Prokes for the helpful insights regarding the heavy-fermion physics. This work has been partially supported by the Ministry of Education and Science of the Russian Federation, Contracts No. 02.A03.21.0006 and No. 02.A03.21.0011

Ferroelectric field effect at ionically conducting oxide interfaces

Tesis de la Universidad Complutense de Madrid, Facultad de Ciencias Físicas, leída el 17-01-2019En este trabajo, realizamos un análisis de la interacción entre la ferroelectricidad, las paredes de dominio ferroeléctrico cargadas y el memristor en las dimensiones reducidas de una unión túnel. Para alcanzar este objetivo, crecemos bicapas de heteroestructuras epitaxiales de manganitas ferromagnéticas.The main findings of this dissertation are summarized here. We performed an analysis of the interplay between ferroelectricity, ferromagnetism, domain structure and memristive response inmagnetic tunnel junctions. In order to reach this objective, we grew epitaxial heterostructures combining ferromagnetic manganites.Fac. de Ciencias FísicasTRUEunpu

Band engineering of graphene using metal mediated oxidation

In the study of materials for electronic devices, there is a continuous search for new materials with useful properties. In the early 2000’s, the 2D semi-metal carbon material graphene was isolated and characterized experimentally, and found to have a variety of desirable electronic properties. Since that time research on graphene and graphene related materials has progressed at an ever growing rate as researchers seek to understand, manipulate, and enhance graphene for use in electronic devices. One arm of this research seeks to manipulate the band structure of graphene such that it behaves like a semiconductor in devices. This thesis reports a study of four graphene systems investigated to attempt to manipulate the electronic structure in graphene; Graphene/Cu, Co/Graphene/Cu, Graphene/Co/SiO2, Co/Graphene/SiO2. The properties of these systems were investigated using various X-ray spectroscopy and surface science techniques. The analysis showed that the band structure of Graphene/SiO2 may be manipulated by depositing cobalt on the graphene surface. At a low concentration, the cobalt is completely oxidized into primarily CoO, and the graphene is not heavily damaged. Oxide groups form on the graphene surface but are found to be proportional to the cobalt thickness below 1 nm. Using X-ray spectroscopy a band gap of up to 0.30 ± 0.10 eV is observed in graphene 2p states when a low concentration of cobalt forms islands on the graphene surface. The mechanism of band gap opening was interpreted using electronic structure calculations to have a contribution from both graphene oxide formation as well as the presence of CoO. These results have implications for graphene electronics and spintronics where magnetic metals can be used to induce a band gap in graphene that is stable at room temperature and under atmospheric exposure

The structural and magnetization studies of Transition Metal-doped ZnO thin films.

Master'sMASTER OF SCIENC

Electron transfer and spin injection in C60-ferromagnetic composites

The magnetic properties of spin doped fullerenes are investigated in hybrid organic/inorganic structures with the aim of establishing the extent to which magnetic states can be induced and controlled in these materials. Volume magnetometry is used to measure a reduction of net magnetization and an increase in coercivity in cobalt which can be understood in terms of a transfer of majority spin electrons from the transition metal d-band into spin polarized hybrid interface states. This is supported by PNR and XAS studies of Co/C60 which reveal AF coupling between Co metal films and a hybrid interfacial region where magnetic ground states are induced in fullerenes through charge transfer. Investigations of hybridization between C60 and the RE-TM alloy CoGd show that the compensation temperature of the ferrimagnet is altered by the presence of C60. PNR measurements of CoGd/C60 MLs reveal interfacial coupling which creates an AF region 1.5 $\pm$ 0.1 nm thick. Magnetometry of Gd/C60 bilayers indicates that hybridization between the metal conduction bands and the C60 LUMO modifies magnetic ordering in Gd. This is supported by the observation of novel features in the temperature dependence of magnetization and resistivity in the composite. XAS of Gd/C60 bilayers shows a large peak in the carbon K-edge at 282 eV which is attributed to interfacial hybridization. It is shown that PL quenching in C60 is greater over Co than Au which is attributed to the greater electron transfer between Co and C60. PL quenching is proposed as an effective way to measure magnetic coupling and electron transfer in interfaces. Raman spectra are recorded in C60 junctions during spin polarised transport. The Ag(2) peak splitting is shown to depend on the polarisation of injected current acting as an effective probe of triplet formation in C60. Finally, XAS at the carbon K-edge is recorded during spin transport. A suppression of the LUMO to zero and increase in the intensity of the 282 eV peak occurs after removal of external bias and is shown to be reversible and repeatable under cycles of grounding and charge injection. A proposed mechanism involving the redistribution of charge following the removal of bias which causes electrons to become trapped in interfacial states is suggested