Molecular spintronic devicesfrom molecular spin valves to spin-oleds

Resumen

The investigation herein described belongs to the molecular spintronics field and it has been motivated by the desire to deepen in the knowledge of molecular spin valves and spin-OLED devices, through the incorporation of new materials and the study of spin precession in the molecular layers. This manuscript outlines the research carried out during the Ph.D. period and it is divided into six chapters. The basic concepts in molecular spintronics are reviewed in the first chapter of the dissertation. There, the notions that will be relevant in the following chapters are emphasized. Chapter two is devoted to the study of thin films of five new molecular compounds based on quinolines: NaYClq, NaTbClq, NaDyClq, NEtDyClq, and KNEtDyClq. An exhaustive multi-technique analysis formed by thermogravimetry, elemental analysis, infrared spectroscopy, mass spectrometry and ac magnetometry demonstrates that only the sodium derivatives are thermally stable and can be sublimated retaining their molecular integrity and magnetic properties. Such a feature has allowed us to prepare films of these sodium derivatives showing low roughness values and high substrate coverage that make them suitable for their incorporation into molecular spintronic devices. Finally, the magnetic sublimable molecules are deposited as thin films on ferromagnetic substrates. Interestingly, a molecular blocking is observed in the FC-ZFC curves probably caused by the activation of the single molecule magnet behavior induced by the ferromagnetic substrate. In the third chapter of this dissertation, we have fabricated spin valves using these sublimable sodium derivatives as spin collector layer. Thus, negative magnetoresistance is observed with the compounds NaYClq and NaDyClq as molecular layers. The two configurations of the devices studied are Co (15 nm) / AlOx (1 nm) / NaYClq / NiFe (15 nm) and Co (15 nm) / AlOx (1 nm) / NaDyClq / NiFe (15 nm) where the molecular layers nominal thicknesses are in the range [8, 25] nm. We demonstrate that the interfacial spin polarization and magnetoresistance in a molecular spin valve can be modified by specific details of the metal-molecule interaction. The positive MR observed in the device with structure NiFe (15 nm)/ AlOx (1 nm) / NaDyClq / Co (15 nm) confirms that the spinterface is NaDyClq / NiFe. The hybridization has been studied by means of X-ray absorption spectroscopy. Chapter four deals with the study of two spin valves based on polyoxometalates formed by stacks of LSMO (20 nm) / DODA3PMo12O40 (100 nm) / Co (25 nm) and LSMO (20 nm) / DODA3PMo12O40 (80 nm) / MoOx (3 nm) / Co (25 nm). Both types of devices are designed taken into account the energy level alignment at the interfaces, determined by UPS spectroscopy. The thickness of the MoOx layer has been chosen taking into account the spin-polarized UPS spectra. The incorporation of the MoOx layer has resulted in a considerable improvement of the device performance since magnetoresistance signal remains up to voltages as high as 3.5 V. In chapter five we design and study a spin-OLED with the configuration: LSMO (20 nm) / PEIE (1 nm) / F8BT (45 nm) / MoOx (3 nm) / Co (25 nm) where PEIE = polyethylenimine ethoxylated and F8BT = poly(9,9-dioctylfluorene-alt-benzothiadiazole). All the layers are formed by commercial materials. In addition, the interfaces between the magnetic electrodes and the organic semiconductor emitting layer have been carefully engineered to have a proper energetic band alignment. The device shows MEL effect in a wide range of temperatures and voltages with a maximum of 2.4 % at 9 V at 20 K. The Hanle effect constitutes the main subject of chapter six. This effect is considered the litmus test to demonstrate that the magnetoresistance in molecular spin valves origins in spin polarized currents. The spin-OLED is a suitable device to study spin precession by means of the Hanle effect since the light emission ensures that the spin-polarized charge is being transported through the frontier orbitals (HOMO and LUMO) of the molecular semiconductor. Remarkably, we measure the absence of Hanle effect in the light and resistance of the spin-OLED: LSMO (20 nm) / ZnO (1.8 nm) / N965 (1 nm) / F8BT (65 nm) / MoOx (3 nm) / Co (15 nm) at different temperatures, voltages and angles. Our results give us a strong hint of exchange as spin transport mechanism in molecular materials. o-----------------------------------------o La investigación llevada a cabo durante el periodo de tesis doctoral y que se describe en este manuscrito pertenece al campo de la espintrónica molecular. Ha sido motivada por el deseo de incorporar nuevos materiales moleculares a dispositivos espintrónicos y ahondar en la comprensión de la inyección y el transporte de espín en este tipo de capas, a través del estudio de dispositivos como la válvula de espín molecular y el spin-OLED. En resumen, en la investigación llevada a cabo durante esta tesis doctoral se han abordado varios temas actuales del campo de la espintrónica molecular como son la spinterface, la magnetoresistencia a alto voltaje, los mecanismos de transporte en las válvulas de espín, el transporte en el HOMO y el LUMO del material molecular en los spin-OLEDs y el estudio de los mecanismos de transporte de espín mediante el efecto Hanle.