Organo-metallic structures for spintronic applications

The revolution in (semi)conducting organic materials has been one of the highlights in physics over the past decade. Molecular and polymeric thin films are projected to be used as active elements in a wide range of electronic and optoelectronic applications. Among the main driving forces for such plastic electronics are the low-cost processing and the chemical tunability. Potential applications include ultrathin organic light emitting diodes for (flexible) flat displays, field-effect transistors, sensors and many other. Although intensively studied both in industrial and academic environments, the intrinsic limits of molecular materials is an open issue. Pushing the limits of these materials and devices is a major scientific challenge with enormous implications for the electronics industry. In this thesis we explored novel hybrid molecular - metallic structures with a magnetic functionality for "spintronic" applications. Spintronics is a new branch of electronics in which electron spin, in addition to charge, is manipulated to yield a desired outcome. The spintronic devices are particularly attractive for memory devices (MRAM’s) and magnetic sensors applications. It has been suggested that molecular materials would provide an attractive alternative, not only in view of the general advantages of plastic electronics, but particularly, also because of the intrinsically low spin-orbit scattering due to the low mass of atoms involved. Despite these challenging opportunities, from materials and preparation point of view a large number of issues still have to be solved. Some of them have been addressed in this thesis. One of the crucial requirements for the realization of molecular spintronics is to obtain control over the ordering and morphology of molecular layers. This aspect is generally considered as one of the decisive parameters for achieving molecular electronics with high carrier mobilities. Since spintronic devices are extremely sensitive to the magnetic properties of the outermost atomic layers, our choice was to work with deposition of molecules under ultra-high-vacuum environment, rather than using "wet" techniques under ambient atmosphere. Obtaining ordered organic molecules on ferromagnetic materials represents a challenge, since the high reactivity of these type of substrates tends to decompose molecules, such as happening for thiols, or lowers surface mobility as to hinder molecular ordering. In Chapter 3 of this thesis we investigated the structural properties of two novel molecular - ferromagnetic systems. We demonstrated that long-range ordering of these molecules can be obtained when the substrate is exposed to small amounts of oxygen (Perylene- tetracarboxylic- dianhydride (PTCDA) - Ni(111)) or when a proper molecule-substrate combination is chosen (PTCDA - Co, pentacene - Co, and pentacene - Ni(111)). Very promising is our finding that pentacene tends to grow in an almost layer-by-layer fashion, producing ordered terraces of few ¹m in lateral size even on polycrystalline Co. Another aspect addressed in this thesis is the electronic properties of thin molecular films in bulk and at interfaces with ferromagnetic metals. Proper functionality of the molecular spintronic devices requires appropriate electronic properties. These are determined, on the one hand, by intra-molecular properties such as transport gap, electron affinity, as well as inter-molecular overlap of molecular orbitals. Also the alignment of the energy levels of the molecular systems with respect to the Fermi level of the metal and the nature of interaction at these interfaces play an important role for the charge injection into the molecular films. We analyzed the electronic properties of thin pentacene films and of its interfaces with Co and Ni(111) by means of ultraviolet photoelectron spectroscopy (UPS). We found a difference of 1.4 eV between the ionization potential of the gas phase and the solid state, which we attribute to a change in the local environment and charge redistribution in pentacene. Despite the fact that the ionization potential of pentacene is very close to the work function of the two studied metals, an increased barrier for the hole injection at these two interfaces was found. We attributed these observations to hybridization between molecules and substrate. Besides the production of large area, pinhole-free and well-ordered layers, a strict requirement consists of preventing interdiffusion when depositing top electrodes on a organic film. While this process has been studied extensively for polymer LEDs, the requirements may be even more stringent in the present case, since diffused atoms may act as spin scattering centers. We studied the magnetic properties of Co layers deposited by two different deposition methods (magnetron sputtering and evaporation) on PTCDA. We demonstrated that the presence of the molecular film influences the magnetic properties of Co (such as magnetic moment and switching behavior). This might provide an attractive way of establishing different switching fields for top and bottom electrodes. Moreover, we have evidence that Co particles interdiffuse more strongly into the molecular film when sputter deposition is used instead of evaporation. As a potential application of organic materials in spintronics, we investigated the so-called magnetic tunnel junctions, with a barrier made out of molecular constituents. We have been able to produce a promising magnetoresistance (MR) of 7% at 4 K with junctions based on [2,2’; 6’,2"] terpyridine-4-yloxy-hexanoic acid (TERPY) deposited in UHV conditions, although still hampered by a poor reproducibility, severe interdiffusion and a full quenching of MR above 30 K.