Charge and spin transport in memristive organic LSMO/Alq3/AlOx/Co spin valves

In this thesis I studied La0.7Sr0.3MnO3/Alq3/AlOx/Co organic spin valves, which are multifunctional devices showing an interesting interplay between magnetoresistive effects and memristive switching. In particular this work aims at elucidating the elusive mechanisms for spin injection and transport in this archetypal structure. While spin injection in organic materials was demonstrated by different spectroscopic techniques, the origin of magnetoresistive effect in organic spin valves is still debated. In fact, the Hanle effect, considered to be the only reliable proof for spin transport across the organic spacer layer, has not been observed in such a device, yet. I investigated the thickness and temperature dependence of charge transport and magnetoresistive properties, and demonstrated the absence of the Hanle effect. Moreover I studied how the resistance and magnetoresistance of our devices were affected by memristive switching, which turned out to be a fundamental tool to enlighten the comprehensive picture. Two clearly distinguishable conduction regimes have been found for non magnetoresistive and magnetoresistive devices. The former is compatible with models for charge transport in organic materials, the latter can be described by an equivalent circuit where metallic paths and hopping channels act in parallel. In the framework of this model, a coherent description for the interplay between MR and memristive switching can be given. SV signals can be explained as tunnel magnetoresistance (TMR) or ballistic magnetoresistance (BMR) occurring across shortened regions of the organic bulk, which is an explanation compatible with the absence of Hanle effect. This work demonstrates that SV signals can be explained without resorting to spin injection and transport into the organic layer

Oxygen impurities link bistability and magnetoresistance in organic spin valves

Vertical cross-bar devices based on manganite and cobalt injecting electrodes and metal-quinoline molecular transport layer are known to manifest both magnetoresistance and electrical bistability. The two effects are strongly interwoven, inspiring new device applications such as electrical control of the magnetoresistance and magnetic modulation of bistability. To investigate the full device functionality, we first identify the mechanism responsible for electrical switching by associating the electrical conductivity and the impedance behavior with chemical states of buried layers obtained by in operando photoelectron spectroscopy. These measurements revealed that a significant fraction of oxygen ions migrates under voltage polarity, resulting in a modification of the electronic properties of the organic material and of the oxidation of interfacial layer with ferromagnetic contacts. Variable oxygen doping of the organic molecule represents the key element for correlating bistability and magnetoresistance and our measurements provide the first experimental evidence in favor of the impurity band model describing the spin transport in organic semiconductors in similar devices

Spin injection in the doped bad metal SrTiO3

In this paper, we demonstrate the capability to establish spin-polarized currents in doped SrTiO3 (STO). The results are based on the study of charge and spin transport in STO layers doped by the reversible electromigration of oxygen atoms in resistive-switching La0.7Sr0.3MnO3/STO/Co vertical stacks. The formation of oxygen vacancies inside STO results in a metallic conductivity at temperatures <200–250 K, above which a transitionto an insulating like behavior is detected. A detailed theoretical analysis shows that the behavior of the metallic phase in our samples corresponds to the well-known state of the thermodynamically doped STO featuring the so-called bad metal behavior. Thus, our findings introduce this class of unconventional materials as valuable candidates for innovative spintronic devices

Revascularized giant aneurysm of the anterior communicating artery after surgery and embolization, occluded by placement of a Leo+Baby intracranial stent. A case report.

Balt (Montmorency, France) recently manufactured the Leo+Baby dedicated intracranial stent for arteries with a calibre between 1.5 and 3.10 mm. We describe a patient with a partially thrombosed giant sacciform aneurysm of the anterior communicating artery treated without success by surgery and coil embolization subsequently occluded by placement of a Leo+Baby stent (Balt, Montmorency, France). A 56-year-old man presented with a giant aneurysm in the anterior communicating artery region. Following successive surgical intervention and embolization procedures the patient was referred to us with a revascularized aneurysm measuring 15×9×8 cm. To stabilize the endovascular occlusion a combined treatment was scheduled with coil embolization and stent deployment after dual antiplatelet therapy started five days before the interventional procedure. Treatment was undertaken two weeks later under general anaesthesia and total heparinization. A microcatheter was inserted into the aneurysmal sac and four metal coils were released for a total of 61cm obtaining almost complete occlusion of the aneurysm from the circulation. A Leo+Baby stent (2.5×18 mm) was subsequently deployed across the aneurysm neck. At follow-up angiography two months later the aneurysm appeared substantially excluded from the arterial circulation except for a small portion in the neck. Six months later, four months after suspending antiplatelet therapy, follow-up angiography disclosed the complete exclusion of the aneurysm from the circulation. Deployment of the new ministent through the same microcatheter used to release the coils made the interventional procedure simpler and faster

Disease progression or pseudoprogression after concomitant radiochemotherapy treatment: Pitfalls in neurooncology

Although radionecrosis has been exhaustively described in depth in the neurooncological literature, its diagnosis is still a challenging issue because its radiological pattern is frequently indistinguishable from that of tumor recurrence. This review discusses the causes of radionecrosis and the potential effect of adjuvant chemotherapy concomitant with radiotherapy on its rate and onset. The potential pitfalls in clinical studies attempting to make a differential diagnosis between radionecrosis and disease progression are also discussed

Low intrinsic carrier density LSMO/Alq3/AlOx/Co organic spintronic devices

The understanding of spin injection and transport in organic spintronic devices is still incomplete, with some experiments showing magnetoresistance and others not detecting it. We have investigated the transport properties of a large number of tris-(8-hydroxyquinoline)aluminum-based organic spintronic devices with an electrical resistance greater than 5 MΩ that did not show magnetoresistance. Their transport properties could be described satisfactorily by known models for organic semiconductors. At high voltages (>2 V), the results followed the model of space charge limited current with a Poole-Frenkel mobility. At low voltages (∼0.1 V), that are those at which the spin valve behavior is usually observed, the charge transport was modelled by nearest neighbor hopping in intra-gap impurity levels, with a charge carrier density of n0 = (1.44 ± 0.21) × 1015 cm−3 at room temperature. Such a low carrier density can explain why no magnetoresistance was observed

Hanle effect missing in a prototypical organic spintronic device

We investigate spin precession (Hanle effect) in the prototypical organic spintronic giant magnetoresistance device La0.7Sr0.3MnO3/tris(8-hydroxyquinoline)/AlOx/Co. The Hanle effect is not observed in measurements taken by sweeping a magnetic field at different angles from the plane of the device. As possible explanations we discuss the tilting out of plane of the magnetization of the electrodes, exceptionally high mobility, or hot spots. Our results call for a greater understanding of spin injection and transport in such devices