Indium Tin Oxide optical access for magnetic tunnel junctions in hybrid spintronic-photonic circuits.

The all-optical magnetization reversal of magnetic layers, by picosecond optical pulses, is of particular interest as it shows the potential for energy-efficient and fast magnetic tunnel junction (MTJ) elements. This approach requires memory elements that are optically and electronically accessible, for optical writing and electronic read-out. In this paper, we propose the integration of indium tin oxide (ITO) as a transparent conducting electrode for magnetic tunnel junctions in integrated spintronic-photonic circuits. To provide light with sufficient energy to the MTJ free layer and allow electrical read-out of the MTJ state, we successfully integrated indium tin oxide as a top transparent electrode. The study shows that ITO film deposition by physical vapor deposition with conditions such as high source power and low O2 flow achieves smooth and conductive thin films. Increase in grain size was associated with low resistivity. Deposition of 150 nm ITO at 300 W, O2 flow of 1 sccm and 8.10-3 mbar vacuum pressure results in 4.8 × 10-4 Ω.cm resistivity and up to 80% transmittance at 800 nm wavelength. The patterning of ITO using CH4/H2 chemistry in a reactive ion etch process was investigated showing almost vertical sidewalls for diameters down to 50 nm. The ITO based process flow was compared to a standard magnetic tunnel junctions fabrication process flow based on Ta hard mask. Electrical measurements validate that the proposed process based on ITO results in properties equivalent to the standard process. We also show electrical results of magnetic tunnel junctions having all-optical switching top electrode fabricated with ITO for optical access. The developed ITO process flow shows very promising initial results and provides a way to fabricate these new devices to integrate all-optical switching magnetic tunnel junctions with electronic and photonic elements.

[Moyamoya disease: Diagnosis, clinical features, evolution and treatment in 10 patients]. / Maladie de Moyamoya : aspects diagnostiques, cliniques, évolutifs et thérapeutiques chez 10 patients.

INTRODUCTION: Moyamoya disease is a rare chronic cerebrovascular disease. The objective of this article is to report the different clinical and radiological presentations and describe treatments and clinical course of this disease. METHODS: We searched for patients with Moyamoya disease diagnosed at a French university hospital. The diagnosis was based on arteriographic records showing uni- or bilateral stenosis of distal intracranial internal carotid arteries or middle cerebral arteries associated with a classic collateral network imparting a puff of smoke aspect. Data about clinical and radiological symptoms were analyzed for all identified patients. RESULTS: Ten patients were recorded between 2009 and 2014 including one child and nine adults. The initial presentation was intracerebral hemorrhage in two patients, ischemic stroke in six, and either exercice-related transient ischemic attacks or syncope in two. Recurrent events were noted in four patients. Four patients had one or several recurrent vascular events. Eight patients were given medical treatment and none underwent surgery. Secondary Moyamoya syndrome was suspected in two patients, all the others one were considered idiopathic. CONCLUSION: Moyamoya disease is a rare but potentially severe illness. The initial presentation is more frequently an ischemic stroke; recurrences are frequent. The diagnosis is based on arteriography, which is also recommended to search for a cause.

Endovascular treatment of symptomatic intracranial atheromatous stenosis: a single center study of 21 consecutive cases.

OBJECTIVES: This retrospective single-center study evaluated the technical success as well as the periprocedural and long-term complications of angioplasty with stenting of symptomatic intracranial atheromatous stenosis. PATIENTS AND METHODS: From January 2005 to December 2010, 21 patients were treated by angioplasty with stent implantation for symptomatic atheromatous intracranial stenosis greater or equal to 50% at least 7 days after a stroke. RESULTS: The median population age was 65 years (range: 41-88 years), and 76% (16/21) of the candidates had a history of transient ischemic attack (TIA) or stroke before the event qualifying them for an endovascular procedure. In addition, 86% (18/21) were taking antithrombotic medication at the time of the qualifying event. Technical success was obtained in all cases. The periprocedural stroke or death rate was 9.5% (2/21), and there was no other stroke or death after 30 days during the mean follow-up of 1.4 years. CONCLUSION: Endovascular treatment of symptomatic intracranial atherothrombotic stenosis can lead to severe complications, particularly during the periprocedural period, but it also represents the only alternative treatment for patients who fail with medical therapy. Future studies need to focus on improvement of periprocedural complications. Essentially, this should include more rigorous selection of candidates as well as a better understanding of the pathophysiological mechanism(s) of the ischemic phenomenon related to stenosis.

Spintronic memristors for neuromorphic circuits based on the angular variation of tunnel magnetoresistance.

In this study, a new type of compact magnetic memristor is demonstrated. It is based on the variation of the conductivity of a nano-sized magnetic tunnel junction as a function of the angle between the in-plane reference layer magnetization and a free layer exhibiting an isotropic in-plane coercivity. The free layer magnetization is rotated by two spin transfer torque contributions: one originating from the in-plane magnetized reference layer and the other one from an additional perpendicular polarizer integrated in the stack. Thanks to a proper tuning of the relative influence of these two torques, the magnetization of the free layer can be rotated step by step clockwise or anticlockwise in a range of angle between 0° (parallel configuration) and 180° (anti-parallel configuration) by sending pulses of current through the stack, of one or opposite polarity. The amplitude of the rotation steps and therefore of the conductance variations depends on the pulse amplitude and duration. In this way, we achieve monotonous variations of the resistance with the voltage polarity through the application of pulses in the ns range. We also retrieve the analytical expression of critical current density which is found to be in good agreement with the experimental results. The thermal stability of the intermediate resistance levels and the role of Joule heating are also discussed.

Thermal robustness of magnetic tunnel junctions with perpendicular shape anisotropy.

The concept of Perpendicular Shape Anisotropy STT-MRAM (PSA-STT-MRAM) has been recently proposed as a solution to enable the downsize scalability of STT-MRAM devices beyond the sub-20 nm technology node. For conventional p-STT-MRAM devices with sub-20 nm diameters, the perpendicular anisotropy arising from the MgO/CoFeB interface becomes too weak to ensure thermal stability of the storage layer. In addition, this interfacial anisotropy rapidly decreases with increasing temperature which constitutes a drawback in applications with a large range of operating temperatures. Here, we show that by using a PSA based storage layer, the source of anisotropy is much more robust against thermal fluctuations than the interfacial anisotropy, which allows considerable reduction of the temperature dependence of the coercivity. From a practical point of view, this is very interesting for applications having to operate on a wide range of temperatures (e.g. automotive -40 °C/+150 °C).

A highly thermally stable sub-20 nm magnetic random-access memory based on perpendicular shape anisotropy.

A new approach to increase the downsize scalability of perpendicular STT-MRAM is presented. It consists of significantly increasing the thickness of the storage layer in out-of-plane magnetized tunnel junctions (pMTJ) as compared to conventional pMTJ in order to induce a perpendicular shape anisotropy (PSA) in this layer. This PSA is obtained by depositing a thick ferromagnetic (FM) layer on top of an MgO/FeCoB based magnetic tunnel junction (MTJ) so that the thickness of the storage layer is of the order of or larger than the diameter of the MTJ pillar. In contrast to conventional spin transfer torque magnetic random access memory (STT-MRAM) wherein the demagnetizing energy opposes the interfacial perpendicular magnetic anisotropy (iPMA), in these novel memory cells, both PSA and iPMA contributions favor the out-of-plane orientation of the storage layer magnetization. Using thicker storage layers in these PSA-STT-MRAMs has several advantages. Due to the PSA, very high and easily tunable thermal stability factors can be achieved, even down to sub-10 nm diameters. Moreover, a low damping material can be used for the thick FM material thus leading to a reduction of the write current. The paper describes this new PSA-STT-MRAM concept, practical realization of such memory arrays, magnetic characterization demonstrating thermal stability factor above 200 for MTJs as small as 8 nm in diameter and possibility to maintain the thermal stability factor above 60 down to 4 nm diameter.