Controlling magnetoresistance by oxygen impurities in Mq3-based molecular spin valves

The understanding of magnetoresistance (MR) in organic spin valves (OSVs) based on molecular semiconductors is still incomplete after its demonstration more than a decade ago. While carrier concentration may play an essential role in spin transport in these devices, direct experimental evidence of its importance is lacking. We probed the role of charge carrier concentration by studying the interplay between MR and multilevel resistive switching in OSVs. The present work demonstrates that all salient features of these devices, particularly the intimate correlation between MR and resistance, can be accounted for by the impurity band model, based on oxygen migration. Finally, we highlight the critical importance of carrier concentration in determining spin transport and MR in OSVs and the role of interface-mediated oxygen migration in controlling the OSVs response

Conditions for the growth of smooth La0.7Sr0.3MnO3 thin films by pulsed electron ablation

We report on the optimisation of the growth conditions of manganite La0.7Sr0.3MnO3 (LSMO) thin films prepared by Channel Spark Ablation (CSA). CSA belongs to pulsed electron deposition methods and its energetic and deposition parameters are quite similar to those of pulsed laser deposition. The method has been already proven to provide manganite films with good magnetic properties, but the films were generally relatively rough (a few nm coarseness). Here we show that increasing the oxygen deposition pressure with respect to previously used regimes, reduces the surface roughness down to unit cell size while maintaining a robust magnetism. We analyse in detail the effect of other deposition parameters, like accelerating voltage, discharging energy, and temperature and provide on this basis a set of optimal conditions for the growth of atomically flat films. The thicknesses for which atomically flat surface was achieved is as high as about 10-20 nm, corresponding to films with room temperature magnetism. We believe such magnetic layers represent appealing and suitable electrodes for various spintronic devices.Comment: original paper, thin film optimization, 25 pages, 9 figure

Electron transport and noise spectroscopy in organic magnetic tunnel junctions with PTCDA and Alq3 barriers

Isidoro Martinez, Juan Pedro Cascales, Jhen-Yong Hong, Minn-Tsong Lin, Mirko Prezioso, Alberto Riminucci, Valentin A. Dediu, Farkhad G. Aliev, "Electron transport and noise spectroscopy in organic magnetic tunnel junctions with PTCDA and Alq3 barriers", Spintronics IX, Proc. SPIE 9931 (4 November 2016); doi: 10.1117/12.2237721. Copyright 2016 Society of Photo-Optical Instrumentation Engineers (SPIE). One print or electronic copy may be made for personal use only. Systematic reproduction and distribution, duplication of any material in this paper for a fee or for commercial purposes, or modification of the content of the paper are prohibitedProceedings of IX Spintronics Conference ( San Diego, California, United States)The possible influence of internal barrier dynamics on spin, charge transport and their fluctuations in organic spintronics remains poorly understood. Here we present investigation of the electron transport and low frequency noise at temperatures down to 0.3K in magnetic tunnel junctions with an organic PTCDA barriers with thickness up to 5 nm in the tunneling regime and with 200 nm thick Alq 3 barrier in the hopping regime. We observed high tunneling magneto-resistance at low temperatures (15-40%) and spin dependent super-poissonian shot noise in organic magnetic tunnel junctions (OMTJs) with PTCDA. The Fano factor exceeds 1.5-2 values which could be caused by interfacial states controlled by spin dependent bunching in the tunneling events through the molecules. 1 The bias dependence of the low frequency noise in OMTJs with PTCDA barriers which includes both 1/f and random telegraph noise activated at specific biases will also be discussed. On the other hand, the organic junctions with ferromagnetic electrodes and thick Alq 3 barriers present sub-poissonian shot noise which depends on the temperature, indicative of variable range hoppingSupport by UAM-Santander collaborative project (2015/ASIA/04) as well as by the Spanish MINECO (MAT2012-32743 and MAT2015-66000-P) grants and the Comunidad de Madrid through NANOFRONTMAG (S2013/MIT-2850) is also gratefully acknowledged. J.P.C. acknowledges support from the Fundacion Seneca (Region de Murcia) posdoctoral fellowship (19791/PD/15

Studio dei difetti elettricamente attivi in strutture contenenti punti quantici di InAs in una matrice di GaAs

In this work, InAs(QD)/GaAs(001) structures are analyzed in order to identify the conditions for plastic strain relaxation via dislocations. To this aim samples with a continuously varying coverage, going from 1.5 to 2.9 ML, have been studied mainly by means of electrical characterization and with the fundamental support of structural TEM analysis, PL measurements and AFM. In the whole coverage range small-sized coherent QDs are formed with diameters and densities that increase up to 15 nm and 2E11 cm−2, respectively. At the same time, for coverage above 2.4 ML, large-sized QDs with diameters of 25 nm and densities ranging from 2E8 to 1.5E9 cm−2 coexist with small-sized QDs. We explain the formation of large-sized QDs as the unavoidable consequence of ripening, as predicted for highly lattice-mismatched systems under thermodynamic equilibrium conditions, when the coverage of the epitaxial layer exceeds a critical value. The fraction of ripened islands which plastically relax increases with coverage, leading to the formation of V-shaped defects at the interface between QDs and upper confining layers that propagate toward the surface. The relaxation of those incoherent islands substantially affects the properties of QD structures by (i) reducing the free carrier concentration near the QD plane, (ii) formation of deep levels with typical features related to extended structural defects, and (iii) considerably quenching the QD photoluminescence intensity. Moreover a preliminary work on Scanning Capacitance Microscopy measurements on these structures have been carried out. The first tests demonstrates a sample ageing due to many hours of ambient atmosphere exposition.In questo lavoro sono state analizzate strutture di InAs(QD)/GaAs(001) per identificare le condizioni in cui avviene il rilascio plastico dello strain. Con questo scopo sono stati cresciuti campioni con un coverage gradato da 1.5 a 2.9ML. Questo studio è stato effettuato principalmente con tecniche di caratterizzazione elettrica, coadiuvate da analisi strutturali tramite TEM, misure di PL e AFM. In tutto il range di coverage esplorato abbiamo riscontrato la presenza di piccoli quantum dots di tipo coerente con diametro e densità che arrivano rispettivamente a 15nm e 2e11cm-2. Contemporaneamente, per coverage superiori a 2.4ML, sono state osservate isole di maggiori dimesioni con diametro fino a 25nm e densità che vanno da 2e8 a 1.5e9 cm-2, che coesistono con i quantum dots più piccoli. Abbiamo spiegato la formazione di questi QDs di dimensioni maggiori, tramite l'inevitabile fenomeno del ripening, come predetto dalla teoria in sistemi con alto mismatch reticolare ed in equilibrio termodinamico, quando si supera un valore di coverage critico. La frazione di isole ripened che rilasciano lo strain in modo plastico, aumenta con il coverage, portando alla formazione di difetti a V, all'interfaccia tra QDs e lo strato di confinamento superiore. Questi difetti si propagano verso la superficie. Il rilascio di strain inficia le proprietà delle strutture a QDs (i) riducendo la concentrazione di portatori liberi vicino al piano dei QDs, (ii) formando livelli profondi che mostrano le caratteristiche tipiche dei livelli associati a difetti estesi e, (iii) riducendo in modo significativo l'intensità di fotoluminescenza dei QDs. Inoltre, durante il periodo di tesi, è stato fatto un lavoro preliminare di implementazione della microscopia a scansione di capacità su questi campioni. I primi test hanno mostrato che l'esposizione all'aria dei campioni ne compromette la misurabilità

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

Silicon nanostructures for photonics applications

Low dimensional silicon is used to improve the optical properties of silicon and enable silicon photonics: few recent developments in the fields are here described