1,720,997 research outputs found
Electric field control of magnetic properties and electron transport in BaTiO3-based multiferroic heterostructures
Full text linkIn this paper, we report on a purely electric mechanism for achieving the electric control of the interfacial spin polarization and magnetoresistance in multiferroic tunneling junctions. We investigate micrometric devices based on the Co/Fe/BaTiO3/La0.7Sr0.3MnO3 heterostructure, where Co/Fe and La0.7Sr0.3MnO3 are the magnetic electrodes and BaTiO3 acts both as a ferroelectric element and tunneling barrier. We show that, at 20 K, devices with a 2 nm thick BaTiO3 barrier present both tunneling electroresistance (TER = 12 ± 0.1%) and tunneling magnetoresistance (TMR). The latter depends on the direction of the BaTiO3 polarization, displaying a sizable change of the TMR from -0.32 ± 0.05% for the polarization pointing towards Fe, to -0.12 ± 0.05% for the opposite direction. This is consistent with the on-off switching of the Fe magnetization at the Fe/BaTiO3 interface, driven by the BaTiO3 polarization, we have previously demonstrated in x-ray magnetic circular dichroism experiments
Blocking Temperature Engineering in Exchange-Biased CoFeB/IrMn Bilayer
Full text linkIn this paper, we report on the magnetic and chemical characterization of the exchange-biased CoFeB/IrMn bilayers, grown
by magnetron sputtering on a Si-based platform and capped by either a Ru or MgO/Ru overlayer. For Ru capping, the locking temperature monotonously increases with the IrMn thickness within the investigated range (3.5–8 nm). On the contrary, for MgO/Ru capping, the exchange bias is inhibited below 6 nm, whereas above 6 nm, the magnetic behavior is the same of Ru-capped films. The chemical analysis reveals a significant dependence of the Mn content from the capping layer for thin IrMn films (2.5 nm), whereas the difference disappears when IrMn becomes thick (7 nm). Our work suggests that a non-uniform composition of the IrMn films directly affects the exchange coupling at the IrMn/CoFeB interface
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Full text linkWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
koamabayili/VECTRON-author-checklist: VECTRON author checklist
No full textWe have done our best to complete the author checklist relating to the use of animals in the hut study. Note that the objective for the hut study was to evaluate the IRS treatment applications for residual efficacy against Anopheles mosquitoes, including the local An. coluzzii mosquito population. Cows were only used to attract mosquitoes into the huts and no tests were carried out directly on the cows. The author checklist is intended for use with studies where experiments are carried out on animals, which is why we have had such difficulty in completing this for the hut study, as many of the questions do not relate to how the cows were used
Voltage control of magnetic anisotropy in metal-oxide heterostructures
No full textIl comportamento di dispositivi aventi dimensioni nanometriche dipende in larga misura dalle superfici e interfacce, poiché una frazione significativa di materiale si trova al bordo o in contatto con altri materiali. In presenza di metalli e ossidi, le proprietà chimiche, elettroniche, meccaniche e magnetiche sono determinate dell'interfaccia metallo-ossido. La possibilità di controllare le proprietà di questa interfaccia con un campo elettrico è da un lato promettente dal punto di vista tecnologico, e dall'altro interessante dal punto di vista teorico. In particolare, il controllo elettrico delle proprietà magnetiche è un campo che è stato recentemente spinto dall'introduzione sul mercato delle Magnetic Random Access Memories (MRAM), e potrebbe permettere di risolvere il loro limite principale, che è la potenza dissipata durante l'operazione di scrittura. Il vantaggio di un approccio basato sul controllo dell'anisotropia magnetica mediante un campo elettrico sta nel fatto che, invece di usare una corrente che dissipa molta potenza per scrivere un bit, come viene fatto nella tecnologia MRAM a stato dell'arte, si potrebbe usare un campo elettrico in un isolante. L'applicazione del campo elettrico può invertire la magnetizzazione di un elettrodo magnetico “libero” in una MTJ, scrivendo un bit “zero” o “uno” al solo costo energetico richiesto per caricare un capacitore. In aggiunta a questo potenziale miglioramento della tecnologia esistente, il controllo delle proprietà magnetiche con un campo elettrico potrebbe portare alla creazione di nuovi tipi di dispositivi, come ad esempio dispositivi magnonici basati sull'elaborazione di onde di spin tramite la creazione di strutture e ambienti magnetici riconfigurabili elettricamente. In questa tesi discuterò due possibili strade verso il controllo elettrico dell'anisotropia magnetica, ovvero l'uso di multiferroici artificiali (CoFeB/BaTiO3) e di sistemi magneto-ionici (CoFeB/GdOx).
I multiferroici artificiali sono sistemi dove un materiale ferromagnetico e un materiale ferroelettrico sono accoppiati all'interfaccia, in modo tale da poter controllare le proprietà del ferromagnete cambiando la polarizzazione del ferroelettrico, il che viene chiamato effetto magneto-elettrico inverso. In particolare, in questa tesi tratto l'eterostruttura CoFeB/BaTiO3. In questo sistema, il campo coercitivo magnetico di uno strato ultrasottile di CoFeB può essere controllato dalla polarizzazione ferroelettrica di un film spesso di titanato di bario (BTO), inducendone una variazione relativa del 60%. Anche se l'effetto della deformazione indotta dalla piezoelettricità del BTO non può essere del tutto escluso, l'origine principale del fenomeno viene dalla carica di polarizzazione indotta all'interfaccia dello strato di CoFeB ultrasottile dai momenti di dipolo elettrico del titanato di bario. La modulazione del campo coercitivo magnetico può essere usata per invertire la magnetizzazione elettricamente, poiché uno stato di magnetizzazione “stabile” quando il campo coercitivo è elevato, può diventare “instabile” se il campo coercitivo viene ridotto dall'effetto della polarizzazione ferroelettrica, in presenza di un opportuno campo magnetico statico. Questa inversione della magnetizzazione assistita elettricamente può essere effettuata per campi magnetici positivi o negativi, e la configurazione di campo coercitivo è non-volatile. Un campo magnetico applicato su un'area grande può essere usato in combinazione con una tensione per invertire dispositivi selezionati in un chip di memoria. Comunque, non abbiamo visto un riorientamento dell'asse facile di anisotropia magnetica da perpendicolare al piano a parallelo al piano, il che limita le potenziali applicazioni in dispositivi di memoria. Per questa ragione abbiamo deciso di studiare un altro sistema, basato sull'effetto magneto-ionico, dove è possibile osservare un riorientamento dell'asse facile magnetico.
L'effetto magneto-ionico permette di controllare le proprietà magnetiche di uno strato sottile di metallo ferromagnetico, mediante un'ossidazione controllata in tensione. Questo avviene poiché gli ioni O2- possono essere spostati da un campo elettrico in un conduttore ionico a stato solido, e possono così modificare l'anisotropia magnetica di un vicino strato ferromagnetico mediante ossidazione. Questo effetto è stato dimostrato recentemente nel Co/GdOx. Nel corso di questa tesi mostro che l'effetto magneto-ionico può essere ottenuto anche in CoFeB/GdOx. L'uso del CoFeB è motivato dalla sua importanza tecnologica, poiché è usato come elettrodo ferromagnetico nelle MRAMs CoFeB/MgO/CoFeB. Le proprietà magnetiche del CoFeB nel sistema CoFeB/GdOx dipendono dalla tensione applicata ai capi del GdOx, attraverso l'effetto magneto-ionico. Abbiamo rilevato questo effetto sia in Ta/CoFeB con anisotropia magnetica nel piano del campione, sia in Pt/CoFeB, con anisotropia magnetica perpendicolare al piano. In particolare, l'anisotropia magnetica perpendicolare al piano è rafforzata quando il CoFeB è ridotto, mentre diminuisce quando il CoFeB è ossidato. Abbiamo ottenuto un cambio dell'asse facile magnetico indotto dalla tensione in Pt/CoFeB a ±2V, grazie al quale abbiamo potuto definire un limite inferiore per la variazione di densità di energia magnetica all'interfaccia pari a 0.2 mJ m-2. Abbiamo anche osservato una modulazione elettrica delle proprietà magnetiche del CoFeB nel CoFeB/Ta, ma in questo caso non abbiamo osservato un riorientamento dell'asse magnetico.
La possibilità di riorientare l'asse magnetico basata su un effetto magneto-ionico è interessante dal punto di vista delle applicazioni. Per testare la fattibilità di questo approccio e con l'obiettivo di dimostrare l'inversione della magnetizzazione di uno strato magnetico indotta elettricamente, abbiamo fabbricato delle MTJs Co/GdOx/Co, in una configurazione tale per cui funzionassero come sensori di campo magnetico. Il GdOx costituisce una buona barriera per gli elettroni, che la attraversano per effetto tunnel, come dimostrato dal comportamento della conduttanza per diversi spessori di GdOx, che viene ben descritto dal modello di Brinkman. Le giunzioni Co/GdOx/Co appena fabbricate presentano una magnetoresistenza tunnel (TMR) del 1.9% a 300 K. è interessante notare che abbiamo osservato salti di resistenza in queste giunzioni (resistive switching), a una tensione simile a quelle usate per l'effetto magneto-ionico, ma la magneto-resistenza tunnel nello stato a bassa resistenza diminuisce a meno dello 0.2%, il che non permette di misurare lo stato magnetico dell'elettrodo. Risultati preliminari mostrano che un annealing in vuoto in presenza di un campo magnetico applicato aumenta il rapporto di magnetoresistenza, il che potrebbe portare alla misura della TMR anche nello stato a bassa resistenza. Questo problema sarà discusso in studi futuri, come anche sarà determinato l'effettivo potenziale dell'effetto magneto-ionico nei dispositivi.The behavior of nanoscale devices largely depends on surfaces and interfaces, since a significant fraction of material lies at the boundaries, in contrast to what happens in macroscopic objects. In the presence of metals and oxides, many chemical, electronic, mechanical and magnetic properties are determined by the characteristics of the metal-oxide interface. The possibility to control the properties of this interface by an electric field is both promising from the technological point of view and interesting from the theoretical one. In fact, the electric control of magnetic properties in metal-oxide heterostructures is a field which recently gained attention thanks to the advent of Magnetic Random Access Memories (MRAMs), and may allow their main limitation, which is the amount of power dissipated during writing, to be overcome. The advantage of an approach based on the control of magnetic anisotropy by an electric field lies in the fact that, instead of using a highly-dissipating current for the writing operation, as in the state-of-art MRAM technology, an electric field may be applied in an insulator, in order to switch the magnetization of a "free" magnetic layer in a MTJ. This, in principle, allows one to write a "zero" or "one" bit, requiring only the energetic cost for charging a capacitor. In addition to this potential improvement of the existing technology, the control of the magnetic properties by a voltage may lead to the creation of new kinds of solid state devices, such as new magnonic devices, based on the processing of spin waves by the creation of magnetic patterns and landscapes that are reconfigurable electrically. In this thesis, two possible routes toward electric control of magnetic anisotropy will be discussed, namely the use of artificial multiferroics (CoFeB/BaTiO3) and of magneto-ionic systems (CoFeB/GdOx).
Artificial multiferroics are systems where a ferromagnetic and a ferroelectric material are coupled at an interface, in such a way that it is possible to control the properties of the ferromagnet by changing the polarization of the ferroelectric material, leading to the Converse Magneto Electric Effect. In particular, in this thesis, we report on the CoFeB/BaTiO3 heterostructure. In this system, the magnetic coercive field of an ultrathin CoFeB layer can be controlled by the ferroelectric polarization of a barium titanate thick film (BTO) inducing a relative variation of 60%. Even if the effect of the strain cannot be completely excluded, the main origin of this phenomenon stems from the charge induced at the interface of the ultra-thin CoFeB layer by the electric dipole moments of the barium titanate. The modulation of the magnetic coercive field can be used to switch the magnetization electrically, since a stable magnetization state when the coercive field is high, becomes unstable if the coercive field is reduced by the effect of the ferroelectric polarization, in the presence of a suitable external magnetic bias field. This electrically assisted switching can be performed for positive, as well as negative, magnetic bias fields and the final state configuration is non-volatile. A magnetic field applied on a large area may be used in combination with a voltage to switch only selected devices in a memory chip. However, no spin reorientation transition was seen in this system, i.e. the change of the magnetic anisotropy axis from perpendicular-to-plane to in-plane, thus limiting its potential application in memory devices. For this reason, we decided to study another system, based on the magneto-ionic effect, where it is possible to detect the spin reorientation transition.
The magneto-ionic effect allows the control of the magnetic properties of a ferromagnetic metallic layer, by a voltage-driven oxidation. This happens since O2- ions can be moved by an electric field in a solid-state ionic conductor, and they can modify the anisotropy of a nearby ferromagnetic layer via oxidation. This effect was recently demonstrated in Co/GdOx. During my thesis I showed that the magneto ionic effect can be achieved also in CoFeB/GdOx. The use of CoFeB is motivated by its technological importance, since it is used as a ferromagnetic electrode in CoFeB/MgO/CoFeB MRAMs. The magnetic properties of CoFeB in the CoFeB/GdOx system depend on the voltage applied across GdOx, via the magneto-ionic effect. We have detected this effect both in Ta/CoFeB with In-plane Magnetic Anisotropy and in Pt/CoFeB with Perpendicular Magnetic Anisotropy. In particular, the perpendicular magnetic anisotropy is weakened when the CoFeB is reduced, while it is strengthened when the CoFeB is oxidized. An electrically driven spin reorientation transition was obtained in Pt/CoFeB at ±2V, by which we set a lower limit for the interface magnetic energy anisotropy variation of 0.2 mJ m-2. We also observed an electrical modulation of the CoFeB magnetic properties in CoFeB/Ta, but in this case no spin reorientation transition was seen.
The existence of a spin reorientation transition based on the magneto-ionic effect is interesting in view of future applications. In order to test the feasibility of such an approach and with the aim of demonstrating the switching of a free layer by the magneto-ionic effect in a device, we fabricated Co/GdOx/Co MTJs, in the magnetic field sensor configuration. GdOx is a good tunnel barrier at room temperature, as indicated by the good fit of the conductance values as a function of the GdOx thickness by the Brinkman model of tunneling. As-fabricated Co/GdOx/Co MTJs present a sizable Tunneling Magneto-Resistance of 1.9% at 300 K. Interestingly, resistive switching was observed in these junctions at a voltage similar to the ones leading to the magneto-ionic effect, but the TMR in the Low-Resistance State is decreased to less than 0.2%, which does not allow to probe the magnetic state of the electrode. Preliminary results show that an annealing in vacuum in the presence of an applied magnetic field improves the TMR ratio, which may allow the detection of the TMR in the LRS. This will be addressed in future studies, as well as the assessment of the potential of the magneto-ionic effect in devices.DIPARTIMENTO DI FISICASolid state physics: advanced spectroscopy, scanning probe microscopy, nanostructure fabrication29GHIRINGHELLI, GIACOMO CLAUDIOTARONI, PAOL
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