Spin-Torque Diode Measurements of MgO-Based Magnetic Tunnel Junctions with Asymmetric Electrodes

We present a detailed study of the spin-torque diode effect in CoFeB/MgO/CoFe/NiFe magnetic tunnel junctions. From the evolution of the resonance frequency with magnetic field at different angles, we clearly identify the free-layer mode and find an excellent agreement with simulations by taking into account several terms for magnetic anisotropy. Moreover, we demonstrate the large contribution of the out-of-plane torque in our junctions with asymmetric electrodes compared to the in-plane torque. Consequently, we provide a way to enhance the sensitivity of these devices for the detection of microwave frequency

A ferroelectric memristor

Memristors are continuously tunable resistors that emulate synapses. Conceptualized in the 1970s, they traditionally operate by voltage-induced displacements of matter, but the mechanism remains controversial. Purely electronic memristors have recently emerged based on well-established physical phenomena with albeit modest resistance changes. Here we demonstrate that voltage-controlled domain configurations in ferroelectric tunnel barriers yield memristive behaviour with resistance variations exceeding two orders of magnitude and a 10 ns operation speed. Using models of ferroelectric-domain nucleation and growth we explain the quasi-continuous resistance variations and derive a simple analytical expression for the memristive effect. Our results suggest new opportunities for ferroelectrics as the hardware basis of future neuromorphic computational architectures

Jonctions tunnel magnétiques et ferroélectriques : nouveaux concepts de memristors.

A memristor is a variable non-volatile nanoresistance which value depends on the quantity of charges that have flown through. This device is very promising as a multi-level binary memory but also as an artificial synapse for brain-inspired computing architecture. During this thesis, we have studied two new concepts of memristor based on purely electronic effects. The first concept, the spintronic memristor, is based on a magnetic tunnel junction in which a domain wall is created. The resistance of the junction depends on the position of the domain wall. The resistance variations are obtained by displacement of the domain wall induced by spin transfer effect. The second concept, the ferroelectricmemristor, is based on a tunnel junction with a ferroelectric barrier. The resistance of such a junction depends on the orientation of the polarization. We show that those junctions exhibit good performances as a binary memory element. The memristive behaviour is obtained by a gradual switching of the polarization. The experimental results bring a proof of those concepts. Unlike other memristors based on mechanisms such as electromigration or phase change, our two concepts based on purely electronic effect are faster and expected to be more reliable.Durant ce travail de thèse, nous avons étudié deux concepts originaux de memristor fondés sur des effets purement électroniques. Un memristor est une nanorésistance variable non-volatile dont la valeur dépend de la quantité de charges qui l'a traversée. Ce composant est particulièrement prometteur pour des applications en tant qu'élément de mémoire binaire multi-niveaux ou en tant que synapse artificielle pour intégration dans des architectures de calculs neuromorphiques. Le premier concept, le memristor spintronique, se base sur une jonction tunnel magnétique dans laquelle une paroi magnétique est introduite. Par l'effet de magnétorésistance tunnel, la résistance de la jonction dépend de la configuration magnétique, et donc de la position de la paroi. La variation de résistance est obtenue en déplaçant la paroi grâce à un courant par effet de transfert de spin. Le deuxième concept, le memristor ferroélectrique, se base sur une jonction tunnel dont la barrière est ferroélectrique. La résistance d'une telle jonction dépend de l'orientation de la polarisation de la barrière ferroélectrique. Nous montrons qu'elle a un fort potentiel en tant qu'élément de mémoire binaire de part la vitesse et l'énergie d'écriture. Le comportement memristif est obtenu par un retournement progressif de la polarisation électrique. Les résultats expérimentaux obtenus apportent la preuve des concepts. Contrairement aux memristors existants basés sur des processus comme l'électromigration ou le changement de phase, ces deux concepts fondés sur des effets purement électroniques sont prometteurs en termes de rapidité et d'endurance

Jonctions tunnel ferroélectriques ou magnétiques (nouveaux concepts de memristor)

Durant ce travail de thèse, nous avons étudié deux concepts originaux de memristor fondés sur des effets purement électroniques. Un memristor est une nanorésistance variable non-volatile dont la valeur dépend de la quantité de charges qui l a traversée. Ce composant est particulièrement prometteur pour des applications en tant qu élément de mémoire binaire multi-niveaux ou en tant que synapse artificielle pour intégration dans des architectures de calculs neuromorphiques. Le premier concept, le memristor spintronique, se base sur une jonction tunnel magnétique dans laquelle une paroi magnétique est introduite. Par l effet de magnétorésistance tunnel, la résistance de la jonction dépend de la configuration magnétique, et donc de la position de la paroi. La variation de résistance est obtenue en déplaçant la paroi grâce à un courant par effet de transfert de spin. Le deuxième concept, le memristor ferroélectrique, se base sur une jonction tunnel dont la barrière est ferroélectrique. La résistance d une telle jonction dépend de l orientation de la polarisation de la barrière ferroélectrique. Nous montrons qu elle a un fort potentiel en tant qu élément de mémoire binaire de part la vitesse et l énergie d écriture. Le comportement memristif est obtenu par un retournement progressif de la polarisation électrique. Les résultats expérimentaux obtenus apportent la preuve des concepts. Contrairement aux memristors existants basés sur des processus comme l électromigration ou le changement de phase, ces deux concepts fondés sur des effets purement électroniques sont prometteurs en termes de rapidité et d endurance.A memristor is a variable non-volatile nanoresistance which value depends on the quantity of charges that have flown through. This device is very promising as a multi-level binary memory but also as an artificial synapse for brain-inspired computing architecture. During this thesis, we have studied two new concepts of memristor based on purely electronic effects. The first concept, the spintronic memristor, is based on a magnetic tunnel junction in which a domain wall is created. The resistance of the junction depends on the position of the domain wall. The resistance variations are obtained by displacement of the domain wall induced by spin transfer effect. The second concept, the ferroelectric memristor, is based on a tunnel junction with a ferroelectric barrier. The resistance of such a junction depends on the orientation of the polarization. We show that those junctions exhibit good performances as a binary memory element. The memristive behaviour is obtained by a gradual switching of the polarization. The experimental results bring a proof of those concepts. Unlike other memristors based on mechanisms such as electromigration or phase change, our two concepts based on purely electronic effect are expected to be faster and more reliable.PARIS-BIUSJ-Physique recherche (751052113) / SudocSudocFranceF

Crossbar operation of BiFeO3/Ce–CaMnO3 ferroelectric tunnel junctions: From materials to integration

Ferroelectric Tunnel Junctions (FTJs) are a candidate for the hardware realization of synapses in artificial neural networks. The fabrication process for a 784 x 100 crossbar array of 500 nm large FTJs, exhibiting effective On/Off currents ratio in the range 50-100, is presented. First, the epitaxial 4 nm-BiFeO3/Ca0.96Ce0.04MnO3//YAlO3 is combined with Ni electrodes. The oxidation of Ni during the processing affects the polarity of the FTJ and the On/ Off ratio, which becomes comparable to that of CMOS-compatible HfZrO4 junctions. The latter have a wider coercive field distribution: consequently, in test crossbar arrays, BiFeO3 exhibits a smaller cross-talk than HfZrO4. Furthermore, the relatively larger threshold for ferroelectric switching in BiFeO3 allows the use application of half-programming schemes for supervised and unsupervised learning. Second, the heterostructure is combined with W and Pt electrodes. The design is optimized for the controlled collapse chip connection to neuromorphic circuits.ISSN:0884-2914ISSN:2044-532

Ferroelectric phase transitions in epitaxial antiferroelectric PbZrO3 thin films

International audienceThe archetypical antiferroelectric, PbZrO3, is currently attracting a lot of interest, but no consensus can be clearly established on the nature of its ground state as well as on the influence of external stimuli over its physical properties. Here, the antiferroelectric state of 45-nm-thick epitaxial thin films of PbZrO3 is established by observing the characteristic structural periodicity of antiparallel dipoles at the atomic scale, combined with clear double hysteresis of the polarization-electric field response related to antiferroelectric–to–ferroelectric phase transitions. Surprisingly, while the antiferroelectric state is identified as the ground state, temperature-dependent measurements show that a transition to a ferroelectric-like state appears in a large temperature window (100 K). Atomistic simulations further confirm the existence, and provides the origin, of such ferroelectric state in the films. Electric-field-induced ferroelectric transitions are also detected by the divergence of the piezoresponse force microscopy response. Using this technique, we further reveal the signature of a ferroelectric ground state for 4-nm-thick PbZrO3 films. Compared with bulk crystals, these results suggest a more complex competition between ferroelectric and antiferroelectric phases in epitaxial thin films of PbZrO3