The 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.

Ferroelectric Tunnel Junctions (FTJs) are a candidate for the hardware realization of synapses in artificial neural networks. The fabrication process for a 784 × 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-BiFeO 3 /Ca 0.96 Ce 0.04 MnO 3 //YAlO 3 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 HfZrO 4 junctions. The latter have a wider coercive field distribution: consequently, in test crossbar arrays, BiFeO 3 exhibits a smaller cross-talk than HfZrO 4 . Furthermore, the relatively larger threshold for ferroelectric switching in BiFeO 3 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. Graphical abstract

Artificial neural networks (ANN) are well known for performing Recognition, Data mining and Synthesis (RMS) tasks. However, the most famous ANNs are software implemented on computers that never take into account the power consumption management. Chip designers are aiming at low-power consumption by developing the neuromorphic engineering field. The goal is to design and produce neural-inspired architectures allowing energy-efficient computation systems. One decade ago, neuromorphic engineering had a renewal of interest, in particular due to the unveiled memristive devices. Indeed, memristors own all the features necessary in order to play the role of plastic synapses in ANNs. Among all memristive technologies, ferroelectric devices present an important advantage for low power systems: their high resistance which implies low current. In this paper, we will present a Verilog-A model of ferroelectric memristors. This model is based on measurements and therefore takes into account the variability of devices in terms of R ON , R OFF and switching characteristics. This realistic model will be helpful for designing neuromorphic systems based on these devices. Finally, we will present some Cadence simulations of learning in small neural networks composed of CMOS neurons and memristive synapses.

In ferroelectric memory devices, information is stored within the polarization direction whose reversal usually occurs by the nucleation and propagation of domains. In ultrathin ferroelectrics, ultrafast dynamics may be achieved by nucleation-limited switching, avoiding the inherently speed-limited propagation of domain walls. Here, we investigate polarization reversal dynamics in ultrathin ferroelectric films by transient current measurements. Thanks to the tunnel electroresistance, the start of polarization reversal induces sharp variations of the transmitted current under voltage pulses. These single-shot measurements show extremely fast switching with durations down to 3 ns that is only limited by the current device geometry. While the OFF-to-ON switching shows finite nucleation times that scale with the pulse amplitude, the ON-to-OFF switching speed cannot be detected under such rectangular pulses. Resorting to triangular pulse excitations allows us to detect the dynamics of this switching direction. Both cases can be interpreted by nucleation switching models following Merz's law.

We present time-resolved measurements of the displacement of magnetic domain-walls (DWs) driven by vertical spin-polarized currents in track-shaped magnetic tunnel junctions. In these structures, we observe very high DW velocities (600 m/s) at current densities below 107 A/cm2. We show that the efficient spin-transfer torque combined with a short propagation distance allows avoiding the Walker breakdown process and achieving deterministic, reversible, and fast (≈1 ns) DW-mediated switching of magnetic tunnel junction elements, which is of great interest for the implementation of fast DW-based spintronic devices.

We present time-resolved, non-averaged measurements of the displacement of magnetic domain-walls (DWs) driven by vertical spin-polarized currents in track-shaped magnetic tunnel junctions. In these structures we observe very high DW velocities (600 m/s) at relatively small current densities (< 10^7 A/cm^2), those velocities being greater than both the typically observed velocities with in-plane injected currents and the predicted velocities in the Walker breakdown DW propagation regime. We show that the efficient spin-transfer torque combined with a short propagation distance can effectively avoid the Walker breakdown process, and achieve deterministic, reversible and fast (~ 1 ns) DW-mediated magnetic switching.

Domain walls, nanoscale transition regions separating oppositely oriented ferromagnetic domains, have significant promise for use in spintronic devices for data storage and memristive applications. The state of these devices is related to the wall position and thus rapid operation will require a controllable onset of domain wall motion and high speed wall displacement. These processes are traditionally driven by spin transfer torque due to lateral injection of spin polarized current through a ferromagnetic nanostrip. However, this geometry is often hampered by low maximum wall velocities and/or a need for prohibitively high current densities. Here, using time-resolved magnetotransport measurements, we show that vertical injection of spin currents through a magnetic tunnel junction can drive domain walls over hundreds of nanometers at ~500 m/s using current densities on the order of 6 MA/cm2. Moreover, these measurements provide information about the stochastic and deterministic aspects of current driven domain wall mediated switching.

Unité Mixte de Physique CNRS/Thales, 1 Av. A. Fresnel, Campus de l’Ecole Polytechnique, 91767 Palaiseau (France) and Université Paris-Sud, 91405 Orsay (France) Université d’Evry-Val d'Essonne, Bd. F. Mitterrand, 91025 Evry cedex (France) Department of Materials Science, University of Cambridge, Cambridge, CB2 3QZ (United Kingdom) Thales Research & Technology, 1 Av. A. Fresnel, Campus de l’Ecole Polytechnique, 91767 Palaiseau (France) National Institute of Advanced Industrial Science and Technology (AIST), Tsukuba, Ibaraki 305-8562 ( Japan) A ferroelectric memristor SUPPLEMENTARY INFORMATION DOI: 10.1038/NMAT3415

Memristors are devices whose dynamic properties are of interest because they can mimic the operation of biological synapses. The demonstration that ferroelectric domains in tunnel junctions behave like memristors suggests new approaches for designing neuromorphic circuits. Memristors are continuously tunable resistors that emulate biological synapses1,2. Conceptualized in the 1970s, they traditionally operate by voltage-induced displacements of matter, although the details of the mechanism remain under debate3,4,5. Purely electronic memristors based on well-established physical phenomena with albeit modest resistance changes have also emerged6,7. Here we demonstrate that voltage-controlled domain configurations in ferroelectric tunnel barriers8,9,10 yield memristive behaviour with resistance variations exceeding two orders of magnitude and a 10 ns operation speed. Using models of ferroelectric-domain nucleation and growth11,12, 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.

This chapter describes the experimental achievement which adds conceptually new features to a standard memristor principle. It shows that electrically controlled magnetoresistance can be achieved in organic devices by combining magnetic bistability, GMR effect, and memristive effects. The chapter discusses the La0.7Ca0.3MnO3/ PrBa2Cu3O7/ La0.7Ca0.3MnO3 (LCMO/ PBCO/ LCMO) magnetic tunnel junctions (MTJs) where the non superconducting cuprate PBCO serves as a barrier. It presents a comprehensive study on the growth and electronic structure of ultrathin europium oxide (EuO) films on Si(001). The chapter also presents spintronic memristor, with resistance changes based on purely electronic phenomena. The chapter demonstrates that tunnel magnetoresistance and resistive switching can be observed simultaneously in nanoscale MTJs with MgO barriers. It focuses on the enhancement of antiferromagnetic coupling in Fe/Si/Fe structures prepared by molecular beam epitaxy. The chapter also shows that resistive switching in manganese-doped zinc oxide (Mn-ZnO) coexists with a switching of the magnetic phase. Controlled Vocabulary Terms antiferromagnetic materials; europium alloys; magnetic tunnelling; magnetoelectronics; tunnelling magnetoresistance

This chapter shows how the tunnel resistance can vary by more than two orders of magnitude upon polarization switching in highly-strained ultrathin BaTiO3 tunnel barriers. This strong electroresistance effect can be probed using a conductive AFM tip as the top electrode, or using solid-state submicron pads. Such ferroelectric tunnel junctions show large, stable, reproducible and reliable tunnel electroresistance, with resistance switching related to ferroelectric polarisation reversal. They thus emerge as an alternative to other resistive memories, with the additional advantage of not being based on voltage-induced migration of matter at the nanoscale, but on a purely electronic mechanism. Importantly, switching can be as fast as a few ns. The chapter presents data on the dynamical response of ferroelectric junctions, and their analysis with standard models of polarization reversal. Controlled Vocabulary Terms ferroelectric materials; magnetic tunnelling; optical polarization

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.

Shifting electrically a magnetic domain wall (DW) by the spin transfer mechanism is one of the future ways foreseen for the switching of spintronic memories or registers. The classical geometries where the current is injected in the plane of the magnetic layers suffer from a poor efficiency of the intrinsic torques acting on the DWs. A way to circumvent this problem is to use vertical current injection. In that case, theoretical calculations attribute the microscopic origin of DW displacements to the out-of-plane (field-like) spin transfer torque. Here we report experiments in which we controllably displace a DW in the planar electrode of a magnetic tunnel junction by vertical current injection. Our measurements confirm the major role of the out-of-plane spin torque for DW motion, and allow to quantify this term precisely. The involved current densities are about 100 times smaller than the one commonly observed with in-plane currents. Step by step resistance switching of the magnetic tunnel junction opens a new way for the realization of spintronic memristive devices.

The discovery that a spin polarized current can exert a large torque on a ferromagnet through a transfusion of spin angular momentum, offers a new way to control a magnetization by simple current injection, without the help of an applied external field. Spin transfer can be used to induce magnetization reversals and oscillations, or to control the position of a magnetic domain wall. In this review, we focus on this last mechanism, which is today the subject of an extensive research, both because the microscopic details for its origin are still debated, but also because promising applications are at stake for non-volatile magnetic memories.

Ferroic-order parameters 1 are useful as state variables in non-volatile information storage media because they show a hysteretic dependence on their electric or magnetic field. Coupling ferroics with quantum-mechanical tunnelling allows a simple and fast readout of the stored information through the influence of ferroic orders on the tunnel current. For example, data in magnetic random-access memories 2 are stored in the relative alignment of two ferromagnetic electrodes separated by a non-magnetic tunnel barrier, and data readout is accomplished by a tunnel current measurement. However, such devices based on tunnel magnetoresistance 3 typically exhibit OFF/ON ratios of less than 4, and require high powers for write operations (>1 × 10 6 A cm −2 ). Here, we report non-volatile memories with OFF/ON ratios as high as 100 and write powers as low as ∼1 × 10 4 A cm −2 at room temperature by storing data in the electric polarization direction of a ferroelectric tunnel barrier. The junctions show large, stable, reproducible and reliable tunnel electroresistance, with resistance switching occurring at the coercive voltage of ferroelectric switching. These ferroelectric devices emerge as an alternative to other resistive memories 4 , and have the advantage of not being based on voltage-induced migration of matter at the nanoscale 5 , 6 , but on a purely electronic mechanism 7 .