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Ultrafast and energy-efficient writing by spin-orbit torque (SOT) prevents device degradation within thermally stable magnetic random-access memory (MRAM) with stable perpendicular magnetic anisotropy (PMA) by separating the read and write paths. However, orthogonality between spin polarization and PMA impedes its application at industrial level because of non-deterministic switching. Previously proposed approaches to break this orthogonality includes structural deformation and external magnetic field control. However, all of those approaches increase the fabrication complexity, are highly sensitive to the SOT current duration and magnitude, or increase the switching energy. By exploiting the precessional nature of the field-like SOT component, we propose toggle PMA SOT-MRAM that achieves field-free and energy-efficient switching with a simple structure, and is robust to variations in the SOT current magnitude and duration. 
The proposed three-terminal structure consists of a magnetic tunnel junction (MTJ), heavy metal, and compensating ferromagnet. A unidirectional SOT current applied through the heavy metal stabilizes the MTJ free ferromagnet magnetization at an excited steady state after crossing the hard axis plane. Switching off the excitation relaxes the magnetization to the nearest easy axis, thus completing a toggle cycle. The next excitory pulse toggles the magnetization back to its initial state. The deformation-free structure provides efficient data read-out and can also be leveraged for directional writing through a simple XOR between the stored and incoming bits. Robust MRAM writing with greater than 50% tolerance to variations in the excitory SOT current magnitude has been demonstrated through micromagnetic simulation. Moreover, room temperature simulations indicate that this switching is robust to thermal noise. The proposed device therefore provides a promising solution for compact, highly-efficient, deformation-free and robust SOT-MRAM with PMA.

Materials Challenges for Memory

Field-Free Toggle Mode Spin-Orbit Torque Driven MRAM with Perpendicular Magnetic Anisotropy

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spin-orbit torque

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The energy cost of storing and moving data is enormous, and is growing at such a rapid pace that within a few years the power demand may overwhelm some developed nations. Non-volatile memory (NVM) and neuromorphic computing are enabling critical advances in this area, but there are many scientific and technological challenges. This conference will cover the materials challenges of different memory technologies, from the fundamental processes and mechanisms, to industry perspectives on materials specifications and manufacturing.

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Exchange Bias (EB), based on the concept of hysteresis (MH) loop shift, investigated almost 70 years back, is one of the most studied phenomenon. The shift in the MH loop has garnered a good amount of interest due to the huge range of applications in spintronic devices. Vertical magnetization shift (VMS) of the MH loops has gained attention recently in exchange bias (EB) regime. But no concrete results have been found yet. We present a novel counter-intuitive approach to realizing the vertical MH loop shift in soft/hard heterostructure. A thin film of 13 unit cells (t= 5nm) of SrRuO3 (SRO) is grown epitaxially by pulsed laser deposition on SrTiO3 (STO) (100) substrate. On top of that, a polycrystalline Ni80Fe20 (Py) thin film of t=2nm is grown by DC/RF sputtering. We observe a clear and significant vertical MH loop shift at 2K after cooling from room temperature (300K) under a bias field (i.e. field cooled-FC) (Figure. a). There is a positive (negative) shift of the MH loop on applying a positive (negative) bias field of around 1T. Essentially, there is no vertical shift for zero field cooling (ZFC). This was only observed when the MH loop is measured with low field range (±500mT). For higher field range (e.g. ±7T) MH measurement, no loop shift is found for both ZFC and FC measurements. There is a threshold to field tracing range (< SRO’s saturation field) below which we can observe a vertical shift phenomenon only. The vertical shifts are observed only below 100K as well which is nearby to the Curie temperature of SRO (TC~160K) below which SRO becomes ferromagnetic. The reason of the shift is also ascribed to the anisotropy play of the two layers. Surprisingly, the coupling between Py and SRO here is exchange spring (ES) type rather than EB type, but simultaneously exhibiting strong EB like shift but in vertical direction. Moreover, the micromagnetic OOMMF simulations are performed, which are in broad agreement with the experimental results (Figure. b). We simulate for the same ratio of thicknesses of the two layers to manifest a generalized ground. The computational results with modelling serve as a verification of the experimental findings qualitatively and quantitatively. We make use of ES tenets to effectuate the novel phenomena of vertical shift. The vertical shift due to the ES interactions between the soft and the hard layer expands the territory of ES applications.

Bias field mediated vertical hysteresis loop shift in Ni80Fe20/SrRuO3 heterostructure for novel memory switching

We report the first results of the growth of nickel and permalloy (Ni80Fe20) circular nanodot arrays with a dot diameter of 300 nm using Talbot lithography and electrochemical deposition. Samples with different thicknesses (50 nm to 400 nm) and, thus, aspect ratios were prepared by the variation of deposition times. The magnetization reversal and the associated magnetic configuration were probed by vibrating sample magnetometry (VSM) and magnetic phase diagram was plotted for both nickel and permalloy nanodot arrays. We observe a transition from a uniform magnetisation in the inplane direction to a vortex state followed by a uniform magnetisation to the out of plane direction, with an increase in thickness of the dots. The magnetic phase diagram in dependence on the dot geometry is in good agreement with micromagnetic simulations and for sputtered structures. We also probe the magnetic vortex states in permalloy nanodot arrays using Kerr microscopy and further aim to magneto-ionically control the magnetic states in our dot arrays.

Magnetic vortex and single domain states in transition metal nanodot arrays prepared by Talbot lithography and electrodeposition

It is an urgent need to design the next generation non-volatile electronic memory with higher density, and resistive random access memory (ReRAM) based on the mechanism of resistive switching is a promising candidate. Here, we study the resistive switching mechanism of Pr0.7Ca0.3MnO3 with strong correlation effects, which has a clockwise bipolar resistive switching property without forming process. When give a positive voltage on the surface of Pr0.7Ca0.3MnO3, it can change from low resistive state (LRS) to high resistive state (HRS), when apply negative voltage, it transforms from HRS into LRS. We also find the transition between LRS and HRS is due to the field induced redox reaction which can change (increase or decrease) the concentration of oxygen vacancies in Pr0.7Ca0.3MnO3. The increase (decrease) of oxygen vacancies in Pr0.7Ca0.3MnO3 can result in the increase (decrease) of electron filling on Mn 3d orbit. And finally lead to a change of Mott band gap in Pr0.7Ca0.3MnO3 with a resistive switching.

A study of Mott-transition based resistive switching in Pr0.7Ca0.3MnO3 film

We developed a novel method to directly integrated few-layer BP which was exfoliated in melamine aqueous by liquid-phase exfoliation (LPE), and then adding cyanuric molecules in the solution, resulting in self-assembling melamine-cyanurate (MCA) by the networks of hydrogen bonds together with embedded few-layer BP sheets. It is noteworthy that we used melamine to facilitate exfoliation of BP bulk in the water under ultrasonication, where high crystalline few-layer BP sheets can be obtained within a half-hour. The as-exfoliated BP sheets presented stable and well dispersed in the aqueous solution. Afterward, we integrated mixed precipitation to a two-terminal RRAM-based memristor as an active layer, exhibiting analog synaptic behaviors under both operations of DC sweeping and AC pulses as emulating artificial synapse.

Analog synaptic behavior of memristor composed of few-layer black phosphorus embedded in supramolecular

Electric-field control of magnetism is a fundamentally different approach towards the implementation of ultralow heat-dissipation memories operated at the nanoscale as it does not involve the use of electric currents and thus minimizes Joule heating effect (resistive heating). Here, we investigate the effects of voltage on arrays of highly porous iron oxide microdisks (8 µm in diameter), obtained combining sol-gel with lithography and wet chemical etching processes, that are virtually non-magnetic in the as-prepared state (OFF state). Electrolyte-gating is then employed to induce a tunable ferromagnetic response in these disks at room temperature, thus accomplishing an OFF-ON switching of magnetism. The changes in magnetic properties are attributed to magnetoelectrically-driven oxygen ion exchange with an anhydrous liquid electrolyte, which is enhanced due to nanoporosity. This causes partial reduction of the various iron oxide phases present in pristine samples to metallic Fe. The effect can be considerably reversed by applying voltage of opposite polarity. These results are appealing for diverse technological applications that require the use of patterned structures with easily tunable magnetic properties, such as magnetic micro-electro-mechanical systems, microfluidic and lab-on-a-chip platforms for biomedical therapies and, ultimately, energy-efficient magnetic memories or neuromorphic computing.

Electric-field control of magnetism in arrays of nanoporous iron oxide microdisks

Memristors are metal/insulator/metal devices with high potential to be implemented as a new generation of non-volatile memories and novel new neuromorphic computing devices. These structures have the property of changing their resistance between different non-volatile states upon the application of external electrical stimuli, a process usually called resistive switching (RS). RS mechanisms are usually associated with the electromigration of defects such as oxygen vacancies (OV), either through the creation/disruption of conductive nanofilaments or by modulating the height of Schottky barriers, formed at metal/insulator interfaces. When the insulating oxide is ferroelectric, the resistive change at Schottky interfaces could be of electronic nature and associated to the switching of the direction of the ferroelectric polarization [1]. Ferroelectric memristors are considered to present a faster and more reliable response, as the physical mechanism does not require electromigration of defects. 
In this work [2] we study the memristive response of PZT and BaTiO3-based ferroelectric memristors. We find in both cases that RS is related to two competing effects acting on metal/ferroelectric Schottky interfaces: the switching of the ferroelectric polarization and OV electromigration. Moreover, we propose that both effects are entangled and the latter is mainly controlled by the depolarizing field, arising from an incomplete screening of ferroelectric bound charges. We model the experimental response with a modified version of the VEOV model [3] that accounts for the presence of ferroelectricity and satisfactorily reproduces several non-trivial aspects of the experimental response. Finally, we show that these systems display coexisting non-volatile and volatile memristive response, an issue that could be useful for the development of neuromorphic devices.

[1] P. W. M. Blom, R. M. Wolf, J. F. M. Cillessen, and M. P. C. M. Krijn, Phys. Rev. Lett. 73, 2107 (1994)
[2] C. Ferreyra, M. Rengifo, M.J. Sánchez, A. S. Everhardt, B. Noheda, and D. Rubi, Phys. Rev. Appl. 14, 044045 (2020)
[3] M. J. Rozenberg, M. J. Sánchez, R. Weht, C. Acha, F. Gomez-Marlasca, and P. Levy, Phys. Rev. B 81, 115101 (2010).

Coexistence of polarization and ionic electromigration effects in the memristive response of ferroelectric oxides

Resistive switching (RS) devices are considered promising among other emerging devices to meet the demand for future memory device applications. Various transition metal oxides have been investigated for their RS properties. Out of different binary transition metal oxides, WO3-x is attractive because of its promising switching result and compatibility with CMOS process technology. While RS properties depend strongly on the defect concentration in the oxides and metal/oxide interfaces, it has been found that in various metal oxides the RS characteristics are also strongly affected by atmospheric moisture content. In this work the RS characteristics of Au/WO3-x/Pt device and its influence by the atmospheric moisture has been investigated. The non-stoichiometry and the adsorbed water molecule on the tungsten oxide’s surface is confirmed by the X-ray photoelectron spectroscopy. The device exhibited rectification and stable bipolar RS characteristics without the need of any forming step. DC conduction mechanism investigated at different temperature suggested the switching is primarily dominated by the Schottky type Au/WO3-x interface. Current conduction through the active layer has been found to be dominated by Schottky emission at low electric field and Poole–Frenkel emission at high electric field. The reverse current has been found to increase significantly on subjecting the device to ambient moisture. Modification of the Schottky barrier due to defect redistribution and molecular dipole at the Au/WO3-x interface have been discussed as a possible mechanism of the observed RS modulation. The modification of Schottky barrier height with ambient moisture reveals an important challenge the WO3-x based interface type resistive switching material faces which must be considered for the devolvement of resistive switching based future memory devices.

Modulation of resistive switching characteristics of WO3-x based MIM structure by ambient moisture

Development of floating gate (FG) non-volatile memories (NVMs) based on nanocrystals (NCs) is of great importance for the continuous increasing requirements for highly efficient data storage.1 NCs by tuning of their size, density & interface quality with embedding oxide make possible the adjusting of device parameters. These NVMs use FG of either metal or semiconductor NCs in oxide as SiO2, HfO2, Al2O3.1–7. In particular, group IV-based NCs of Si, Ge, Sn are of high interest due to the compatibility with CMOS technology.3–8
Here we report on obtaining NVM capacitors with FG of SiGeSn NCs acting as charge storage nodes, being embedded in HfO2. The advantage of using SiGeSn NCs consists in tailoring NC compositions (up to Sn segregation limit), reducing thermal budget by using Sn for NCs formation and at same time obtaining NCs stability by alloying with Si.
We use configuration of trilayer (gate HfO2/FG of SiGeSn NCs in HfO2/tunnel HfO2) that we prepare on Si wafers.4 Trilayers are deposited by magnetron sputtering of HfO2/ SiGeSn-HfO2/HfO2 and annealed for nanostructuring in rapid thermal processor (520–530 oC). XTEM, XRD & Raman spectroscopy are used for complex characterization of layers composition, morphology & structure. NVM structures are capacitor-like, with Al top & bottom contacts. XTEM reveals formation of ~5 nm Ge-rich SiGeSn NCs in FG, with 5% Si content & diamond structure. 2 types of SiGeSn NCs are evidenced (also by Raman) in function of their position in FG. 1st type NCs are positioned inside FG (extending on the whole FG thickness), having a small Sn amount (2%), while 2nd type NCs more rich in Sn (15%) are positioned at FG interfaces with gate & tunnel layers. The SiGeSn NCs in FG produce good NVM performance by C–V hysteresis loops with ~1 V memory window.
1. Chem Rev 120, 3941 (2020) 2. Sci Adv 2, e1501101 (2016)
3. ACS Appl Electron Mater 1, 1852 (2019) 4. Appl Phys Lett 113, 213106 (2018) 
5. Appl Phys Lett 107, 093102 (2015) 6. Nanotechnology 30, 365604 (2019)
7. Appl Surf Sci 542, 148702 (2021) 8. Scripta Mater. 113, 135 (2016)

Trilayer non-volatile memory structures with floating gate of SiGeSn nanocrystals embedded in HfO2

Already used industrially in TFTs in OLED displays, amorphous In-Ga-Zn oxide (IGZO) is a particularly attractive material also for Resistive-random-access-memory (ReRAM) applications, thanks to its low oxygen vacancy migration barrier, size scalability and ease of fabrication at RT by sputtering. In traditional IGZO-based ReRAM, conducting filaments are stochastically formed (usually by a “forming” process) and ruptured to induce switching between high and low resistance states (HRS and LRS). The issues then lie in poor uniformity, unfavorable scaling, limited endurance and stability issues.
In this study, novel IGZO ReRAM devices are investigated with an interface-type mechanism. These devices overcome stability issues. Bipolar Resistive switching (BRS) is achieved by the modulation of Schottky barrier height, through oxygen vacancy migration at the electrode interface.

Interface-controlled Resistive Switching in a-InGaZnO Memristors

In most atomic force microscopy (AFM) based measurements, the electrostatic effect is a common phenomenon, which will affect the measured electromechanical response in contact-based dynamic force microscopy. The electrostatic contributions might result in a misinterpretation of the observed response. The development of piezoresponse force microscopy (PFM) has enabled researches to investigate ferroelectric characteristics on the nanoscale. PFM is not free of being affected by electrostatic effects and leads to misinterpretation. Therefore, it is crucial to explore the original of electrostatic effects in PFM testing. Nevertheless, the relationship between electrostatic effect and nano metal-semiconductor (M-S) junction is barely discussed. Here, we examine how to differentiate ferroelectric from non-ferroelectric of well-known ferroelectric (BiFeO3, BaTiO3), paraelectric (SrTiO3) and semiconducting (Si) systems via various PFM measurement characteristics. The nano M-S junction studied by characterizing the I-V response. In the end, we observed that for different materials should apply different DC voltages to compensate for the excess charge on the surface. The obtained results reveal that the nano M-S junction rules the electrostatic effect between the tip and the sample, and the DC voltage applied to compensate surface charging is related to the rectification behavior due to Schottky contact at the nano M-S junction.

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Bias field mediated vertical hysteresis loop shift in Ni80Fe20/SrRuO3 heterostructure for novel memory switching

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