United States

The rapid development of memory devices has brought the areal density to approach 10 Tb/in2 (i.e., nearly 244 bits or 44 independent memory units in the area of one square inch), corresponding to an average size around 10 mm2 (or 33 mm2) for one memory unit. Thanks to the interfacial lattice coupling and switchable electric polarization, the artificially-designed perovskite oxide multilayers are capable to exhibit magnetically-working units with the characteristic length of only a few nanometers. For examples, the 12-unit-cells nonmagnetic SrTiO3 (or PbTiO3) spacers can make the 9-unit-cells ferromagnetic (La,Sr)MnO3 layers switch independently in a multilayer heterostructure, of which the functional unit is only 21 unit cells or around 8 nm. Also, the paramagentic CaRuO3 spacer can continuously change the bond angle in the adjacent ferromagnetic (La,Ca)MnO3 layer, resulting in multiple tunable coercivities in a coherently-grown multilayer. More importantly, our recent data show that the thickness of those spacers can be further reduced to 3~4 unit cells (1.2-1.6 nm), while the independence of each magnetic layer is still maintained. Our results demonstrate the promising potentials of well-designed heterostructures in high-density memory devices.

Materials Challenges for Memory

Towards High Density Memory Devices Using Perovskite Oxide Heterointerfaces

oxide heterostructure manganite characteristic length

The 65th Annual Conference on Magnetism and Magnetic Materials (MMM 2020) was held in a virtual form. The COVID-19 pandemic, with its associated uncertainty about international travel and the ability to gather in large groups, meant that the Steering Committee decided to cancel the face-to-face meeting that had been planned in Palm Beach, Florida November 2-6, 2020.

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MMM 2020

poster


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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.

The ever-increasing demand for data storage has driven the development of many cryogenic memory storage technologies. Among these, magnetic random-access memory (MRAM) in the form of magnetic tunnel junctions (MTJs) has emerged as a leading candidate. MTJs have proven successful as a means of data storage, but the large resistance of the tunnel barrier prevents this structure from integration with other low-temperature technologies such as superconducting computing.

Use of a low-resistance barrier, such as in a GMR device using current perpendicular to plane geometry (CPP), has potential to solve this issue, however now we are presented with the opposite problem of having too low resistance. Here we present a way to model this problem and thereby provide a solution to find a ‘Goldilocks’ zone for the conductivity ratio of each layer that allows for a reliable and meaningful voltage readout.

Furthermore, we propose a series of materials with a tunable conductivity that is dependent on the initial growth parameters to replace the commonly used transition metals in this endeavor. The rare-Earth nitride (REN) series are ferromagnetic semiconductors with massive tunability and show promise for integration with existing superconducting technologies.

Cryogenic memory storage using tunable ferromagnetic semiconductors

The field of magnetic random-access memory (MRAM) has captivated immense recognition recently given its capability to provide non-volatility along with accelerated read and write performance. Conventional MRAM devices based on spin-orbit torque (SOT)1, spin-transfer torque (STT)2, or current-driven methods employ magnetization switching using external current. The current-based methods accompany thermal issues and thereby surges the power dissipation. An attractive alternative that has emanated lately is a conception of straintronic (STR) based MRAM devices to store information. To carry out magnetization switching in STR devices external small voltage is employed. Currently, in STR-based MRAM devices, the magnetization reversal without external bias magnetic field is not explored. Although external bias is not desired since it increases device size and cost and lessens thermal stability and retention time by lowering the energy barrier between two extremes stable states.
In this work, size-dependent in-plane magnetization switching of elliptical multiferroic magnets of FeGaB on single-crystal PMN-PT (011) piezoelectric are studied using the finite element method (FEM). To ensure minimum thickness (dmin) for multiferroic magnets in-plane induced energy well (∆E) is scaled by kbT, such that if ∆E/ kbT ≤ 70, STR based MRAM device is thermally unstable. To investigate maximum thickness corresponding to self-biasing (dsb), pre-stress equilibrium exchange and demagnetization energy of multiferroic magnets are examined against thickness. Large size magnets strive to diminish the energy by line up magnetization with magnet boundary that ultimately causes vortex state. Oppositely smaller size magnets strive to diminish the energy by line up magnetization along the easy axis (0o or 180o) of the multiferroic magnet. This examination is deciding after all such self-biasing would give superiority to future STR-based MRAM devices because it gets rid of an external magnetic bias field. Even though the thickness range of dmin≤d≤dsb gives an idea about the STR-MRAM device that is stable and self-bias without applying the external magnetic bias field, it is observed that maximum thickness (dmax) for in-plane 180o magnetization switching is limited by critical stress requirement. When external stress more than or equivalent to critical stress is generated using external voltage, in-plane 90o magnetization switching is observed (Fig 1(a)) from the pre-stress state (0o or 180o). The critical stress requirement for fixed thickness and increased cross-sectional area rises. During in-plane 90o magnetization switching exchange and stress anisotropy energies are observed almost negligible and the demagnetization energy rises since the magnetization is switched from the easy axis to the hard axis. To obtain in-plane 180o switching magnetic precession is used. As soon as the magnetization component along the x-axis reaches its maximum negative value, removal of external voltage causes another 90o switching (Fig 1(b)) since the multiferroic magnet makes an effort to reduce its total energy by falling to its nearest easy axis. In-plane fastest 180o magnetization switching time and associated external strain are probed. For fixed cross-sectional area, when the thickness is increased trade-off between the fastest switching time and associated switching energy is observed. For fixed thickness, when the cross-sectional area is increased faster switching time and lesser switching energy is observed. This work validates and verifies the STR-based MRAM device that abolishes the external magnetic bias field and achieves in-plane 180o magnetization switching.
Reference 
1C. Zhang, S. Fukami, H. Sato, F. Matsukura and H. Ohno, Appl. Phys. Lett. 107, 012401 (2015).
2C.F. Pai, L. Liu, Y. Li, H. W. Tseng, D. C. Ralph and R. A. Buhrman, Appl. Phys. Lett. 101, 122404 (2012).


Strain-induced magnetization switching in FeGaB nanomagnets for self-biased MRAM applications

Change of angular dependence of magnetoresistance derivative in CoFeB/Ta/CoFeB platform under nanoparticles Fe/Fe3O4, deposited on sample surface was observed. 
In Р+ state of the CoFeB/Ta/CoFeB platform effective anisotropy of the system “platform-particle” is strongly reduced to the value close to zero. This happens due to compensation of positive anisotropy of the areas not covered by nanoparticles by strong negative anisotropy of those areas covered by nanoparticles.
Comparison between the Atomic Force and Magnetic Force Microscope images and subtraction of corresponding phase contrasts allows visualization of the film magnetization affected by the particles.
Theoretical estimation and OOMMF modeling of the diameter of the CoFeB film area, which magnetization is reversed under the dipolar magnetic field created by the Fe/Fe3O4 particle, are obtained. They are found to be in good agreements with the area observed experimentally. 


Microwave magnetoresistance of CoFeB spin valves covered with nanoparticles

An abrupt reduction in resistance called forming is required for resistance switching phenomenon in ReRAM structures using a binary oxide such as nickel oxide (NiO). In this study, dependence of the forming characteristics on applied voltage, cell size, and ambient temperature were investigated under constant voltage stresses to Pt/NiO/Pt stack structures. Many experimental results suggest that the formation of conductive filaments at forming follows a weakest link theory, and that weakest spots are randomly distributed in the NiO film according to the Poisson statistics. Moreover, at a high temperature of 400K, gradual reduction phenomena in resistance were observed before the forming process, which indicates that defects in the ultrathin layer in the NiO can occur continuously due to electrical stress as the ambient temperature elevate.

Thermal effect of Forming Characteristics in NiO-based ReRAM cells

Emerging memory technologies show the potentials of new memory devices to store more data at a lower cost compared to the industry-standard silicon-based devices. Among these, resistive switching random access memory (RRAM) devices, which are based on the metal-insulator-metal (MIM) structure, have shown potential in non-volatile memory applications due to their characteristics of low-power consumption, superior switching speed, excellent reliability, ultimate scaling potential with the simple fabrication process and adaptive aptitudes for the neuromorphic applications [1, 2]. 
The advantage of the biodegradable materials is non-toxic and eco-friendly but the essential material properties remain the major issues in the scaling of biomass-based RRAMs [3]. Biodegradable magnesium fluoride (MgFx) is an insulator with a large bandgap (11.3 eV) which makes it a favorable candidate for biodegradable RRAM devices. Recently, a couple of reports on MgFx based RRAM devices have shown excellent resistive switching and biodegradable properties [2, 4]. Nevertheless, the conduction mechanism and resistive switching mechanism in the MgFx based RRAM devices are not fully understood. The electrodes in RRAM devices may significantly affect the resistive switching (RS) behavior. The effects of electrode materials and operating environment including cryogenic temperature on the device performance have not been studied yet. 
In this study, we report resistive memory devices employing MgFx as the resistive switching layer that is inserted between the platinum and titanium metal electrodes. The MgFx based RRAM devices exhibit electroforming free bipolar resistive switching characteristics at room temperature and open-air environment. We also report the effects of electrode materials (Top: Pt, Au/Ni, ITO, and Bottom: p+ Si, n+ Si, ITO) on MgFx based RRAM devices. It is found that the difference in workfunctions affects the performances of the devices. We have also analyzed the effect of the operating environment as well as temperature from room temperature down to cryogenic temperature. The detailed performance of the devices, their conduction and switching mechanisms, the effects of the electrode materials, and the operating environment including cryogenic temperature will be presented at the virtual conference.


Effects of the electrode materials and operating environment on magnesium fluoride based bipolar RRAMs

Interfacial exchange coupling and proximity effects in antiferromagnetic/ferromagnetic (AFM/FM) bilayers are the key that control the exchange bias phenomena exploited in most spintronic devices. In both cases, spin fluctuations in the AFM layer during the temperature-driven magnetic phase transition at the Néel temperature (TN) are commonly used to understand the magnitude of the exchange bias field (HE), the onset of HE, referred as the Blocking temperature (TB ≤TN), and the enhancement of the coercive field (HC). Here we show that the magnetic domain structure during the magnetization reversal of the FM layer controls the mentioned effects. Temperature-dependent measurements performed in a V2O3/Co bilayer after different field cooling (FC) procedures reveal that these effects depend strongly on the FC angle and are associated with a different domain structure of a FM layer that has a well-defined uniaxial magnetic anisotropy. Remarkably, a wide temperature window for TB (c.a. 35 K) and up to a factor of two in HE are found. All experimental observations can be explained within the Random-Field Model for the interfacial exchange coupling in AFM/FM bilayers with a fixed (nanometre-sized) AFM domain structure in contact with a variable (angle-dependent) FM domain structure. Our results highlight the importance of the domain structure and magnetization reversal of the FM layer in the exchange coupling phenomena, paving the way to enhance and/or control interfacial effects in future spintronic devices.

Tailoring interfacial phenomena in hybrid V2O3/Co bilayers

This experimental study measures interface spin Seebeck effect (SSE), spin current generation by a temperature gradient, in two antiferromagnetic crystals using optical pump-Terahertz emission spectroscopy. We utilise the absorption of a femtosecond optical pulse in an adjacent Pt thin layer to create a strong temperature discontinuity at the interface, which subsequently generates an incoherent spin current. The Pt layer also converts the spins to a charge current, generating a single-cycle Terahertz electric field pulse. The ultra-fast probing technique enables focusing the interpretation to only the interface effect. Strong THz emissions from both antiferromagnets indicate their excellent interface quality for spin transport. We observe disparate nature of the SSE signal dependence on temperature in the two antiferromagnets, which unambiguously reveals the influence of magnon resonant frequency.

Probing Interfacial Spin Transport in Antiferromagnets using Terahertz Spin Seebeck Effect

Most optoelectronic and memristive devices are fabricated using a vertically aligned configuration, where one or more layers of the active material(s) are “sandwiched” between the metal electrodes. Coplanar electrode architectures on the other hand, may offer many advantages, such as lower power consumption, faster operating speed, greater sensitivity and higher-level of integration. Furthermore, using nanogap (<15 nm)-separated coplanar electrodes, extreme downscale of the active layer thickness can be achieved, resulting in fast response speed photodetectors and low-power resistive switching in memristive devices.
Herein, we show memristive devices based on coplanar nanogap electrodes. These have been fabricated at a wafer scale with an inexpensive nanopatterning technique, compatible with flexible substrates, named adhesion lithography. We demonstrate memristors based on empty and metal oxide, polymer and carbon-nanotube materials, discuss the mechanism dominating their operation and showcase their ability to be integrated in arrays.

Nanogap electrode-based memristive devices

The paper presents the results of numerical modeling and experimental studies of the regularities of the manifestation of the memristor effect of oxide nanodots obtained by the method of local anodic oxidation.

Memristive properties of titanium oxide nanodot structures for resistive memory

High density, high-speed and low power consuming nonvolatile memories are currently being vigorously explored for use in next-generation computation, particularly due to the performance gap between the logic and memory elements of the current computational architecture.[1–3] Of various explored material systems, electrically switchable spontaneous polarization of ferroelectric materials enables a robust nonvolatile memory solution.[4–6] Using ultrathin films of ferroelectric materials as a tunnel barrier in metal/ferroelectric/metal trilayer structure, so called ferroelectric tunnel junctions (FTJ), is being explored widely as a potential nonvolatile memory elements. Unlike to ferroelectric RAM (FeRAM), FTJ offers nondestructive readout, in addition to low operation energy and high operation speed.[7]
In this work,[8] we have demonstrated FTJs with a very large OFF/ON resistance ratio and relatively low resistance area product (RA) with ~ 1 nm thick Zr doped HfO2 (HZO) ferroelectric tunnel barrier. We stabilized ferroelectricity in ultrathin films of rhombohedral HZO (R-HZO) through the substrate-induced compressive strain. The resistance area product at the bias voltage (~ 300 mV) required for one-half of the zero-bias TER ratio is three orders of magnitude lower than reported value with relatively thick ferroelectric barriers. [9, 10] The OFF/ON resistance ratio of 135 achieved with ~ 1 nm barrier, which is enhanced to 105 with increasing the barrier thickness to 2.5 nm, is the highest reported value for HZO barrier based FTJs. These results set the stage for further exploration of Hafnia-based FTJs for non-volatile memory applications.

References
[1] S. Manipatruni, D. E. Nikonov, I. A. Young, Nat.
Phys. 2018, 14, 338.
[2] H. S. P. Wong, S. Salahuddin, Nat. Nanotechnol.
2015, 10, 191.
[3] B. Prasad, Y. L. Huang, R. V. Chopdekar, Z. Chen,
J. Steffes, S. Das, Q. Li, M. Yang, C. C. Lin, T.
Gosavi, D. E. Nikonov, Z. Q. Qiu, L. W. Martin, B.
D. Huey, I. Young, J. Íñiguez, S. Manipatruni, R.
Ramesh, Adv. Mater. 2020, 32, 1.
[4] S. Manipatruni, D. E. Nikonov, C.-C. Lin, T. A.
Gosavi, H. Liu, B. Prasad, Y.-L. Huang, E.
Bonturim, R. Ramesh, I. A. Young, Nature 2019,
565, 35
[5] S. Oh, H. Hwang, I. K. Yoo, APL Mater. 2019,
091109 7.

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