Session S64: Controlling Dielectrics and Ferroics for Devices

Low-Voltage Control of Magnetism in Fe0.49Rh0.51/0.68PbMg1/3Nb2/3O3-0.32PbTiO3 Thin Films Heterostructures

With the rapid development of computing applications, researchers are motivated to develop novel low-energy consumption devices for information processing. Amongst candidate technology for such devices are low-power spin logics. Therein, magnetoelectric heterostructures remain a promising route to address the rising need of processing power. Previous research, however, has mainly utilized piezo-strains from piezoelectric bulk or thick films which require high voltages and energies due to their size.^[1] Here, using 100 nm-thick 0.68PbMg_1/3Nb_2/3O₃-0.32PbTiO₃ thin-films as actuator layers, we discuss the potential for ultra-low-voltage control of magnetism in Fe_0.49Rh_0.51 thin-films via strain. Leveraging routes for epitaxial integration of metallic ferromagnets on oxide thin films, initial studies have demonstrated reversible changes in anomalous Hall resistivity with small voltages applied to the 0.68PbMg_1/3Nb_2/3O₃-0.32PbTiO₃ at room temperature which are attributed to the strain induced ferromagnetic and antiferromagnetic phase transition in Fe_0.49Rh_0.51. Our findings reveal the feasibility of voltage control of magnetism with piezoelectric thin-films and can be used in low-voltage/energy computing.   

Electric-field Controllable Non-Volatile Ferroelastic Strain Response in Ferroelectrics with Thin Film Strain Engineering

Recently, a type of ‘straintronic’ device combining thin film stress of capping layers, and ferroelastic coupling in relaxor ferroelectric Pb(Mg_1/3Nb_2/3)_0.71Ti_0.29O₃ (PMN-PT), has been shown as an effective approach to electrically control strain/electronic phase of 2D MoTe₂ [1]. An evolution from bipolar to non-volatile unipolar strain has been observed in devices under electric-field and temperature cycling, which is different from the typical strain response of PMN-PT alone. Here, we investigate the strain evolution of metal thin films deposited on (110) oriented PMN-PT surface under electric-field and temperature cycling and find that in the [-110] direction, it is strongly dependent on thin film stress. Tensile or compressively stressed films result in opposite ferroelastic unipolar responses under applied electric field, attributed to opposite electric-field biasing from elastic dipole locking in PMN-PT. Temperature cycling modifies thin film stress, which can be used to control the polarity of the unipolar strain response. Our finding expands the potential of the ‘straintronic’ platform where non-volatile strain can be controlled, and NMOS/PMOS style device polarity can be created through thin film strain engineering.

[1] W. Hou, et al. Nat. Nanotechnol. 14, 668–673 (2019)   

Low-voltage Graphene/LiNbO3 memristors

Recent observations of domain wall conductivity in the LiNbO₃ thin films may facilitate the development of electrically-tunable resistive switching devices employing conductive domain walls as functional elements. In LiNbO₃ capacitors, several orders of magnitude modulation of resistance can be achieved by systematically changing the density of injected domain walls using voltage pulse amplitude above the coercive bias. However, a deleterious characteristic of this approach is a high-energy cost of polarization reversal due to high leakage current. Here, we demonstrate a new approach for tuning the device resistance by modulating the conductivity of domain walls rather than changing the domain configuration. Using the LiNbO₃ thin film capacitors with graphene top electrodes, we show that once the device is set to a specific poly-domain state, its resistance can be continuously tuned by application of sub-coercive voltage. The tuning mechanism is based on reversible transition between the conducting and insulating states of the domain walls due to the electrically-induced wall bending near the sample surface. The developed approach provides an energy-efficient way to realize resistive functionality without the need of the domain structure modification by high voltage pulses.   

An ultralow power magnetoelectric nonvolatile memory

Most modern computing is built upon the von Neumann architecture composed of the data storage and data operation in a central processing unit. However, as Moore’s law made exponential gains over the decades, a memory bottleneck emerged, also known as the von Neumann bottleneck. This memory bottleneck can often degrade computing by orders of magnitude below the maximum potential of the computer especially for applications with large data sets. Hence it is of great interest to develop new energy-efficient memory technologies. Spintronic memory based on spin-transfer torque has emerged as a potential on-chip memory but it suffers from high energy dissipation (due to the requirement for a large current) and high voltages (due to the use of tunnel junctions). In contrast, a magneto-electric memory can operate with a capacitive displacement charge and potentially reach a 1-10 aJ/switching operation. Here we show magneto-electric switching of a memory element with a giant magnetoresistance (GMR) readout, operating at ~200 mV. We utilize a combination of phase detuning via isovalent substitution, thickness scaling, and conductive oxide electrode choice to scale the switching energy density to 20 µJ/cm². To the best of our knowledge, this is among the lowest voltage and energy density demonstrated in a spintronic memory element, thus presenting an attractive pathway to ultralow-power electronics.   

Magnetocapacitance Effect and Magnetoelectric Coupling in Type-II Multiferroics RFeWO6 (R=Ho, Dy and Tb)

In this work, we investigated the multiferroic property and magnetoelectric (ME) coupling in HoFeWO₆ with a noncentrosymmetric polar structure (space group Pna2₁). Low temperature XRD measurements down to 8 K did not show a structural phase transition and our DFT calculations reveal a stable structure. With an antiferromagnetic transition at T_N=17.8 K and a ferroelectric transition with the onset at the same temperature, this compound is a type-II multiferroic which shows magnetoelectric (ME) coupling. Field-dependent dielectric measurements show magnetocapacitance (MC) effect with double hysteresis loop at low temperature, which is directly related to its metamagnetic behavior. We further studied the MC effect in other members of this family, in particular, DyFeWO₆ and TbFeWO₆ and compared the results with HoFeWO₆. The observed MC effect and its direct correspondence with magnetization further confirm the ME coupling in these compounds.   

Negative capacitance in the hexagonal-YbFeO3 epitaxial thin films

The management of the heat generation and the power dissipation during information processing are very import issues in highly integrated electronic circuits. The ferroelectric negative capacitance (NC) effects, which was proposed to overcome the fundamental limit posed by the Boltzmann distribution of electrons, are attracting a great deal of interests. This work, we designed a heterostructure based on the hexagonal-YbFeO₃ (h-YbFO) multiferroic thin films to study the transient NC effects. For the first time, room-temperature ferroelectric properties in the epitaxial h-YbFO thin films and a direct observation of NC effects of h-YbFO thin films are reported. The transient NC effects as a function of the load resistance and applied source voltage are systematically investigated, which qualitatively follow the theoretical prediction. The structural requirements for the NC effects have been discussed, e.g. domain structure correlation with dielectric layers, grain size effects, microscopy mechanisms of ferroelectric switching. This work provides a new platform for the study of NC effects, and opens new horizons for understanding and improving the NC effects.   

Stability of the magnetoelectric state in Y-type hexaferrite single crystals

The magnetoelectric (ME) properties of the trigonal Y-type hexaferrites, Ba_2-ySr_yCo₂Fe_12-xAl_xO₂₂, are related to the multiferroic FE3 phase [1,2]. Although this phase has been observed both as a metastable and as a stable phase [3,4,5], stable control over magnetization by electric field has not yet been achieved close to room temperature. This indicates that for applications, the stability of the multiferroic phase as well as the ME state are crucial issues, which are hard to investigate using conventional approaches.
In this presentation, we report the results of the direct (P-H) and inverse (M-E) magnetoelectric effects in Y-type hexaferrites with different Sr doping levels. Using the combination of high electric and magnetic fields, we investigate the isothermal switching between two ME states. We discuss these new findings in terms of a possible way to evaluate the stability of the ME state in multiferroic materials.

[1] T. Kimura, Ann. Rev. Condens. Matter Phys. 3, 93-110 (2012)
[2] T. Kimura et. al., PRL 94, 137201 (2005)
[3] S. Hirose et. al., APL 104, 022907 (2014)
[4] T. Nakajima et. al., PRB 94 195154 (2016)
[5] V. Kocsis et. al., Nat. Comm. 10, 1247 (2019)   

Local probing of jerky switching dynamics in Pb(Zr0.2Ti0.8)O3 thin films

Polarisation switching in ferroelectric materials is governed by the same competition between elasticity and disorder, which describes many systems from earthquake statistics to magnetic domain walls. The movement of the interface follows nonlinear creep and (de)pinning dynamics, with jerky individual events whose energy and size distributions are characterised by universal power laws. In ferroelectrics, domain nucleation and growth can be observed from nano to macroscale, providing a useful system in which this complex fundamental behaviour can be explored.

While past measurements have focused on average responses at the level of the whole sample, recent theoretical studies suggest that spatial resolution of the switching event distribution can provide important new information about the dynamic regime under observation.

Here, using piezoresponse force microscopy to follow polarisation reversal at the nanoscale in two epitaxially grown Pb(Zr0.2Ti0.8)O3 thin films, we show that the probability distribution of switching event sizes follows a power law with different exponents in samples with different disorder landscapes. Furthermore, we show how individual jerks attributed to domain wall motion and domain merging events contribute differently to the global switching dynamics.   

2D Semiconductor Transistors using Layered van der Waals Oxide MoO3 as High-K Gate Dielectric

The search for smaller field effect transistors is leading research towards atomically thin 2D semiconductor materials. Due to the decreasing size of these transistors, there must be an emphasis on finding compatible gate dielectrics that can be equally effective in the 2D regime. MoO₃ is an attractive multi-functional transition metal oxide replacement for gate dielectrics in field effect transistors due to its exfoliatable van der Waals structure in addition to its high dielectric constant. This study demonstrates that as-grown MoO₃ has a high dielectric constant, K, of approximately 35 at room temperature at low frequencies by fabricating parallel plate capacitors from these thin flakes. Mechanically exfoliated MoO₃ flakes are used to create 2D heterostructures with WSe₂, demonstrating that MoO₃ also induces holes in the WSe₂ layer. Most importantly, MoO₃ proves to be a strong gate dielectric when used as a top-gate material for WSe₂ heterostructure devices.   

First-principles investigations of ferroelectric and pyroelectric properties of HfO2

The presence of ferroelectricity in HfO2-based thin films have revitalized the interests in using ferroelectrics at the nanoscale. To understand the origin of ferroelectricity in this silicon-compatible ferroelectric, we investigated the kinetic effects of phase transitions in HfO2 thin films by quantifying the transition barriers between different polymorphs of hafnia with density-functional-theory calculations and variable-cell nudged elastic band technique. We found that the transition from the tetragonal phase to the polar orthorhombic phase is a fast process kinetically under clamping. We further explored the pryoelectricity in HfO2 with both first-principles lattice dynamics and ab initio molecular dynamics calculations. Unlike most conventional pyroelectrics, the large pyroelectricity of HfO2 results from the secondary effect which is intimately related to the negative longitudinal piezoelectric effect.   

Ferroelectricity in [111]-oriented epitaxially strained HfO2 from fist principles

First principles calculations are used to investigate the effect of (111) epitaxial strain in the structural and ferroelectric properties of Hafnia (HfO₂), a silicon compatible high-k dielectric that is already used in high volume semiconductor manufacturing applications. We find that [111]-oriented epitaxial strain lowers the symmetry of the bulk cubic Fm-3m and tetragonal P4₂/nmc phases of HfO₂ to rhombohedral R-3m and monoclinic P2₁/m, respectively. The polar orthorhombic Pca2₁ phase stabilizes a ferroelectric monoclinic P1 structure above -2% (111)-strain, not present in either bulk or (001)-strained form. Under (111)-strain, ferroelectricity displays an out of plane polarization component P~ 35μC/cm². At large compressive strain (>2%), [111]-oriented thin films are paraelectric. We further explore the energy landscape of (111) strained HfO₂ and possible pathways for the stabilization of ferroelectricity.