Electric Field Control Of Magnetism Research Articles

Hybrid improper ferroelectricity and multiferroic in Ruddlesden-Popper structures

Hybrid improper ferroelectricity (HIF) is a secondary ferroelectric ordering induced by the coupling between oxygen octahedral in-plane rotation and out-of-plane tilt in a metal-oxide containing the perovskite structure units. Investigation of HIF will greatly extend the connotation and denotation of ferroelectric physics and material science, and it is expected to develop the room temperature single phase multiferroic material with large polarization and strong magnetoelectric coupling, owing to its intrinsic characteristic of the electric-field control of magnetism through HIF in magnet. In the present paper, the recent primary progress of HIFs and the multiferroics with Ruddlesden-Popper structures is reviewed, and the perspective of the future development is also presented.

Two-dimensional hyperferroelectric metals: A different route to ferromagnetic-ferroelectric multiferroics

Recently, two-dimensional (2D) multiferroics have attracted numerous attention due to their fascinating properties and promising applications. Although the ferroelectric (FE)-ferroelastic and ferromagnetic (FM)-ferroelastic multiferroics have been observed/predicted in 2D systems, 2D ferromagnetic-ferroelectric (FM-FE) multiferroics remain to be discovered since FM insulators are very rare. Here, we proposed for the first time the concept of 2D hyperferroelectric metals, with which the insulating prerequisite for the FM-FE multiferroic is no longer required in 2D systems. We validate the concept of 2D hyperferroelectric metals and 2D metallic FM-FE multiferroics by performing first-principle calculations on 2D CrN and CrB2 systems. The 2D buckled monolayer CrN is found to be a hyperferroelectic metal with the FM ground state, i.e., a 2D FM-FE multiferroic. With the global optimization approach, we find the 2D CrB2 system has an antiferromagnetic (AFM)/planar ground state and a FM/FE metastable state, suggesting that it can be used to realize electric field control of magnetism. Our analysis demonstrates that the spin-phonon coupling and metal-metal interaction are two new mechanisms for stabilizing the out-of-plane electric polarization in 2D systems. Our work not only extends the concept of FE to metallic systems, but also paves a new way to search the long-sought high temperature FM-FE multiferroics.

High-throughput investigation of orientations effect on nanoscale magnetization reversal in cobalt ferrite thin films induced by electric field

The magnetoelectric device concept which enables the non-volatile electric field control of magnetism needs to be investigated for the development of practical information storage devices. In this aspect, the emerging field of magneto-ionics based on the modulation of magnetism by field-driven ion migration is promising because it only requires a simple sample structure in the solid state and has good cyclability. However, the degree of ion migration within the magnetic structure is strongly dependent on the crystal orientations. Since the epitaxial films growing on the commercial single crystal substrates have limited orientations, the ability of magnetism modulated by field-driven ion migration cannot be optimized and understood by using these data. In this work, we utilized the high-throughput synthesis approach, namely, combinatorial substrate epitaxy, which utilizes a polycrystalline substrate. This provides a platform to develop and understand the degree of ionic migration in different orientations of the model system CoFe2O4 (CFO) films. The library of electric driven nanoscale magnetization reversal data of CFO with different orientations was obtained by applying the electric field in the same region of known CFO grain orientations. It was determined from the analysis that the [110] crystal direction exhibits the maximum nanoscale magnetization reversal ratio. This is mainly attributed to the ease Co2+ migration in the [110] direction under the electric field assisted by a Fe3+ and oxygen vacancies.

Real-space imaging of non-collinear antiferromagnetic order with a single-spin magnetometer.

Although ferromagnets have many applications, their large magnetization and the resulting energy cost for switching magnetic moments bring into question their suitability for reliable low-power spintronic devices. Non-collinear antiferromagnetic systems do not suffer from this problem, and often have extra functionalities: non-collinear spin order may break space-inversion symmetry and thus allow electric-field control of magnetism, or may produce emergent spin-orbit effects that enable efficient spin-charge interconversion. To harness these traits for next-generation spintronics, the nanoscale control and imaging capabilities that are now routine for ferromagnets must be developed for antiferromagnetic systems. Here, using a non-invasive, scanning single-spin magnetometer based on a nitrogen-vacancy defect in diamond, we demonstrate real-space visualization of non-collinear antiferromagnetic order in a magnetic thin film at room temperature. We image the spin cycloid of a multiferroic bismuth ferrite (BiFeO3) thin film and extract a period of about 70 nanometres, consistent with values determined by macroscopic diffraction. In addition, we take advantage of the magnetoelectric coupling present in BiFeO3 to manipulate the cycloid propagation direction by an electric field. Besides highlighting the potential of nitrogen-vacancy magnetometry for imaging complex antiferromagnetic orders at the nanoscale, these results demonstrate how BiFeO3 can be used in the design of reconfigurable nanoscale spin textures.

Manipulation of anisotropic magnetoresistance and domain configuration in Co/PMN-PT (011) multiferroic heterostructures by electric field

The electric field manipulation of magnetic anisotropy and domain configuration has been investigated in the artificial multiferroic Co/PMN-PT (011) heterostructure at room temperature. A uniaxial magnetic anisotropy is induced with the application of an electric field, which leads to an electrically switched anisotropic magnetoresistance with tunability as large as ∼29%. Furthermore, the magnetic domain structures of Co films are investigated by magnetic force microscopy under an in situ electric field, which exhibits direct evidence for electric field control of magnetism at the mesoscale. The converse magnetoelectric effect demonstrated in this multiferroic heterostructure has potential to be utilized in magnetoelectric devices with low power consumption.

Electric-field control of magnetism in graphene on chromia

First-principle calculations are used to investigate how an external electric field controls the spin polarization in graphene on chromia, a system of interest in the area of spin field-effect transistors. Both free-standing chromia thin films and graphene-bilayers are considered. The effect of the electric field depends on the thickness of the chromia and ranges from moderately strong and linear effects to very strong nonlinear magnetoelectricity. The graphene modifies and generally enhances the nonlinear magnetoelectric effect. We also find that the external electric field drastically changes the energy-dependent spin polarization in the graphene layers, which is predicted to reach values of up to about 80%.

Large non-volatile tuning of magnetism mediated by electric field in Fe–Al/Pb(Mg1/3Nb2/3)O3–PbTiO3 heterostructure

Electric-field control of magnetism is now an attractive trend to approach a new kind of fast, low-power-cost memory device. In this work, we report a strong non-volatile electric control of magnetism in an Fe–Al/Pb(Mg1/3Nb2/3)O3–PbTiO3 heterostructure. In this system, a 90° rotation of the in-plane uniaxial magnetic anisotropy is exhibited during the increase of the external electric field, which means the easy axis turns into a hard axis and the hard axis turns into an easy one. Additionally, a non-volatile switch of the remanence is observed after a sweeping of the electric field from 0 kV cm−1 to ± 10 kV cm−1, then back to 0 kV cm−1. More interestingly, a 20% non-volatile magnetic state tuning driven by individual pulse electric fields is shown in contrast to large tuning up to 120% caused by pulse electric fields with small assistant pulse magnetic fields, which means a 180° reverse of the magnetization. These remarkable behaviors demonstrated in this heterostructure reveal a promising potential application in magnetic memory devices mediated by electric fields.

Electric-field effects on magnetism of Fe/NCZF/PZT composite thin film

Electric field control of magnetism has many potential applications in magnetic memory storage, sensors and spintronics. Epitaxial heterostructure involving ferroelectric (FE), ferromagnetic (FM) materials is one of the possible routes towards the realization of devices exploiting sizable magnetoelectric effects. In this study, we prepared multiferroic composite ferromagnet/ferrite/ferroelectric thin films on SrTiO3(111) substrate by electron beam evaporation and pulsed laser deposition. This system represents a prototypical example of interface between body-centered-cubic Fe, spinel NCZF (Ni0.25Cu0.05Zn0.7Fe1.96O3.96) and perovskite PZT (PbZr0.52Ti0.48O3), respectively. Magnetoelectric effects measured as a function of bias voltage on these heterostructures have been investigated via magneto-optical Kerr effect (MOKE). The magnetic coercivity (Hc) considerably decreased as a voltage was applied. The Hc values exhibited the similar variation whether the applied voltage was positive or negative and the reversibility was demonstrated. The strain coupling-mediated magnetoelectric effect and local heating effect are supposed to be the possible origins of Hc variation.

Electric field control of magnetism in nickel with coaxial cylinder structure at room temperature by electric double layer gating

Electric-field manipulation of magnetism in nickel coaxial cylinders with an electric double layer capacitor (EDLC) structure, with potential applications in spin capacitors.

Strain and electric-field control of magnetism in supercrystalline iron oxide nanoparticle-BaTiO3 composites.

The manipulation of the magnetism of self-assembled iron oxide nanoparticle (NP) monolayers on top of BaTiO3 (BTO) single crystals is reported. We observe strain induced magnetoelectric coupling (MEC) as shown by measurements of both the magnetization and magneto-electric AC susceptibility (MEACS). The magnetization, coercivity, remanent magnetization and MEACS signal as a function of temperature show abrupt jumps at the BTO phase transition temperatures. Hereby the jump values are opposite for in-plane and out-of-plane measurements. Grazing incidence small angle X-ray scattering (GISAXS) and scanning electron microscopy (SEM) confirm a hexagonal close-packed supercrystalline order of the NP monolayers. Cross-sectional scanning transmission electron microscopy (STEM) experiments provide information about the layer structure of the sample. This work opens up viable possibilities for fabricating energy-efficient electronic devices by self-assembly techniques.

Phase diagram ofSr1−xBaxMnO3as a function of chemical doping, epitaxial strain, and external pressure

We use \textit{ab initio} calculations to systematically study the phase diagram of multiferroic Sr$_{1-x}$Ba$_x$MnO$_3$ ($0 \leq x \leq 1$) as a function of chemical doping, epitaxial strain and external pressure. We find that by replacing Sr with Ba in cubic SrMnO$_3$ and imposing epitaxial strain, the material can be tuned to the vicinity of a first order transition between two multiferroic phases, one antiferromagnetic with a smaller polarization and one ferromagnetic with a larger polarization. A giant effective magneto-electric coupling and cross-field control (electric field control of magnetism or magnetic field control of polarization) can be achieved in the vicinity of the transition. The dependence of the theoretically computed transition point on the choice of exchange correlation functionals is determined and is found to be non-negligible. We also show that the perovskite structure of BaMnO$_3$ can be stabilized relative to its hexagonal polymorphs at pressures larger than 20 GPa.

Predicting room-temperature multiferroic phase of NaLaFeMO6 (M = Ru, Re, Os) with strong magnetoelectric coupling

Discovering multiferroic materials is one of the most important questions for facilitating devices based on the electric field control of magnetism. The great challenge is to create room temperature multiferroic materials with strongly coupled ferroelectric and ferromagnetic (or ferrimagnetic) orderings. Here we present simple, chemically intuitive design rules to identify a class of bulk magnetoelectric materials based on the “bicolor” layering of P21 double ferrimagnetic perovskites, e.g., NaLaFeMO6 (M = Ru, Re, Os). Using density functional theory calculations, we elucidated the origin of the ferroelectricity and show that it is a general consequence of the layering of any bicolor P21 double ferrimagnetic perovskites. Our calculations showed that the ferroelectric polarization is up to several μC/cm2 and the net magnetic moments are 3, 2, and 3 μB for NaLaFeRuO6, NaLaFeReO6, and NaLaFeOsO6, respectively in these proposed materials. More importantly, we highlighted the existence of a low-energy ferroelectric switching path, along which the magnetization is also likely reversed, evidencing the strong magnetoelectric coupling existing in this system. In addition, the critical temperature of polarization and magnetization are both well above room temperature. It is expected that this work can encourage the designing and experimental implementation of a large class of strong magneto-electric multiferroic materials with large magnetization and electric polarization above room temperature.

Magneto-electric coupling in antiferromagnet/ferroelectric Mn2Au/BaTiO3 interface

Within the crucial issue of the electric field control of magnetism, the use of antiferromagnets (AFMs) coupled to ferroelectrics is much less explored than the ferromagnets counterpart, although the first choice might lead to better performances and larger stability with respect to external perturbations (such as magnetic fields). Here, we explore the possibility to control the magnetic anisotropy of a Mn2Au layer by reversing the ferroelectric polarization of BaTiO3 in Mn2Au/BaTiO3 interfaces. By means of a thorough exploration of many possible geometry configurations, we identify the two most stable, corresponding to compressive and tensile strain at the interface. The first appears to be easy-axis, while the second—easy-plane—with a large induced moment on the interface Ti atom. The reversal of ferroelectric polarization changes the anisotropy by approximately 50%, thus paving the way to the control of AFM properties with an electric field.

Integrated Magnetics and Multiferroics for Compact and Power-Efficient Sensing, Memory, Power, RF, and Microwave Electronics

The coexistence of electric polarization and magnetization in multiferroic materials provides great opportunities for realizing magnetoelectric (ME) coupling, including electric field control of magnetism, or vice versa, through a strain-mediated ME coupling in layered magnetic/ferroelectric multiferroic heterostructures. Strong ME coupling has been the enabling factor for different multiferroic devices, which, however, has been elusive, particularly at RF/microwave frequencies. In this paper, most recent progress on new integrated multiferroic devices for sensing RF and microwave electronics will be presented, including novel RF Nano-electro-mechanical systems ME resonators with picotesla sensitivity for dc magnetic fields and novel gigahertz magnetic and multiferroic integrated inductors with a wide operation frequency range of $0.3\sim 3$ GHz, a high quality factor close to 20, and a voltage tunable inductance of 50% $\sim 150$ %. At the same time, we will also demonstrate other tunable RF devices, including integrated non-reciprocal tunable bandpass filter with ultrawideband isolation more than 13 dB. These novel magnetics and multiferroic devices show great promise for applications, such as compact, lightweight, and power-efficient sensing, memory, RF, and microwave integrated electronics.

In Situ Nanoscale Electric Field Control of Magnetism by Nanoionics.

Direct, nonvolatile, and reversible control of nanomagnetism in solid-state ferromagnetic thin films is achieved by controlling the chemical composition of the film through field-driven ion redistribution. The electric field-driven de-intercalation/intercalation of lithium ions can result in ≈100% modulation of the magnetization and drives domain wall motion over ≈100 nm. High-speed and multilevel magnetic information storage is further demonstrated.

Electric field control of magnetization in Cu2O/porous anodic alumina hybrid structures at room temperature

Cu2O nanoporous films are deposited on porous anodic alumina (PAA) substrates by DC-reactive magnetron sputtering. This paper focuses on voltage driven magnetization switching in Cu2O/PAA (CP) composite films prepared by DC-reactive magnetron sputtering. By applying a dc electric field, the magnetization of the CP composite films can be controlled in a reversible and reproducible way and shows an analogous on-off behavior. The magnitude of the change in the magnetization was about 75 emu/cm3 as the electric field was switched on and off. Resistive switching behavior was also observed in as-prepared CP composite films. Further analysis indicated that the formation/rupture of conducting filaments composed of oxygen vacancies is likely responsible for the changes in the magnetization as well as in the resistivity. Such reversible change of magnetization controlled by an electric field at room temperature may have applications in spintronics and power efficient data storage technologies.

The unique effect of in-plane anisotropic strain in the magnetization control by electric field

The electric field control of magnetization in both (100)- and (011)-Pr0.7Sr0.3MnO3/Pb(Mg1/3Nb2/3)0.7Ti0.3O3(PSMO/PMN-PT) heterostructures were investigated. It was found that the in-plane isotropic strain induced by electric field only slightly reduces the magnetization at low temperature in (100)-PSMO/PMN-PT film. On the other hand, for (011)-PSMO/PMN-PT film, the in-plane anisotropic strain results in in-plane anisotropic, nonvolatile change of magnetization at low-temperature. The magnetization, remanence and coercivity along in-plane [100] direction are suppressed by the electric field while the ones along [01-1] direction are enhanced, which is ascribed to the extra effective magnetic anisotropy induced by the electric field via anisotropic piezostrains. More interestingly, such anisotropic modulation behaviors are nonvolatile, demonstrating a memory effect.

Research Update: Magnetoionic control of magnetization and anisotropy in layered oxide/metal heterostructures

Electric field control of magnetization and anisotropy in layered structures with perpendicular magnetic anisotropy is expected to increase the versatility of spintronic devices. As a model system for reversible voltage induced changes of magnetism by magnetoionic effects, we present several oxide/metal heterostructures polarized in an electrolyte. Room temperature magnetization of Fe-O/Fe layers can be changed by 64% when applying only a few volts in 1M KOH. In a next step, the bottom interface of the in-plane magnetized Fe layer is functionalized by an L10 FePt(001) underlayer exhibiting perpendicular magnetic anisotropy. During subsequent electrocrystallization and electrooxidation, well defined epitaxial Fe3O4/Fe/FePt heterostructures evolve. The application of different voltages leads to a thickness change of the Fe layer sandwiched between Fe-O and FePt. At the point of transition between rigid magnet and exchange spring magnet regime for the Fe/FePt bilayer, this induces a large variation of magnetic anisotropy.

Linear magnetoelectricity at room temperature in perovskite superlattices by design

Discovering materials that display a linear magnetoelectric (ME) effect at room temperature is a challenge. Such materials could facilitate devices based on the electric field control of magnetism. Here we present simple, chemically intuitive design rules to identify a class of bulk magnetoelectric materials based on the ``bicolor'' layering of $Pbnm$ ferrite perovskites, e.g., ${\mathrm{LaFeO}}_{3}/{\mathrm{LnFeO}}_{3}$ superlattices, Ln = lanthanide cation. We use first-principles density functional theory calculations to confirm these ideas. We elucidate the origin of this effect and show it is a general consequence of the layering of any bicolor $Pbnm$ perovskite superlattice in which the number of constituent layers are odd (leading to a form of hybrid improper ferroelectricity). Our calculations suggest that the ME effect in these superlattices is larger than that observed in the prototypical magnetoelectric materials ${\mathrm{Cr}}_{2}{\mathrm{O}}_{3}$ and ${\mathrm{BiFeO}}_{3}$. Furthermore, in these proposed materials, the strength of the linear ME coupling increases with the magnitude of the induced spontaneous polarization which is controlled by the La/Ln cation radius mismatch. We use a simple mean field model to show that the proposed materials order magnetically above room temperature.

Magnetoelectric Random Access Memory-Based Circuit Design by Using Voltage-Controlled Magnetic Anisotropy in Magnetic Tunnel Junctions

We introduce a magnetoelectric junction driven by voltage-controlled magnetic anisotropy (VCMA-MEJ) as a building block for a range of low-power memory applications. We present and discuss specifically two applications, magnetoelectric random access memory (MeRAM) and ternary content-addressable memory (TCAM). The MEJ differs from a magnetic tunnel junction (MTJ) in that electric field is used to induce switching in lieu of substantial current flow in MTJ. Electric field control of magnetism can dramatically enhance the performance of magnetic memory devices in terms of switching energy efficiency and switching speed. The development of such an energy-efficient and ultrafast memory has a potential to change the paradigm of a hierarchical memory system in the conventional computer architecture. By combining speed, low power, and high density, electric-field-controlled magnetic memory merges features of multiple separate memory technologies used in today's memory hierarchy. The performance of a VCMA-MEJs-based MeRAM, especially in the case of one access transistor associated with one MEJ (1T-1R) structure, is evaluated by comparing it with that of phase-change RAM, resistive RAM, and spin transfer torque RAM. MeRAM can achieve ultrafast switching (<1 ns), low switching energy (∼1 fJ), and compact cell size of <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"><tex-math>$6\;F^2$</tex-math></inline-formula> with a shared source region, as well as nonvolatility. For another application, we propose the VCMA-MEJ-based TCAM, which will be referred to as MeTCAM, consisting of 4T-2MEJs. Since MeTCAM fully exploits the low power and high density features of the VCMA effect both in write and search operation modes, it obtains a fast searching speed (0.2 ns) with the smallest cell area ( <inline-formula xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink"> <tex-math>$44\;F^2 $</tex-math></inline-formula> ) compared to previous works.