Electric-field manipulation of magnetization in an insulating dilute ferromagnet through piezoelectromagnetic coupling

Using the spin degree of freedom to realize the information storage and processing is a promising way to overcome the challenges of high chip heat loss and quantum size effect in classic semiconductor devices of nanoscale, which has developed to be a hot research area of spintronics. The manipulation of magnetism is one of the core topics in the field of spintronics. Various routes have been developed to control the magnetism, such as magnet, strain, chemical doping, current, and light. With the advantages of reversibility, low-energy consumption, and good compatibility with semiconductor industry, manipulation of magnetism using an electric field attracts increasing research interest and shows enormous potential for application. Actually, the possibility of tuning the magnetism by an electrical way was firstly proposed by the Maxwell’s equation and the thought of the electric field control of magnetism could date back to the 1960s. After the experimental realization of the electric field control of magnetism in diluted magnetic semiconductor (In,Mn)As by Ohno et al. in 2000, such a technology has been utilized to manipulate the magnetic properties of different magnetic metals, oxides, topological insulators, and two-dimensional materials. The physical mechanisms behind the electric field control of magnetism also attract a lot of attention. In this review, we summarize the recent progress of electric field control of magnetism in different materials and devices based on their mechanisms: Carrier-density, strain, exchange coupling, orbital reconstruction, and ion migration. Various magnetic properties, like Curie temperature, magnetic moment, magnetic anisotropy, coercivity, and exchange bias have been effectively modulated by the electric field. Among them, the electric field controls of magnetic anisotropy, coercivity, and exchange bias are thought to be promising routes for magnetization switching based on the pure electric field. Meanwhile, the electric field is also used to control the different magnetoresistance and spin-related transport properties. The spin transfer torque and spin orbit torque are two kinds of torques that are generated by electrical current and can be utilized to switch the magnetization. Using an electric field to control these two torques attracts more and more attention. On the other hand, a lot of efforts have been made to control spintronic devices like magnetic tunnel junction, multi-ferroic tunnel junction, and racetrack memory by the electric field for pursuing the next generation nonvolatile memory technologies with low energy consumption, high density, and high speed. A large number of researches have been done in the area of electric field control of magnetism, whereas, it is still a developing topic with a lot of open questions in the performances, mechanisms, and practical applications. More efforts need to be done in the area of electric field control of magnetism, to reveal the coupling between various mechanisms, to increase the working temperature of more and more spintronic devices to or beyond room temperature, to realize the electric field control of magnetism in low dimensional magnetic systems, magnetic nanostructures, antiferromagnet, and magnetic insulator, to switch the magnetization using much lower or even no current density with the assistance of electric field at room temperature and clarify the corresponding dynamic process, and to apply the electric field control of magnetism in neurocomputing and logical operation.

Artificial multiferroics consist of materials systems engineered to have a coupling between multiple order parameters at the interface, such as between magnetic and ferroelectric order (magnetoelectric coupling) which enable the electric field control of magnetism suitable for applications in energy efficient storage or sensor devices. In this thesis we investigate two types of magnetoelectric coupling, namely, strain-mediated and charge-mediated, with a goal of characterizing their dynamic behaviour. For strain-mediated coupling, we considered a system consisting of Co dots fabricated on a ferroelectric BaTiO3 thin film, where application of an electric field led to a change in magnetic domain structure induced by the piezo-strain; however, we find that the process is stochastic as a consequence of a strong pining of the Co magnetization induced by the high surface roughness of BaTiO3 making it unsuitable for pump and probe dynamical characterization. A second type of system investigated consists of perpendicular magnetic anisotropy (PMA) structures deposited on a silicon nitride membrane gate dielectric, where we used the charge screening effects to modulate the charge carrier density at the metallic/silicon nitride interface. We studied two types of tri-layer structure (i) Pt/Co/Pt/Si3N4 and (ii) Pt/Co/Ta/Si3N4, where the Co thickness is chosen to be at spin reorientation transition. For Pt/Co/Pt, we find the presence of a charge mediated magnetoelectric coupling in the form of domain nucleation and domain wall fluctuations dependency with the electric field; from the latter we estimate a change in energy barrier height of about 10 %. For Ta/Co/Pt heterostructures a net Dzyaloshinskii-Moriya interaction (DMI) is expected and the goal was to investigate the possibility to control the DMI and/or skyrmions with applied electric fields. For these structures we observe the presence of out-of-plane spin structures in an in-plane dominant magnetized surroundings. The out-of-plane spin structures resemble a Neel type skyrmion with a dimension from 200 nm to 2 µm at room temperature under no external magnetic field. We demonstrate that such out-of-plane spin structures can be manipulated by changing the anisotropy of the system with electric fields. The measured capacitive rise time of a 200 nm thick silicon nitride membrane is ~140 ns making it suitable for high frequency characterization; however, we find that the presence of charge traps and/or charge defects in the silicon nitride membranes preclude a systematic control of the magnetization. In this context, we characterize the dielectric time response of different dielectrics, including stoichiometric silicon nitride membranes, AlN, Al2O3, BaTiO3 and MgO grown by physical vapour deposition (PVD) methods. We find that all dielectrics have a significant density of charge defects and/or charge traps. From capacitance vs frequency characterization, we find that the capacitance decreases with increasing frequency; since the mobility of carrier charges such as electrons is independent of the measuring frequency and we measure a higher capacitance at lower frequency, it is likely that we are also moving ions or possible vacancies with the applied electric field along with bound electrons, as ionic mobility with electric field is slower than electron mobility. Our results suggest the importance of characterizing and optimizing the dielectric time response for high frequency charge mediated magnetoelectric devices.

Electric-field control of magnetism has recently received much attention because of low-power consumption, which has potential applications in low-power multifunction devices. Ferromagnetic/ferroelectric multiferroic heterostructure is a useful way to realize the electric-field control of magnetism. Strain-mediated magnetoelectric coupling with large magnetoelectric coupling coefficient at room temperature is one of the current research hotspot. In this paper, we give an overview of recent progress of strain-mediated magnetoelectric coupling in multiferroic heterostructures.This review paper consists of five parts:introduction of multiferroics, electric-field control of magnetism in multiferroic heterostructures, electrical control of magnetization reversal, electric-field control of magnetic tunnel junctions, and the future prospects of multiferroic heterostructures. The basic concepts of multiferroics and background of magnetoelectric coupling effect are introduced in the first part.In the second part, a brief review of the recent work on the Pb(Mg1/3Nb2/3)0.7Ti0.3O3 (PMN-PT) based multiferroic heterostructures is given. The PMN-PT has a FE domain structure, which plays a vital role in electric-field control of magnetism, especially the 109 domain switching. For PMN-PT (001), the importance of 109 domain switching on the nonvolatile electrical control of magnetism is discussed. For PMN-PT (011), it is shown how to obtain nonvolatile strain which induces magnetic easy axis to be rotated by 90. The work on electric-field modulation of ferromagnetic material with perpendicular magnetic anisotropy is also mentioned.Electric-field control of magnetization reversal is still a challenge and remains elusive. Combination of strain-mediated magnetoelectric coupling and exchanging bias is a promising method to reverse magnetization by electric field, and the exchange-biased system/ferroelectric structures are given in the third part. There are also some theoretical attempts and proposals to realize the electrical control of 180 magnetization reversal. Then the method to manipulate magnetic tunnel junctions by electric field is given through integrating multiferroics and spintronics. Further outlook of the multiferroic heterostructures is also presented finally.

By taking advantage of the coupling between magnetism and ferroelectricity, ferromagnetic (FM)/ferroelectric (FE) multiferroic interfaces play a pivotal role in manipulating magnetism by electric fields. Integrating the multiferroic heterostructures into spintronic devices significantly reduces energy dissipation from Joule heating because only an electric field is required to switch the magnetic element. New concepts of storage and processing of information thus can be envisioned when the electric-field control of magnetism is a viable alternative to the traditional current based means of controlling magnetism. This article reviews some salient aspects of the electric-field effects on magnetism, providing a short overview of the mechanisms of magneto-electric (ME) coupling at the FM/FE interfaces. A particular emphasis is placed on the ME effect via interfacial magneto-elastic coupling arising from strain transfer from the FE to FM layer. Recent results that demonstrate the electric-field control of magnetic anisotropy, magnetic order, magnetic domain wall motion, and etc are described. Obstacles that need to be overcome are also discussed for making this a reality for future device applications.

Room-temperature multiferroicity and magnetoelectric couplings in (Co0.75Al0.25)2(Fe0.75Mg0.25)O4 spinel films

A polycrystalline ZnO film is grown on a silicon substrate by the pulsed laser deposition method, and the electric field-induced magnetization in ZnO using an optical cantilever beam magnetometer setup is studied. The magnetization vs bipolar dc electric field measurements reveal the occurrence of magnetization switching in the ZnO film. The magnetization switching in the presence of an electric field is ascribed to the converse magnetoelectric (ME) coupling that takes place between the electrical and the magnetic order parameters existing in the ZnO film. We have found the strain-driven magnetization change as evidenced by the butterfly shape of the magnetization vs the electric field curve. A saturation magnetization of 13.31 MA/m is obtained. Moreover, a significant value of the ME coupling coefficient (α) (1.61 × 10−7 s/m) has also been reported in this article. The emergence of electric field-induced magnetization in a single polycrystalline ZnO film is regarded to be a very promising aspect in designing high-density energy-efficient spintronic and different multifunctional devices.

Using the molecular-field approximation a form of the Landau-Ginsburg equation supplemented by boundary conditions for the case of disordered dilute ferromagnetic thin film has been derived.The spatial and temperature dependences of the magnetization below the ordering temperature of pure and dilute ferromagnetic rhin film bave been obtained. It bas been shown that the magnetization in both cases decreases near surfaces of the film on the distance approximately equal to the correlation length. When the correlation length is bigger than the thickness of the film, the magnetization changes clearly across the whole film. For temperatures very close to the Curie temperature our results are consistent with those obtained by Kokowska et al. [1] and Wojtczak [4], where the iteration procedure was applied to Valenta equations. Spatial and temperature dependence of the magnetization of pure and disordered semi-infinite ferromagnet bave been obtained in the limit when the thickness of the film tends to infinity. The magnetization below the ordering temperature in the case of pure and disordered dilute three-dimensional ferromagnet has also been considered. The magnetization of the disordered dilute ferromagnetic media is smaller than the magnetization of pure ferromagnetic media.

Review: 154 refs.

Electric field-induced magnetoresistance in spin-valve/piezoelectric multiferroic laminates for low-power spintronics

Room-temperature electric field control of magnetization is of interest for the potential application of low consumption nonvolatile memory. Here, we report the realization of electric field modulation of magnetization in a single phase multiferroic material Ba0.5Sr1.5Co2Fe11AlO22. In this work, we use static and dynamic measurements to reveal the magnetoelectric coupling between magnetization (M) and polarization (P). At the temperature of 10 K, the first order magnetoelectric (ME) term is dominated and the magnetization linearly changes with the electric field. While at room temperature, the quadratic term become apparent, leading to symmetric butterfly-like P–H and M–E curves. Our study reveals that an alternating longitudinal conical (ALC) phase induced exchange interaction may exist in Y type hexaferrite Ba0.5Sr1.5Co2Fe11AlO22 and contribute to the ME coupling mechanism.

The relationships between four basic circuit variables—voltage (v), current (i), charge (q), and magnetic flux (ϕ)—have defined three fundamental circuit elements: resistors, capacitors, and inductors. From a viewpoint of symmetry, there is a fourth fundamental circuit element defined by the relationship between charge and magnetic flux. Historically, a device called a memristor was considered to be the fourth element, but it has caused intense controversy because a memristor is conceived based on a nonlinear i–v relationship rather than a direct q-ϕ relationship. Alternatively, a direct correlation between charge (q) and magnetic flux (ϕ) can be built up by employing the magnetoelectric (ME) effects; i.e. magnetic field control of electric polarization and electric field control of magnetization. In this review, recent progress on the principle and applications of the fourth circuit element based on ME effects is summarized. Both the fourth linear element and nonlinear memelement, termed transtor and memtranstor, respectively, have been proposed and experimentally demonstrated. A complete relational diagram of fundamental circuit elements has been constructed. A transtor with a linear ME effect can be used in a variety of applications, such as energy harvesters, tunable inductors, magnetic sensors, gyrators, transformers, etc. A memtranstor, showing a pinched hysteresis loop, has great potential in developing low-power nonvolatile electronic devices. The principle is to utilize the states of the ME coefficient α E = dE/dH, instead of resistance, magnetization, or electric polarization, to store information. Both nonvolatile memory and logic devices can be implemented by using memtranstors, which provides a route toward logic-in-memory computing systems. In addition, artificial synaptic devices that are able to mimic synaptic behaviors have also been realized using memtranstors. The fourth circuit element and memelement based on ME effects provide extra degrees of freedom to broaden circuit functionalities and develop advanced electronic devices.

Electric field control of magnetism has become increasingly important for the next generation of graphene-based spintronic devices. We predict that the magnetic moment induced by chemisorbed H atoms on the top layer of a few-layer graphene system is tunable by an external electric field. Through accurate first-principles electronic structure calculations, we show that this magnetoelectric effect is negligible in one-layer graphene, but becomes pronounced in bilayer and trilayer graphene, saturating in magnitude in quadrilayer graphene. The effect is due to shifting of the Dirac cone of the pure graphene layers relative to the bands of the hydrogenated layer, induced by the external field. The calculated magnetoelectric coefficient (α) has values comparable to those found for ferromagnetic films or perovskite interfaces. The value of α was also used to identify a half-metallic state at low gate bias, which suggests a new class of spin-polarized materials based on hydrogenated multilayer graphene. Our results point to an experimentally feasible way to create a magnetoelectric coupling in graphene using the interplay between covalent functionalization and electric fields.

Magnetoelectric (ME) coupling is the key ingredient for realizing the cross-control of magnetism and ferroelectricity in multiferroics. However, multiferroics are not only rare, especially at room-temperature, in nature but also the overwhelming majority of known multiferroics do not exhibit highly-desired switching of the direction of magnetization when the polarization is reversed by an electric field. Here, we report group theory analysis and ab initio calculations demonstrating, and revealing the origin of, the existence of a novel form of ME coupling term in a specific class of materials that does allow such switching. This term naturally explains the previously observed electric field control of magnetism in the first known multiferroics, i.e., the Ni–X boracite family. It is also presently used to design a switchable near room-temperature multiferroic (namely, LaSrMnOsO6 perovskite) having rather large ferroelectric polarization and spontaneous magnetization, as well as strong ME coupling.

Interfaces often exhibit unique electronic and magnetic properties that are not present in their bulk constituents. Understanding the atomic-level structure and properties of the interface is crucial for their technological applications. In this article, we report a first-principles study of the γ-FeSi2/Si(001) interface to unravel the atomic-level structure property relationship. An external electric field is included in our model to tune the properties of the interface. Based on our calculations, we found a modest application of an electric field (> 0.15 eV/A) could stabilize a sixfold and sevenfold coordinated, spin-active interface over a nonmagnetic (eightfold coordinated) interface-providing direct evidence of electric field control of magnetism at the interface. The sixfold as well as sevenfold coordinated structures are shown to favor antiferromagnetic spin ordering arising from the Fe(d)-Si(p)-Fe(d) super-exchange interaction. The distinct non-linear response of the interface structure to the applied electric field can be attributed to the different electronic and magnetic structures at the interface; the sixfold exhibits the highest polarizability over the other coordinated structures.

Mediating nonvolatile magnetism by purely electrical methods is a crucial challenge for the development of high-density and low energy consumption information storage. Research into most material systems, including ferromagnetic /ferroelectric heterostructures and, especially, magnetostriction/lead magnesium niobate-lead titanate (PMN-PT), has focused mainly on the nonvolatile strain-mediated converse magnetoelectric (ME) effect. The recent discovery of a type of FeAl/lead indium niobate (PIN)-PMN-PT heterostructure has provided an approach for electrical control of magnetism at room temperature. We demonstrate selective control of nonvolatile magnetism in (001)- and (111)-oriented FeAl/PIN-PMN-PT. In order to illustrate the behavior of electric field control of magnetism in (111)-oriented PIN-PMN-PT, a converse ME coupling mechanism is presented via polarization switching models. This mechanism indicates that the magnetization process is related to both the direction of polarization switching and the strain-mediated effect. By controlling the piezoresponse force microscopy (PFM) scan direction and analyzing the color contrast of the PFM images, we confirm the validity of the proposed models of polarization switching and demonstrate the validity of the mechanism. The results of this study may offer a method for the selective control of magnetism in nonvolatile multilevel data storage.