Spatially Resolved Electric-Field Manipulation of Magnetism for CoFeB Mesoscopic Discs on Ferroelectrics

Multienzyme-Like Nanozymes: Regulation, Rational Design, and Application

Nanomaterials with more than one enzyme-like activity are termed multienzymic nanozymes, and they have received increasing attention in recent years and hold huge potential to be applied in diverse fields, especially for biosensing and therapeutics. Compared to single enzyme-like nanozymes, multienzymic nanozymes offer various unique advantages, including synergistic effects, cascaded reactions, and environmentally responsive selectivity. Nevertheless, along with these merits, the catalytic mechanism and rational design of multienzymic nanozymes are more complicated and elusive as compared to single-enzymic nanozymes. In this review, the multienzymic nanozymes classification scheme based on the numbers/types of activities, the internal and external factors regulating the multienzymatic activities, the rational design based on chemical, biomimetic, and computer-aided strategies, and recent progress in applications attributed to the advantages of multicatalytic activities are systematically discussed. Finally, current challenges and future perspectives regarding the development and application of multienzymatic nanozymes are suggested. This review aims to deepen the understanding and inspire the research in multienzymic nanozymes to a greater extent.

Twisted Integration of Complex Oxide Magnetoelectric Heterostructures via Water-Etching and Transfer Process

HIGHLIGHTS

The (001)-oriented ferromagnetic La0.67Sr0.33MnO3 films are stuck onto the (011)-oriented ferroelectric single-crystal 0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 substrate with 0° and 45° twist angle. By applying a 7.2 kV cm-1 electric field, the coexistence of uniaxial and fourfold in-plane magnetic anisotropy is observed in 45° Sample, while a typical uniaxial anisotropy is found in 0° Sample. Manipulating strain mode and degree that can be applied to epitaxial complex oxide thin films have been a cornerstone of strain engineering. In recent years, lift-off and transfer technology of the epitaxial oxide thin films have been developed that enabled the integration of heterostructures without the limitation of material types and crystal orientations. Moreover, twisted integration would provide a more interesting strategy in artificial magnetoelectric heterostructures. A specific twist angle between the ferroelectric and ferromagnetic oxide layers corresponds to the distinct strain regulation modes in the magnetoelectric coupling process, which could provide some insight in to the physical phenomena. In this work, the La0.67Sr0.33MnO3 (001)/0.7Pb(Mg1/3Nb2/3)O3-0.3PbTiO3 (011) (LSMO/PMN-PT) heterostructures with 45º and 0º twist angles were assembled via water-etching and transfer process. The transferred LSMO films exhibit a fourfold magnetic anisotropy with easy axis along LSMO < 110 >. A coexistence of uniaxial and fourfold magnetic anisotropy with LSMO [110] easy axis is observed for the 45° Sample by applying a 7.2 kV cm-1 electrical field, significantly different from a uniaxial anisotropy with LSMO [100] easy axis for the 0° Sample. The fitting of the ferromagnetic resonance field reveals that the strain coupling generated by the 45° twist angle causes different lattice distortion of LSMO, thereby enhancing both the fourfold and uniaxial anisotropy. This work confirms the twisting degrees of freedom for magnetoelectric coupling and opens opportunities for fabricating artificial magnetoelectric heterostructures.

Large in-plane piezo-strain enhanced voltage control of magnetic anisotropy in Si-compatible multiferroic thin films

Voltage control of magnetic anisotropy (VCMA) in Si-compatible ferroelectric/ferromagnetic multiferroic thin films is promising to enable power-efficient and integrated magnetic memories. However, their VCMA effect is weak and is always smaller than that of the bulk counterparts. Here, we achieve a more substantial VCMA effect in thin films than in the bulk, benefiting from the large in-plane piezo-strain mediated magnetoelectric coupling under strong fields. Si-compatible ferroelectric Pb(Zr,Ti)O₃ (PZT) thin films with large breakdown strength of up to 3.2 MV cm^-1 are fabricated to further construct multiferroic thin films. Since conventional methods fail to measure the VCMA effect under strong fields, we establish a micro-ferromagnetic resonance method based on micro-fabrication. An enhanced VCMA effect is demonstrated in PZT/CoFeB thin films, whose voltage-induced effective magnetic field (H_eff) could experimentally reach 26.1 Oe, which is much stronger than that in bulk control samples "PZT ceramic/CoFeB" (2.6 Oe) and "PMN-PT single crystal/CoFeB" (18.5 Oe) as well as previous reports. Theoretically, the H_eff in thin films could be > 60 Oe near the breakdown strength, resulting from a giant in-plane piezo-strain S₃₁ < -0.3%, which is comparable to that of the best ferroelectric single crystals. Si-compatible multiferroic thin films with enhanced VCMA will be a useful platform for developing integrated magnetic and spintronic devices.

Using Dipole Interaction to Achieve Nonvolatile Voltage Control of Magnetism in Multiferroic Heterostructures

Nonvolatile electrical control of magnetism is crucial for developing energy-efficient magnetic memory. Based on strain-mediated magnetoelectric coupling, a multiferroic heterostructure containing an isolated magnet requires nonvolatile strain to achieve this control. However, the magnetization response of an interacting magnet to strain remains elusive. Herein, Co/MgO/CoFeB magnetic tunnel junctions (MTJs) exhibiting dipole interaction on ferroelectric substrates are fabricated. Remarkably, nonvolatile voltage control of the resistance in the MTJs is demonstrated, which originates from the nonvolatile magnetization rotation of an interacting CoFeB magnet driven by volatile voltage-generated strain. Conversely, for an isolated CoFeB magnet, this volatile strain induces volatile control of magnetism. These results reveal that the magnetization response to volatile strain among interacting magnets is different from that among isolated magnets. The findings highlight the role of dipole interaction in multiferroic heterostructures and can stimulate future research on nonvolatile electrical control of magnetism with additional interactions.

Nano-magnetic tunnel junctions controlled by electric field for straintronics

The magnetic tunneling junction (MTJ) controlled by electric field as an alternate approach for energy efficiency is the highlight for nonvolatile RAM, while there is still a lack of research on resistance manipulation with the electric field in nanoscale MTJs. In this study, we integrated nanoscale MTJs on the (011) orientated Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ (PMN-PT) ferroelectric substrates and systematically investigated the magnetoresistance as a function of the magnetic field and electric field. A single domain state of the nanoscale MTJ was demonstrated by the experimental result and theoretical simulation. Afterward, the obvious electric field control of R-H curves was obtained and explained by the competition between magnetoelastic energy and shape anisotropy. More importantly, simulation results also predicted that the switching pathway of magnetic moments under the magnetic field is strongly dependent on the applied electric field, displaying the electric field control of chiral switching in the nano-MTJ. Our work is a milestone in the realization of the emerging dubbed straintronics field.

Magnetoelectric Memory Based on Ferromagnetic/Ferroelectric Multiferroic Heterostructure

Electric-field control of magnetism is significant for the next generation of large-capacity and low-power data storage technology. In this regard, the renaissance of a multiferroic compound provides an elegant platform owing to the coexistence and coupling of ferroelectric (FE) and magnetic orders. However, the scarcity of single-phase multiferroics at room temperature spurs zealous research in pursuit of composite systems combining a ferromagnet with FE or piezoelectric materials. So far, electric-field control of magnetism has been achieved in the exchange-mediated, charge-mediated, and strain-mediated ferromagnetic (FM)/FE multiferroic heterostructures. Concerning the giant, nonvolatile, and reversible electric-field control of magnetism at room temperature, we first review the theoretical and representative experiments on the electric-field control of magnetism via strain coupling in the FM/FE multiferroic heterostructures, especially the CoFeB/PMN–PT [where PMN–PT denotes the (PbMn_1/3Nb_2/3O₃)_1−x-(PbTiO₃)_x] heterostructure. Then, the application in the prototype spintronic devices, i.e., spin valves and magnetic tunnel junctions, is introduced. The nonvolatile and reversible electric-field control of tunneling magnetoresistance without assistant magnetic field in the magnetic tunnel junction (MTJ)/FE architecture shows great promise for the future of data storage technology. We close by providing the main challenges of this and the different perspectives for straintronics and spintronics.

Strain-Mediated Giant Magnetoelectric Coupling in a Crystalline Multiferroic Heterostructure

Multiferroic heterostructures based on the strain-mediated mechanism present ultralow heat dissipation and large magnetoelectric coupling coefficient, two conditions that require endless improvement for the design of fast nonvolatile random access memories with reduced power consumption. This work shows that a structure consisting of a [Pb(Mg_1/3Nb_2/3)O₃]_0.7-[PbTiO₃]_0.3 (001) substrate on which a crystalline FeGa(001)/MgO(001) bilayer is deposited exhibits a giant magnetoelectric coupling coefficient of order 15 × 10^-6 s m^-1 at room temperature. That result is a 2-fold increment over the previous highest value. The spatial orientation of the magnetization vector in the epitaxial FeGa film is switched 90° with the application of electric field. The symmetry of the magnetic anisotropy is studied by the angular dependence of the remanent magnetization, demonstrating that poling the sample generates a switchable uniaxial magnetoelastic anisotropy in the film that overcomes the native low 4-fold magnetocrystalline anisotropy energy. Magnetic force microscopy shows that the switch of the easy axis activates the displacement of domain walls and the domain structures remain stable after that point. This result highlights the interest in single-crystalline structures including materials with large magnetoelastic coupling and small magnetocrystalline anisotropy for low-energy-consuming spintronic applications.

Electric-field-driven non-volatile multi-state switching of individual skyrmions in a multiferroic heterostructure

Electrical manipulation of skyrmions attracts considerable attention for its rich physics and promising applications. To date, such a manipulation is realized mainly via spin-polarized current based on spin-transfer torque or spin-orbital torque effect. However, this scheme is energy consuming and may produce massive Joule heating. To reduce energy dissipation and risk of heightened temperatures of skyrmion-based devices, an effective solution is to use electric field instead of current as stimulus. Here, we realize an electric-field manipulation of skyrmions in a nanostructured ferromagnetic/ferroelectrical heterostructure at room temperature via an inverse magneto-mechanical effect. Intriguingly, such a manipulation is non-volatile and exhibits a multistate feature. Numerical simulations indicate that the electric-field manipulation of skyrmions originates from strain-mediated modification of effective magnetic anisotropy and Dzyaloshinskii-Moriya interaction. Our results open a direction for constructing low-energy-dissipation, non-volatile, and multistate skyrmion-based spintronic devices.

Full voltage manipulation of the resistance of a magnetic tunnel junction

One of the motivations for multiferroics research is to find an energy-efficient solution to spintronic applications, such as the solely electrical control of magnetic tunnel junctions. Here, we integrate spintronics and multiferroics by depositing MgO-based magnetic tunnel junctions on ferroelectric substrate. We fabricate two pairs of electrodes on the ferroelectric substrate to generate localized strain by applying voltage. This voltage-generated localized strain has the ability to modify the magnetic anisotropy of the free layer effectively. By sequentially applying voltages to these two pairs of electrodes, we successively and unidirectionally rotate the magnetization of the free layer in the magnetic tunnel junctions to complete reversible 180° magnetization switching. Thus, we accomplish a giant nonvolatile solely electrical switchable high/low resistance in magnetic tunnel junctions at room temperature without the aid of a magnetic field. Our results are important for exploring voltage control of magnetism and low-power spintronic devices.

Unusual Behaviors of Electric-Field Control of Magnetism in Multiferroic Heterostructures via Multifactor Cooperation

Electric-field control of magnetism (EFCM) is very important for the exploration of high-density, fast, and nonvolatile random-access memory with ultralow energy consumption. Here, we report the electric-field-induced ferroelectric phase transitions in Pb(Mg_1/3Nb_2/3)_0.82Ti_0.18O₃ (PMN-0.18PT) and symmetry breaking of EFCM behaviors for corresponding directions in multiferroic heterostructures composed of amorphous ferromagnetic Co₄₀Fe₄₀B₂₀ (CoFeB) and PMN-0.18PT. We uncover a new mechanism behind the unusual phenomena, involving coupling between CoFeB and PMN-0.18PT via complex cooperation of electric-field-induced ferroelectric phase transitions, competition of different ferroelectric domains, and internal electric field in PMN-0.18PT. The deterministic EFCM with reversible and nonvolatile nature opens up a new avenue for exploring EFCM in multiferroic heterostructures and is also significant for applications.

Ferroelectric field manipulated nonvolatile resistance switching in Al:ZnO/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 heterostructures at room temperature

Resistance switching was obtained in Al:ZnO/Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ heterostructures at room temperature by applying an external electric field. The modulation of the resistance is more pronounced in the thinner samples, indicating that it is an interfacial effect. In addition, the resistance of Al:ZnO films is significantly reduced by the photoexcited carriers when illumination is applied. The results indicate that the carrier density in the Al:ZnO films is modulated under external electric fields, due to the accumulation and depletion of charge at the interface between Al:ZnO and Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃. Hence, reversible and nonvolatile resistance states can be achieved by the ferroelectric field effect, and it is expected that multilevel storage will be realized.

Integration of Oxide Semiconductor Thin Films with Relaxor-Based Ferroelectric Single Crystals with Large Reversible and Nonvolatile Modulation of Electronic Properties

We report the fabrication of 0.71Pb(Mg_1/3Nb_2/3)O₃-0.29PbTiO₃ (PMN-0.29PT)-based ferroelectric field effect transistors (FeFETs) by the epitaxial growth of cobalt-doped tin dioxide (SnO₂) semiconductor thin films on PMN-0.29PT single crystals. Using such FeFETs we realized in situ, reversible, and nonvolatile manipulation of the electron carrier density and achieved a large nonvolatile modulation of the resistance (∼330%) of the SnO₂:Co films through the polarization switching of PMN-0.29PT at 300 K. Particularly, combining the ferroelectric gating with piezoresponse force microscopy, X-ray diffraction, Hall effect, and magnetoresistance (MR), we rigorously disclose that both sign and magnitude of the MR are intrinsically determined by the electron carrier density, which could modify the s-d exchange interaction of the SnO₂:Co films. Furthermore, we realized multilevel resistance states of the SnO₂:Co films by combining the ferroelectric gating with ultraviolet light illumination, demonstrating that the FeFETs have potential applications in multistate resistive memories and electro-optical devices.