Control of magnetism through electric-field-driven migration of ions, referred to as magneto-ionics (MI), holds promise for the development of non-volatile energy-efficient memory storage, as well as spintronic, neuromorphic and magnetoelectric devices. Here, we study the MI phenomena in 350 nm thick Ni55Co45 oxide films with varying degrees of porosity, obtained by electrodeposition of the parent Ni-Co metallic alloy on metallized Si substrate and subsequent annealing in air. Annealing at 450 °C of the film electrodeposited from a P-123-containing electrolyte with Ni and Co sulfate salts yields a Ni-Co oxide that partially retains its mesoporosity. This sample exhibits a higher MI response compared to a low-porosity (nearly dense) Ni-Co oxide film, indicating that an increased surface-to-volume ratio enhances MI. Comprehensive characterization of the mesoporous Ni-Co oxide-coated Si/Ti/Au sample reveals that annealing not only oxidizes the top ≈100 nm of the Ni-Co film but also induces silicon diffusion. MI phenomena occur via O2- migration out of and into the top Ni-Co oxide layer under negative and positive biasing, respectively. While the system shows some irreversibility, endurance improves significantly as cycling frequency increases, evidencing the potential of this material for voltage-tunable memory applications.

Magneto-ionics-as the voltage-driven control of magnetic properties through ionic motion and redox processes-offers a promising route toward energy-efficient spintronic devices. Exchange bias, being the unidirectional anisotropy arising from interfacial coupling between antiferromagnets and ferromagnets, plays a central role in spintronics. Here, we demonstrate reversible, room-temperature magneto-ionic generation, suppression, and modulation of exchange bias within a 50 nm-thick antiferromagnetic, magneto-ionically active NiCoO layer. Instead of relying on field cooling to set exchange bias, an applied magnetic field during the growth promotes alignment of the antiferromagnetic spin sublattices, producing a preferential unidirectional orientation. Gating drives oxygen-ion migration along columnar grain boundaries, partially reducing NiCoO and forming ferromagnetic NiCo clusters that couple to the antiferromagnetic matrix. The exchange bias can be controlled by tuning the Ni/Co ratio, which adjusts the Néel temperature, and by varying the actuation time and voltage amplitude which control ferromagnetic cluster size. Micromagnetic simulations reveal that the exchange bias originates from the interfacial uncompensated spins exhibiting partial ferromagnetic-like behavior. This single-layer approach, together with the voltage-controlled formation and tuning of exchange bias without heat treatments, simplifies fabrication and offers a framework for low-power antiferromagnetic spintronic devices.

Magnetoelectric materials enable low-power memory devices by leveraging the electric control of magnetization. The discovery of ferroelectricity in doped hafnia has unlocked further opportunities since the distinct ferroelectric switching mechanism in this material can enable robust and multilevel modulation of magnetization by electric field, if combined with appropriate magnetic materials. Here, we demonstrate a 5% electric field-induced modulation of the saturation magnetization in a cobalt layer, driven by ferroelectric switching of an adjacent epitaxial La(1%):Hf0.5Zr0.5O2 film. Dichroic imaging with synchrotron radiation confirms that ferroelectric switching induces a magnetic change. We show that the response time is faster than 500 ns (limited by the setup time resolution threshold) and that energy consumption is 6 nJ. This low energy consumption is mainly enabled by the absence of relevant leakage current contribution (10 nA/cm(2) at 500 mV). The found response time and energy-efficient behavior point to the presence of an electronically driven modulation of magnetism (i.e., conventional magnetoelectric effects), which is confirmed by theoretical calculations and compositional analysis. Additionally, a multilevel magnetoelectric response is observed, enabling neuromorphic-like behavior. The demonstration of magnetoelectric coupling in a system based on CMOS-compatible materials offers a viable route toward the development of low-power beyond von-Neumann technologies.

Optical switching of ferroelectric polarization is of interest for wireless and energy-efficient control of logic states. So far, this phenomenon has been widely demonstrated only in ferroelectric perovskites, while studies on other emerging ferroelectrics remain limited. In this regard, the paradigmatic example of a technologically relevant ferroelectric material is HfO2. However, HfO2 has a very wide bandgap, limiting light absorption. So far, the proposed strategies to enhance light absorption in HfO2-based systems are detrimental to ferroelectric properties, i.e., bandgap lowering or on-purpose defect introduction, which reduce switchable polarization and increase the presence of leakage currents. Here, we show that good ferroelectric properties, i.e., sizeable polarization (up to 15 μC cm−2), low leakage current (under 10–6 A cm−2), high endurance (up to 108 cycles) and fast switching (< 50 ns), can be achieved in epitaxial Hf0.5Zr0.5O2 films through an alternative strategy, BaTiO3 capping. While ferroelectric properties are remarkable, we demonstrate that the presence of BaTiO3 allows light absorption and the concomitant electric field generation, as supported by density functional theory calculations, which enables optical switching of polarization in Hf0.5Zr0.5O2 under 405 nm illumination. It is observed that optical switching is more efficient in films with thicker BaTiO3 capping layer. The high polarizability of BaTiO3 contributes to minimizing degradation in the ferroelectric response of the system. The results presented here indicate that appropriate designs can be followed to obtain optical switching of polarization in ferroelectric HfO2 while preserving main functional properties.

Voltage control of magnetism via magneto-ionics, where ion transport and/or redox processes drive magnetic modulation, holds great promise for next-generation memories and computing. This stems from its non-volatility and ability to precisely tune both the magnitude and speed of magnetic properties in a potential energy-efficient manner. However, expanding magneto-ionics to incorporate novel mobile ions or even multiple ion species is crucial for unlocking new phenomena and enabling multifunctional capabilities. Here, we demonstrate voltage-driven multi-ion transport in an FeBO system with increasing oxygen content, progressively transitioning from an electrostatic-like response to a more pronounced electrochemical (magneto-ionic) behavior. The voltage-driven transport of both B and Fe is activated by oxidation state tuning, owing to the larger electronegativity of oxygen. Such charge-transfer effects allow multi-ion magneto-ionics, where O ions move oppositely to Fe and B ions. These results pave the way for programmable functionalities by leveraging elements with different electron affinities through charge-transfer engineering.

Perovskite relaxor ferroelectrics and antiferroelectrics have been the workhorse materials for energy storage electrostatic devices. Recently, there has been growing interest in ferroelectric HfO2, which is highly compatible with fabrication processes of the electronics industry and scalable down to the ultrathin limit. However, wake-up and fatigue phenomena in ferroelectric HfO2 can limit the stability of energy storage capacity upon cycling. Interface engineering using simple AxOy (Ir, Ce, Ru, Al, La and Zr) binary oxides as capping layers is a promising strategy to mitigate these effects, but previous works have focused on optimizing high remanent polarization for memory applications. In contrast, for energy storage applications, the optimal parameters differ from those required for memory devices. Therefore, alternatives to AxOy capping layers must be investigated. Here, we demonstrate that the integration of the perovskite ferroelectric BaTiO3 with the fluorite ferroelectric Hf0.5Zr0.5O2 yields highly stable energy storage density and enhanced efficiency over cycling and at high temperatures. We study a set of samples combining epitaxial Hf0.5Zr0.5O2 grown on Si(001) with polycrystalline BaTiO3 of various thicknesses. The ferroelectricity of Hf0.5Zr0.5O2 is preserved and the efficiency is enhanced without compromising the high breakdown voltage. The device performance is optimal for a BaTiO3 thickness of 10 nm, exhibiting an energy storage density of 100 J/cm3, efficiency of 80 % and breakdown field of 12 MV/cm. We attribute this remarkable performance to the distinct electrical properties of BaTiO3 compared with previously investigated AxOy capping layers. The investigations are carried out on a system that combines memory and energy storage properties, enabling the design of prospective devices in which energy recovery and memory functionalities are integrated at the chip level.

Magnetic straintronics, the strain-mediated control of magnetic anisotropy, has emerged as a key direction for next-generation energy-efficient technologies. In multiferroic heterostructures, magnetoelectric coupling is typically achieved by applying an electric field on a ferroelectric phase, inducing strain through the converse piezoelectric effect, which is then transferred to the adjacent ferromagnetic phase. As an alternative, strain can be remotely modulated through the photostrictive effect induced by light. Although light-driven control of magnetic anisotropy has been explored, the optical modulation of more complex phenomena such as exchange bias remains largely unaddressed. Here, we demonstrate significant light-induced modulation of exchange bias and magnetization switching at room temperature in a Pb(Mg1/3Nb2/3)O3-Pb(Zr,Ti)O3 (PMN-PZT)/Fe80Ga20(FeGa)/Ir20Mn80 (IrMn) multiferroic heterostructure, driven by visible-light-photostriction. A measurable reduction in the exchange bias field is observed (Delta H EB approximate to 21 Oe) under the illumination of visible light of 405 nm. The magnetization state correlates with the light intensity, enabling multi-level states with light power densities as low as 0.1 W cm-2. These findings suggest a promising route toward low-power, multistate, and wireless opto-magnetic memory applications.

Nanolaminates based on ferroelectric polycrystalline doped HfO2 have gained interest because those compounds show enhanced functional properties. Here, we achieve coexisting improvement of remanent polarization and dielectric permittivity in wake-up-free epitaxial Hf0.5Zr0.5O2/HfO2 nanolaminates with different numbers of HfO2 nanolayers if compared with Hf0.5Zr0.5O2 single films of equivalent thickness or other reported polycrystalline nanolaminates. Comprehensive structural characterization reveals that the origin of the enhancement must be the larger amount of the orthorhombic phase in the nanolaminates. The retention of nanolaminates is greater than that of Hf0.5Zr0.5O2 single films; however, fatigue is larger and ferroelectric switching is slower in the nanolaminates compared with single layers. The present work reveals nanolamination in high-quality films as a strategy to increase dielectric permittivity without important degradation of other functional properties.

Control of magnetism through voltage-driven ionic processes (i.e., magneto-ionics) holds potential for next-generation memories and computing. This stems from its non-volatility, flexibility in adjusting the magnitude and speed of magnetic modulation, and energy efficiency. Since magneto-ionics depends on factors like ionic radius and electronegativity, identifying alternative mobile ions is crucial to embrace new phenomena and applications. Here, the feasibility of C as a prospective magneto-ionic ion is investigated in a Fe-C system by electrolyte gating. In contrast to most magneto-ionic systems, Fe-C presents a dual-ion mechanism: Fe and C act as cation and anion, respectively, moving uniformly in opposite directions under an applied electric field. This leads to a 7-fold increase in saturation magnetization with magneto-ionic rates larger than 1 emu cm-3 s-1, and a 25-fold increase in coercivity. Since carbides exhibit minimal cytotoxicity, this introduces a biocompatible dimension to magneto-ionics, paving the way for the convergence of spintronics and biotechnology.

The ferroelectric phase of HfO2 is metastable, and its stabilization in thin films strongly depends on both doping and microstructure. Optimally doped films exhibit high ferroelectric polarization; however, it typically decreases rapidly with increasing film thickness. In contrast to these usual results, a few exceptions report ferroelectricity in thick films with specific dopants, such as Y or La, crystallized under conditions that favor granular microstructure. Disentangling the roles of microstructure and doping on the thickness-dependent stability of ferroelectricity remains essential, and identifying highly effective dopant atoms is of high relevance. In this work, Y-doped epitaxial films of various thicknesses up to about 100 nm are prepared to determine phase evolution and ferroelectric polarization. Films deposited on SrTiO3(001) and SrTiO3(110) substrates exhibit robust ferroelectric response across the entire thickness range, in contrast to equivalent La-doped films. Coexistence of monoclinic (paraelectric) and orthorhombic (ferroelectric) phases is observed, with columnar grains revealed by scanning transmission electron microscopy, demonstrating that the microstructure of epitaxial HfO2 films can be preserved beyond 10 nm. The observed columnar grain structure indicates that the robustness of ferroelectricity in Y-doped films results from the high effectiveness of Y, supporting its use in devices requiring thick ferroelectric layers.

Magneto-ionics, which refers to the modification of the magnetic properties of materials through electric-field-induced ion migration, is emerging as one of the most promising methods to develop nonvolatile energy-efficient memory and spintronic and magnetoelectric devices. Herein, the controlled generation of ferromagnetism from paramagnetic Co-Ni oxide patterned microdisks (prepared upon thermal oxidation of metallic microdisks with dissimilar Co-Ni ratios, i.e., Ni25Co75 and Ni50Co50) is demonstrated under the action of voltage. The effect is related to the partial reduction of the oxide phases to their metallic forms. Samples richer in Co show stronger magneto-ionic activity, which manifests in lower-onset threshold voltages, faster switching rates, and larger values of the attained saturation magnetization. By means of scanning electron microscopy, a cobalt segregation phenomenon has been experimentally observed upon thermal oxidation, which has been theoretically discussed from the diffusivities' viewpoint. X-ray diffraction characterization has revealed transitions between purely mixed Ni and Co oxides, in the OFF state, to a mixture of oxide and metallic phases, in the ON state, because of the oxygen ion motion outward/inward the Co-Ni oxide microdisks, depending on the voltage polarity. Ab initio calculations reveal that the energy barrier for oxygen vacancy migration is lower in CoO than in NiO, in agreement with the obtained magneto-ionic responses. The observation of magneto-ionic effects in patterned disks (and not only in archetypical continuous films) is a step further for the practical utilization of this phenomenon in real miniaturized devices.

Ferroelectric doped hafnium oxide constitutes, at present, an intensively investigated candidate material to develop outperforming non-volatile memory devices. We report reading and writing of the ferroelectric polarization with light in Hf0.5Zr0.5O2/Nb:SrTiO3 structures, where light is absorbed at the interface between the two materials, thereby enabling both processes. Reading of ferroelectric polarization is accomplished through the induced short-circuit photocurrents, which is a pathway towards voltage-less-non-destructive reading. Optical writing allows remote and contact-less switching of ferroelectric polarization, without the need for external voltages. The presence or absence of a Pt capping layer is crucial for the aforementioned read/write operations. If top Pt is present, photocarriers flow, resulting in short-circuit photocurrent, whose magnitude is modulated by the induced depolarization field. Instead, if Pt is removed, photocarriers are accumulated at the top surface, eventually producing an optically induced switching of polarization, as revealed by piezoelectric force microscopy observations. These results are highly appealing for the design of novel energy-efficient ferroelectric memory devices actuated with light.

Solid-state magneto-ionic (MI) effects have shown promise for energy-efficient nanoelectronics, where ionic migration may be used to achieve atomic-scale control of interfaces in magnetic nanostructures. To date, magneto-ionics have been mostly explored in oxygen-based systems [1-4], while there is a surge of interest in alternative ionic systems due to their different ionic migration mechanisms and characteristics [5-7]. We have recently demonstrated effective MI control of magnetic functionalities using a variety of ionic species, particularly nitrogen. In nitride-based Ta/CoFe/MnN/Ta films, the chemically induced MI effect is combined with the electric field driving of nitrogen to electrically manipulate exchange bias [8]. Upon field-cooling the heterostructure, ionic diffusion of nitrogen from MnN into the Ta layers occurs. A significant exchange bias is observed, which can be further enhanced by ~ 20% after voltage conditioning (Figs. 1a-1e). This enhancement can be reversed by voltage conditioning with an opposite polarity. Nitrogen migration within the MnN layer and into the Ta capping layer causes the enhancement in exchange bias, which is observed in polarized neutron reflectometry studies. We have also achieved all-nitride-based magneto-ionic systems [9]. Thin films of (001)-ordered Mn 4 N are grown by sputtering Mn onto Mn 3 N 2 seed layer on Si (100) substrate. Nitrogen ion migration across the Mn 3 N 2 /Mn layers leads to a continuous evolution of the layers to Mn 3 N 2 /Mn 4 N, Mn 2 N/Mn 4 N, and eventually Mn 4 N alone (Figs. 1f-1g). Furthermore, we have demonstrated MI control of the exchange bias effect in an all-nitride Mn 4 N/MnN x system, where the field-trained exchange field can be varied up to ten times by introducing or extracting nitrogen from the nitride system. This is achieved by adjusting the nitrogen gas partial pressure during deposition or varying annealing temperature after deposition. These effects demonstrate contrasts with oxygen-based MI effects in terms of operating principles, switching speed, and reversibility. Such MI systems are valuable platforms to gain quantitative understanding at buried interfaces. They also offer potentials for device applications based on electric modulation of magnetic functionalities. This work has been supported in part by the NSF (ECCS-2151809, DMR-2005108, DMR-1828420), SRC/NIST SMART Center, and KAUST. [1] U. Bauer et al. , Nat. Mater. 14 , 174 (2015). [2] C. Bi et al. , Phys. Rev. Lett. 113 , 267202 (2014). [3] D. A. Gilbert et al. , Nat. Commun. 7 , 11050 (2016). [4] G. Chen et al. , Sci. Adv. 6 , eaba4924 (2020). [5] A. J. Tan et al. , Nat. Mater. 18 , 35 (2019). [6] G. Chen et al. , Phys. Rev. X 11 , 021015 (2021). [7] J. de Rojas et al. , Nat. Commun. 11 , 5871 (2020). [8] C. J. Jensen et al. , ACS Nano 17 , 6745 (2023). [9] Z. J. Chen et al. , Appl. Phys. Lett. 123 , 082403 (2023). Figure 1

Magnetoelectric multiferroics, either single-phase or composites comprising ferroelectric/ferromagnetic coupled films, are promising candidates for energy efficient memory computing. However, most of the multiferroic magnetoelectric systems studied so far are based on materials that are not compatible with industrial processes. Doped hafnia is emerging as one of the few CMOS-compatible ferroelectric materials. Thus, it is highly relevant to study the integration of ferroelectric hafnia into multiferroic systems. In particular, ferroelectricity in hafnia, and the eventual magnetoelectric coupling when ferromagnetic layers are grown atop of it, are very much dependent on quality of interfaces. Since magnetic metals frequently exhibit noticeable reactivity when grown onto oxides, it is expected that ferroelectricity and magnetoelectricity might be reduced in multiferroic hafnia-based structures. In this article, we present excellent ferroelectric endurance and retention in epitaxial Hf0.5Zr0.5O2 films grown on buffered silicon using Co as the top electrode. The crucial influence of a thin Pt capping layer grown on top of Co on the ferroelectric functional characteristics is revealed by contrasting the utilization of Pt-capped Co, non-capped Co and Pt. Magnetic control of the imprint electric field (up to 40% modulation) is achieved in Pt-capped Co/Hf0.5Zr0.5O2 structures, although this does not lead to appreciable tuning of the ferroelectric polarization, as a result of its high stability. Computation of piezoelectric and flexoelectric strain-mediated mechanisms of the observed magnetoelectric coupling reveal that flexoelectric contributions are likely to be at the origin of the large imprint electric field variation.

Magneto-ionic gating, a procedure that enables the modulation of materials' magnetic properties by voltage-driven ion motion, offers alternative perspectives for emerging low-power magnetic storage and spintronic applications. Most previous studies in all-solid-state magneto-ionic systems have focused on the control of interfacial magnetism of ultrathin (i.e., 1–3 nm) magnetic films, taking advantage of an adjacent ionic conducting oxide, usually GdOx or HfOx, that transports functional ionic species (e.g., H+ or O2−). Here, we report on room-temperature OFF–ON ferromagnetism by solid-state magneto-ionics in relatively thick (25 nm) patterned CoOx films grown on an yttria-stabilized zirconia (YSZ) layer, which acts as a dielectric to hold electric field and as an O2− ion reservoir. Upon negatively biasing, O2− ions from the CoOx tend to migrate toward the YSZ gate electrode, leading to the gradual generation of magnetization (i.e., OFF-to-ON switching of a ferromagnetic state). X-ray absorption and magnetic circular dichroism studies reveal subtle changes in the electronic/chemical characteristics, responsible for the induced magnetoelectric effects in such all-oxide heterostructures. Recovery of the initial (virtually non-magnetic) state is achieved by application of a positive voltage. The study may guide future development of all-solid-state low-power CMOS-compatible magneto-ionic devices.

Magneto-ionics relies on the voltage-driven transport of ions to modify magnetic properties. As a diffusion-controlled mechanism, defects play a central role in determining ion motion and, hence, magneto-ionic response. Here, the potential of ion implantation is exploited to engineer depth-resolved defect type and density with the aim to control the magneto-ionic behavior of Co3O4 thin films. It is demonstrated that through a single implantation process of light ions (He+) at 5 keV, the magneto-ionic response of a nanostructured 50 nm thick Co3O4 film, in terms of rate and amount of induced magnetization, at short-, mid-, and long-term voltage actuation, can be controlled by varying the generated collisional damage through the ion fluence. These results constitute a proof-of-principle that paves the way to further use ion implantation (tuning the ion nature, energy, fluence, target temperature, or using multiple implantations) to enhance performance in magneto-ionic systems, with implications in ionic-based devices. Light-ion implantation is exploited to engineer depth-resolved defect type and density along nanostructured 50 nm thick Co3O4 films. This allows for the control of the magneto-ionic response of the Co3O4 thin films in terms of rate and amount of induced magnetization, at short-, mid-, and long-term voltage actuation, by varying the generated collisional damage through ion fluence. image

FeRh shows an antiferromagnetic to ferromagnetic phase transition above room temperature, which permits its use as an antiferromagnetic memory element. However, its antiferromagnetic order is sensitive to small variations in crystallinity and composition, challenging its integration into flexible devices. Here, we show that flexible FeRh films of high crystalline quality can be synthesized by using mica as a substrate, followed by a mechanical exfoliation of the mica. The magnetic and transport data indicate that the FeRh films display a sharp antiferromagnetic to ferromagnetic phase transition. Magnetotransport data allow for the observation of two distinguishable resistance states, which are written after a field-cooling procedure. It is shown that the memory states are robust under the application of magnetic fields of up to 10 kOe.

Narrow bandgap ferroelectrics are receiving a renewed interest for photovoltaic applications aiming at exploiting new functionalities arising from bulk photovoltaic effect (BPE). We report on the photovoltaic response of vertical capacitors of ferroelectric hexagonal LuMnO3 films sandwiched between semitransparent top electrodes (Pt, Co, Ti) and a common bottom (Pt) bottom electrode. Our results show that the presence of electrodes, other than their optical transmittance, crucially determines the imprint in the ferroelectric layer and ultimately the sensitivity of short circuit current density (Jsc) to the ferroelectric polarization direction. The use of ultrathin (7 nm) Pt top electrodes allowed to obtain a large Jsc (up to 100 mA/cm2) and an open circuit voltage of Voc approximate to 0.52 V, with a responsivity of 2 x 10-3 A/ W. Yet, polarization back-switching due to imprint largely washes out the dependence of Jsc on the direction of the polarization and thus, at first sight, Jsc seemingly appears to be ruled by conventional photovoltaic response. However, a pioneering analysis of the light-polarization dependent photosensitivity, allowed to disen-tangle for the first time, a genuine contribution of BPE from a ubiquitous Fresnel-like contribution arising from interfaced optical media.