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The preparation and manipulation of crystalline yet bendable functional complex oxide membranes has been a long-standing issue for a myriad of applications, in particular, for flexible electronics. Here, we investigate the viability to prepare magnetic and crystalline CoFe2O4 (CFO) membranes by means of the Sr3Al2O6 (SAO) sacrificial layer approach using chemical deposition techniques. Meticulous chemical and structural study of the SAO surface and SAO/CFO interface properties have allowed us to identify the formation of an amorphous SAO capping layer and carbonates upon air exposure, which dictate the crystalline quality of the subsequent CFO film growth. Vacuum annealing at 800 °C of SAO films promotes the elimination of the surface carbonates and the reconstruction of the SAO surface crystallinity. Ex-situ atomic layer deposition of CFO films at 250 °C on air-exposed SAO offers the opportunity to avoid high-temperature growth while achieving polycrystalline CFO films that can be successfully transferred to a polymer support preserving the magnetic properties under bending. Float on and transfer provides an alternative route to prepare freestanding and wrinkle-free CFO membrane films. The advances and challenges presented in this work are expected to help increase the capabilities to grow different oxide compositions and heterostructures of freestanding films and their range of functional properties., This work was funded by MICIN/AEI/10.13039/501100011033/FEDER through the projects Severo Ochoa FUNFUTURE CEX2019-00917-S and PID2020-114224RB-I00. We also acknowledge the financial support from the 2020 Leonardo Grant for Researchers and Cultural Creators BBVA Foundation, the i-link A20346-CSIC project, the National Key RD Program of China (2018YFA0305800), and the Beijing Outstanding Young Scientist Program (BJJWZYJH01201914430039). The project that gave rise to these results received the support of a fellowship from the “la Caixa” Foundation LCF/BQ/DI19/11730026. D.Z. acknowledges the funding and financial support of the Fund of Chemical Industries (Kekulé fellowship) for his Ph.D. research. The authors are thankful to R. Solanas for his help with the RHEED measurements. This work has been done in the framework of the doctorate in material science of the Autonomous University of Barcelona., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

The magnetic properties of cobalt metal nanowires grown by electrodeposition in porous membranes depend largely on the synthesis conditions. Here, we focus on the role of electrolyte additives on the magnetic anisotropy of the electrodeposited nanowires. Through magnetometry and internal field nuclear magnetic resonance (IF NMR) studies, we compared both the magnetic and crystalline structures of 50 and 200 nm diameter Co nanowires synthesized in the presence or absence of organic additives. The spectral characteristics of IF NMR were compared structurally to X-ray diffraction patterns, and the anisotropy of the NMR enhancement factor in ferromagnetic multidomain structures to magnetometry results. While the magnetic behavior of the 50 nm nanowires was dominated, as expected, by shape anisotropy with magnetic domains oriented on axis, the analysis of the 200 nm proved to be more complex. 59Co IF NMR revealed that the determining difference between the samples electrodeposited in the presence or in absence of organic additives was not the dominant crystalline system (fcc or hcp) but the coherent domain sizes and boundaries. In the presence of organic additives, the cobalt crystal domains are smaller and with defective grain boundaries, as revealed by resonances below 210 MHz. This prevented the development in the Co hcp part of the sample of the strong magnetocrystalline anisotropy that was observed in the absence of organic additives. In the presence of organic additives, even in nanowires as wide as 200 nm, the magnetic behavior remained determined by the shape anisotropy with a positive effective magnetic anisotropy and strong anisotropy of the NMR enhancement factor., NMR equipment at ESPCI Paris is funded in part by the Paris Region. This project has received funding from the European Union′s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No 754387. Work at Georgetown University has been supported by the US NSF (ECCS-1933527). The acquisition of a Magnetic Property Measurements System (MPMS3) at GU, which was used in this investigation, was supported by the US-NSF (DMR-1828420). Bruno Bresson (ESPCI) is thanked for performing the SEM analysis., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Magneto-ionics is an emerging actuation mechanism to control the magnetic properties of materials via voltage-driven ion motion. This effect largely relies on the strength and penetration of the induced electric field into the target material, the amount of generated ion transport pathways, and the ionic mobility inside the magnetic media. Optimizing all these factors in a simple way is a huge challenge, although highly desirable for technological applications. Here, we demonstrate that the introduction of suitable transition-metal elements to binary nitride compounds can drastically boost magneto-ionics. More specifically, we show that the attained magneto-ionic effects in CoN films (i.e., saturation magnetization, toggling speeds, and cyclability) can be drastically enhanced through 10% substitution of Co by Mn in the thin-film composition. Incorporation of Mn leads to transformation from nanocrystalline into amorphous-like structures, as well as from metallic to semiconducting behaviors, resulting in an increase of N-ion transport channels. Ab initio calculations reveal a lower energy barrier for CoMn-N compared to Co-N that provides a fundamental understanding of the crucial role of Mn addition in the voltage-driven magnetic effects. These results constitute an important step forward toward enhanced voltage control of magnetism via electric field-driven ion motion., Financial support by the European Research Council (MAGIC-SWITCH 2019-Proof of Concept Grant, agreement no 875018; REMINDS 2021-ERC-Advanced Grant, agreement no 101054687), the European Union’s Horizon 2020 research and innovation program (European Training Network, ETN/ITN Marie Skłodowska-Curie grant no 861145; and Integrated Infrastructure, RADIATE, grant no 824096), the Spanish Government (PID2020-116844RB-C21 and PDC2021-121276-C31), the Generalitat de Catalunya (2017-SGR-292), and the KU Leuven (BOF program) is acknowledged. We acknowledge the technical support from the MiNa Laboratory at IMN in Madrid, who received funding from the CM (project S2018/NMT-4291 TEC2SPACE), MINECO (project CSIC13-4E-1794), and EU (FEDER, FSE). A.Q. acknowledges financial support from the Spanish Ministry of Science, Innovation and Universities through the “Severo Ochoa” Programme for Centers of Excellence in R&D (FUNFUTURE CEX2019-000917-S) and the “Juan de la Cierva─Formación” contract (FJC2019-039780-I). J.S. thanks the Spanish “Fábrica Nacional de Moneda y Timbre” (FNMT) for fruitful discussions. E.M. is a Serra Húnter Fellow., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

Electric-field-controlled magnetism can boost energy efficiency in widespread applications. However, technologically, this effect is facing important challenges: mechanical failure in strain-mediated piezoelectric/magnetostrictive devices, dearth of room-temperature multiferroics, or stringent thickness limitations in electrically charged metallic films. Voltage-driven ionic motion (magneto-ionics) circumvents most of these drawbacks while exhibiting interesting magnetoelectric phenomena. Nevertheless, magneto-ionics typically requires heat treatments and multicomponent heterostructures. Here we report on the electrolyte-gated and defect-mediated O and Co transport in a Co3O4 single layer which allows for room-temperature voltage-controlled ON-OFF ferromagnetism (magnetic switch) via internal reduction/oxidation processes. Negative voltages partially reduce Co3O4 to Co (ferromagnetism: ON), resulting in graded films including Co- and O-rich areas. Positive bias oxidizes Co back to Co3O4 (paramagnetism: OFF). This electric-field-induced atomic-scale reconfiguration process is compositionally, structurally, and magnetically reversible and self-sustained, since no oxygen source other than the Co3O4 itself is required. This process could lead to electric-field-controlled device concepts for spintronics., Financial support by the European Research Council (SPINPORICS 2014-Consolidator Grant, Agreement No. 648454), the Spanish Government (Projects MAT2017-86357-C3-1-R and associated FEDER), the Generalitat de Catalunya (2017-SGR-292) and the European Union’s Horizon 2020 research and innovation programme under the Marie Skłodowska-Curie Grant Agreement No. 665919 is acknowledged. E.P. is grateful to MINECO for the “Ramon y Cajal” contract (RYC-2012-10839). The ICN2 is funded by the CERCA programme/Generalitat de Catalunya. ICN2 also acknowledges the support from the Severo Ochoa Program (MINECO, Grant SEV-2013-0295). Work at UCD is supported by the US NSF (DMR1610060 and ECCS-1611424).

Manipulation of exchange bias with electric field is appealing to boost energy efficiency in spintronic devices. Here, this effect is shown at room temperature in Ir20Mn80/Fe80Ga20/Ta layers grown onto «011»-oriented PMN-32PT single crystals. After magnetic field-cooling (FC) along the [01-1] and [100] in-plane directions of PMN-32PT and upon allowing the system to relax through consecutive hysteresis loops (training effect), the exchange bias field (HEB) is measured under the action of voltage (out-of-plane poling). Depending on the applied voltage (magnitude and sign), HEB can either increase or decrease with respect to its value at 0 V. The relative variations of HEB are 24% and 5.5% after FC along the [01-1] and [100] directions, respectively. These results stem from strain-mediated magnetoelectric coupling. The applied electric field causes changes in the coercivity and the squareness ratio of the films, suggesting a reorientation of the effective magnetic easy axis in Fe80Ga20. However, larger HEB values are observed when the squareness ratio is lower. It is claimed that the effect of voltage is equivalent to an in-plane component of an applied magnetic field oriented perpendicular to the cooling field direction. Perpendicular in-plane magnetic fields have been shown to induce an increase in exchange bias in some ferromagnetic/antiferromagnetic systems due to partial recovery of the untrained antiferromagnetic state. Remarkably, here, this effect is directly induced with voltage, therefore enhancing energy efficiency., This work was supported by TÜBİTAK (The Scientific and Technological Research Council of Turkey) through the Project No. 1059B192000016. Partial financial support by the European Research Council (MAGIC-SWITCH 2019-Proof of Concept Grant, Agreement No. 875018), the European Union's Horizon 2020 research and innovation programme (European Training Network, ETN/ITN Marie Skłodowska-Curie, Agreement No. 861145), the Spanish Government (Nos. Severo Ochoa FUNFUTURE-CEX2019-000917-SMCIN, AEI/10.13039/501100011033-, MAT2017-86357-C3-1-R, PID2019-107727RB-I00, PID2020-116844RB-C21, and PDC2021-121276-C31), the Generalitat de Catalunya (No. 2017-SGR-292), and the European Regional Development Fund (No. MAT2017-86357-C3-1-R) is acknowledged. E.M. is a Serra Húnter Fellow. A.Q. acknowledges the Spanish Ministry of Science, Innovation and Universities for the Juan de la Cierva Formación Contract (No. FJC2019-039780-I)., With funding from the Spanish government through the ‘Severo Ochoa Centre of Excellence’ accreditation (CEX2019-000917-S).

A synergetic approach to enhance magnetoelectric effects (i.e., control of magnetism with voltage) and improve energy efficiency in magnetically actuated devices is presented. The investigated material consists of an ordered array of Co–Pt microdisks, in which nanoporosity and partial oxidation are introduced during the synthetic procedure to synergetically boost the effects of electric field. The microdisks are grown by electrodeposition from an electrolyte containing an amphiphilic polymeric surfactant. The bath formulation is designed to favor the incorporation of oxygen in the form of cobalt oxide. A pronounced reduction of coercivity (88%) and a remarkable increase of Kerr signal amplitude (60%) are observed at room temperature upon subjecting the microdisks to negative voltages through an electrical double layer. These large voltage-induced changes in the magnetic properties of the microdisks are due to (i) the high surface-area-to-volume ratio with ultranarrow pore walls (sub-10 nm) that promote enhanced electric charge accumulation and (ii) magneto-ionic effects, where voltage-driven O2– migration promotes a partial reduction of CoO to Co at room temperature. This simple and versatile procedure to fabricate patterned “nano-in-micro” magnetic motifs with adjustable voltage-driven magnetic properties is very appealing for energy-efficient magnetic recording systems and other magnetoelectronic devices., This work was funded by the European Research Council under the SPIN-PORICS 2014-Consolidator Grant (Agreement No. 648454), the Generalitat de Catalunya (2017-SGR292 project), the Spanish Government (MAT2017-86357-C3-1-R, MAT2014-57960-C3-1-R, and associated FEDER projects), and the European Union’s Horizon 2020 research and innovation program under the Marie Skłodowska-Curie grant agreement No. 665919. J.F. and E.P. acknowledge the Juan de la Cierva (IJCI-2015-27030) and Ramon y Cajal (RYC-2012-10839) fellowships, respectively, from MINECO. The ICN2 is funded by the CERCA program/Generalitat de Catalunya. ICN2 also acknowledges the support from the Severo Ochoa Program (MINECO, grant SEV-2013-0295)