Significant Strain‐Induced Orbital Reconstruction and Strong Interfacial Magnetism in TiNi(Nb)/Ferromagnet/Oxide Heterostructures via Oxygen Manipulation

Dynamical manipulation of oxygen ion (O^2?) at metal/oxide heterointerfaces is widely demonstrated to tailor numerous physical and chemical properties and facilitate creating novel functionalities significantly. The traditional works mainly focus on electric control of O^2? dynamical behavior and related interface characteristics. Here, an alternative strategy is reported to modulate O^2? transport and interfacial magnetism via a significant strain induced by shape memory effect, which is different from the conventional magnetoelastic coupling mechanism. By driving the martensite to austenite transition in TiNi(Nb) shape memory alloy substrates, a significant and tunable strain is exerted on Pt/Co/MgO heterostructure, which promotes interfacial O^2? migration in a nonvolatile manner. The O^2? migration induces an orbital reconstruction of Co to tune the orbital magnetism noticeably, which strengthens the interfacial magnetic anisotropy energy by two times to a striking value of 0.95 erg cm^?2. Besides, the overall magnetic anisotropy is broadly tunable from in‐plane to perpendicular direction by an elaborate strain engineering with changing Co thickness. This work develops a nonelectrical oxygen manipulation for tailoring ion‐controlled interfacial properties universally and also clarifies the magnetoionic coupling origin for enriching the oxygen‐related orbital physics and functional device applications.     

Ten States of Nonvolatile Memory through Engineering Ferromagnetic Remanent Magnetization

Emerging nonvolatile multilevel memory devices have been regarded as a promising solution to meet the increasing demand of high‐density memory with low‐power consumption. In particular, decimal system of the new computers instead of binary system could be developed if ten nonvolatile states are realized. Here, a general remanent magnetism engineering method is proposed for realizing multiple reliable magnetic and resistance states, not depending on a specific material or device structure. Especially, as a proof‐of‐concept demonstration, ten states of nonvolatile memory based on the manipulation of ferromagnetic remanent magnetization have been revealed in both Co/Pt magnetic multilayers with strong perpendicular magnetic anisotropy and MgO‐based magnetic tunneling junctions at room temperature. Considering ferromagnets have been one of the key factors that enabled the information revolution from its inception, this state‐of‐the‐art remanent magnetism engineering approach has a very broad application prospect in the field of spintronics.     

Enhanced Magnetic Anisotropy and Orbital Symmetry Breaking in Manganite Heterostructures

Manipulating magnetic anisotropy in complex oxide heterostructures has attracted much attention. Here, three interface‐engineering approaches are applied to address two general issues with controlling magnetic anisotropy in the La_2/3Sr_1/3MnO₃ heterostructure. One is the paradox arising from the competition between Mn_3d–O_2p orbital hybridization and MnO₆ crystal field. The other is the interfacial region where the nonuniform Mn? O bond length d and Mn? O? Mn bond angle θ disturb the structural modulation. When the interfacial region is suppressed in the interface‐engineered samples, the lateral magnetic anisotropy energy is increased eighteen times. The d‐mediated anisotropic crystal filed that overwhelms the orbital hybridization causes the lateral symmetry breaking of the Mn 3d_x²_?y² orbital, resulting in enhanced magnetic anisotropy. This is different from the classic Jahn–Teller effect where the lateral symmetry is always preserved. Moreover, the quantitative analysis on X‐ray linear dichroism data suggests a direct correlation between Mn 3d_x²_?y² orbital symmetry breaking and magnetic anisotropy energy. The findings not only advance the understanding of magnetic anisotropy in manganite heterostructures but also can be extended to other complex oxides and perovskite materials with correlated degrees of freedom.     

Tuning a Binary Ferromagnet into a Multistate Synapse with Spin–Orbit‐Torque‐Induced Plasticity

Ferromagnets with binary states are limited for applications as artificial synapses for neuromorphic computing. Here, it is shown how synaptic plasticity of a perpendicular ferromagnetic layer (FM1) can be obtained when it is interlayer exchange‐coupled by another in‐plane ferromagnetic layer (FM2), where a magnetic field‐free current‐driven multistate magnetization switching of FM1 in the Pt/FM1/Ta/FM2 structure is induced by spin–orbit torque. Current pulses are used to set the perpendicular magnetization state, which acts as the synapse weight, and spintronic implementation of the excitatory/inhibitory postsynaptic potentials and spike timing‐dependent plasticity are demonstrated. This functionality is made possible by the action of the in‐plane interlayer exchange coupling field which leads to broadened, multistate magnetic reversal characteristics. Numerical simulations, combined with investigations of a reference sample with a single perpendicular magnetized Pt/FM1/Ta structure, reveal that the broadening is due to the in‐plane field component tuning the efficiency of the spin–orbit torque to drive domain walls across a landscape of varying pinning potentials. The conventionally binary FM1 inside the Pt/FM1/Ta/FM2 structure with an inherent in‐plane coupling field is therefore tuned into a multistate perpendicular ferromagnet and represents a synaptic emulator for neuromorphic computing, demonstrating a significant pathway toward a combination of spintronics and synaptic electronics.     

Magnetic Anisotropy Switch: Easy Axis to Easy Plane Conversion and Vice Versa

The rational design of the magnetic anisotropy of molecular materials constitutes a goal of primary importance in molecular magnetism. Indeed, the applications of molecular nanomagnets, such as single‐molecule magnets and molecular magnetic refrigerants, depend on the full control over this property. Axially anisotropic magnetic systems are frequently classified as easy axis or easy plane, depending on whether the lowest energy is obtained by application of a magnetic field parallelly or perpendicularly to the unique axis. Here, the magnetic anisotropy of three lanthanide complexes is studied as a function of magnetic field and temperature. It is found that for two of these the type of magnetic anisotropy switches as a function of these parameters. Thus, this paper experimentally demonstrates that the magnetic anisotropy is not uniquely defined by the intrinsic electronic structure of the systems in question but can also be reversibly switched using external stimuli: temperature and magnetic field.     

Study on the mechanism of perpendicular magnetic anisotropy in Ta/CoFeB/MgO system

The mechanism of perpendicular magnetic anisotropy (PMA) in a MgO-based magnetic tunnel junction (MTJ) has been studied in this article. By comparing the magnetic properties and elementary composition analysis for different CoFeB-based structures, such as Ta/CoFeB/MgO, Ta/CoFeB/Ta and Ru/CoFeB/MgO structures, it is found that a certain amount of Fe-oxide existing at the interface of CoFeB/MgO is helpful to enhance the PMA and the PMA is originated from the interface of CoFeB/MgO. In addition, Ta film plays an important role to enhance the PMA in Ta/CoFeB/MgO structure.     

Electric Field‐Induced Oxidation of Ferromagnetic/Ferroelectric Interfaces

Composite multiferroics are a new class of material where magneto‐electric coupling is achieved by creating an interface between a ferromagnetic and a ferroelectric compound. The challenge of understanding the chemical and magnetic properties of such interface is a key to achieve good magneto‐electric coupling. The unique possibilities offered by isotope sensitive techniques are used to selectively investigate the interface's chemistry and magnetism in Fe/BaTiO₃ and Fe/LiNbO₃ systems during the application of an electric field. With a large enough electric field, a strong oxidation of Fe is triggered, which creates a magnetically dead interface. This leads to an irreversible decrease of the magneto‐electric coupling properties. Material parameters are identified that determine under which electric field the interface may be modified. The results are confirmed on the two systems and are expected to be widespread in this new class of hybrid material.     

Deterministic Current-Induced Perpendicular Switching in Epitaxial Co/Pt Layers without an External Field

Current-induced spin-orbit torques (SOTs) have emerged as a powerful tool to control magnetic elements and non-uniform magnetic textures such as domain walls and skyrmions. SOT-induced switching of perpendicular magnetization generally requires an external field to break the rotational symmetry of the spin-orbit effective fields responsible for the deterministic reversal. The proposed mechanisms to eliminate this requirement often rely on complex multilayer structures that necessitate laborious optimization in the material and spin transport properties, making them less attractive for applications. Herein, current-induced, external field-free switching of an epitaxial MgO/Pt/Co trilayer with an extremely large perpendicular anisotropy in excess of 3 Tesla is reported. It is found that switching occurs due to the interplay of strong SOTs, local anisotropy fluctuations, and the Dzyaloshinkii-Moriya interaction inherent to this epitaxial system. Given that these layers constitute the base stack of a magnetic tunnel junction, this switching mechanism offers the most technologically viable path toward devices such as field-free SOT-based magnetic random-access memories.     

Fabrication of Co2Fe(Al,Si) and Co2Fe(Al,Si)/MgO on Ge(111) Substrate and Its Magnetic Properties

We investigated the interfacial effects on magnetic properties in Co₂Fe(Al,Si)/Ge (CFAS/Ge) and CFAS/MgO/Ge systems to demonstrate the effects of the interface structure on magnetic properties. CFAS and CFAS/MgO were deposited on the i-Ge(111) substrate. In-situ reflection high energy electron diffraction (RHEED) patterns showed epitaxially grown CFAS and MgO on Ge(111). According to the X-ray diffraction (XRD) ϕ-scan of CFAS(220), we determined that the crystallographic orientation relationships were CFAS(111)<–110>// Ge(111)<–110> and CFAS(111)<–110>//MgO(111)<–110>Ge(111)<–110>. The magnetic properties were measured by the vibrating sample magnetometer (VSM) and the saturation magnetization M_s value of CFAS with 2-nm thick MgO reached the value of L2₁ ordered one. A uniaxial magnetic anisotropy behavior was observed both in CFAS/Ge and CFAS/MgO/Ge structures after annealing. We confirmed the behavior did not only originate from the CFAS/Ge interface but also CFAS/MgO and the ordering structure.     

Synthesis of Interfacially Active and Magnetically Responsive Nanoparticles for Multiphase Separation Applications

A novel interfacially active and magnetically responsive nanoparticle is designed and prepared by direct grafting of bromoesterified ethyl cellulose (EC‐Br) onto the surface of amino‐functionalized magnetite (Fe₃O₄) nanoparticles. Due to its strong interfacial activity, ethyl cellulose (EC) on the magnetic nanoparticles enables the EC‐grafted Fe₃O₄ (M‐EC) nanoparticles to be interfacially active. The grafting of interfacially active polymer EC on magnetic nanoparticles is confirmed by zeta‐potential measurements, diffuse reflectance infrared Fourier‐transform spectroscopic (DRIFTS) characterization, and thermogravimetric analysis (TGA). Scanning electron microscopy (SEM) images show a negligible increase in particle size, confirming the thin silica coating and grafted EC layer. The magnetization measurements show a marginal reduction in saturation magnetization by silica coating and EC grafting of original magnetic nanoparticles, confirming the presence of coatings. The M‐EC nanoparticles prepared in this study show excellent interfacial activity and highly ordered features at the oil/water interface, as confirmed using the Langmuir–Blodgett technique and atomic force microscopy (AFM). The magnetic properties of M‐EC nanoparticles at the oil/water interface make the interfacial properties tunable by or responsive to an external magnetic field. The occupancy of M‐EC at the oil/water interface allows rapid separation of the water droplets from emulsions by an external magnetic field, demonstrating enhanced coalescence of magnetically tagged stable water droplets and a reduced overall volume fraction of the sludge.     

Modulating the Ferromagnet/Molecule Spin Hybridization Using an Artificial Magnetoelectric

Spin‐polarized charge transfer at the interface between a ferromagnetic (FM) metal and a molecule can lead to ferromagnetic coupling and to a high spin polarization at room temperature. The magnetic properties of these interfaces can not only alter those of the ferromagnet but can also stabilize molecular spin chains with interesting opportunities toward quantum computing. With the aim to enhance an organic spintronic device's functionality, external control over this spin polarization may thus be achieved by altering the ferromagnet/molecule interface's magnetic properties. To do so, the magnetoelectric properties of an underlying ferroelectric/ferromagnetic interface are utilized. Switching the ferroelectric polarization state of a PbZr_0.2Ti_0.8O₃ (PZT) bottom layer within a PZT/Co/FePc‐based (Pc ‐ phthalocyanine) device alters the X‐ray magnetic circular dichroism of the Fe site within the phthalocyanine molecular top layer. Thus, how to electrically alter the magnetic properties of an interface with high spin polarization at room temperature is demonstrated. This expands electrical control over spin‐polarized FM/molecule interfaces, which is first demonstrated using ferroelectric molecules, to all molecular classes.     

Ionic Modulation of Interfacial Magnetism in Light Metal/Ferromagnetic Insulator Layered Nanostructures

Ferromagnetic insulator thin film nanostructures are becoming the key component of the state‐of‐the‐art spintronic devices, for instance, yttrium iron garnet (YIG) with low damping, high Curie temperature, and high resistivity is explored into many spin–orbit interactions related spintronic devices. Voltage modulation of YIG, with great practical/theoretical significance, thus can be widely applied in various YIG‐based spintronics effects. Nevertheless, to manipulate ferromagnetism of YIG through electric field (E‐field), instead of current, in an energy efficient manner is essentially challenging. Here, a YIG/Cu/Pt layered nanostructure with a weak spin–orbit coupling interaction is fabricated, and then the interfacial magnetism of the Cu and YIG is modified via ionic liquid gating method significantly. A record‐high E‐field‐induced ferromagnetic resonance field shift of 1400 Oe is achieved in YIG (17 nm)/Cu (5 nm)/Pt (3 nm)/ionic liquid/Au capacitor layered nanostructures with a small voltage bias of 4.5 V. The giant magnetoelectric tunability comes from voltage‐induced extra ferromagnetic ordering in Cu layer, confirmed by the first‐principle calculation. This E‐field modulation of interfacial magnetism between light metal and magnetic isolator may open a door toward compact, high‐performance, and energy‐efficient spintronic devices.     

Magnetic Anisotropy of Cr7Ni Spin Clusters on Surfaces

The problem of the experimental and theoretical determination of magnetic anisotropy in isolated molecular spin clusters is addressed here. To this end, the case of molecular Cr₇Ni rings sublimated in ultrahigh vacuum conditions and assembled in an ordered fashion on Au(111) surface is addressed and investigated using X‐ray magnetic dichroism (XMCD) and theoretical calculations. Fixing the experimental conditions at a temperature T = 8 K and a magnetic field of 5 T, the angular‐dependence of the dichroic signal reveals an easy‐axis anisotropy for the Ni magnetization along the direction perpendicular to the ring while the magnetization of the whole Cr₇Ni molecule is preferentially aligned within the ring plane. These features are well reproduced by spin Hamiltonian simulations, which reflect the character of the S = 3/2 first excited multiplet, dominating at T = 8 K and 5 T. Density functional theory (DFT) calculations show that local spin orbit interactions determine an easy axis anisotropy at the Ni site while the Cr magnetic moment turns out to be more isotropic. This is the first direct observation of the interplay between the single ion and the overall magnetic anisotropy in complex (polynuclear) molecular systems.     

A Strain‐Mediated Magnetoelectric‐Spin‐Torque Hybrid Structure

Magnetization dynamics induced by spin–orbit torques in a heavy‐metal/ferromagnet can potentially be used to design low‐power spintronics and logic devices. Recent computations have suggested that a strain‐mediated spin–orbit torque (SOT) switching in magnetoelectric (ME) heterostructures is fast, energy‐efficient, and permits a deterministic 180° magnetization switching. However, its experimental realization has remained elusive. Here, the coexistence of the strain‐mediated ME coupling and the SOT in a CoFeB/Pt/ferroelectric hybrid structure is shown experimentally. The voltage‐induced strain only slightly modifies the efficiency of SOT generation, but it gives rise to an effective magnetic anisotropy and rotates the magnetic easy axis which eliminates the incubation delay in current‐induced magnetization switching. The phase field simulations show that the electric‐field‐induced effective magnetic anisotropy field can reduce the switching time approximately by a factor of three for SOT in‐plane magnetization switching. It is anticipated that such strain‐mediated ME‐SOT hybrid structures may enable field‐free, ultrafast magnetization switching.     

Long‐Range Nonvolatile Electric Field Effect in Epitaxial Fe/Pb(Mg1/3Nb2/3)0.7Ti0.3O3 Heterostructures

One of the ideal candidates of using electric field to manipulate magnetism is the recently developed multiferroics with emergent coupling of magnetism and electricity, particularly in synthesizing artificial nanoscale ferroelectric and ferromagnetic materials. Here, a long‐range nonvolatile electric field effect is investigated in Fe/Pb(Mg_1/3Nb_2/3)_0.7Ti_0.3O₃ heterostructure using the dependence of the magnon‐driven magnetoelectric coupling on the epitaxial Fe thin film (4–30 nm) thickness at room temperature using measurements based on the ferromagnetic resonance. The magnon‐driven magnetoelectric coupling tuning of the ferromagnetic resonance field shows a linear response to the electric field, with a resonance field shift that occurs under both positive and negative remanent polarizations, and demonstrates nonvolatile behavior. Moreover, the spin diffusion length of the epitaxial Fe thin film of ≈9 nm is obtained from the results that the change of the cubic magnetocrystalline anisotropy field under different electric fields varies with Fe thickness. These results are promising for the design of future multiferroic devices.     

Electrical Manipulation of Orbital Occupancy and Magnetic Anisotropy in Manganites

Electrical manipulation of lattice, charge, and spin is realized respectively by the piezoelectric effect, field‐effect transistor, and electric field control of ferromagnetism, bringing about dramatic promotions both in fundamental research and industrial production. However, it is generally accepted that the orbital of materials are impossible to be altered once they have been made. Here, electric field is used to dynamically tune the electronic‐phase transition in (La,Sr)MnO₃ films with different Mn⁴⁺/(Mn³⁺ + Mn⁴⁺) ratios. The orbital occupancy and corresponding magnetic anisotropy of these thin films are manipulated by gate voltage in a reversible and quantitative manner. Positive gate voltage increases the proportion of occupancy of the orbital and magnetic anisotropy that were initially favored by strain (irrespective of tensile and compressive), while negative gate voltage reduces the concomitant preferential orbital occupancy and magnetic anisotropy. Besides its fundamental significance in orbital physics, these findings might advance the process towards practical oxide‐electronics based on orbital.     

Dynamic Interface Formation in Magnetic Thin Film Heterostructures

Magnetic thin film heterostructures have been widely studied for fundamental interests in the emergence of novel phenomena associated with the heterointerface formation as well as their promising practical potential. Combining X‐ray magnetic circular dichroism with scanning tunneling microscopy, it is shown for fcc Fe thin films grown on Cu(001) with Mn overlayers (Mn/Fe thin film heterostructures) that the interfacial factors dominating the electronic and magnetic properties of the entire system dynamically change with the amount of the Mn overlayer. Element specific magnetization curves of the Fe layer exhibit a two‐step spin reorientation transition from out‐of‐plane to in‐plane direction by increasing the Mn coverage. The atomic‐scale characterizations of structural and electronic properties in combination with the first‐principles calculations successfully unravel the roles of the entangled interfacial factors and clarify the driving forces of the transition. The first step of the transition at a low Mn coverage is dominantly induced by the formation of FeMn disordered alloy at the heterointerface, and the electronic hybridization with the interfacial FeMn ordered alloy is dominant as the origin of the second step of the transition at a high Mn coverage.     

Mechanical Strain‐Tunable Microwave Magnetism in Flexible CuFe2O4 Epitaxial Thin Film for Wearable Sensors

Purely mechanical strain‐tunable microwave magnetism device with lightweight, flexible, and wearable is crucial for passive sensing systems and spintronic devices (noncontact), such as flexible microwave detectors, flexible microwave signal processing devices, and wearable mechanics‐magnetic sensors. Here, a flexible microwave magnetic CuFe₂O₄ (CuFO) epitaxial thin film with tunable ferromagnetic resonance (FMR) spectra is demonstrated by purely mechanical strains, including tensile and compressive strains, on flexible fluorophlogopite (Mica) substrates. Tensile and compressive strains show remarkable tuning effects of up‐regulation and down‐regulation on in‐plane FMR resonance field (H_r), which can be used for flexible tunable resonators and filters. The out‐of‐plane FMR spectra can also be tuned by mechanical bending, including H_r and absorption peak. The change of out‐of‐plane FMR spectra has great potential for flexible mechanics‐magnetic deformation sensors. Furthermore, a superior microwave magnetic stability and mechanical antifatigue character are obtained in the CuFO/Mica thin films. These flexible epitaxial CuFO thin films with tunable microwave magnetism and excellent mechanical durability are promising for the applications in flexible spintronics, microwave detectors, and oscillators.     

Flexible Janus Nanoribbons Array: A New Strategy to Achieve Excellent Electrically Conductive Anisotropy,Magnetism, and Photoluminescence

A new type of flexible Janus nanoribbons array with anisotropic electrical conductivity, magnetism, and photoluminescence has been successfully fabricated by electrospinning technology using a specially designed parallel spinneret. Every single Janus nanoribbon in the array consists of a half side of Fe₃O₄ nanoparticles/polyaniline/polymethylmethacrylate (PMMA) conductive‐magnetic bifunctionality and the other half side of Tb(BA)₃phen/PMMA insulative‐photoluminescent characteristics, and all the Janus nanoribbons are aligned to form array. Owing to the unique nanostructure, the conductance along with the length direction of nanoribbons reaches up to eight orders of magnitude higher than that along with perpendicular direction, which is by far the most excellent conductive anisotropy for anisotropic conductive materials. The Janus nanoribbons array is also simultaneously endowed with magnetic and photoluminescent characteristics. The obtained Janus nanoribbons array will have important applications in the future subminiature electronic equipments owing to its high electrical anisotropy and multifunctionality. Furthermore, the design concept and fabrication technique for the flexible Janus nanoribbons array provide a new and facile approach for the preparation of anisotropic conductive films with multifunctionality.     

Toward Flexible Spintronics: Perpendicularly Magnetized Synthetic Antiferromagnetic Thin Films and Nanowires on Polyimide Substrates

The successful fabrication of ultra‐thin films of CoFeB/Pt with strong perpendicular magnetic anisotropy and antiferromagnetic interfacial interlayer coupling on flexible polyimide substrates is demonstrated. Despite an increased surface roughness and defect density on the polyimide substrate, magnetic single layers of CoFeB still show sharp coercive switching. Magnetic Kerr imaging shows that the magnetization reversal is dominated by a greater density of nucleation sites than the identical film grown on Si. These layers maintain their magnetic characteristics down to a radius of curvature of 350 ± µm. Further, antiferromagnetically (AF) Ruderman‐Kittel‐Kasuya‐Yoshida (RKKY) coupled bilayers of CoFeB were fabricated which are robust under bending and the coupling strength is successfully modulated via interlayer engineering. Finally, a perpendicular synthetic antiferromagnetic (SAF) thin film grown on a polyimide substrate is patterned into straight 10 µm long nanowires down to 210 nm in width that displayed the robust switching characteristics of the thin film. These are extremely promising results for the fabrication of robust, flexible, magneto‐electronic, non‐volatile memory, logic, and sensor devices.