Flexible Spintronics Research Articles

Enormous and Tunable Bulk Charge/Spin Photovoltaic Effect in Piezoelectric Binary Materials T-IV-VI and T-V-V.

Understanding the nonlinear response of light and materials is crucial for fundamental physics and next-generation electronic devices. In this work, we have investigated the second-order nonlinear bulk photovoltaic (BPV) and bulk spin photovoltaic (BSPV) effects in the piezoelectric binary materials T-IV-VI and T-V-V (IV = Ge, Sn; VI = S, Se; and V = P, As, Sb, Bi). The independent nonzero conductivity tensors of charge current are derived for these binaries through the symmetry analysis, along with the mechanism for generating pure spin current. These binaries, with their unique folded structure, exhibit significant charge and spin currents under illumination. Furthermore, we find that strain engineering can effectively modulate charge/spin currents by influencing charge density distribution and built-in electric field due to the piezoelectric effect. Our research suggests that the piezoelectric binary materials possess enormous and tunable charge/spin currents, underscoring their potential for applications in nonlinear flexible optoelectronics and spintronics.

Aged‐Precursor‐Assisted Growth of Ferrimagnetic 2D Cr9Se13 with Anomalous Elasticity

Abstract2D magnetic materials with distinct mechanical properties are of great importance for flexible spintronics. However, synthesizing 2D magnets with atomic thickness is challenging and their mechanical properties remain largely unexplored. Here, the growth of a ferrimagnetic 2D Cr9Se13 with anomalous elasticity is reported by an aged‐precursor‐assisted method. The obtained 2D Cr9Se13 exhibits an out‐of‐plane ferrimagnetic order with a coercivity larger than those of conventional magnetic materials. Noteworthy, it presents decent breaking strength and a Young's modulus of 52 ± 8 GPa that is among the smallest of the 2D family. This exceptional elasticity is attributed to the unique internal voids in Cr9Se13, as evidenced by the formed edge dislocations under strain. This work not only offers a facile method to synthesize 2D magnets but also develops avenues for obtaining 2D materials with desired mechanical properties, paving the way for future flexible spintronics.

Observation of room-temperature ferromagnetism in copper-based graphene induced by stress engineering

Two-dimensional (2D) room-temperature ferromagnetism is highly sought after for its great application potential but is challenged by chemical or magnetic structural instability. If room-temperature ferromagnetism can be realized in structurally stable 2D graphene, it will have a broad application prospect and market in flexible spintronics and wearable artificial intelligence. Although symmetry breaking such as vacancies, edges can introduce localized magnetic moments in intrinsically antimagnetic graphene, their weak coupling can only hold long-range ordering at low temperatures. Here, a robust ferromagnetic order has been observed in large-area graphene (centimeter scale) through mechanical folding and successive preprocesses of rapid cooling and heating method, where the Curie temperature of ∼6-layer graphene is close to 100 K and that of ∼10-layer graphene is above room temperature. We propose that the observed ferromagnetism is possibly attributed to the surface sp3-type bonds emergent in the structure phase transition caused by strain effects.

Heat-Assisted Magnetization Switching in Flexible Spin-Orbit Torque Devices.

Heat-assisted magnetic anisotropy engineering has been successfully used in selective magnetic writing and microwave amplification due to a large interfacial thermal resistance between the MgO barrier and the adjacent ferromagnetic layers. However, in spin-orbit torque devices, the writing current does not flow through the tunnel barrier, resulting in a negligible heating effect due to efficient heat dissipation. Here, we report a dramatically reduced switching current density of ∼2.59 MA/cm2 in flexible spin-orbit torque heterostructures, indicating a 98% decrease in writing energy consumption compared with that on a silicon substrate. The reduced driving current density is enabled by the dramatically decreased magnetic anisotropy due to Joule dissipation and the lower thermal conductivity of the flexible substrate. The large magnetic anisotropy could be fully recovered after the impulse, indicating retained high stability. These results pave the way for flexible spintronics with the otherwise incompatible advantages of low power consumption and high stability.

Solar-Powered Switch of Antiferromagnetism/Ferromagnetism in Flexible Spintronics.

The flexible electronics have application prospects in many fields, including as wearable devices and in structural detection. Spintronics possess the merits of a fast response and high integration density, opening up possibilities for various applications. However, the integration of miniaturization on flexible substrates is impeded inevitably due to the high Joule heat from high current density (1012 A/m2). In this study, a prototype flexible spintronic with device antiferromagnetic/ferromagnetic heterojunctions is proposed. The interlayer coupling strength can be obviously altered by sunlight soaking via direct photo-induced electron doping. With the assistance of a small magnetic field (±125 Oe), the almost 180° flip of magnetization is realized. Furthermore, the magnetoresistance changes (15~29%) of flexible spintronics on fingers receiving light illumination are achieved successfully, exhibiting the wearable application potential. Our findings develop flexible spintronic sensors, expanding the vision for the novel generation of photovoltaic/spintronic devices.

Strain-tuned full spin polarization and ndoping of phosphorene via the phosphorene/Co[formula omitted]Sn[formula omitted]S[formula omitted] heterojunction

Phosphorene’s desired bandgap, high carrier mobility and weak spin–orbit coupling qualify it as an exceptional semiconductor spin channel material. These traits underscore the importance of examining phosphorene’s electronic structure and that of half-metal heterojunctions (HJs) in innovating spintronic device design. This paper methodically investigates the electronic structure and Schottky barrier (SB) of a van der Waals HJ, comprising phosphorene and the half-metal Co3Sn3S2, subjected to biaxial mechanical strain via a first-principles method. The investigation unveils that phosphorene, as an n-type semiconductor, possesses substantial spin polarization. Remarkably, phosphorene undergoes three states – metallic, half-metallic, and semiconducting – subject to varying strain ratios (E). Furthermore, we discern that phosphorene’s spin polarization, peaking at 100% for −2%<E<1%, is tunable with strain. The SB heights (SBHs) also exhibit a consistent escalation with strain: for −5%≤E<1%, the electron SBH is 0 eV, which increments from 0.017 eV to 0.021 eV for 1%≤E≤3%. Intriguingly, the spin-down band alignment of the HJ transitions from type-II to type-I at E= 3%. Our Bader charge analysis validates that the n-doping of phosphorene is primarily due to electron acceptance from Co3Sn3S2. The orbital hybridizations between phosphorene and Co3Sn3S2 leads to complete spin polarization of phosphorene. Collectively, these observations confirm phosphorene’s capability to attain n-type doping with 100% spin polarization. Moreover, our findings suggest the possibility to modulate both the bandgap of phosphorene and the SBHs of the HJ with strain. This highlights the potential of the phosphorene/Co3Sn3S2 HJ in advancing the field of low-dimensional, flexible spintronics.

Self-Assembled La0.67Sr0.33MnO3:CeO2 Vertically Aligned Nanocomposite Thin Films on Flexible Mica

Vertically aligned nanocomposite (VAN) thin film has attracted tremendous research interests owing to its multifunctionality, enhanced physical properties and multi-field coupling. However, VAN has rarely been demonstrated in flexible form, which hinders its further application in flexible devices. In this work, La0.67Sr0.33MnO3-CeO2 (LC) VAN film has been deposited on flexible mica with or without a buffer layer. The LC nanocomposite films show high quality following textured growth and form a typical, vertically aligned nanostructure. Magnetic, transport and magnetoresistance properties have been explored for flexible nanocomposite thin films. Furthermore, flexible LC films maintain their properties after numerous mechanical bending tests, presenting promising future applications in flexible electronics and spintronics.

Enhanced spin–orbit torques in strained NiFe/Pt bi-layers on flexible substrate

Magnetization manipulation caused by spin–orbit torque is one of the central themes investigated in spintronics domain. The spin–orbit torque efficiencies can be manoeuvred by application of mechanical strain. In this direction, this study furthers the effort in understanding the evolution of spin–orbit torque efficiencies in mechanically strained ferromagnet (FM)/heavy metal(HM) (Ni81Fe19/Pt) bi-layer films on flexible kapton substrate, by the use of spin torque ferromagnetic resonance (ST-FMR) measurement technique. We report a tensile strain induced enhancement in damping-like and field-like spin–orbit torque efficiencies. The effective damping-like spin torque conductivity increased by 42% on the application of 0.312% tensile strain, which is encouraging from the magnetization switching application point of view. In order to investigate the driving factors behind the enhanced damping-like spin torque conductivity, ab-initio calculations were also carried out. Our findings provide insights into the workings of strain in FM/HM bi-layer stack and is a step forward in the implementation of flexible spintronics devices.

Co/Pd-based spin-valves with perpendicular magnetic anisotropy on flexible substrates. Direct deposition vs transfer-and-bonding approaches

Flexible spintronics is an emerging field of research that has received increasing attention due to the additional functionalities that are allowed (lightweight, flexibility, shape-ability, wearability) with respect to conventional rigid systems. In this work, different strategies for the fabrication of flexible spintronic devices with perpendicular magnetic anisotropy are compared, i.e., transfer-and-bonding approaches exploiting wet and dry lift-off methods, and direct deposition on flexible substrates. To evaluate the potential of the proposed strategies, Co/Pd-based giant magneto-resistive spin-valves including a synthetic antiferromagnet reference electrode were investigated. Such stacks represent a demanding model system, owing to the large number of interfaces whose quality strongly affects the overall magnetic and electric performances. The advantages and drawbacks of the different strategies are discussed to provide crucial indications for the development of flexible spintronic devices of any complexity. Based on the results, the most suitable option for achieving high-quality heterostructures on large area surfaces via direct deposition is using polyethylene naphthalate (Teonex®) tapes, provided that the processing and operating temperatures are relatively low (<525 K). On the other hand, if the process requires higher temperatures, the dry lift-off method exploiting the low adhesion between an Au underlayer and the SiOx/Si(100) substrate is the preferred alternative.

Current-induced spin polarization in Janus WSSe monolayer

Janus engineering of transition metal dichalcogenide monolayers breaks the vertical mirror symmetry and induces huge built-in polarization and large Rashba spin-orbit coupling. By performing first-principles calculations, we find that the Rashba spin-orbit coupling induces sizable current-induced spin polarization in both the conduction and valence bands in the Janus WSSe monolayer, which is comparable to that in perovskite oxide interfaces. By constructing the low-energy $\mathbf{k}\ifmmode\cdot\else\textperiodcentered\fi{}\mathbf{p}$ Hamiltonian from the invariant theory, we find a sign change of the current-spin conductivity at chemical potential $\ensuremath{\mu}\ensuremath{\approx}\ensuremath{-}0.73$ eV, which indicates the flip of spin polarization and is attributed to the competition between the intrinsic Rashba spin-orbit coupling and the hexagonal warping of the valence bands. Our numerical and analytic results suggest that the current-induced spin polarization in the Janus WSSe monolayer is insensitive to temperature and impurity concentration but can be effectively tuned by external biaxial or uniaxial strain. The Janus WSSe monolayer could be a promising candidate to realize flexible two-dimensional spintronics devices.

Extreme Enhanced Curie Temperature and Perpendicular Exchange Bias in Freestanding Ferromagnetic Superlattices.

Most recently, the freestanding of an epitaxial single-crystal oxide has been greatly developed to its fundamental concerns and the possibility of integration with metal, two-dimensional, and organic materials for more promising functionalities. In an artificial ferromagnetic oxide heterostructure and superlattice, the release of the substrate constraint can induce a reasonable transformation of the magnetic structure because the change of the lattice field occurs. In this study, we have comprehensively investigated the evolution of magnetic properties of (La0.7Ca0.3MnO3/SrRuO3)n [(LCMO/SRO)n] ferromagnetic superlattices while they are epitaxially on SrTiO3 and freestanding. It is found that the Curie temperature and the perpendicular exchange bias of the freestanding superlattices exhibit extreme sensitivity to the interface number and the thickness of LCMO and SRO, which can maximumly reach ∼293 K and ∼1150 Oe. These enhanced and bulk-beyond magnetic behaviors originate from the interfacial magnetic transition from ferromagnetic to antiferromagnetic via the charge reconstruction with the assistance of strain. Our study provides not only a reference for designing a high-performance flexible ferromagnetic architectural superlattice but also a deep understanding of the interfacial effect in freestanding ferromagnetic heterostructures benefiting flexible spintronics.

Modulating Exchange Bias, Anisotropic Magnetoresistance, and Planar Hall Resistance of Flexible Co/MnN Epitaxial Bilayers on Mica by Bending Strain.

The integration of ferromagnetic/antiferromagnetic bilayers with exchange bias effect on flexible substrates is crucial for flexible spintronics. Here, the epitaxial Co/MnN bilayers are deposited on mica by facing-target sputtering. A large in-plane exchange bias field (HEB) of 1800 Oe with a coercive field (HC) of 2750 Oe appears in the Co (3.8 nm)/MnN (15.0 nm) bilayer at 5 K after field cooling from 300 to 5 K. Effective interfacial exchange energy Jeff of the Co/MnN bilayer is 0.83 erg/cm2. The strain-induced maximum increase of HEB and HC reaches 18% and 21%, respectively, in the Co(3.8 nm)/MnN(15.0 nm) bilayer. Strain-modulated HEB is attributed to the change of interfacial exchange coupling between Co and MnN layers. HEB is inversely proportional to Co thickness but independent of MnN thickness. The change of HEB is less than 5% after 100 bending cycles, indicating mechanical durability. The out-of-plane exchange bias also appears since Co spins are not fully reversed due to the strong pinning effect. Anisotropic magnetoresistance (AMR) and planar Hall resistance (Rxy) show obvious hysteresis due to HEB. Exchange bias-induced phase difference of AMR and Rxy almost remains unchanged at different bending strains. The results provide the basis for understanding the bending strain tailored exchange bias.

Large and Robust Exchange Bias in Co/CoO film: Implication for Flexible Spintronics

AbstractDeveloping a robust and flexible exchange bias (EB) system is crucial for wearable spintronic devices. In this work, large and robust EB effect is demonstrated in flexible Co/CoO thin films grown by magnetron sputtering. The obtained EB field is as large as 6172 Oe at low temperatures on a flexible substrate. Based on the atomistic spin models, the unexpectedly large EB field is explained by the decrease of ferromagnetic grain size and the increase of ferromagnetic /antiferromagnetic contact area. Furthermore, the strain‐dependent coercivity and EB effect are observed in flexible Co/CoO films. Finally, the flexible Co/CoO thin film shows no degradation (even a slight increase) of performances in all figures of merit after 500 bending cycles, indicating excellent mechanical durability. This study sheds light on Co/CoO as a robust EB material for flexible spintronics.

Bending Switch of Gigahertz La0.67Sr0.33MnO3 Thin Film Under Spin Wave Excitation

Spin waves, used for information processing and transmission, promise huge potential in the applications of the spintronic devices with low loss, minimal sizes, and high frequency. More attentions have been focused on all-magnon data processing utilizing traveling spin waves and moving domain walls. Bending control of spin wave is an effective modulation method for magnetic properties, and easily attuned to the trends of flexible magnon spintronics in future. In this work, we demonstrate the excitation of spin waves in the flexible La0.67Sr0.33MnO3 thin film by mechanical bending, which can be taken as the states for On and Off in microwave single-pole single-throw switches. Besides, the microwave magnetisms of the unbent and bending La0.67Sr0.33MnO3 thin films have been studied. The bending of La0.67Sr0.33MnO3 thin films does not change the microwave ferromagnetic resonance field. Our results demonstrate that the controllable spin wave state by mechanical bending has enormous potential for future flexible magnon spintronics.

Photoelectron-induced quantitative regulation of ferromagnetism in Permalloy at room temperature for photovoltaic flexible spintronics

Flexible spintronics has recently sparked an upsurge due to the growing demand for miniaturization, high-speed, integration and energy-saving in portable and wearable devices. However, the stress/strain during the substrate deformation process is inevitable for flexible spintronic devices and may be available to assist the switching of the magnetization, accordingly. Therefore, combined with the previously discovered high energy efficient sunlight controlled magnetization switching, we propose a bending-insensitive photovoltaic flexible spintronic device constructed by PET/Ta/Permalloy/(PC71BM: PTB7-Th)/Pt heterostructure. The bent device achieved a 281 Oe of maximal ferromagnetic resonance (FMR) field shift by photoelectrons in a reversible manner under the sunlight soaking at room temperature. And the magnetic change as a function of the external light radiation was precisely determined. These findings provide a feasible way to combine the flexible substrate and photoelectrons for energy-saving and precise manipulation of magnetism in bendable spintronic devices.

Quantum transport in CVD graphene synthesized with liquid carbon precursor

Large-area high-quality graphene enabled by chemical vapor deposition (CVD) can possibly pave the path for advanced flexible electronics and spintronics. CVD-grown method utilizing liquid carbon precursor has recently been demonstrated as an appealing choice for mass graphene production, thanks to its low cost and safe operation. However, the quality of the graphene film has been the major obstacle for the implementation of the liquid-precursor-based CVD method. Here we report the growth of centimeter-scale easily-transferable single-layer graphene (SLG) using acetone as a liquid carbon precursor. The dry-transfer technique was used to prepare the graphene device. The typical mobility of the dry-transferred SLG device is as high as 12 500 cm2 V−1 s−1 at room temperature. Thanks to the high quality of the device, the robust quantum Hall effect can survive up to room temperature. The excellent device quality also enables us to observe the Shubnikov–de Haas oscillation in the low magnetic field regime and systemically study the leading scattering mechanism. We extracted both the transport scattering time τ t and the quantum scattering time τ q over a wide range of carrier density. The ratio of the scattering times suggests that the charged-impurity resided near the surface of the graphene restricted the device performance.

Magnetization Reversal and Domain Structures in Perpendicular Synthetic Antiferromagnets Prepared on Rigid and Flexible Substrates

Ferromagnetic (FM) layers separated by nonmagnetic metallic spacer layers can exhibit Ruderman-Kittel-Kasuya-Yosida (RKKY) coupling, which may lead to a stable synthetic antiferromagnetic (SAF) phase. In this article, we have studied magnetization reversal by varying the number of bilayer stacks [Pt/Co] as well as thicknesses of Ir space layer t\(_{Ir}\) on rigid Si(100) and flexible polyimide substrates. The samples with t\(_{Ir}\) = 1.0 nm show a FM coupling, whereas samples with t\(_{Ir}\) = 1.5 nm show an antiferromagnetic (AFM) coupling between the FM layers. At t\(_{Ir}\) = 2.0 nm, it shows a bow-tie shaped hysteresis loop indicating a canting of magnetization at the reversal. Higher anisotropy energy compared to the interlayer exchange coupling (IEC) energy is an indication of the smaller relative angle between the magnetization of lower and upper FM layers. We have also demonstrated the strain-induced modification of IEC as well as magnetization reversal phenomena. The IEC shows a slight decrease upon application of compressive strain and increase upon application of tensile strain, which indicates the potential of SAFs in flexible spintronics.

A novel anisotropic saturation magnetization phenomenon in flexible Mn-doped BiFeO3 thin films for wearable device

Pure inorganic flexible thin film with anisotropic magnetism is crucial for high temperature flexible/wearable spintronic devices and magnetic sensors. Here, a pure inorganic flexible Mn-doped BiMn0.05Fe0.95O3 (BMFO) thin film with a novel anisotropic saturation magnetization phenomenon is demonstrated on (001)-oriented fluorophlogopite (F-Mica) single-crystalline substrates by sol–gel method. The results of XRD indicate that the intensity ratios of the (101) and (110) direction increases with the increase in thicknesses. And an obvious anisotropy behavior in magnetization was observed in the thicker flexible BMFO thin films, and the anisotropic magnetization is slowly weakened as the thickness decreases. Meanwhile, the saturation magnetization (Ms) of both in-plane and out-of-plane increases with the increase in the thickness. These results indicate that the thickness can modulate the anisotropic magnetism for the flexible BMFO thin films. The maximum value of (Ms-in-plane-Ms-out-of-plane) is obtained in the flexible BMFO sample with the thickness of 550 nm. Moreover, the real nonvolatile ferroelectric polarization with Pr = 58.22 μC/cm2 was obtained in the flexible BMFO sample with the thickness of 336 nm. These flexible BMFO thin films with anisotropic saturation magnetism, mechanical anti-fatigue characters and well ferroelectric properties are promising for the applications in high temperature flexible spintronics and magnetic sensors.

Mechanically enhanced magnetism in flexible semitransparent CuFe2O4/mica epitaxial heterostructures

Mechanical strain tuning of magnetic properties is crucial for developing flexible electronic and spintronic devices. Here, (111)-oriented CuFe2O4 (CuFO) thin films are epitaxially grown on flexible mica substrates, and the influence of the mechanical strain on magnetic properties is investigated by altering the curvature of the CuFO/mica structures. Both the in-plane and out-of-plane saturation magnetic moments increase monotonously with the decrease of bending radius, which can be ascribed to the mechanical strain-induced Jahn-Teller distortion and much easier neighboring ferromagnetic domain switching in the CuFO films. Particularly, the residual magnetization can be dramatically enhanced by mechanical strain with a large gauge factor of 321.25, demonstrating effective strain tuning of magnetism. Moreover, the CuFO/mica structures exhibit strain tunable parallel magnetic anisotropy and excellent mechanical antifatigue properties. This work implies a great potential for promissing applications in flexible spintronics and electronics.

Flexible Multiferroic Heterostructure Based on Freestanding Single‐Crystalline BaTiO3 Membranes for Spintronic Devices

AbstractOne of the key concepts in flexible/freestanding spintronics is the effective control of ultra‐thin film magnetism, and at the same time assures that the functional properties will not be compromised when gating methods are applied. Traditionally, strain application is through magnetoelectric effect while using the gate voltage to transfer the strain to the magnetism layer, which can lead the noise in the heterostructure. On the other hand, the knowledge of freestanding magnetism changes largely lacks quantitative analysis. Coupling the freestanding characteristic such as large strain with important magnetic properties can further expand the freestanding spintronics storage area. In this work, the high‐quality freestanding multiferroic heterostructures of SrTiO3(001)/Sr3Al2O6/BaTiO3/Ta/(Co/Pt)5 with different Co thicknesses have been successfully prepared with both pulsed laser deposition and magnetism sputtering processes. The ultra‐flexible freestanding structure shows a strong strain gating effect with bending, that the ferromagnetic resonance (FMR) shifts around 440 Oe at room temperature with tensile strain. Upon decreasing the temperature to BaTiO3 phase transition range from T‐phase to O‐phase and O‐phase to R‐phase, respectively, a sharp FMR shift can be observed for both in‐plane and out‐of‐plane measurements. This work suggests that the tunable flexible spintronics are achievable through freestanding mechanisms and shows the promising future of freestanding spintronics.