Reliable Spin Valves of Conjugated Polymer Based on Mechanically

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Reliable Spin Valves of Conjugated Polymer Based on Mechanically Transferrable Top Electrodes Shuaishuai Ding, Yuan Tian, Hanlin Wang, Zhang Zhou, Wenbo Mi, Zhenjie Ni, Ye Zou, Huanli Dong, Hongjun Gao, Daoben Zhu, and Wenping Hu ACS Nano, Just Accepted Manuscript • Publication Date (Web): 09 Nov 2018 Downloaded from http://pubs.acs.org on November 9, 2018

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Reliable

Spin

Valves

of

Conjugated

Polymer

Based

on

Mechanically

Transferrable Top Electrodes

Shuaishuai Ding,†, § Yuan Tian, †,* Hanlin Wang, †, Zhang Zhou, £,§ Wenbo Mi,⊥ Zhenjie Ni, †, Ye Zou,†, Huanli Dong,†, Hongjun Gao,£ Daoben Zhu,†, and Wenping Hu†,‡,* †Beijing

National Laboratory for Molecular Sciences, Key Laboratory of Organic Solids, Institute of

Chemistry, Chinese Academy of Sciences, Beijing 100190, China ‡Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Sciences, Tianjin University, Tianjin 300072, China £Beijing

National Laboratory for Condensed Matter Physics and Institute of Physics, Chinese Academy of

Sciences, Beijing 100190, China ⊥ Tianjin

Key Laboratory of Low Dimensional Materials Physics and Preparation Technology, School of

Science, Tianjin University, Tianjin 300354, China §University

of Chinese Academy of Sciences, Beijing 100049, China

KEYWORDS: organic spin valves, polymer spintronics, organic spintronics integration, metal penetration, spin-interface.

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ABSTRACT: Organic spintronic devices present one of the most appealing technologies for future spintronic devices by taking advantage of the spin degree of freedom. Conjugated polymers are attractive to the exemplified device of organic spin valves (OSVs) due to their weak spin-orbit coupling, solutionprocessability, low production cost and mechanical flexibility. However, the performance of polymer SVs are a matter of debate as the evaporated top ferromagnetic (FM) electrode will penetrate into the organic layer during typical fabrication process, especially in the device with organic layer thickness of nanometers. It will cause a severe problem in controllable and reproducible spin-manipulations not to mention the clarification of spin-dependent transport mechanism. Here, a universal, simple, and low-cost method based on transferred electrode is developed for polymer spin valve (SV) with stable-and-reliable-state operation. It is demonstrated in an OSV device with vertical structure of La2/3Sr1/3MnO3 (LSMO)/P3HT/AlOx/Co/Au that this approach not only builds a damage-free interface between magnetic electrodes and organic spacer layer, but also can be generalized for other devices with delicate active layers. Furthermore, a multi-state writing and reading prototype is achieved on the premise of robust and quick magnetic response. The results reveal the importance of spinterface and effective thickness of organic layer in fundamental spintronic research and may lead to the strong potential in future flexible, large-area, and robust organic multifunctional circuits.


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Spintronics is a principle that exploits the magnetic nature of electrons and manipulates electronics with the spin degree of freedom, which shows great potential in non-volatile data storage and quantum computing.1-3 As a principal spintronics prototype of hard-disk drive read heads, spin-valve (SV) has been highly attractive and hold the premise in the next generation flexible- and opto-electronics.4 It is a sandwiched structure composed of two soft FM electrodes with different coercivity decoupled by a nonmagnetic interlayer.5,6 Defined as MR=(Rap-Rp)/ Rp,7,8 magnetoresistance (MR) is a relative change in electrical resistance of SV caused by different spin-dependent scatterings respect to the relative alignment of magnetization (parallel or antiparallel) in FM electrodes when external magnetic field sweeps. A considerable variation can be detected only if the spin information can be reserved during the spin transport process in nonmagnetic layer. The introducing of organic materials into the field of spintronics is highly preferable due to their weak spinscatterings induced by low spin-orbit coupling.8,9 Especially, the detected spin relaxation time of organics (in the range of μs)10 is much longer than that of their inorganic counterparts by orders of magnitude.2,11 Moreover, π-conjugated polymers benefit from abundance of supply, solution-processability, low cost, lightweight, and mechanical flexibility,12 serving as good candidates in large scale optoelectronic applications.13 Thus, the comprehensive investigations of π-conjugated polymers based OSVs will not only provide some insights into the fundamental spintronics research but also boost the development for the multifunctional integration of organic circuits manipulated by light, electric, and magnetic field. However, delicate organic materials often suffer from the unexpected performance of MR inversion since the first vertical OSV device achieved by Xiong et al.14 in 2004. Even in the same device configuration with identical fabrication procedures, both positive and negative MR could be observed.15,16 Formation of spinhybridization-induced polarized states17 between the first molecular layer and the top electrode interface11,18 is considered as one of the most probable reason. Both the metal-organics contact and metal penetration in organic materials could lead to a spin dependent hybrid-interface-state, while the latter is dominant and uncontrollable in most experiments,15,16 which impair spin injection and transport in organic layers.19,20 In some cases, a proper formation of interface state is beneficial for spin injection. However, it is quite different 3 ACS Paragon Plus Environment

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for individual device since the damage to organic layer caused by metal penetration during direct evaporation is a random process, let alone for the irreversibility and severe decay in performance for fragile organic materials.21,22 The magnetic atoms induced by direct evaporation can even cause pinholes and inclusion in the organic layer over a distance of ~100 nm,23 in which the effective thickness of organic layer can’t be clearly defined and the contributions of such dead layer and pure organic materials in spintronics are hard to distinguish. This uncontrollable interface state and irreversible damages to organic layer caused by metal penetration will not only make the spin mechanism research more complex and debatable, but also limit the practical application of organic spin valves due to poor quality and performance control. Therefore, the key point towards a reliable and stable OSV device is to obtain a metal-penetration-free interface with little damage to the organics. To date, it is still difficult to suppress the top metallic electrode penetration, the damage involved by ‘hot’ atoms to spinterface and organic layer during the traditional evaporation process16 in OSV fabrication. Insertion of thin tunneling barrier (e.g. AlOx) between organic layer and electrodes seems to be an efficient way to reduce such metal penetration,23-25 however, the thickness of AlOx is hard to control. Thick AlOx might hinder the effective spin transport7 while thin AlOx can’t fully prevent metal penetration into fragile organic layers. Another strategy is using a low-temperature fabrication method by cooling the substrate with liquid N2 during the deposition while special experiences and high costs in high-vacuum equipment reformation are needed.9 An indirect deposition method with assistance of inert gas is not an option for the same consideration.26 A successful approach named buffer layer assisted growth (BLAG) with top Co electrode exhibits an extraordinary MR ratio of up to ~300%. Nevertheless, even such a skillful method is incapable to hinder the ill-defined layer completely when the organic layer thickness is lower than 23 nm.27 All these existing methods are either expensive and exclusive with high threshold of technique or hard to completely prevent the metal penetration, limiting their practical application for most laboratories and further industrial integration. Hence, for majority researchers, a universal, low-cost, and straightforward method that eliminates interfacial diffusion is desired for the achievement of reliable and stable OSV device. 4 ACS Paragon Plus Environment

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Herein we report a general approach for architecture of top FM electrodes in polymer SV devices to a stable-and-reliable-state operation. In our method, the mechanically transferrable top electrode is thermally deposited on a water-solvable polymer poly(sodium 4-styrenesulfonate) (PSSNa), peeled off and transferred onto the target polymer/bottom electrode, forming a mechanical contact at the top interface. This damagefree strategy to metal penetration justified in both organic28 and inorganic electronics29 before, has advantages of generality, practicability, low manufacturing cost and simple technology in organic spintronics device fabrication comparing to methods mentioned above. Optical microscope image, X-ray photoelectron spectroscopy (XPS) and M-H characterization show a well-preserved quality of as-transferred FM electrode, which is applicable to the OSV device. As a noted classical material with better compability in solutionprocess, P3HT is taken in a traditional vertical spin valve structure as an example to manifest our method is capable to obtain real MR signals output stably and reliably with a clear evaluation of the effective thickness in organic layer. A multi-state writing and reading prototype is further achieved on the premise of such robust and quick magnetic response. Our research provides an advanced strategy to improve OSV device reliability hindered by the sensitive spinterface and thermal damages to organics due to metal penetration. It also offers a potential advance to solution accessible OSV units in mass production, benefiting the future application in large-scale of organic integrated circuits as well. Results and Discussion The device fabrication steps are sketched in Figure 1a, b. Briefly, 1 nm Al followed by immediate oxidation, 10 nm Co, and 60 nm Au were subsequently evaporated on the spin-coated water-soluble poly (4styrene sulfonate) (PSS) layer. Then a polystyrene (PS) membrane was spin-coated onto the top electrodes (60 nm Au/10 nm Co/ ~1 nm AlOx) to facilitate the transfer process and encapsulate the device. By a poly(dimethylsiloxane) (PDMS) assisted transfer technique, it is easy to completely peel-off the top electrode in water and transfer it onto the target P3HT/LSMO substrate. Detailed fabrication methods were shown in the Experimental Section. Once batch fabrication of evaporated top electrodes is done, such a simple transfer 5 ACS Paragon Plus Environment

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procedure could be finished within 15 minutes (including baking time) even in the laboratory with only the most basic and fundamental equipment, i.e., a spin coater and a hot-stage. As schematically illustrated in Figure 1c, the as-prepared polymer SV has a typical vertical cross-bar structure with the following layers: PS capping layer/Au (60 nm)/Co (10 nm)/AlOx (~1nm)/P3HT (~20 nm)/LSMO (100 nm). LSMO with high spin polarization and stability to oxygen/water14 serves as source while Co as detector of the spin-polarized transport in the P3HT spacer (Figure S1, Supporting Information). A semi-oxidized AlOx is inserted to physically prevent the direct contact with the chemically active Co. Optical microscope images in Figure 1dg declare the completeness and flatness of the top electrodes before and after transfer process, partly conforming the validity of the method since the magnetic properties will be substantially reduced in wrinkled electrodes. To study the influence of fabrication method and transfer procedure in top FM electrode quality, a detailed investigation about of Co electrodes is shown in Figure 2. The magnetization of all three kinds of electrodes, including AlOx/thermal evaporated (TE) Co, AlOx/e-beam evaporated (EB) Co, and e-beam evaporated Co without AlOx, have declined to some extent after peel-off and transfer procedure (Figure S2, Supporting Information). The quality of thermal evaporated Co is worse than those of e-beam evaporated ones in both before and after transfer, indicating a looser structure in TE electrode, which could be attributed to the less energy and heat accumulation for atoms reconstruction during the thermal evaporation. In the presence of AlOx the magnetic properties of e-beam evaporated Co is preserved as much as possible during the transfer procedure (Figure 2a-b), which is crucial for the spin detection. XPS spectra for different layer surfaces during transfer process are depicted in Figure S3, Supporting Information. No peaks were detected in inverted AlOx/Co/Au/PS membrane in Na 1s spectra (Figure 2c) compared to the standard PSS sample, indicating minimal PSS residual during the peel-off process. The remove of PSS residual is necessary so that we can detect the nature of organic layer itself instead of a spin transport in organic layer/PSS buffer layer heterostructure. In Al 2p spectra, the peak position of inverted AlOx/Co sample shifts toward lower bindingenergy than fully oxidized Al evaporated on Si substrate (Figure 2d). This result points out that the 6 ACS Paragon Plus Environment

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transferred thin AlOx layer probably deviates from the standard Al2O3,30 indicating an actually electrically leaky state with transport properties close to bad metal. Such leaky AlOx barrier is crucial for achieving effective spin transport in vertical OSVs.7 To demonstrate the gentle, ‘low-energy’ materials integration, we mechanically peeled off the transferred metal electrodes from P3HT interface after the device fabrication. Elemental mapping based on energy-dispersive x-ray spectroscopy (EDS) (Figure S4, Supporting Information) confirmed that the mechanical contact without direct chemical bonding of Co and P3HT molecule can highly suppress the spin dependent hybrid-interface-state induced by metal penetration, since most of Co could be removed after peeling off the electrodes (Figure 2e). Compared to those directly evaporated approach, we provide a gentle method to form a mechanically-formed electric contact since there is no extra energy to impel the metal atoms to defuse into the organic layer like the evaporation one. Theoretically, it is compatible to any thermally fragile organic materials that would be easily damaged by aggressive fabrication in tradition procedure, dramatically expanding the range of organic spintronic research in materials. The optical microscopy images (Figure 1d-g), M-H characterizations (Figure 2a-b), XPS spectrum (Figure 2c-d) and EDS mapping (Figure 2e) together testify the protection of AlOx and the quality of transferred magnetic electrodes, which is the foundation to obtain reliable and stable MR signal. Figure 3a displays device resistance as the temperature dropped from 300 K to 2 K for the 20nm-thick P3HT device (Figure S1, Supporting Information) at current I = 0.01 μA. The stability and reliability are manifested by comparing the R-T curves with same experimental settings fabricated in different batches (Figure S5). Such a uniform resistance is hard to achieve in traditional evaporated samples even in the same batch.15,16 In order to explore the current transport mode in P3HT-based SVs, the differential conductance (dI-dV) characteristics of the OSV under different temperatures were recorded (Figure 3b). The dI-dV curves are almost temperature independent and nearly parabolic, indicating the multi-step-tunneling31 dominated conducting mode at low temperature zone, in which the spin-polarized carriers transport via the intermediate states inside the HOMO-LUMO gap. This tendency is consistent with the resistance changes shown in Figure 3a, considering the P3HT layer is thin enough to form a conducting mode by multi-step tunneling. Note that 7 ACS Paragon Plus Environment

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the absence of zero-bias dip in dI-dV curve is a proof of well-defined interface without magnetic impurities or pinholes between the transferred electrodes and P3HT layer.32 Figure 3c represents the typical traces of resistance versus magnetic field of the OSV with 20 nm P3HT measured at T = 2 K. Both the square shape and the coercive force is consistent with the M-H loop shown in Figure 2b. The rapid response and steep change at both parallel and antiparallel state suggest that the device is reliable and well stabilized, which were preserved at different measurement conditions (Figure S6, Supporting Information). It should be pointed out that the junction resistance of all the fabricated OSV device is about two or three orders of magnitude higher than that of LSMO/AlOx/Co/Au32 without P3HT layer, indicating the MR mainly contributed from organic semiconductor material. The influence of anisotropic magnetoresistance (AMR) effect of LSMO and Co FM electrodes can be ruled out considering their relative low resistance value.12 The small step at low coercive fields around antiparallel state might possibly be induced by magnetic coupling between FM electrodes4 at such thin layer of 20 nm P3HT. The transferred TE Co could also be applicable for such OSV device (Figure S7, Supporting Information) with less stability and quality control. Figure 3d summarizes the MR ratios as functions of applied current and temperature, which is consistent with previously reported data.33 Such a steep decrease of MR is partly a consequence of weakened surface spin polarization of the LSMO electrode34 with increased temperature. Our primary goal here is to introduce a low-cost, simple and universal method to get reliable and stable MR signals so that many kinds of organic materials could be involved, while room temperature MR can also be expected with carefully optimized LSMO electrodes.35 The reliable OSV device with rapid magnetic response is intrinsically related to the superior fabrication method. In order to gain more insight on the importance of the clear interface and the effective thickness, three typical MR curves with different interfaces between FM electrodes and P3HT layer and the effective thickness of P3HT are summarized in Figure 4. Thick polymer device with direct-evaporated Co usually exhibits MR response with triangle background curves (Figure 4a). Especially in the interval from antiparallel to parallel state there exists several intermittent spiny noises (Figure S6, Supporting Information), 8 ACS Paragon Plus Environment

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indicating an unstable magnetic response. Furthermore, the curved shape of MR suggests that either the antiparallel or parallel state is not well stabilized, most likely due to the penetrated Co conducting filament 16 in P3HT layer generated during direct thermal evaporation (Figure 4b). It is deducted that the device exhibits incoherent transport behavior dominated by variable range hopping (VRH) 36 or multiple trapping and release (MTR)

37

with sacrificing the effective thickness of the P3HT. In some areas where the layer thickness is

strongly shortened, for instance, through pinholes / filament caused by thermal irradiation, the device might display a parallel current path prone to show tunneling magnetoresistance (TMR).38 In this situation both the spin diffusion length and magnetic response performance might be overestimated in the presence of “dead layer”. Especially, the direct thermal evaporation method is not applicable to deal with thin film whose thickness is comparable to that of sacrificed “dead layer” thickness (Figure 4d). Such kind of device usually shows peculiar negative MR response (Figure 4c, Figure S9, Supporting Information) ascribed to the severely damaged interface induced by metal penetration.11,

15, 16

The greatest advantage of transfer

technology is that it avoids the issues of pinholes caused by artificial and external-induced mental filament, which means reliable and stable MR signals can be expected (Figure 4e) on any substrates and materials only if the organic semiconductor layer is continuous and flat (Figure 4f). Moreover, the spin diffusion length, another key parameter in OSV performance evaluation, can be valuated more precisely in this method since the effective transport thickness is nearly equal to the actual thickness of organic layer. External traps and defects induced by thermal damages to the organic layer can be limited with less spin and charge carriers scattering, in which MR performance will be more controllable with precise evaluation. By introducing transferred top electrodes, no pseudo-stochastic behavior is observed in the interval from the parallel to antiparallel state due to the inhibited magnetic pinning sites, which usually occurs in traditional evaporation method by atoms migration from the top electrodes. The forward bias current and resistance output were monitored by continuously alternating positive and negative magnetic fields throughout the stability test, as plotted in Figure S10, Supporting Information. On the premise of stably and reliably operated device with sharp and quick MR response, the resistance value will keep steady state at constant 9 ACS Paragon Plus Environment

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applied bias current and external magnetic field. We note that the stability is hard to reproduce in those devices with noisy and triangle MR curves (Figure 4a, Figure S8, Supporting Information) since metal filament might migrate irreversibly during the measurement. Based on the cycle stability results, a multi-state writing (W) and reading (R) operation was realized on the P3HT-based OSV device (Figure 5). When the device is initialized by a negative magnetic field (-1000 Oe), the relative change of resistance value (R1) is consistent with the change of applied magnetic field (W), as illustrated in the red area of left component, while a positive initialized magnetic field (+1000 Oe) will cause an opposite resistance response (R4). The resistance signals are significantly distinguishable under different applied bias current (Wi), and take this device for example, 6 distinct sets of resistance states (Ri) were formed. Besides, the relative change of resistance was highly symmetric at the same bias current, such success is credited with the perfect MR response curve. The tested results are mostly stable under low operating bias current and magnetic field, demonstrating its potential application in the field of information processing with low consumption.


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Conclusion In conclusion, a universal method of transferred top electrode is proposed for the fabrication of metal penetration-free vertical OSV device. Instead of MR overestimation due to the existence of the dead layer, our device fabricated by transferred electrode not only shows the reliable and pure spin related MR signal, but also can evaluate the effective thickness of organic layer more precisely without the dead layer contribution from metal filament and thermal irradiation. Based on its reliability, a multi-state writing and reading prototype was established. Compared to traditional memristor, it is operated by spin-polarized electrodes so that the magnetic field can act as a second input line, integrating both storage and processing operations within the same device. Furthermore, various potential applications in large-area operational organic spintronics can be expected due to the solution processability and simple fabrication process demonstrated in this work. Experimental Section Materials: The following materials and solvents were used as received without further purification: polystyrene (PS) from Aldrich Chemical Co.; poly(styrene sulfonic acid) sodium salt (PSS, MW 70,000) from Alfa Aesar; regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) from TCI Chemicals, 1,2Dichlorobenzene (99%, pure) and N-butyl acetate (99+%, extra pure) from Acros Organics, and poly(dimethylsiloxane) (PDMS) (Sylgand 184) from Dow Corning. Other normal chemicals were purchased from Beijing Chemical Works. Fabrication of Top Self-encapsulated Ferromagnetic Electrodes: N-type Si wafer containing 300 nm thick SiO2 was cleaned successively with deionized water, boiled piranha solution (sulfuric acid (98%): hydrogen peroxided ≧ 3:1), deionized water and isopropanol. Then the substrate was dried under a stream of N2 gas. The PSS (30mg mL-1 in deionized water) was spin coated at 2,500 r.p.m. for 60 s to form a water-soluble layer. 1 nm Al was deposited on PSS-coated substrate by thermal evaporation at a deposition rate of about 11 ACS Paragon Plus Environment

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0.1 Å s-1. An electrically leaky AlOx barrier was obtained by subsequent oxidation in a stream of oxygen (400 sccm) for 5 min at room temperature. Co FM layers (10 nm thickness) and top Au film (60 nm thickness) were grown by e-beam evaporation (pbase = 1 × 10−7 Torr) through a shadow mask at a rate of 0.3 Å s-1. For the fabrication of support membrane during transfer process, the PS/N-butyl acetate solution (80mg mL-1) was spin-coated onto the prepared substrate at a spinning speed of 2,000 r.p.m. for 30 s, forming a soft selfencapsulated protect layer on the electrodes. Device Fabrication of Organic Spin Valve: The 500-μm-wide LSMO strip electrode with a thickness of 100 nm was fabricated by a DC facing-target magnetron sputtering method as reported before.15 It is stable in oxygen atmosphere and water, and can be repeatedly utilized after clean without apparent degradation.14 10 mg P3HT powders were dissolved in 1 mL 1,2-Dichlorobenzene and stirred at 80 °C for 30 min to form dispersive solutions. After filtration through a 0.22 μm syringe filter, 50 μL P3HT solution was spin coated onto the SrTiO3/LSMO substrate at 5,000 r.p.m. for 60 s and subsequently baked at 120 ℃ for 5 min. Finally, the top FM electrodes were transferred onto the P3HT layer by a PDMS-assisted transfer technique. PDMS-assisted Transfer Technique: The self-encapsulated top Co electrodes were mechanically scratched at the border of PS membrane by a knife to facilitate the PSS dissolution. For transfer holder fabrication, hollow PDMS molds were prepared by mixing base and curing agent in a 10:1 weight ratio and baking at 70 ℃ overnight. The PDMS transfer holder was attached onto the PS membrane and then the entire substrate was put in water. Once the PSS was dissolved and the membrane was floated on water surface, the transfer process was done by moving the target electrodes onto the organic semiconductor layer with the assistance of PDMS holder and followed by a baking step at 70 ℃ for 10 min. Next the PDMS mold was removed by cutting the edge of PS membrane in the hollow area and then peeling it off carefully from the substrate with tweezers. Characterizations: Optical microscope images were taken with Olympus BX51 optical microscope at 10x. Surface topographies were imaged using a Veeco atomic force microscope operating in tapping mode. 12 ACS Paragon Plus Environment

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Elemental distribution was conducted using a scanning electron microscope-energy dispersive spectrometer (SEM-EDS, SU8010, Hitachi, Japan) combination. The hysteresis loop measurements of FM electrodes were performed using a vibrating sample magnetometer (Quantum Design, PPMS). The XPS-spectra were acquired using an Axis Ultra spectrometer (Kratos Analytical, UK) with a monochromatic Al Kα source. All the electrical characteristics were carried out with standard four-probe method in Quantum Design PPMS with a closed-cycle helium cryostat. The wire-bonding and measurement procedure were reported elsewhere in detail.15


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FIGURES

Figure 1. Organic spin valve fabrication and structure. a-b) Fabrication process of organic spin valve. Top ferromagnetic electrodes are transferred onto the polymer surface by a sacrificial water-soluble layer. c) Cross-sectional schematic diagram of the device structure and molecular structure used in experiments. d) Evaporated top electrodes onto spin-coated PSS. e) Self-encapsulated electrodes by PS supporting film. f) Bottom electrodes LSMO and spin-coated P3HT. g) Top view of the spin valve device fabricated by a transferred top ferromagnetic electrode. All of the fabricated devices have a fixed junction area of 500 × 500 μm2. Scale bar: 100 μm.


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Figure 2. Characterization of top Co electrodes. Magnetic hysteresis loops for top ferromagnetic electrodes a) before and b) after transfer, fabricated by e-beam evaporation with AlOx and measured at T = 2 K, respectively. c) Na 1s XPS spectra for different layers during transfer process. d) Comparison of Al 2p XPS spectra between inverted top electrodes after transfer and fully oxidized AlOx on Si substrate. e) EDS mapping of Co element. The electrodes were transferred as described in experimental section, then partial of the electrodes were removed by peeling off the PDMS stamps.


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Figure 3. Electrical and magnetotransport measurement in LSMO/P3HT/AlOx/Co/Au spin valve. a) Resistance characteristics with temperature ranged from 300 K to 2 K at a constant current I = 0.01 μA. b) dIdV curves for the device at different temperatures. It shows parabolic curves near zero-bias. c) Typical square MR response curve measured at T = 2 K, I = 0.15 μA. The black line and red line represent the MR value while the magnetic field is swept from positive to negative and back from negative to positive, respectively. d) Temperature dependence of MR at I = 0.01 μA and current dependence of MR at T = 2 K.


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Figure 4. Schematic illustration of metal penetration influence in MR response. Typical MR response and related interface between polymer and top ferromagnetic electrodes of a-b) thick polymer with directevaporated Co, c-d) thin polymer with direct-evaporated Co, and e-f) thin polymer with transferred Co, respectively.


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Figure 5. Write (W)–read (R) cycles performed on LSMO/P3HT/AlOx/Co/Au device with different current at 2 K. The multi-state non-volatility was proved by the retained magnetic response sequence after the application of initial writing magnetic field. The grey area represents the initialization of magnetic field. The corresponding resistance response results (R1 to R6) recorded at different writing bias current (W1 to W6) I = 0.01 μA, 0.02 μA, and 0.03 μA are highlighted in red, green, and blue area, respectively. ASSOCIATED CONTENT The authors declare no competing financial interests. Supporting Information. The Supporting Information is available free of charge on the ACS Publications website. Figure S1−S10 (PDF)


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AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]; [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. ACKNOWLEDGMENT The authors are grateful for the financial support from the Ministry of Science and Technology of China (2016YFB0401100, 2017YFA0204503), the National Natural Science Foundation of China (51725304, 51633006, 51703159, 51733004, 91433115), National Program for Support of Top-notch Young Professionals and the Strategic Priority Research Program of Chinese Academy of Sciences (Grant No. XDB12000000). The authors acknowledge the Laboratory of Microfabrication, Institute of Physics, CAS for their assistance in electrode fabrication. REFERENCES (1)

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Table of Contents


Reliable Spin Valves of Conjugated Polymer Based on Mechanically

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