Anisotropic magnetoresistance in NiFe based polymer spin valves

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Anisotropic magnetoresistance in NiFe based polymer spin valves Shuaishuai Ding, Yuan Tian, Huanli Dong, Daoben Zhu, and Wenping Hu ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b20659 • Publication Date (Web): 10 Jan 2019 Downloaded from http://pubs.acs.org on January 12, 2019

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ACS Applied Materials & Interfaces

Anisotropic magnetoresistance in NiFe based polymer spin valves Shuaishuai Ding,

†,§

Yuan Tian,*,† Huanli Dong,† 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 § University of Chinese Academy of Sciences, Beijing 100049, China ‡ Tianjin Key Laboratory of Molecular Optoelectronic Sciences, Department of Chemistry, School of Sciences, Tianjin University, Tianjin 300072, China ABSTRACT: Precise evaluation of magnetoresistance (MR) with identified transport mechanism is one of the key points in organic spin valve (OSV) devices. To investigate the origin of spin-valve like signal in polymer spin valve with vertical structure of NiFe/P3HT/AlOx/Co, the magnetic response measurements in rotated magnetization direction were established in different well-designed device configurations.

We

magnetoresistance

identified (AMR)

and

the

significant

further

influence

induced

of

anisotropic

tunneling

anisotropic

magnetoresistance (TAMR) in NiFe electrode contributing to the total MR signal. These findings suggest that the mechanisms responsible for the transport mode in polymer spin valve device also strongly depend on the interface between the ferromagnetic (FM) electrodes

ACS Paragon Plus Environment

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ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

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and organic semiconductor layer. Even for the spin-valve resembled MR response with distinct parallel and antiparallel states, carefully control experiments such as temperature-dependent and angle-dependent measurements should always be performed to track down the injected spin-polarized carriers. Our thorough experiments and analyses may shed light on the effective MR signal evaluation in OSVs and spin related parameters with transport mechanism identification. KEYWORDS: Conjugated polymer, spintronics, spin valve, magnetoresistance, anisotropic magnetoresistance (AMR), tunneling anisotropic magnetoresistance (TAMR). INTRODUCTION As a booming interdisciplinary field, organic spintronics draw much attention from material, chemistry, and physics communities for bringing in spin degree of freedom and strong potential applications in future flexible, large-area, and robust organic multifunctional circuits modulated by light, electric, and magnetic field.1-5 Focused on the spin injection, transport and manipulations, several spintronic devices with great performance and new functionalities have been reported.4,


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Organic spin valve, which is composed of two ferromagnetic (FM) electrodes separated by a non-magnetic organic spacer, is a typical prototype for research in organic spintronics.13-17 Related to the switching of FM electrodes between parallel and antiparallel magnetization configuration, the electron spin can either tunneling over the organic layer or actually injecting and consequently transporting through it, forming the tunneling magnetoresistance (TMR) or giant magnetoresistance (GMR), respectively.18-19 However, it is still under debate whether the spin is dominantly conserved throughout the whole organic layer,20 since side magnetoresistance (MR) response induced by magnetocrystalline anisotropy with FM electrodes can cause signal mixture or crosstalk in OSVs.21 For example, the resistance of a FM electrode itself relies on the angle between the current and the magnetization direction, and this so called anisotropic magnetoresistance (AMR) effect widely exists in the FM metal system.22 In the meanwhile, a frequently overlooked but highly relevant effect of the tunneling anisotropic magnetoresistance (TAMR),23 can also cause a spin-valve-like signal by modulating the tunneling resistance of the injection contact when a suitable magnetocrystalline anisotropy is fulfilled.21, 24 These additional effects may induce an overestimation of effective spin signal and as-resulted spin related parameters (e.g. spin diffusion time and length), therefore a detailed investigation on the derivation of MR response is necessary. Here we take the classical polymer spin valve with a vertical structure of NiFe/P3HT/AlOx/Co/Au as an illustration of the importance to carefully distinguish the influence of magnetocrystalline anisotropy in FM electrodes from the real spin-valve signal. Several

well-designed

device

configurations,

including

NiFe

electrode

alone,

NiFe/AlOx/Co/Au, NiFe/P3HT/Au were introduced to investigate the influence of AMR effect, TMR effect, and TAMR effect in NiFe based polymer spin valve, respectively. A


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phenomenological model is proposed to give a full explanation of different contributions in the MR response, especially in two identical polymer spin valves with distinct conducting mode. Our investigation provides a complete analysis of MR response in polymer spintronic devices, which is crucial for the effective magnetic response evaluation. EXPERIMENTL SECTION Device Fabrication. The NiFe/P3HT/AlOx/Co vertical spin valve device with a junction area of 500 × 500 μm2 was composed of a 12-nm-thick NiFe bottom electrode, a spin-coated organic P3HT spacer, a ~1-nm-thick AlOx tunneling barrier and a 10-nm-thick Co top electrode with a 15-nm-thick Au capping layer. N-type Si wafer containing 300 nm thick SiO2 was cleaned successively with deionized water, boiled piranha solution (sulfuric acid (98%): hydrogen peroxide ≧ 3:1), deionized water and isopropanol. Then the substrate was dried under a stream of N2 gas. NiFe thin films of zero-magnetostrictive composition (81% Ni and 19% Fe by weight) were deposited through a shadow mask by a magnetron sputtering method (Kurt J Lesker Lab 18 Sputtering System) at a rate of 0.5 Å/s on the substrates. The electronic-grade regioregular P3HT used in this study was purchased from Rieke Metals, Inc. and used without further purification. 10 mg


4

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P3HT powders were dissolved in 1 mL 1,2-Dichlorobenzene and stirred at 80 °C for 2 h to form dispersive solution. After filtration through a 0.22 μm syringe filter, 50 μL P3HT solution was spin coated onto the NiFe/SiO2/Si substrate and subsequently baked at 120 ℃ for 5 min. Except for additional statement, the rotational

speed

was

set

at

3000

rpm.

The

corresponding

thickness

characterizations at different experimental settings were shown in Figure S1 and S2, respectively. Next, 1-nm Al was thermally evaporated and then oxidized in a stream of oxygen (400 sccm) for 30 min at room temperature to form an AlOxbased buffer layer. The function of AlOx is to partly resist the metal penetration into P3HT layer and further to decouple the interaction between top ferromagnetic electrode and the first organic layer. Subsequently 10 nm Co was evaporated at an extra-slow rate of less than 0.1 Å/s through a shadow mask by thermal evaporation. It is worth mentioning that to avoid the destruction of metal filament in organic layer, the initial rate was controlled at less than 0.1 Å/s for the first 1 nm Co, then the deposition rate was accelerated to 0.5 Å/s so that Co atoms at the surface could be molten by new deposited Co atoms and form immobile


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clusters before penetrating into P3HT.25 To prevent the oxidation, the entire device was finally covered with 15 nm Au at a rate of 0.5 Å/s by thermal evaporation. Other structures tested in the experiment, including NiFe ferromagnetic electrodes, NiFe/AlOx/Co tunneling junction, and NiFe/P3HT/Au TAMR device, were all fabricated by the above methods but with some related steps left out. Device Characterization and Measurement Setup. The roughness and height of NiFe ferromagnetic electrodes was characterized by the Veeco AFM in tapping mode. The hysteresis loop measurement was performed at 10 K using Physical Properties Measurement System (PPMS) -vibrating sample magnetometer (VSM) module (Quantum Design, Inc.). All the electrical characteristics were carried out by a Horizontal Rotator Option with standard four-probe method in PPMS with a closed-cycle helium cryostat. The wire-bonding procedure were reported elsewhere in detail.26 The magnetic field was always applied parallel to the substrate when the sample holder was rotated in plane under the control of PPMS. The I-V curves were carried out with a Keithley 4200 semiconductor characterization system by using the probe station at room temperature.


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RESULTS AND DISCUSSION The NiFe based polymer spin valve device under investigation has a standard vertical transport structure. Figure 1a shows the schematic representation of the sample. NiFe and Co electrodes act as the source and detector of the spin transport in the P3HT spacer separately. An ultrathin AlOx is inserted at the interface between organic layer and top electrode to prevent further metal penetration of Co atoms. An in-plane magnetic field B sweeps back and forth along the angle θ. Especially, when θ = 0° the magnetic field is applied along the easy axis of NiFe strip. In spin valve device, the resistance of the device changes with applied magnetic field of a dependence on the angle between the respective magnetization orientations of the two ferromagnetic electrodes.18 The magnetic response is commonly defined as MR, which is determined by the relative value of resistance in the antiparallel state (RAP) and the resistance in the parallel state (RP).19 When RAP > RP the MR is positive, otherwise it is negative.17, 27

Two typical magnetic response curves of NiFe/P3HT/AlOx/Co polymer spin

valves in the same fabrication procedure with magnetic field in the plane at 0° are shown in Figure 1b-c. They are symmetric with respect to the magnetic field but have opposite MR signs. The resistance of the device with positive MR (Figure 1b) is much larger than that of the one with negative MR (Figure 1c). However, it is quite unexpected that the MR sign of the


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former device changes gradually from positive to negative as the temperature increased (Figure S3) while that of the latter one remains unaltered (Figure S4). According to the classical Jullière’s model for tunneling magnetoresistance,27-28 the ideal MR sign is expected to be positive considering both of the spin polarizations at two interfaces is positive. To investigate the origin of the abnormal negative MR sign, we designed the following control experiments. We now focus on the influence of the NiFe FM electrode itself. The strip of the magnetron sputtering fabricated NiFe with dimensions of 12 nm (thickness) × 500 μm (width) × 10 mm (length) shows a flatness with root-mean-square roughness Rq = 1.24 nm for an area of 10 × 10 μm2 according to the atomic force microscope (AFM) measurement (Figure 2a). Such roughness is mainly induced by the small islands on NiFe stripe with average height ranged from 59 nm (Figure S5). Magnetic hysteresis loop of NiFe thin film with B along its easy axis measured at 10 K is shown in Figure 2b. The M-H curves exhibited standard square shape with coercive field of 40 Oe. MR curves with the magnetic field in-plane at 0°, 45°, and 90° are plotted in Figure 2c, 2d, and 2e, respectively. Spin-valve like negative signals with distinct switch can be observed when θ = 0° while positive signals is


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found when θ = 90°. In the configuration with only one FM electrode, we ascribe this phenomenon to the well-known anisotropic magnetoresistance (AMR) of NiFe, which is coincidence with the previous study.29 If the magnetization orientation is parallel to the current, then the electron scattering cross-section is increased due to spin-orbit coupling effect, and thus a high resistance is obtained at the saturation field. As the relative angle between magnetization M and electrical field E changes, the electron cloud deformation changes the size of scattering cross-section, giving rise to the different magnetic response to the saturation field. In particular, when M⊥E a symmetric positive magnetic response is observed because of the smallest scattering effect. Angle dependence of MR at the saturated magnetization state, in which MRAngle is defined as (R(θ)-RMin)/RMin, shows a strongly uniaxial symmetry (Figure 2f). Such an angular dependence is a clear signature of AMR in NiFe FM electrodes since the mean free path on the angle θ between the magnetization and the electron velocity is associated with cos2θ.22 To clarify the anisotropy effect of the conventional TMR, which the TMR ratio changes as the magnetic field rotates with respect to the crystallographic reference axis of FM electrode,30 we exam the angle-dependent magnetic response in a magnetic tunneling junction (MTJ) with vertical structure of NiFe/AlOx/Co (Figure 3). Nevertheless, unlike traditional TMR effect that the observed MR sign does not change for the two orthogonal orientations,31-32 our MTJ device shows an extremely similar angle-dependent magnetic response compared to the one of the NiFe electrode. This is reasonable considering our actual experimental settings and the main explanations are as follows: i) The energy of sputtered atoms (Ni, Fe) arrived over the target surface is three or four orders of magnitude higher than that of the evaporated one.31 In order to avoid the metal penetration into the fragile organic materials, the top FM electrode Co must be thermally evaporated onto the P3HT layer, leading to a looser structure than the sputtered NiFe


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electrode. As a result, the magnetocrystalline anisotropy of Co could be almost neglected compared to the one of the NiFe. ii) It is noted that even in the MTJ with fully oxidized AlOx at a thickness of several ten nanometers, a low junction resistance of a few ohms is still observed due to the presence of pinholes.33 In our case, the thickness of AlOx is precisely controlled to facilitate the effective spin transport in organic layer,34 also leading to a severe penetration problem. Therefore, the TMR effect itself is too weak to exhibit total positive MR in our MTJ. As a consequence, the strong angle-dependent magnetic response in NiFe electrode is dominated in NiFe/AlOx/Co configuration. Figure 4 shows the angular-dependent electrical characterization of the NiFe/P3HT/AlOx/Co polymer spin valve configuration with the same design in Figure 1a. A positive MR response (Figure 4a) similar to the experimental results shown in Figure 1b is observed when the magnetic field is applied along the easy axis of NiFe strip, and it has at least two distinct components. The first component is the switching events caused by the spinvalve operation of two FM electrodes, which is in accordance with the coercive field of NiFe (Figure 2b) and Co electrodes (Figure S6), respectively. While the second one is a continuous decreased background of the resistance with increasing magnetic field which is likely induced by the magnetic saturation of electrodes. At this condition the MR curves are considered to be approximately symmetrical with respect to the sign of external magnetic field. However, such a symmetry is broken when the relative orientation between magnetic field and NiFe strip is changed. Two MR curves measured at θ = 45° and θ = 90° are recorded in Figure 4b and Figure 4c, respectively. Though small positive MR signals still can be detected in accordance with the switching point of FM electrodes at low field, the device resistance is quite distinct between


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positive and negative magnetic field applied as high as 2000 Oe. Such a striking difference in resistance is unexpected in a classical spin valve device in which two directions should be equivalent since such a high magnetic field is far beyond the coercive field of NiFe electrodes. We infer it could be related to some anisotropic spin transport property. Within this perspective, the resistance at saturation magnetic field (2000 Oe) for different magnetization directions is measured (Figure 4d). Unlike ordinary TMR or GMR at high field that always show the same resistance value via angle-dependent scan,21 a biaxial symmetry is found in the anisotropic resistance distribution, suggesting an extra component of the device resistance. The symmetry about y-axis can be indexed into the AMR in NiFe strip, as is shown in Figure 2f and Figure 3d. As for the symmetry about x-axis, a more complicated anisotropic spin related effect should be included. For example, an effect of TAMR could be added to the total MR signal, since it may reflect the anisotropy of the density of states (DOS) in FM electrodes.35 In order to confirm the origin of the biaxial symmetry and to investigate the possible artifacts of the AMR effect in NiFe itself and further induced a TAMR effect, we compare the results of junctions with a vertical structure of NiFe/P3HT/Au (Figure 5), in which the TMR or GMR effect could be completely excluded due to the absence of the encounter FM electrode as a spin detector. This is because the spin-dependent detecting is unnecessary while the charge injection from NiFe to P3HT is the only essential for the TAMR effect.20 A standard spin-valve-like switching magnetic response is clearly observed in the sandwich configuration with only one FM electrode when θ = 45° (Figure 5b). By excluding the intrinsic AMR in NiFe and any TMR or GMR component in the device, we ascribe this phenomenon to the contribution of TAMR effect induced by spin-orbit coupling (SOC) in FM electrode with crystalline anisotropy.24 Since the Au electrode is nonmagnetic, the electron structure of NiFe electrode should be the primary suspect. When the


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contact between the NiFe electrode and organic material has a tunneling component, the DOS of the tunneling matrix element could be strongly influenced by the change of the magnetization vector via SOC effect.21, 24 Thus the change in tunneling resistance of the device is determined exactly by the interfacial magnetization direction of the FM electrode.31 The resistance switching can be either positive or negative and its amplitude will be enhanced by the SOC effect and the magnetic anisotropy. It is noted that the spin-polarized carrier injection is irrelevant to the spinvalve-like switching. Instead, a biaxial magnetocrystalline anisotropy in FM electrode caused by symmetry breaking (e.g. by step edges at the surface), could be responsible for this phenomenon.21 In this system, the tunneling matrix element in NiFe is different along two separate easy axes, therefore two distinct resistance state is achieved in accordance with each easy axis. The coercive fields of the two easy axes vary as the magnetic field scans along the different directions (Figure 5a, 5b and 5c), and the corresponding angle-dependent device resistance at the saturation field verified the existence of magnetocrystalline anisotropy in NiFe and related TAMR effect (Figure 5d). Particularly, the spin-valve-like signal faded away and a single-step switching appeared when the magnetic field is along one of the easy axes (Figure 5c). Based on the above experimental results and analyses, we can draw a conclusion that the magnetic response shown in the classical NiFe based polymer spin valve is a competing consequence of the multiple effects. Take the thickness of the P3HT layer into consideration (Figure S1 and S2), any direct tunneling transport through the organic semiconductor could be excluded. Organic magnetoresistance (OMAR) effect can also be ruled out since it usually occurs at elevated voltages (e.g. a few V),36 while in our experiment the applied voltage bias is lower than a few mV. Due to the poor reproducibility of the device resistance (Figure 1b, Figure 1c, and Figure 4) caused by random metal penetrating process during the thermal evaporation and uneven


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P3HT surface (Figure S7), the dominant mechanism varies depending on the quality of the organic layer and top FM electrodes. Given the dense islands on the surface of NiFe electrodes (Figure 2b and Figure S5), as well as the estimated thickness of polymer interlayer (Figure S1 and S2), the spin transport mechanism would be much more complex. Thus, the I-V tests of the polymer spin valve devices with different interlayer thickness were performed to provide more details for the spin transport behaviors (Figure S8 and S9). By comparing the order of the resistance magnitudes shown in Figure 1b-c with results in Table S1, symmetrical and almost linear I-V characteristics with ohmic contacts are usually deduced in thinner P3HT layer. Considering the weak temperature dependence of the background resistance (Figure S3 and Figure S4), we infer a major contribution of MR is the TMR effect induced by tunneling from NiFe stripe to the counter FM electrodes through pinholes. The total resistance is mainly composed of the TMR and normal charge transport via organic layer, and the former is a magnetization dependent term that account for the change in MR value while the latter is irrelevant. Instead of a common interpretation that a reduced MR in device with effective thicker organic layer (Figure 4a, in contrast to Figure 1b) is induced by the finite spin diffusion length,15 it is more likely the contribution of TMR through pinholes that account for a total resistance is inhibited in our polymer spin valve. Thus the total MR is reduced by the parallel current path by either multi-step tunneling or charge transport through the organic layer.20 Moreover, the angle-dependent resistance in NiFe/P3HT/Au (Figure 5d) confirmed the contribution of TAMR to the spin valve signal. As the


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organic film becomes thicker, the influence of TAMR effect will be greatly suppressed due to the weakened tunneling transport component.24 It is worth mentioning that when the Co atoms penetrating through the organic layer and severely impair the semiconductor conductivity, the spin valve device shows a stably inversed MR (Figure S4). Such a performance is similar to the AMR and TMR effect induced by NiFe electrode in a wide temperature range. Analogical negative MR response were also found in its counterpart (Figure S3) when the temperature is above 50 K, and we ascribe the vanished positive MR signal to the shortened spin relaxation length at the higher temperature zone. Mechanisms for the explanation of spin-valve like signal, including the AMR, TMR, GMR, TAMR, or an intermixing of them, strongly depend on the effective contact interface between the FM electrodes and the organic semiconductor layer,20 which should be carefully distinguished case by case. CONCLUSION In this study, we have demonstrated the multiple effects coexisted in the performance of NiFe based polymer spin valve. Especially, we address the major contribution of the NiFe anisotropy in the MR response in system. Considering the poor reproducibility of total device resistance mainly caused by top electrode metal penetration, the precise evaluation of MR response and as-resulted parameters for individual device should be done with more cautiousness. Both the temperature-dependent and angle-dependent measurements are effective to


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recognize the origin of a spin-valve-like signal. Meanwhile, these results can also offer potential of utilizing the interface magnetic anisotropy between FM electrodes and the organic layer to realize multifunctional device with multistate memory cells as long as the new technologies toward controllable interface is developed.


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FIGURES

Figure 1. (a) Schematic illustration of the NiFe/P3HT/AlOx/Co spin valve. The angle θ defines the direction of the applied in-plane magnetic field relative to the long direction

of NiFe stripe. Inset shows the molecular structure of the organic semiconductor P3HT. Typical magnetic response of the NiFe-based polymer spin valve devices with the (b) normal positive MR and the (c) abnormal negative MR. The above two distinct curves were taken from nearby devices on the same substrate.


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Figure 2. Characterization of a NiFe thin film and its corresponding angular dependent magnetic response. (a) AFM image of the 12 nm thick NiFe thin film with root-mean-square roughness of Rq = 1.24 nm. (b) Magnetic hysteresis loop

of the NiFe thin film with the coercive field of 40 Oe measured at 10 K. Magnetic response curves of the NiFe thin film at different angles of the magnetic field measured at 10 K with a bias current of 0.5 μA: (c) θ = 0°, (d) θ = 45°, and (e) θ = 90°, respectively. The black/red line represent the resistance value while the magnetic field is swept from positive/negative to negative/positive. (f) θ-scan of the NiFe resistance measured at T = 10 K and I = 1 μA in a saturation magnetic field B = 2000 Oe.


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Figure 3. Angular dependent magnetic response of a NiFe/AlOx/Co tunneling

junction. Corresponding the magnetic response curves at different angles of the magnetic field measured at 10 K with a bias current of 0.5 μA: (a) θ = 0°, (b) θ = 45°, and (c) θ = 90°, respectively. The black/red line represent the resistance value while the magnetic field is swept from positive/negative to negative/positive. (d) θ-scan of the NiFe/AlOx/Co resistance measured at T = 10 K and I = 0.5 μA in a saturation magnetic field B = 2000 Oe.


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Figure 4. Angular dependent magnetic response of a NiFe/P3HT/AlOx/Co polymer spin valve device. Corresponding magnetic response curves at different angles of

the magnetic field measured at 10 K with a bias current of 0.05 μA: (a) θ = 0°, (b) θ = 45°, and (c) θ = 90°, respectively. The black/red line represent the resistance value while the magnetic field is swept from positive/negative to negative/positive. (d) θscan of the NiFe/P3HT/AlOx/Co resistance measured at T = 10 K and I = 0.5 μA in a saturation magnetic field B = 2000 Oe.


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Figure 5. Angular dependent magnetic response of a NiFe/P3HT/Au organic TAMR device. Corresponding magnetic response curves at different angles of the magnetic field measured at 10 K with a bias current of 1 μA: (a) θ = 0°, (b) θ = 45°, and (c) θ = 90°, respectively. The black/red line represent the resistance value while the magnetic field is swept from positive/negative to negative/positive. (d) θ-scan of the NiFe/P3HT/Au resistance measured at T = 10 K and I = 1 μA in a saturation magnetic field B = 2000 Oe. To maximum reduce Au penetrations, the P3HT were spin coated at a speed of 2000 rpm.


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ASSOCIATED CONTENT Supporting Information The following files are available free of charge. Supporting file contains the temperature-dependent MR measurements of the NiFe based polymer spin valve (PDF). 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. Notes The authors declare no competing financial interest. ACKNOWLEDGMENT The authors are grateful for the financial support from the Ministry of Science and Technology of China (2016YFB0401100, 2017YFA0204503), the National


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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 Zhiwei Song in National Center for Nanoscience and Technology for his assistance in NiFe electrode fabrication.


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Anisotropic magnetoresistance in NiFe based polymer spin valves

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