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Room-temperature magnetoresistance in Ni78Fe22/C8-BTBT/Ni78Fe22 nanojunctions fabricated from magnetic thin-film edges using a novel technique

Molecular spintronic devices are gaining popularity because the organic semiconductors with long spin relaxation times are expected to have long spin diffusion lengths. A typical molecular spintronic device consists of organic molecules sandwiched between two magnetic layers, which exhibits magnetoresistance (MR) effect. Nanosized devices are also expected to have a high spin polarization, leading to a large MR effect owing to effective orbital hybridization. However, most studies on nanosized molecular spintronic devices have investigated the MR effect at low temperatures because of the difficulty in observing the MR effect at room temperature. Here we focus on high-mobility molecules expected to show long spin diffusion lengths, which lead to the observation of the MR effect in nanoscale junctions at room temperature. In this study, we fabricate magnetic nanojunctions consisting of high-mobility molecules, 2,7-dioctyl[1]benzothieno[3,2-b][1]benzothiophene (C8-BTBT), sandwiched between two Ni78Fe22 thin films with crossed edges. Transmission electron microscopy (TEM) images reveal that C8-BTBT molecular layers with smooth and clear interfaces can be deposited on the Ni78Fe22 thin-film edges. Consequently, we observe a clear positive MR effect, that is, RP < RAP, where RP and RAP are the resistances in the parallel (P) and antiparallel (AP) configurations, respectively, of two magnetic electrodes in the Ni78Fe22/C8-BTBT/Ni78Fe22 nanojunctions at room temperature. The obtained results indicate that the spin signal through the C8-BTBT molecules can be successfully observed. The study presented herein provides a novel nanofabrication technique and opens up new opportunities for research in high-mobility molecular nano-spintronics.


Introduction
Molecular spintronics is an emerging eld of research that combines spintronics and organic materials. A notable aspect of molecular spintronics is the ability to achieve long spin coherent length in organic materials mainly composed of light elements, such as C, H, and O, owing to the long spin relaxation times originating from the weak spin-orbit coupling associated with such materials. Typical molecular spintronic devices are organic spin valves (OSVs) which consist of organic molecules sandwiched between two magnetic layers. [1][2][3][4][5][6][7][8][9][10][11][12][13][14] These types of devices show a large magnetoresistance (MR) effect. The resistance of such devices changes according to the conguration of the magnetization vectors in both magnetic layers adjacent to the molecular layer. The MR ratio is dened as MR ¼ (R AP − R P )/ R P , where R P and R AP are the resistances in the parallel (P) and antiparallel (AP) congurations, respectively, of the two magnetic electrodes. An MR ratio of 40% was attained at 11 K in Co/tris-(8-hydroxy-quinoline)aluminum (Alq 3 )/La 0.67 Sr 0.33 MnO 3 (LSMO) devices. 1 Two other studies have reported MR ratios of 15.6% at 2 K in poly(3-hexylthiophene) (P3HT)-based devices 7 and 7.5% at 10 K in fullerene (C 60 )-based devices. 3 Furthermore, because of their nanosized junction area, Co/Alq 3 /LSMO devices were shown to have an MR ratio as high as 300% at 2 K. 15 The large MR effect in nanoscale junctions was attributed to effective orbital hybridization, which originates from the spinhybridization-induced polarized states (SHIPS) 15 at the metalmolecule interface. In another study, a Ni/single 1,4-3-phenyldithiolate (1,4-tricene-dithiolate)/Ni junction has been shown to exhibit an MR ratio of 600%, which was predicted by calculating the current-voltage (I-V) characteristics in both P and AP congurations using a combination of density functional theory (DFT) and non-equilibrium Green's function (NEGF) method, considering a local approximation to the exchange correlation potential. 16 However, most of the studies on nanoscale devices are limited to low-temperature experiments. 15,17 Previously, we demonstrated the fabrication of Ni nanojunctions with an area of 17 Â 17 nm 2 ; each junction consisted of two Ni thin lms directly in contact with edges crossing each other. 18 To fabricate these nanojunctions, polyethylene naphthalate (PEN) organic lms were used as substrates for the Ni thin lms. However, the use of PEN lms limits process exibility owing to their low glass transition temperature (T g ¼ 120 C). In particular, this prohibits the use of thermal treatments that require temperatures higher than 120 C for surface/ interface engineering and crystal structure optimization. In addition, PEN lms cannot be practically used because a typical manufacturing process, such as CMOS-LSI, includes a thermal treatment stage. To overcome this problem, we had proposed a fabrication method that uses low-soening point (LSP) glass substrates. In this method, a metal thin lm was deposited onto an LSP glass, aer which another LSP glass was pressed onto the deposited metal thin lm using a thermal-pressing technique. Because T g of the utilized LSP glass substrate is greater than 450 C, the fabricated electrode can be heated to temperatures as high as 450 C. Thus, the formation of Co thin-lm electrodes with smooth Co edges, sandwiched between the LSP glasses, has been successfully demonstrated using the proposed method. 19 This technique can also be applied to NiFe alloys, which are excellent materials characterized by long-term stability and high oxidation resistance in air. [20][21][22] They also exhibit superior mechanical properties, such as workability and plasticity. Therefore, NiFe alloys can be used to manufacture thin-lm electrodes of the proposed nanojunctions.
We focus on 2,7-dioctyl [1]benzothieno [3,2-b][1]benzothiophene (C8-BTBT) as the organic material, which is sandwiched between the thin-lm edges. The high mobility of C8-BTBT molecules makes the material attractive for the fabrication of organic thin-lm transistors (OTFTs). [23][24][25][26] The reported maximum mobility of a C8-BTBT lm is 43 cm 2 V −1 s −1 . 23 The carrier mobility is one of the important factors that determines the spin-transport properties of organic molecules in a device. The reported high mobilities are mainly measured in eld effect transistor (FET) with the current ow parallel to the growth plane of molecules. By contrast, the current ows in the vertical direction to the growth plane of molecules in typical OSVs, including our proposed devices. The previous study reported that the vertical mobility to the oriented direction is only 0.3 times lower than the parallel one in the molecules with pconjugated electrons. 27 This study indicates that there is no signicant difference between the parallel and vertical mobilities. Although high-mobility molecules could have large spin coherent lengths, low-mobility molecules, such as Alq 3 and bathocuproine (BCP) ($10 −5 to 10 −6 cm 2 V −1 s −1 ), [28][29][30] are usually used in spintronic devices. 1,2,4,8,11,12,15,17 By contrast, recent studies have reported that high-mobility molecules, such as P(NDI2OD-T2) ($0.2-0.85 cm 2 V −1 s −1 ) 31 and PTDCNTVT-320 ($0.8-1.46 cm 2 V −1 s −1 ), 32 can be used in OSVs. 6,9,14 Thus, C8-BTBT could be potentially applicable to high-performance molecular spintronic devices. Such excellent devices will be scaled down for future integration, where one recorded bit may even reach the nanometer scale.
In this study, we propose a fabrication technique for metal/ molecule/metal nanojunctions consisting of C8-BTBT molecules sandwiched between two Ni 78 Fe 22 thin lms with crossed edges (Fig. 1

Fabrication
The procedure followed for the fabrication of the nanojunctions is illustrated in Fig. 2. First, LSP glass substrates of size 10 Â 10 Â 2 mm 3 were prepared and chamfered on both sides ( Fig. 2(a)). The glass composition was SiO 2 -B 2 O 3 -Al 2 O 3 -Na 2 O-CaO, and the glass deformation temperature was 503 C. Ni 78 Fe 22 thin lms with thicknesses of 10-50 nm were then deposited onto the glass substrates by sputtering ( Fig. 2(b)). A base pressure in the range of 3.0-9.0 Â 10 −5 Pa was maintained prior to sputtering. The growth rate of the Ni 78 Fe 22 thin lms was 3 nm min −1 at a sputtering power of 60 W (1.0 kV and 60 mA) under Ar pressure of 3.0-5.0 Â 10 −2 Pa. Au thin lms with a thickness of 14 nm were sputtered onto the chamfered edges of the deposited Ni 78 Fe 22 thin lms (Fig. 2(c)). Subsequently, glass substrates of the same composition were stacked on the fabricated Ni 78 Fe 22 thin lms by thermal pressing in a N 2 atmosphere at a temperature of 513 C and pressure of 0.25-1.0 MPa ( Fig. 2(d)). The obtained samples were cut in half using a high-speed cutting machine (Mecatome T202, Meiwafosis Co., Ltd.; Fig. 2(e)), and their cross-sectional surfaces were rst subjected to a two-step mechanical polishing (MP) treatment using Al 2 O 3 -based emeries and diamond paste, then a two-step chemical mechanical polishing (CMP) procedure using Al 2 O 3 and SiO 2 slurries ( Fig. 2(f); the detailed polishing conditions are described in ESI †). Next, C8-BTBT powder was dissolved in 1,2-dichlorobenzene (DCB) to form a 10 mg mL −1 solution, and C8-BTBT thin lms were deposited on the polished glass/Ni 78 Fe 22 /glass substrate by the off-center spin-coating (OCSC) method 23 ( Fig. 2(g); the OCSC method is illustrated in detail in Fig. S1 †). Finally, another polished glass/Ni 78 Fe 22 /glass sample was carefully stacked on the C8-BTBT lms by pressing under a pressure of 0.05 MPa. The fabricated nanojunction is illustrated in Fig. 2(h). The reproducibility of the fabrication method is described in ESI. †

Measurement
The interfacial features of the polished glass/Ni 78 Fe 22 /glass samples were examined using transmission electron microscopy (TEM; TECNAI Osiris, FEI) and energy-dispersive Xray spectroscopy (EDS). Cross-sectional TEM specimens were prepared using the focused ion beam (FIB) technique. The thickness of the Ni 78 Fe 22 thin lms was measured using TEM. The surface morphologies and roughness of the polished glass/ Ni 78 Fe 22 /glass substrates with and without C8-BTBT were analyzed using atomic force microscopy (AFM; Nanocute, SII Nano Technology Inc.). The chemical compositions and structures of C8-BTBT molecules were identied using Fourier transform infrared (FT-IR; FT/IR-4X, JASCO Corporation) spectroscopy. The electrical properties of the Ni 78 Fe 22 thin-lm edges, with and without C8-BTBT, were evaluated using a conductive AFM (c-AFM) system equipped with a Rh-coated cantilever (Si-DF3-R). Magnetization curves of the Ni 78 Fe 22 thin lms were measured using magneto-optic Kerr effect spectroscopy (MOKE; BH-PI920-HU, NEOARK). The MR curves of the fabricated nanojunctions were evaluated using a fourprobe method under a magnetic eld of up to AE190 Oe at room temperature.    glasses. Films with smooth and clear interfaces can be observed to have successfully formed. Precise formation of a 42 nm thick Ni 78 Fe 22 thin lm is important because this thickness directly determines the junction area; that is, 42 nm thick lms can produce nanojunctions with an area of 42 Â 42 nm 2 . Fig. 3(b) shows high-angle annular dark eld (HAADF)-EDS mapping images of the same sample. The lms exhibit smooth and clear interfaces without any diffusion of Ni or Fe atoms into the bulk glass. In some earlier studies, metal atoms were found to diffuse into the LSP materials aer deposition. [33][34][35][36][37] By contrast, in this study, metal diffusion into the bulk of the LSP materials did not occur at the Ni 78 Fe 22 /glass interface. The results indicate that glass/Ni 78 Fe 22 /glass can be successfully fabricated by thermal pressing. Fig. 3(c) shows an AFM image of the polished crosssectional surfaces of the glass/Ni 78 Fe 22 (42 nm)/glass substrate. The roughness of the polished surface measured over the same scanning area of 10 Â 20 mm 2 , shown in Fig. 3(c), does not exceed 0.77 nm. The observed Ni 78 Fe 22 thin-lm edge exhibits a convex shape with a height of approximately 5.5 nm, which ensures good contact between the Ni 78 Fe 22 edges and the sandwiched material (C8-BTBT in this study) in the nanojunctions. The heights of the projected edges can be controlled during the second step of the CMP process using SiO 2 slurries. Fig. 3(d) shows a c-AFM image of the same specimen, indicating uniform electrical conduction along the Ni 78 Fe 22 edges. Fig. 3(e) shows the I-V characteristics obtained at an arbitrary position on the Ni 78 Fe 22 edges under optimized polishing conditions. The I-V curve is ohmic, suggesting that the Ni 78 Fe 22 edge is not oxidized. In our previous study, contrast enhancement of the Co edge was observed in the scanning electron microscope (SEM) images of glass/Co (14 nm)/glass specimens, which was attributed to the high conductivity of Co. 19 However, we did not observe uniform electrical conduction and ohmic behavior during c-AFM studies of Co samples, suggesting the formation of a thin Co oxide layer on the Co edge surface. Therefore, the obtained results in this study indicate that Ni 78 Fe 22 can offer better long-term stability and oxidation resistance than Co. The AFM and c-AFM results reveal that the Ni 78 Fe 22 thin lms sandwiched between the LSP glasses can be used as electrodes in the proposed nanojunctions.

Structural and electrical characterization of C8-BTBT thin lms
Aer the formation of the C8-BTBT lms on the polished surface of the glass/Ni 78 Fe 22 (42 nm)/glass substrates, the roughness of the surface of the C8-BTBT lm is measured to be approximately 0.82 nm over a scanning area of 10 Â 20 mm 2 using AFM. Uniform electrical conduction along the Ni 78 Fe 22 edges is observed in the c-AFM images. The details of AFM and c-AFM studies are described in Fig. S2. † In addition, the measured FT-IR spectra are in good agreement with the reference experimental data for C8-BTBT, [38][39][40][41] indicating the formation of high-quality C8-BTBT thin lms. The detailed measurement conditions and results of the FT-IR study are presented in the ESI. † Fig. 4(a) shows the cross-sectional TEM image obtained for the 2 nm thick C8-BTBT lm on the polished surface of the glass/Ni 78 Fe 22 (42 nm)/glass substrates. A lm with smooth and clear interfaces is successfully fabricated on the Ni 78 Fe 22 edges. Here, we note that there is a slight curvature in the interface between C8-BTBT lms and glass/Ni 78 Fe 22 (42 nm)/glass substrates. This curvature is caused by the structural change of LSP glasses due to the heat generated during TEM observation. The shape of the LSP glasses, which are used for sandwiching Ni 78 Fe 22 thin lms, changes with time during TEM observation. Fig. 4(b) shows the local I-V characteristics obtained by c-AFM for the C8-BTBT lms on the polished Ni 78 Fe 22 edges. The I-V curve exhibits a clear nonlinear behavior, which can be explained in terms of the space charge limited current (SCLC). 42,43 According to Child's law, the transport carriers injected from a metal layer into the trap-free organic layer can exhibit two different regimes: ohmic conductivity (I f V) at low voltages and square-law behavior (I f V 2 ) at higher voltages, which corresponds to SCLC. 42,43 As shown in Fig. 4(b), the obtained experimental results are in good agreement with the theoretical calculation based on Child's law. The obtained results indicate the successful formation of C8-BTBT lms on the Ni 78 Fe 22 edge.  44 This result indicates that high-quality Ni 78 Fe 22 thin lms can be formed on LSP glasses with regard to magnetic properties. Fig. 5(b) shows the magnetization curve of the Ni 78 Fe 22 lms sandwiched between two LSP glasses. The coercivities of Ni 78 Fe 22 thin lms increase to 29 and 68 Oe by increasing the applied pressures to 0.15 and 0.25 MPa, respectively, during the thermal pressing process. According to the random anisotropy model, the enhancement in the coercivity is attributed to the increase in the crystal grain size. 22 The different coercivities can produce the P and AP congurations, leading to the observation of MR effect. These results indicate that Ni 78 Fe 22 thin lms sandwiched between LSP glasses can be used as electrodes for the proposed nanojunctions.  (Fig. S4(a) †), which corresponds to the resistance of electrode short circuit. The resistance of Ni 78 Fe 22 /C8-BTBT (2 nm)/Ni 78 Fe 22 nanojunctions is 135.2 U (Fig. S4(b) †), which is larger than the resistance of Ni 78 Fe 22 /Ni 78 Fe 22 . Here, the measured resistance (135.2 U) is much smaller than the value estimated using Ohm's law. In our proposed method, we can fabricate the molecular layer without any interdiffusion of Ni or Fe atoms because the top layer is not deposited on molecules by thermal evaporation or sputtering method. Therefore, Ni or Fe atoms cannot penetrate easily into molecules. This means that the low resistance is not due to the formation of conductive path through Ni or Fe atoms. In addition, during the measurement of I-V characteristics, the bias voltage is only $5 mV as shown in Fig. S4 and S5. † Since the metal laments are generally caused by a high voltage from 0.5 to a few volts, 45 the low resistance is not attributed to the formation of short-circuit path originating from the metal laments. Although the reason for the low resistance is not yet clear, one possibility is the effect of quantized conductance. [46][47][48][49][50][51][52][53][54][55][56][57] Another possibility is that the resistance can change depending on the experimental method, such as nanoindentation, nanoimprinting, lithography, or our newly proposed method using thin-lm edges, and materials, i.e., Alq 3 or C8-BTBT, in nanoscale devices. The resistance is also sensitive to the shape, such as wires, rings, rectangles, ellipses and circles. In fact, the resistance in nanoscale magnetic tunnel junctions (MTJs) can change depending on experimental methods, materials and shape. 58  nanopillar MTJs with a 200 Â 70 nm 2 ellipsoidal area fabricated using electron-beam (EB) lithography is small at $200 U. 61 Other examples include the resistance in Co 60 Fe 20 B 20 /Al-oxide (0.6-1.0 nm)/Co 60 Fe 20 B 20 nano-ring MTJs with an outer diameter of 100 nm and ring-width of 25 nm (corresponding to an effective junction area of 76 Â 76 nm 2 ) fabricated using EB lithography is as small as 285-337 U, 58 and also the resistance in CoFeB/MgO (0.93 nm)/FeB nano-MTJs with a diameter of 21.7 nm fabricated using EB lithography is 12 kU. 62 Thus, the resistance ranges from $100 U to $MU in nanoscale devices. Therefore, it could be possible to show such a small resistance in our proposed nanoscale devices.

Electrical properties and MR effect of nanojunctions
Next  Fig. 5(b). This is consistent with the MR curve shown in Fig. 6(a). Moreover, as shown in Fig. 6(b), no positive MR effect is observed in the Ni 78 Fe 22 /C8-BTBT (2 nm)/Ni 78 Fe 22 nanojunctions. The resistance ($135 U) is larger than the measured resistance ($82 U) of the Ni 78 Fe 22 /Ni 78 Fe 22 nanojunctions. This means that the electrical conductivities through the 2 nm thick C8-BTBT molecules can be successfully observed in Fig. 6(b). However, the behavior of the MR curve of the Ni 78 Fe 22 /C8-BTBT (2 nm)/Ni 78 Fe 22 junctions is similar to that of the Ni 78 Fe 22 / Ni 78 Fe 22 junctions. This indicates that the observed MR is due to the AMR in the interfacial Ni 78 Fe 22 close to the C8-BTBT molecules. Here, the AP conguration of magnetization is not realized in the interfacial Ni 78 Fe 22 because of the magnetic coupling between the two edges of the Ni 78 Fe 22 thin lms. This suggests that with thicker C8-BTBT layers suppressing the magnetic coupling, the P and AP congurations could be realized in the interfacial Ni 78 Fe 22 , leading to a positive MR effect. Fig. 7 shows the MR curves of the Ni 78 Fe 22 /C8-BTBT (4 nm)/ Ni 78 Fe 22 nanojunction at room temperature. A positive MR effect, that is, R AP > R P , is clearly observed. The positive MR cannot be explained by AMR effect. The observation of the positive MR is also one of the evidence to rule out the possibility of electrode short circuit. These reasons are described in ESI. † Thus, the observation of the positive MR effect indicates that a spin signal through C8-BTBT molecules can be successfully observed. This also indicates that the top and bottom Ni 78 Fe 22 electrodes are not magnetically coupled at the junction interface. The magnetic coupling between the two Ni 78 Fe 22 edges can be suppressed by the larger molecular spacing ($4 nm). Here, we note that the observed MR curves are slightly different from what is expected based on the magnetization curves shown in Fig. 5(b). Such incongruence between the MR curves and magnetization states has also been reported in other studies. 1,7,9,14 For example, in Alq 3 -based MR devices, the top electrode (Co) and bottom electrode (LSMO) showed coercivities of 150 and 30 Oe, respectively, which are not in good agreement with the values 170 and 70 Oe, respectively, estimated from the observed MR effect. 1 In another study, the MR effect was observed in Ni/benzene-1,4-dithiol (BDT)/Ni single molecular junctions using mechanically controllable break junction (BJ) method. 55 However, it is difficult to compare the MR effect and corresponding magnetization states of electrodes in the BJ method. By contrast, we can precisely investigate both the MR effect and magnetization curves simultaneously by using our proposed method. The results of the investigation show that, because the magnetization states of the Ni 78 Fe 22 thin-lm edges adjacent to C8-BTBT molecules are different from those of the Ni 78 Fe 22 thin lms obtained by MOKE measurements (Fig. 5(b)), the MR curves show slightly different behaviors, as expected. In addition, the AMR effect can be observed during magnetic-eld switching between the P and AP congurations.
This also provides strong evidence for attributing the observed positive MR effect to the spin signal through the C8-BTBT molecules. A positive MR effect is also observed at different applied voltages, as shown in Fig. S7. † However, it is difficult to distinguish whether the observed spin signal originates from the tunneling or hopping mechanism. To clarify the mechanism, it is necessary to investigate the temperature dependence of resistance, which will be a future work.

Conflicts of interest
There are no conicts to declare.