Semiconductor-to-metallic spin-filtering and positive and negative magnetoresistance effects in C3N with nickel electrodes

Physica E 118 (2020) 113861

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Semiconductor-to-metallic spin-filtering and positive and negative magnetoresistance effects in C3N with nickel electrodes Jing Zeng a, b, *, Yanhong Zhou c a

College of Physics and Electronic Engineering, Hengyang Normal University, Hengyang, 421002, People’s Republic of China Hunan Provincial Key Laboratory of Intelligent Information Processing and Application, Hengyang, 421002, People’s Republic of China c College of Science, East China Jiao Tong University, Nanchang, Jiangxi, 330013, People’s Republic of China b

A R T I C L E I N F O

A B S T R A C T

Keywords: C3N Spin-filtering Magnetoresistance Nonequilibrium Green’s functions density functional theory

Carbon-based spintronic devices have attracted extensive attention recently because they have long spin relax­ ation times and lengths. As a new member of the carbon material family, C3N may have great application prospects in the field of spintronics. Though a previous work proposed the model of spintronic devices based on bare C3N nanoribbons, these devices are difficult to implement experimentally. This is due to the fact that edge reconstruction will occur in the bare C3N nanoribbons. In this work, we systematically investigate the spinpolarized properties of 2D C3N sheet and its corresponding hydrogen-passivated nanoribbons with ferromag­ netic Ni electrodes. Semiconductor-to-metallic spin-filtering and positive magnetoresistance effect are observed in 2D C3N sheet stacked on top of the Ni electrodes. Especially impressive is the enhancement of spin-polarization efficiency and the emergence of negative magnetoresistance effect after the 2D C3N sheet is converted into hydrogen-passivated nanoribbons. These results indicate that C3N has strong potential for nanoscale spintronics applications.

1. Introduction Spintronic devices are an important part of computer hard drives, and have important application values in high-performance electronic circuitry [1–6]. With the continuous improvement of integration, spin­ tronic devices will inevitably move towards miniaturization. Therefore, the search of low-dimensional spintronic devices has attracted an explosive interest. One of the most important goals of low-dimensional spintronic devices is the ability to realize spin-filtering and magneto­ resistance effects [1,6–10]. At present, scientists have found that two types of low-dimensional materials have the potential to achieve these functions, namely single molecules and atomically thin monolayers [11–15]. Especially for 2D thin monolayers, they have been designed into a variety of spintronic devices and exhibit excellent spin transport properties. For example, Klein et al. [13] investigated the conductance of the layered magnetic insulator CrI3 coupled with graphite, and found that antiparallel-to-parallel reorientation of Cr spins gives rise to large magnetoresistance. Piquemal-Banci et al. [14] designed 2D magnetic tunnel junctions making use of hexagonal boron nitride (h-BN) barrier. Their results showed that magnetoresistances in Co/h-BN/h-BN/Co junction and Co/h-BN/Fe junction can reach 12% and 50%,

respectively. Chen et al. [15] studied the spin-polarized transport properties of phosphorene with Ni electrodes, and found that magne­ toresistance and spin-injection efficiency can be improved by applying mechanical deformations. These recent research results indicate that atomically thin monolayers are ideal candidates for a new generation of spintronic devices. C3N is a new type of carbon-based material discovered in recent years [16], and has been found to have important application prospects in many fields, such as sensors [17], photoelectronic devices [18], ion batteries [19,20], metal-free catalyst [21], etc. Many studies have shown that the low-dimensional carbon-based materials not only have high structural stability, but also have long spin relaxation times and lengths [22]. So the application of C3N in spintronics is expected. However, both 2D C3N sheet and hydrogen-passivated C3N nanoribbons do not have inherent magnetic moments. For this reason, Ren et al. [23] proposed a design scheme based on bare C3N nanoribbons, and found that these nanoribbons exhibit high spin-polarization ratio by applying external fields. However, Tagani et al. [24] indicated that the edge reconstruction may occur in the bare C3N nanoribbons. Thus, it is difficult to construct spintronic devices based on such nanoribbons. The stability of these nanoribbons can be improved when their edges are

* Corresponding author. College of Physics and Electronic Engineering, Hengyang Normal University, Hengyang, 421002, People’s Republic of China. E-mail address: [email protected] (J. Zeng). https://doi.org/10.1016/j.physe.2019.113861 Received 9 August 2019; Received in revised form 29 November 2019; Accepted 3 December 2019 Available online 4 December 2019 1386-9477/© 2019 Elsevier B.V. All rights reserved.

J. Zeng and Y. Zhou

Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113861

practical spintronic devices based on C3N. Our results show that stacking C3N on the top of Ni electrodes is a promising strategy for realizing its possible application in the field of spintronics. 2. Method and model In Fig. 1(a), we present the two-probe model of 2D C3N-based magnetic tunnel junction. The dark (light) orange regions indicate the electrodes (electrode extensions) part of the two-probe system. The purple region represents the central scattering region of the device. In the electrode region, the 2D C3N is adsorbed on the top of the Ni(111) surface. Here the direction of the spin-transport is oriented along a zigzag direction. The unit cell of the electrode is made up of 1 � 1 orthorhombic unit cell of C3N and 2 � 2 orthorhombic unit cells of Ni (111). By calculation, the lattice constant mismatching is around 3%. This value is reasonably small, which is based on the fact that C3N is still dynamically stable when the pressure reaches 14% [25]. In order to obtain the equilibrium geometry of the electrode region, we adopt a method used in the previous reports [26,27]. In detail, we fix the Ni (111), and move the C3N sheet along the zigzag (armchair) direction with a slight step of 1/5, 2/5, 3/5, 4/5, and 5/5 (1/10, 2/10, 3/10, 4/10, 5/10, 6/10, 7/10, 8/10, 9/10, and 10/10) in the corresponding lattice constant. Thus, we obtain fifty initial configurations. All initial config­ urations are relaxed, and the electrode part of Fig. 1(a) shows the structure with the lowest energy. The scattering region only includes the 2D C3N sheet, which acts as the tunnel channel in the system. Through cutting C3N sheet along the zigzag direction, we can also obtain zigzag C3N nanoribbons (ZCNNRs) with different edge atoms. Following pre­ vious convention, the nanoribbons in Fig. 1(b)–(d) are as called 4-ZCNNR-CNCN, 4-ZCNNR-CCCC, and 5-ZCNNR-CCCN, respectively. As a previous report has confirmed that the edge reconstruction may occur in the bare edges of C3N nanoribbons, the C or N atoms at both edges are terminated by one hydrogen atom in these nanoribbons. Similar to the 2D C3N, ZCNNRs-based two-probe models are also constructed, which is shown in Fig. 1(b)–(d). The electronic and spin-transport properties of these systems are calculated using density-functional theory in combination with the Keldysh nonequilibrium Green’s technique, as implemented in the ATOMISTIX TOOLKIT package [28–30]. For the electronic properties of the bulk’s structures, the double- ξ polarized basis set, a Monkhorst-Pack K-mesh of (7, 7), and 150 Ry for cutoff energy are used. The local spin density approximation with the Perdew-Zunger (LSDA-PZ) parametri­ zation is employed to quantify the exchange-correlation potential, which has been proved to give excellent results for the carbon-based sheet with the Ni(111) substrate [31]. For the spin-transport properties of the two-probe systems, the double- ξ polarized basis set is used for 2D C3N sheet and ZCNNRs, and the single- ξ polarized basis set is used for Ni atoms. LSDA-PZ and a mesh cutoff of 150 Ry are adopted. A 5 � 1 � 200 (3 � 1 � 200) K-point mesh is employed for the self-consistent calculations of the 2D C3N-based (ZCNNR-based) two-probe system. In the transmission calculation, a 11 � 1 (7 � 1) K-point sampling in the x and y directions is used for the 2D C3N-based (ZCNNR-based) two-probe system. The spin-resolved trans­ mission coefficient is computed from Ref. [32]. � � � � � � Tσ ðE; Vb Þ ¼ Tr Im Σ Rlσ ðE; Vb Þ GRσ ðE; Vb Þ � Im Σ Rrσ ðE; Vb Þ GAσ ðE; Vb Þ (1)

Fig. 1. (a) The schematic view of 2D C3N-based two-probe system. The upper (lower) panel shows a side (top) view of the two-probe system. (b) The sche­ matic view of 4-ZCNNR-CNCN-based two-probe system. (c) The schematic view of 4-ZCNNR-CCCC-based two-probe system. (d) The schematic view of 5ZCNNR-CCCN-based two-probe system. The green, black, and blue balls indi­ cate Ni, C, and N atoms, respectively.

where σ represents spin-up or spin-down states; GR and GA are the retarded and advanced Green functions of the device’s scattering region, respectively.

passivated by hydrogen atoms. However, the magnetic moments in these nanorbbons disappear after edges hydrogenation [24]. In order to explore the application of hydrogenated C3N nanorbbons in spintronics, an effective strategy is to stack hydrogenated C3N nanorbbons on the magnetic bulk electrodes. However, there are few studies focusing on this aspect. Whether stacking the C3N sheet and its corresponding hydrogen-passivated nanoribbons on the top of magnetic electrodes is an effective way for realizing their application in spintronics remains an open question. In the present work, we report a sensible model of

3. Results and discussion We first investigate the stabilities and electronic properties of 2D C3N/Ni(111) system by only considering the electrode part. For the corresponding adsorption configuration displayed in Fig. 1(a), an 2

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Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113861

increase in the C3N sheet. This 0.631 e increase gives rise to an interesting phenomenon that the energy gap of the free C3N monolayer disappears, and a strong spin-polarized characteristic is observed near the Fermi level [see Fig. 2(a)]. This result is very similar to 2D insulating hexagonal boron nitride coupled with ferromagnetic electrode [14]. Further anal­ ysis shows that C and N atoms in the C3N sheet have almost equal con­ tributions to the spin-polarization of gap states. Moreover, we also calculate the magnetic moment in the C3N sheet. Result shows that its total magnetic moment increases from 0 in the free-standing case to 0.055 μB in the adsorption case. For the ZCNNRs/Ni(111) systems, it is known that the monohydrogen-terminated ZCNNRs are nonmagnetic, and show metallic or semiconducting properties depending on their edge atoms. Our calculation indicates that free-standing 4-ZCNNR-CCCC and 5-ZCNNR-CCCN are metal, and 4-ZCNNR-CNCN is semiconductor of 0.91 eV, which is in agreement with the previous report [24]. Similar to the 2D C3N/Ni(111) system, the gap state with strong spin-polarized feature is also found in 4-ZCNNR-CNCN with the semiconducting prop­ erty, as shown in Fig. 2(b). This can be attributed to the fact that the 4-ZCNNR-CNCN/Ni(111) interface exists a very strong chemisorption, which is reflected in an obvious increase (~0.615 e) of the charges of 4-ZCNNR-CNCN. As a result, the strong hybridization between the Ni surface and 4-ZCNNR-CNCN gives rise to the orbital mixing and electron sharing at the interface, which eventually leads to the occurrence of the gap sate [34]. For the 4-ZCNNR-CCCC (5-ZCNNR-CCCN)/Ni(111) sys­ tem, it is found from Table 1 that the binding energy of 4-ZCNNR-CCCC (5-ZCNNR-CCCN) on the Ni(111) surface is higher than that of 4-ZCNNR-CNCN. Thus, stronger charge-transfer doping effect may occur in the 4-ZCNNR-CCCC (5-ZCNNR-CCCN)/Ni(111) system. Results in Table 1 confirm our inference. We can find that amount of charges transfer from the Ni(111) substrate to 4-ZCNNR-CCCC and 5-ZCNNR-CCCN is about 0.676 and 0.791 e, respectively. Thus, the cor­ responding sequence for electron injection capability is 4-ZCNNR-CNCN/Ni(111)< 4-ZCNNR-CCCC/Ni(111) < 5-ZCNNR-CCCN /Ni(111). Moreover, we also note an interesting feature that for the 4-ZCNNR-CCCC (5-ZCNNR-CCCN)/Ni(111) system, the strong spin-polarized characteristic near the Fermi level is dominated by the p-orbitals of C and N atoms at the negative energy region, which is con­ trary to the case of 4-ZCNNR-CNCN/Ni(111) system. Total magnetic moments in these nanoribbons are also calculated. Result shows the low magnetic moment belongs to 4-ZCNNR-CCCC (0.004 μB ) while the large magnetic moment belongs to 4-ZCNNR-CNCN (0.081μB ). We further investigate spin-polarized quantum transport properties of all systems by constructing two-probe configurations. For conve­ nience of explanation, the two-probe configurations shown in Fig. 1(a)– (d) are named as M1-M4, respectively. By applying the external

Table 1 The average distance d (Å) between 2D C3N (ZCNNR) and the Ni(111) surface, binding energy Eb (eV), and the amount of charges transfer from Ni(111) to 2D C3N (ZCNNR) in 2D C3N/Ni(111), 4-ZCNNR-CNCN/Ni(111), 4-ZCNNR-CCCC/ Ni(111), and 5-ZCNNR-CCCN/Ni(111), respectively. Structures

d(Å)

Eb (eV)

2D C3N/Ni(111) 4-ZCNNR-CNCN/Ni(111) 4-ZCNNR-CCCC/Ni(111) 5-ZCNNR-CCCN/Ni(111)

2.06 2.07 2.09 2.09

4.79 7.02 7.80 8.65

Charge transfer (e) 0.631 0.615 0.676 0.791

important feature is that the 2D C3N sheet appears small corrugations because nitrogen atoms are slightly away from the Ni(111) surface. As a result, the carbon atoms have the possibility to bond with the Ni atoms, and chemisorption is existent in the interface. From Table 1, it is noted that for the 2D C3N/Ni(111) system, the average distance d between the C3N sheet and the Ni(111) surface is about 2.06 Å. Conversion of 2D C3N sheet into ZCNNRs does not produce obvious changes, and the corre­ sponding average distance for 4-ZCNNR-CNCN/Ni(111) [4-ZCNNRCCCC/Ni(111) and 5-ZCNNR-CCCN/Ni(111)] is 2.07 Å [2.09 Å and 2.09 Å]. These values are very similar to the case of graphene on the top of Ni surface [31], and indicate that the C3N sheet and its corresponding nanoribbons may have a strong adsorption capacity on the Ni(111) surface. To confirm this inference, we further calculate the binding en­ ergies of C3N sheet (ZCNNRs) adsorbed on the surface of the Ni(111) using the formula, Eb ¼ EC3 N ​ sheet ​ ðZCNNRÞ=Nið111Þ

EC3 N ​ sheet ​ ðZCNNRÞ

ENið111Þ ;

(2)

where EC3 N ​ sheet ​ ðZCNNRÞ=Nið111Þ is the total energy of the relaxed 2D sheet or ZCNNR on the Ni(111) surface. EC3 N ​ sheet ​ ðZCNNRÞ and ENið111Þ are total energies of perfect 2D sheet (ZCNNR) and the Ni(111) substrate, respectively. We can see clearly from Table 1 that the binding energy of 2D C3N on the surface is about 4.79 eV, indicating strong adsorption and good stability. Especially impressive is that the binding energies are higher for the ZCNNRs/Ni(111) systems (in the range 7.02–8.65 eV). Thus, disassociating the 2D C3N and ZCNNRs from the Ni(111) surfaces is very difficult under general operational conditions [33]. In order to investigate the electronic properties of 2D C3N (ZCNNRs)/ Ni(111) systems, in Fig. 2, we plot projected density of states (PDOS) on C and N atoms. The strong bonding between C and Ni atoms facilitates the charge transfer, which may lead to the changes of electronic properties in 2D C3N and ZCNNRs. Thus, we also compute the amount of charges transfer by adopting Mulliken population analysis [see Table 1]. Calcu­ lations show that for the 2D C3N/Ni(111) system, there is a ~0.631 e

Fig. 2. DOS of 2D C3N/Ni(111), 4-ZCNNR-CNCN/Ni(111), 4-ZCNNR-CCCC/Ni(111), and 5-ZCNNR-CCCN/Ni(111) systems. 3

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the 2D C3N and its quasi-one-dimensional nanoribbons have strong potential for nanoscale spintronics applications. We take M1 and M4 as examples to investigate the mechanisms of spin-polarization, PMR, and NMR effects. In Fig. 3(a) and (b), we first present the spin-resolved transmission probability of M1 at the parallel and antiparallel magnetic configurations. It is found from Fig. 3(a) that for the spin-up state of M1↑↑, a high transmission peak can be found at the occupied side, but it is far away from the Fermi level. At the Fermi level, the transmission coefficient is low (~0.0327), as shown in Table 2. For the spin-down state of M1↑↑, however, a relatively high transmission coefficient (~0.0485) is observed at the Fermi level. As a result, a weak spin-polarization effect appears in M1↑↑. To understand the origin of this spin-polarization effect, in Fig. 4(a) and (b), we plot the positiondependent local DOS from left electrode extension to right electron extension in M1↑↑. Compared Fig. 4(a) with Fig. 4(b), one obvious feature is that the spin-down state has a larger density of states in the electron extension regions, This hints that the spin-down state has lower contact resistance between the C3N sheet and the Ni(111) surface, and thus higher electron injection ability. Meanwhile, we also note that the electron transmission mode of the spin-up and spin-down states is the barrier tunneling due to the computed Fermi levels going through the band gap of 2D C3N sheet [35]. However, the barrier is lower in the spin-down state, which is based on the fact that the stronger spin-down electron injection ability results in the changes of electronic states of those atoms close to the electrode extension regions. This feature gives rise to the difference of spin-up and spin-down coefficients, which eventually leads to the emergence of spin-polarization effect in M1↑↑. Next we further explore the origin of PMR effect in M1. It is found from the lower panels of Fig. 4(a) and (b) that the spin-polarized trans­ mittance for the spin-down channel weakens obviously when the rela­ tive orientation of the spins for two Ni(111) electrodes goes from parallel to antiparallel. Clearly, the corresponding transmission coeffi­ cient has been reduced from 0.0485 to 0.0267 at the Fermi energy, giving rise to an increase in the resistance of M1↑↓. Thus, we also present the position-dependent local DOS in the spin-down state of M1↑↓ to explore the origin of this phenomenon [see Fig. 4(c)]. It is noted from Fig. 4(b) and (c) that for the spin-down state, the width of the potential barrier increases when the magnetic configuration of Ni(111) is changed from parallel to antiparallel. Furthermore, it is also noted that the right electrode extension region in the spin-down state of M1↑↓ presents lower density weights than that in M1↑↑. This is due to fact that the down electrons in the right electrode is the minority channel due to the change of magnetization direction in the right bulk Ni(111). So it is predicted that the resistance will rise for the spin-down state of M1↑↓. In order to understand the spin-transport mechanisms in M4, in Fig. 3 (c) and (d), we plot the spin-dependent transmission spectra of M4↑↑ and M4↑↓. It is found that for M4↑↑, the spin-down transmission coefficient at the Fermi level is higher (the corresponding transmission coefficient values can be found in Table 2). This leads to a typical spin-polarization phenomenon. To investigate the origin of this spin-polarization effect,

Table 2 The transmission coefficient, spin-polarization efficiency, and magnetoresis­ tance in the corresponding two-probe systems. The ↑↑ and ↑↓ indicate the magnetic configurations of Ni(111) electrodes are parallel and anti parallel, respectively. Property

M1

M2

M3

M4

T↑↑ ð ↑Þ

0.0327

0.0065

0.6519

0.2070

T↑↑ ð↓Þ

0.0485

0.0105

1.2549

0.7659

T↑↓ ð ↑Þ

0.0404

0.0067

1.3066

0.8015

T↑↓ ð↓Þ

0.0267

0.0076

1.0817

0.3906

SPE (↑↑) SPE (↑↓) MR

19% 20% 21% (PMR)

24% 6.3% 19% (PMR)

32% 9.4% 25% (NMR)

57% 34% 23% (NMR)

magnetic field, these devices can be set as parallel (↑↑) and antiparallel (↑↓) magnetic configurations. Thus the corresponding two-probe con­ figurations are marked as M1↑↑-M4↑↑ (M1↑↓- M4↑↓) when the Ni elec­ trodes’ magnetic configuration is set in parallel (antiparallel). In Table 2, we present the spin-polarization efficiency (SPE) of M1↑↑-M4↑↑ (M1↑↓- M4↑↓) at zero bias, which is based on the formula � � � � �T↑↑ ð↑Þ T↑↑ ð↓Þ� �T↑↓ ð↑Þ T↑↓ ð↓Þ� �¼� � SPE ¼ �� (3) T↑↑ ð↑Þ þ T↑↑ ð↓Þ� �T↑↓ ð↑Þ þ T↑↓ ð↓Þ� where T↑↑ ð ↑Þ and T↑↑ ð↓Þ [T↑↓ ð ↑Þ and T↑↓ ð↓Þ] represents the spin-up and spin-down transmission coefficients of the device with the parallel (antiparallel) magnetic configuration, respectively. Clearly, for M1↑↑, the SPE is ~19%. When the magnetic configuration of Ni electrodes is set in parallel, conversion of 2D C3N to quasi-one-dimensional nano­ ribbons produces higher SPE. The maximum SPE belongs to M4↑↑, and it can reach ~57%, showing a significant increase in magnitude compared with that of M1↑↑. However, the situation is different when the magnetic configuration of these systems is changed from parallel to antiparallel. For M1↑↓, its SPE is ~20%. Compared with that of M1↑↓, the SPE is reduced in M2↑↓ and M3↑↓ but increased in M4↑↓.We also further calculate the magnetoresistance (MR) ratio of M1-M4. For M1 and M2, their resistance values rise when the magnetic configuration is changed from parallel to antiparallel, resulting in positive MR (PMR) effect. Here the PMR is defined by the equation PMR ¼

½T↑↑ ð↑Þ þ T↑↑ ð↓Þ� ½T↑↓ ð↑Þ þ T↑↓ ð↓Þ� T↑↓ ð↑Þ þ T↑↓ ð↓Þ

(4)

However, for M3 and M4, the change of magnetic configuration gives rise to a decrease in the resistance value. Thus negative MR (NMR) effect appears, which is calculated by the expression NMR ¼

½T↑↓ ð↑Þ þ T↑↓ ð↓Þ� ½T↑↑ ð↑Þ þ T↑↑ ð↓Þ� ½T↑↑ ð↑Þ þ T↑↑ ð↓Þ�

(5)

From Table 2, we can find that at the zero bias, MR ratio of all sys­ tems exceeds 20%, with the exception of M2. These results indicate that

Fig. 3. (a) [(b), (c), and (d)] is the spin-resolved transmission of M1↑↑ (M1↑↓, M4↑↑, and M4↑↓) at zero bias. 4

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Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113861

Fig. 5. (a) [(b) and (c)] is the first transmission eigenstate in the spin-up state of M4↑↑ [the spin-down state of M4↑↑ and the spin-up state of M4↑↓ ] at the Fermi level and Γ point. (d) PDOS of upper and lower edge atoms in 5ZCNNR-CCCN. The projected regions of upper and lower edge atoms can be found at the top of the picture.

(~99.2% for the spin-up state, and ~98.2% for the spin-down state). Thus, we plot the transmission eigenstate of the first channel in the spinup (spin-down) state of M4↑↑ [see Fig. 5(a) and (b)]. Clearly, the dif­ ference of edge atoms in 5-ZCNNR-CCCN gives rise to a nearly vanishing eigenstate amplitude in the upper part, and a large density weights in the lower part. To explore the origin of the uneven distribution of eigenstate amplitudes, we further plot PDOS of upper and lower edge atoms in Fig. 5(d). Clearly, the electronic states near the Fermi level are domi­ nated by lower edge atoms. While for upper edge atoms, it is dominant in a higher or lower energy region. Therefore, the atoms in the lower part forms an effective π transmission channel, and dominate the elec­ tron transport in the both spin states of M4↑↑. Further observation shows that for the first transmission eigenstate of the spin-up state of M4↑↑, the corresponding charge density shows the spherical features, indicating a localized eigenchannel. For the spin-down state of M4↑↑, however, the charge density with strong polarization characteristics is observed, and a relatively delocalized transport channel is formed [see Fig. 5(b)]. We also plot the pathways of the electron transmitting for the both spin states in Fig. 6(a) and (b). It can be seen clearly that the electron transmission is mainly dominated by the atoms in the lower edge of 5ZCNNR-CCCN. Here two different transmission modes are found: (1) chemical bonds transmission; (2) electron-hopping transmission (The vertical black arrow in the bottom of 5-ZCNNR-CCCN demonstrates electron hopping between two atoms). Obviously, the chemical bonds transmission plays a leading role in the process of electron transfer. This is reflected in the fact that an effective current channel is formed via the chemical bonds between the lower edge atoms, as shown in Fig. 6(a) and (b). Meanwhile, it is found that the magnitude of the pathway in the spin-down state is stronger. Thus, in the transmitted region, the spindown electrons show a stronger ability to cross the boundary between 5-ZCNNR-CCCN and the right bulk Ni(111), which leads to an obvious spin-polarization phenomenon. Next we further investigate the NMR effect observed in M4, which is an interesting physical phenomenon in the field of spintronics. From the upper panels of Fig. 3(c) and (d), we are surprised to find that the transmission coefficient in the spin-up state increases obviously after the magnetic configuration is tuned. This gives

Fig. 4. (a) [(b) and (c)] is the position-dependent local DOS from left electrode extension to right electron extension in the spin-up state of M1↑↑ [the spin-down states of M1↑↑ and the spin-down states of M1↑↓ ].

we compute the transmission eigenvalues in the spin-up and spin-down states of M4↑↑. Results show that the both spin-up and spin-down states have multiple eigenchannels. However, further analysis indicates that only the first channel for both spin states is of significant weight 5

J. Zeng and Y. Zhou

Physica E: Low-dimensional Systems and Nanostructures 118 (2020) 113861

rise to a decrease in the resistance of M4↑↓. To explain the origin of this phenomenon, we also present the corresponding transmission eigenstate and electron transmission pathway in the spin-up state of M4↑↓. Compared Fig. 5(c) with Fig. 5(a), it can be seen clearly that the po­ larization feature of charge density become more obvious when the magnetic configuration of bulk Ni electrodes goes from parallel to antiparallel, resulting in strong hybridization in the right 5-ZCNNRCCCN/Ni(111) interface. The corresponding electron transmission pathway also indicates that for the spin-up state of M4↑↓, there is a more efficient chemical bonds channel at the bottom edge of 5-ZCNNR-CCCN, and thus more electrons are injected into the right Ni(111), as shown in Fig. 6(a) and (c). As a consequence, the conductance in the spin-up state of M4↑↓ increases obviously, and eventually leads to a reduction in resistance of M4↑↓. 4. Conclusion By using first-principles quantum transport calculation, we investi­ gate the electronic and spin-transport properties of 2D C3N sheet and its corresponding nanoribbons. The results show that the energy gaps of the free C3N sheet and 4-ZCNNR-CNCN disappear, and strong spin-polarized characteristics are observed near the Fermi level when they are stacked on the top of the Ni(111), giving rise to an interesting semiconductor-tometallic spin-filtering phenomenon. Moreover, we find spinpolarization transport features in all constructed two-probe systems. Especially impressive is that the PMR and NMR effects are observed. The PMR effect in the 2D C3N-based two-probe system mainly originates from the increase of the width of the potential barrier in the spin-down state when the magnetic configuration of Ni(111) is changed from parallel to antiparallel. While for the NMR effect in the 5-ZCNNR-CCCNbased two-probe configuration, it originates from the fact that for the spin-up state, the polarization feature of charge density become more obvious after the magnetic configuration is tuned, resulting in strong hybridization in the right 5-ZCNNR-CCCN/Ni(111) interface. Our re­ sults confirm that C3N has important applications in future spintronic devices. Author statement The corresponding author of this manuscript confirm that all authors listed are aware that the manuscript is now being considered at Physica E: Low-dimensional Systems and Nanostructures. Declaration of competing interest The authors declared that they have no conflicts of interest to this work. Acknowledgements This work was supported by the Hunan Provincial Natural Science Foundation of China (No. 2019JJ40006), by Scientific Research Fund of Hunan Provincial Education Department (No.18B368), by the Science and Technology Plan Project of Hunan Province (No. 2016TP1020), by the Science and Technology Development Plan Project of Hengyang City (Nos. 2017KJ159 and 2018KJ121), and by the National Natural Science Foundation of China (Nos. 11804093, 61764005). References [1] T. Song, X. Cai, M.W.-Y. Tu, X. Zhang, B. Huang, N.P. Wilson, K.L. Seyler, L. Zhu, T. Taniguchi, K. Watanabe, M.A. McGuire, D.H. Cobden, D. Xiao, W. Yao, X. Xu, Science 360 (2018) 1214–1218. [2] Z.-Q. Fan, W.-Y. Sun, X.-W. Jiang, Z.-H. Zhang, X.-Q. Deng, G.-P. Tang, H.-Q. Xie, M.-Q. Long, Carbon 113 (2017) 18. [3] L.-P. Tang, L.-M. Tang, H. Geng, Y.-P. Yi, Z. Wei, K.-Q. Chen, H.-X. Deng, Appl. Phys. Lett. 112 (2018), 012101.

Fig. 6. (a) [(b) and (c)] is the transmission pathway at the Fermi level for the spin-up state of M4↑↑ [the spin-down state of M4↑↑ and the spin-up state of M4↑↓ ]. The arrow color means the electron-transport direction. The arrow volume means the magnitude of the pathway. The vertical black arrow in the bottom of 5-ZCNNR-CCCN demonstrates electron hopping between two atoms. 6

J. Zeng and Y. Zhou

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