3MnO3

Article pubs.acs.org/JPCC

Large Spatial Spin Polarization at Benzene/La2/3Sr1/3MnO3 Spinterface: Toward Organic Spintronic Devices Qian Zhang,† Li Yin,† Wenbo Mi,*,† and Xiaocha Wang*,‡ †

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology, Faculty of Science, Tianjin University, Tianjin 300354, China ‡ Tianjin Key Laboratory of Film Electronic & Communicate Devices, School of Electronics Information Engineering, Tianjin University of Technology, Tianjin 300384, China

ABSTRACT: The fascinating spinterfaces between benzene and La2/3Sr1/3MnO3 (LSMO) are investigated based on the firstprinciples calculations. LSMO is a traditional high spin-polarized material used as the electrode in organic spintronic devices. However, the understanding of spin characteristics at organic/LSMO interfaces is unclear. Therefore, we study the benzene/ LSMO spinterfaces to clarify the spin behaviors at organic/LSMO interfaces because benzene is the basic unit of organic molecules. It is found that SrO terminations have weak binding with benzene, showing the inert properties. More importantly, the positive spatial spin polarization expands into the benzene layer and vacuum layer in all of the models. To further make sure the credibility of the results, both the effect of the ordering of Sr atoms and the adsorption density of benzene are considered in our calculations. There is no spatial spin-polarization inversion above the benzene plane in all of the models. The results indicate that the organic layer can carry large spin polarization at organic/LSMO spinterfaces.

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INTRODUCTION In the past few decades, the properties of strontium-doped lanthanum manganites has been studied in various spintronic devices, such as the tunnel junctions and spin valves.1−3 The properties of the La1−xSrxMnO3 (LSMO) including the halfmetallicity and ferromagnetic order are significantly composition-dependent, which is necessary for spin injection in the spintronic devices, just appears when 0.17 < x < 0.5,1,4,5 while it carries the high Curie temperature of 370 K. Especially, the organic spintronic devices based on LSMO attract numerous investigators, and lots of efforts were devoted.3,6−10 The large magnetoresistance (MR) and spin injection were gained in the ferromagnet/organic/LSMO heterostructures, while the large MR mostly occupied below 200 K.3,11,12 The room-temperature MR appears in the LSMO/Alq3/Al2O3/Co multilayer, but it is very small.13 Therefore, the novelties of the organic/LSMO interface fascinate us. We have noted that lots of first-principles calculations focus on the details of interfaces between organic and ferromagnetic or antiferromagnetic metals, such as the benzene/Fe14 and benzene/Mn15 interfaces. Moreover, a large abundance of experiments16−18 on the spin-related interface, i.e., the spinterface,19 have been studied. In those structures, the organic molecules modify the spin-related properties at interfaces, for example, the spatial spin-polarization (SSP) © XXXX American Chemical Society

inversion. The chemical diversity of organics endue the spintronic devices more methods to control magnetic properties of the interface, such as the ligand modifying20,21 and the isomers transitions.22 The SSP inversion due to the hybridization between p and d orbitals is the common phenomenon at the interface between organic and magnetic metal.14,15 The MR in organic spin valves, carrying peculiar properties, is tightly bound up with the materials of electrodes. When Fe4N, Co, Fe, CoFeB, or Ni80Fe20 serve as the electrode,23−26 the MR is small; however a spin valve with LSMO electrodes3,11,12 often achieves a large MR. The MR is dependent significantly on the temperature. The large MR almost appears at low temperature;3,11,12 however, the roomtemperature MR is rarely in an LSMO organic spin valve. The much peculiar properties fascinate us to investigate the mystery of the spinterface in this work. To make the present work able to explain the common spinterface between the organic molecule and LSMO, we select the benzene molecule, which is the basic unit of large organic materials (such as the metal− phthalocyanine27 and metal−tetraazaprophyrin18), as the adsorbed molecule. Additionally, the effect of the ordering of Received: February 2, 2016

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DOI: 10.1021/acs.jpcc.6b01165 J. Phys. Chem. C XXXX, XXX, XXX−XXX

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The Journal of Physical Chemistry C

Figure 1. Structures and DOS of the O-LSMO and the D-LSMO. The LaO, MnAO, SrO, and MnBO layers are labeled in (a). Part (b) shows the half-metal properties of the LSMO.

Figure 2. Structures of different adsorption models. In the graphic, partial O octahedrons near the surface are drawn to make the reader under the rotation cleanly. The dashed line is the lattice.

Sr atoms and the adsorption density of benzene are considered as the supplement to our conclusion. In this work, the

mysteries of spinterface and mechanism of the large MR in organic/LSMO interface are discussed. B


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Figure 3. Charge difference of different adsorption models. The charge difference graphic gives the top and side views of the models. The charge accumulation (depletion) is in yellow (blue).

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keep less symmetry than that in O-LSMO (see Figure 1b).32 In this work, the main calculation is based on the O-LSMO; the D-LSMO is as the supplement. The half-metallicity can be guaranteed for both the O-LSMO and the D-LSMO models with U = 2.0 eV for Mn atoms (see Figure 1c). The bulk OLSMO has two LaO layers, one SrO layer, and three MnO layers (see Figure 1a). The Mn atoms located between the LaO layer and the SrO layer are named MnA, while the ones located between the LaO layer and the LaO layer are named MnB. Therefore, the O-LSMO(001) surface has four different terminations: MnAO, MnBO, LaO, and SrO terminations. In each termination, there are three adsorption sites: with the MnA (MnB, La, or Sr) atom beneath the C atom, C−C bonds, or the center of the benzene molecule, named as top, bridge, or center models (see Figure 2), respectively. The C atoms binding directly with metal atoms are named C_t in the top model, and C_b in the bridge model; the unbinding C atoms are just named C_u atoms (see Figure 2). Whether for the O-LSMO or D-LSMO, the substrate has seven atomic layers, and the bottom four atomic layers are fixed at the position in bulk LSMO. The O-LSMO models have a (2 × 2) periodicity at the surface (001).

CALCULATION DETAILS AND MODELS In this work, all of the calculations are based on the density functional theory28 performed by the Vienna Ab initio Simulation Package code.29 The generalized gradient approximation in the parametrization of Perdew−Burke−Ernzerhof is used by the plane-wave basis.30 The pseudopotential is built by the projector augmented wave method.31 For the Mn atom, we select U = 2.0 eV, which guarantees the half-metallicity of LSMO.32 The plane-wave basis is converged by the energy cutoff of 500 eV, and the models are relaxed until the force is weaker than 0.03 eV/Å. The van der Waals force based on DFT-D2 is considered in our calculations.33 To study the SSP distribution, the spin-polarized scan tunneling microscopy is calculated.34 The LSMO with a space group of P4mm has a lattice constant of a = 3.87 Å,35 and in this work, we select the LSMO with x = 1/3. As we know, we cannot exactly determine the relative position between La and Sr atoms. Therefore, we construct the ordered model (O-LSMO), i.e., one-third of the layers of La atoms are modified by Sr atoms (see Figure 1a)36,37 and the disordered LSMO model (D-LSMO), i.e., the Sr atoms C


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The Journal of Physical Chemistry C Table 1. Average Moment and Charge of Mn Atoms in Different Layersa moment (μB) MnAO

MnBO

LaO

SrO

I III V VII I III V VII II IV VI II IV VI

Bader (e)

top

bridge

center

surface

top

bridge

center

surface

3.44 3.27 3.35 3.64 3.54 3.28 3.43 3.46 3.83 3.70 3.65 3.42 3.55 3.51

3.44 3.27 3.35 3.63 3.54 3.28 3.43 3.46 3.82 3.70 3.65 3.42 3.55 3.51

3.59 3.17 3.34 3.64 3.68 3.17 3.39 3.47 3.88 3.65 3.64 3.42 3.54 3.51

3.59 3.17 3.34 3.64 3.61 3.19 3.40 3.46 3.84 3.70 3.65 3.43 3.54 3.51

11.28 11.20 11.26 11.29 11.28 11.21 11.21 11.30 11.36 11.29 11.32 11.27 11.27 11.30

11.28 11.20 11.26 11.29 11.28 11.21 11.21 11.30 11.36 11.29 11.32 11.27 11.27 11.29

11.30 11.20 11.28 11.29 11.29 11.23 11.20 11.33 11.36 11.30 11.36 11.25 11.22 11.31

11.29 11.18 11.26 11.30 11.30 11.18 11.21 11.30 11.39 11.26 11.32 11.27 11.29 11.29

a

The table gives the average moment value of atoms in the same layer. The top layer is defined as I, and the bottom layer is defined as VII. MnA and MnB carry 3.50 and 3.53 μB in bulk O-LSMO, respectively.

Table 2. Charge and Moment of Special Atoms, in the MnAO and MnBO Terminations MnAO terminations Bader (e) moment (μB)

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Mn C Mn C

MnBO terminations

MnA-t

MnA-b

MnA-c

MnB-t

MnB-b

MnB-c

11.26 4.09 3.32 −0.02

11.26 4.11 3.33 −0.02

11.26 4.06 3.65 0.00

11.26 4.11 3.47 −0.01

11.26 4.12 3.45 −0.01

11.28 4.06 3.73 0.00

On the basis of the Bader analysis theory,38−40 the charges of partial atoms are calculated. The average moment and charge of Mn atoms in the same layer are shown in Table 1. The average moment of Mn atoms in the same layer keeps similar, it even keeps equal in the top and bridge models with the MnAO, MnBO, and SrO terminations. The strong binding between benzene and O-LSMO for the top and bridge models can decrease the moment of Mn atoms at the first layer comparing to the clean surface models, especially in the MnAO and MnBO terminations; see the moment in Table 1. However, the moment of Mn atoms is increased significantly, when the Mn atom sitting under the center of benzene in both MnAO and MnBO terminations; see the Table 2. The charge of the Mn atoms in the LaO terminations increases relative to MnAO and MnBO terminations (see Table 1). The odd phenomenon is caused by the dangling bonds of La atoms. The LaO termination loss so numerous O atoms which may capture lots of charge from La atoms, as to redistribute excess charge leading to the charge increase of O and Mn atoms. Adsorption energy (Eabs) of different models is calculated (see Figure 4a), so as to confirm the adsorption type. Eabs is defined by Eabs = −(Esystem − Ebenzene − ELSMO), where the Esystem, Ebenzene, and ELSMO are the total energy of the system, benzene molecule, and the O-LSMO substrate, respectively. All of the adsorptions are classified as the exothermic adsorptions. The LaO termination adsorptions have the larger Eabs than others, indicating that the LaO termination adsorptions can be formed easily. In the MnBO termination models, they carry similar adsorbed energies, implying that the different adsorbed sites may appear simultaneously at the benzene/MnBO interface. To investigate SSP distribution of the space, we define the SSP as

RESULTS AND DISCUSSION The adsorption models are displayed in Figure 2, and the O octahedrons are in different colors to clarify their rotation. The O atoms sited at the center position move toward the second layers, and then, the O octahedrons are rotated along the plane which is perpendicular to the surface in top and bridge models; see Figure 2a,b,d,e,g,h,j,k. The rotations are significant in the MnBO and LaO terminations, while those are tiny in the MnAO and SrO terminations. For the center models, the strong distortions just appear in the MnBO and LaO terminations; the O octahedron rotates along the plane parallel to the O-LSMO surface in the MnBO termination, while the rotation appears in both the planes parallel and perpendicular to the O-LSMO surface in the LaO termination. The H atoms have no significant upward shift in all of adsorption models, keeping in the plane of C atoms. To study the binding between benzene and O-LSMO, charge differences are calculated, as shown in Figure 3. The charge difference is defined by Δρ = ρC6H6/LSMO − ρC6H6 − ρLSMO, where ρC6H6/LSMO, ρC6H6, and ρLSMO are the charge densities of the full system, isolated benzene, and the LSMO surface, respectively. The charge accumulation (depletion) is in yellow (blue); see Figure 3. Except for the LaO termination, benzene cannot bind strongly with the O-LSMO substrate in center models, whereas, in the top and bridge models, benzene binds strongly with the O-LSMO substrate, no matter in MnAO, MnBO, and LaO terminations. However, for SrO terminations, benzene just can bind weakly with the O-LSMO substrate in the top model, even no binding between benzene and OLSMO at the bridge and center adsorption models. It turns out that the LaO and MnBO terminations are more active than the other terminations; however, the SrO termination is inert. D


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Therefore, we just give the MnAO-top and MnBO-top graphics. Even the SrO terminations show 100% SSP in the whole space. The binding mechanism of the benzene/O-LSMO can be further extracted from the spin-resolved density of states (DOS). Therefore, the DOS graphics about the related binding atoms are shown, so as to illustrate origination of the positive SSP above benzene and the binding between benzene and OLSMO. Figure 5 shows the DOS of MnAO and MnBO terminations, respectively. For the MnAO terminations, we know that C atoms pz orbitals mix strongly with Mn atoms dz2 orbitals for spin-up states in the energy of −1.7 to 0.6 eV, whether at the top or bridge adsorptions (see Figure 5a). Those hybridization leads to the positive net spin for benzene, which indicates that strong spin injection happens at the interface. Additionally the strong hybridization between C_t pz and Mn_t dz2 atoms appears near the energy level of 3.7 eV for the spin-up states for MnAO-top models. In the MnAO-center models, although we cannot see the strong charge accumulation between the benzene and O-LSMO, the DOS of benzene is affected significantly; i.e., a little positive net spin appears at the Fermi energy for the benzene. MnBO-top has the similar behaviors at the Fermi energy (see Figure 5b). In the MnBObridge adsorption, the C_u atoms which do not bind with Mn atoms just show little net spin. In the MnBO-center adsorption, all the C atoms keep the localized properties, i.e., the strong and narrow peaks. We display the DOS graphic of the LaO and SrO terminations, in Figure 6. The benzene pz orbitals in LaO or SrO terminations occupy the lower energy level rather than that in other terminations; see Figure 6. In Figure 6a, we note that benzene pz orbitals mix with the La f orbitals nearby the Fermi energy. In the top model, besides the hybridization with the La f orbitals, benzene pz orbitals strongly mix with La dz2 and dxz + dyz at 1.9 eV. However, in the bridge model, the C_b pz orbitals hybridize significantly with the dxz + dyz orbitals of La_b atoms nearby the Fermi energy, and the C_u pz orbitals mix with dz2

Figure 4. Adsorption energy of different adsorbed models based on OLSMO substrate and the spatial spin polarization for MnAO-top and MnBO-top models. The height related to the benzene is about 3.0 Å.

Pspace =

ns↑(r , z , ε) − ns↓(r , z , ε) ns↑(r , z , ε) + ns↓(r , z , ε)

(1)

where the n↑(↓) (r, z, ε) is the spin-up (down) charge density in s real space with the energy interval of [ε, EF], at positions r and a distance z from the surface.15 Here, we select the ε = EF − 0.4 eV in this work.15 The SSP data are projected to the plane which is parallel to the benzene plane, and the distance between the data plane and benzene molecule is about 3 Å. The SSPs of MnAO-top and MnBO-top models are shown in the Figure 4b. The positive SSP above the benzene plane cannot be inversed by the hybridization between the pz orbitals of the organic molecule and d orbitals of the metal. The large positive SSP distributions of other adsorptions have no significant difference.

Figure 5. DOS of the MnAO and MnBO terminations. Part (a) shows the three models with MnAO termination, and (b) shows the three models with MnBO termination. E


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Figure 6. DOS of the LaO and SrO terminations. Part (a) shows the three models with LaO termination, and (b) shows the three models with SrO termination.

Figure 7. DOS of the 1 ML, 0.5 ML, and 0.25 ML models. Part (a) is the DOS of the O atom at the surface for the 0.5 ML models with different models. Part (b) is the DOS of 1 ML model, and (c) is the DOS of 0.25 ML.

orbitals strongly. In the center model, the benzene pz orbitals hybridize with the La f orbitals strongly. From the above analysis, benzene has strong hybridization with the LaO terminations, while the benzene does not show the strong spin-polarized at the Fermi energy. In Figure 6b, nearby the Fermi energy, benzene shows no spin-polarized states in the SrO terminations. The properties of localized orbitals of benzene, i.e., the narrow and strong peaks, are preserved, and there is no strong hybridization between benzene p and Sr s, p orbitals nearby the Fermi energy. Additionally, the binding between the benzene and Sr atoms is weak in the bridge and center models (see Figure 3j,k). However, the benzene pz orbitals hybridize with Sr dxz + dyz and dz2 orbitals at ∼0.8 eV in top and bridge models. The SrO

termination is so inert, so that SSP cannot be tailored by the benzene adsorptions. From the DOS analysis, the bridge and top models with MnAO or MnBO termination show strong positive spin polarization at the Fermi energy, illustrating that the C atoms’ positive spin polarization contributes to the positive SSP. C atoms have little positive spin polarization at the Fermi energy in DOS graphics for LaO and SrO terminations, indicating that the positive SSP above the benzene originates mainly from the O-LSMO substrate. To extend our conclusion, the effect of ordering of Sr atoms and adsorption densities of benzene are studied as the supplemental calculations. For studying the effect of adsorption densities of benzene, we select the MnBO-top not only because F


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Figure 8. DOS and SSP graphs of the benzene adsorbed on D-LSMO, and the model of the LSMO/benzene/LSMO spin valve. Part (a) is the DOS of LSO-top and MnO-top models. Part (b) is the SSP graphics. The SSP is the data plane above the benzene molecule; the height of the data plane is 3 Å relative to the benzene molecule plane. Part (c) is the model of the LSMO/benzene/LSMO spin valve, and the SSP distribution along z.

MnO-top adsorption is that two Mn atoms are located under the C atoms in the MnO termination. The benzene shows no positive spin polarization at the Fermi energy in the DOS graphic, and the benzene pz orbitals hybridized strongly with dz2, dxz+yz, and f orbitals of La atoms at 0.6 eV for the LSO-top model (see Figure 8a). In the MnO-top model, the spin-up states of C_t pz orbitals hybridize strongly with Mn_t dz2 orbitals at the Fermi energy. The SSP of LSOtop can keep the positive value, but the SSP value is not large as the O-LSMO adsorption. That is, the disorder dopant of Sr atoms will decrease the SSP injection (see Figure 8b). However, MnO-top adsorption keeps a large positive SSP above the benzene plane (see Figure 8b). From the above analysis, strong SSP injection happens at the benzene/LSMO interface, whether for O-LSMO or D-LSMO. The LSMO is one of the mostly potential magnetic electrodes in organic spin valves owing to the strong interfacial SSP injection. The SSP is inversed above the benzene molecule plane in the benzene/Fe4N41 and benzene/Fe14 due to p−d hybridization, while that does not at the benzene/LSMO interface. The preservation of the positive SSP above the benzene implies that the spin transport at the benzene/LSMO interface is strong. Therefore, the organic spin valves based on LSMO will carry large MR,3,11,12 whereas ones based on Fe4N or Fe do not.23,24 Because of the potential application of the benzene/LSMO interface in the spin valve, the spin valve based on the interface

it has a large net spin (see Figure 5) but also because the surface of substrates keeps large spin polarization in the MnBO termination (see Figure 7a). First, we construct the substrate with √2 × √2 and 2√2 × 2√2 periodicity, respectively. To distinguish those models, the √2 × √2, 2 × 2, and 2√2 × 2√2 periodic models are named as 1 ML, 0.5 ML, and 0.25 ML models, respectively. The DOS graphs of models with different densities of benzene are displayed in Figure 7b. In the 0.25 and 1 ML models, benzenes show the 100% spin polarization; i.e., the strong spin injection happens (see Figure 7b). Then, the adsorption density of benzene at LSMO will not influence the essential properties, i.e., the strong spin injection. The positive spin polarization is not influenced significantly, retaining the large positive SSP elongation. For studying the effect of ordering of Sr atoms, we build the adsorption models based on the D-LSMO. The half-metal properties of bulk DLSMO can be guaranteed in our work (see Figure 1c). The adsorption models are based on the D-LSMO (001) surface with seven atomic layers, i.e., three unit cells. There are two types termination for the D-LSMO: the LSO termination, which has La, Sr, and O atoms at the surface, and the MnO termination, which just has Mn and O atoms at the surface. We select the two adsorbed positions: LSO-top and MnO-top adsorptions. LSO-top adsorption is that two La atoms are located under the C atoms in the LaSrO termination; G


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C.; Heckmann, O.; Hricovini, K. Effects of Three-Dimensional Band Structure in Angle- and Spin-Resolved Photoemission From HalfMetallic La2/3Sr1/3MnO3. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 165120. (5) Ma, C.; Yang, Z.; Picozzi, S. Ab Initio Electronic and Magnetic Structure in La0.66Sr0.33MnO3: Stain and Correlation Effects. J. Phys.: Condens. Matter 2006, 18, 7717−7728. (6) Xu, W.; Szulczewski, G. J.; LeClair, P.; Navarrete, I.; Schad, R.; Miao, G.; Guo, H.; Gupta, A. Tunneling Magnetoresistance Observed in La0.67Sr0.33MnO3/Organic Molecule/Co Junctions. Appl. Phys. Lett. 2007, 90, 072506. (7) Wang, X. Z.; Ding, X. M.; Li, Z. S.; Zhan, Y. Q.; Bergenti, I.; Dediu, V. A.; Taliani, C.; Xie, Z. T.; Ding, B. F.; Hou, X. Y.; Zhang, W. H.; Xu, F. Q. Modification of the Organic/La0.7Sr0.3MnO3 Interface by in Situ Gas Treatment. Appl. Surf. Sci. 2007, 253, 9081−9084. (8) Arisi, E.; Bergenti, I.; Dediu, V.; Loi, M. A.; Muccini, M.; Murgia, M.; Ruani, G.; Taliani, C.; Zamboni, R. Organic Light Emitting Diodes with Spin Polarized Electrodes. J. Appl. Phys. 2003, 93, 7682−7683. (9) Majumdar, S.; Majumdar, H. S.; Laiho, R.; Ö sterbacka, R. Comparing Small Molecules and Polymer for Future Organic SpinValves. J. Alloys Compd. 2006, 423, 169−171. (10) Bergenti, I.; Dediu, V.; Arisi, E.; Mertelj, T.; Murgia, M.; Riminucci, A.; Ruani, G.; Solzi, M.; Taliani, C. Spin Polarised Electrodes for Organic Light Emitting Diodes. Org. Electron. 2004, 5, 309−314. (11) Dediu, V.; Murgia, M.; Matacotta, F. C.; Taliani, C.; Barbanera, S. Room Temperature Spin Polarized Injection in Organic Semiconductor. Solid State Commun. 2002, 122, 181−184. (12) Wang, F. J.; Yang, C. G.; Vardeny, Z. V.; Li, X. G. Spin Response in Organic Spin Valves Based on La2/3Sr1/3MnO3 Electrodes. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 245324. (13) Dediu, V.; Hueso, L. E.; Bergenti, I.; Riminucci, A.; Borgatti, F.; Graziosi, P.; Newby, C.; Casoli, F.; De Jong, M. P.; Taliani, C.; Zhan, Y. Room-Temperature Spintronic Effects in Alq3-Based Hybrid Devices. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 78, 115203. (14) Atodiresei, N.; Brede, J.; Lazić, P.; Caciuc, V.; Hoffmann, G.; Wiesendanger, R.; Blügel, S. Design of the Local Spin Polarization at the Organic-Ferromagnetic Interface. Phys. Rev. Lett. 2010, 105, 066601. (15) Caffrey, N. M.; Ferriani, P.; Marocchi, S.; Heinze, S. AtomicScale Inversion of Spin Polarization at an Organic-Antiferromagnetic Interface. Phys. Rev. B: Condens. Matter Mater. Phys. 2013, 88, 155403. (16) Rizzini, A. L.; Krull, C.; Balashov, T.; Kavich, J. J.; Mugarza, A.; Miedema, P. S.; Thakur, P. K.; Sessi, V.; Klyatskaya, S.; Ruben, M.; Stepanow, S.; Gambardella, P. Coupling Single Molecule Magnets to Ferromagnetic Substrates. Phys. Rev. Lett. 2011, 107, 177205. (17) Stepanow, S.; Mugarza, A.; Ceballos, G.; Moras, P.; Cezar, J. C.; Carbone, C.; Gambardella, P. Gaint Spin and Orbital Moment Anisotropies of a Cu-Phthalocyanine Monolayer. Phys. Rev. B: Condens. Matter Mater. Phys. 2010, 82, 014405. (18) Klanke, J.; Rentschler, E.; Medjanik, K.; Kutnyakhov, D.; Schönhense, G.; Krasnikov, S.; Shvets, I. V.; Schuppler, S.; Nagel, P.; Merz, M.; Elmers, H. J. Beyond the Heisenberg Model: Anisotropic Exchange Interaction between a Cu-Tetraazaporphyrin Monolayer and Fe3O4(100). Phys. Rev. Lett. 2013, 110, 137202. (19) Sanvito, S. Molecular Spintronics: The Rise of Spinterface Science. Nat. Phys. 2010, 6, 562−564. (20) Atodiresei, N.; Caciuc, V.; Lazić, P.; Blügel, S. Engineering the Magnetic Properties of Hybrid Organic-Ferromagnetic Interfaces by Molecular Chemical Functionalization. Phys. Rev. B: Condens. Matter Mater. Phys. 2011, 84, 172402. (21) Friedrich, R.; Caciuc, V.; Kiselev, N. S.; Atodiresei, N.; Blügel, S. Chemically Functionalized Magnetic Exchange Interactions of Hybrid Organic-Ferromagnetic Metal Interfaces. Phys. Rev. B: Condens. Matter Mater. Phys. 2015, 91, 115432. (22) Wang, Y.; Che, J. G.; Fry, J. N.; Cheng, H. P. Reversible Spin Polarization at Hybrid Organic-Ferromagnetic Interfaces. J. Phys. Chem. Lett. 2013, 4, 3508−3512.

is designed to increase the efficiency (see Figure 8c). When the moments of LSMO electrodes are keeping the parallel orientation, the two benzene/LSMO interfaces carry SSP with the same sign, leading to the low resistance states, whereas, when the LSMO electrodes are keeping the antiparallel moments, the interfaces carry the opposite sign SSP, leading to the high resistance states. The strong SSP injection at benzene/LSMO interfaces induces the large MR, increasing the efficiency of the organic spin valve. Additionally, the similar spin valve with the same materials as the two electrodes has been realized in CoFeB/MgO/CoFeB.42 That switch between low resistance and high resistance has significant potential for multifunctional spintronic devices.

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CONCLUSION The O-LSMOs with different terminations show significant distinct spin properties, such as the strong spin injection happens in both MnAO and MnBO terminations and benzene shows the strong hybridization between C pz orbitals and La f orbitals; however, the SrO terminations show inert properties. Moreover, the adsorption density influences the essential properties weakly; the effect of disordered dopant of the Sr decreases the SSP, but keeping the positive sign, in the LSOtop models. However, the SSP above the benzene plane keeps the same sign with LSMO substrates, whatever the terminations are. Those results indicate that the organic/LSMO structure, especially the benzene/LSMO structure, can keep the strong spin transport in the organic layer. The discovery of large MR in the LSMO/organic/FM structure is due to the spread of SSP. Therefore, this paper shows the spinterface dependence on the different terminations, to guide the different spintronics device development, and displays the efficient spin valve models.

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (W.M.). *E-mail: [email protected] (X.W.). Author Contributions

Q.Z. and W.M. designed the outline of the manuscript and wrote the main manuscript text. L.Y. and X.W. contributed detailed discussions and revisions. All the authors reviewed the manuscript. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work is supported by the National Natural Science Foundation of China (51171126) and the Program for New Century Excellent Talents in University (NCET-13-0409). It is also supported by the High Performance Computing Center of Tianjin University, China.

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REFERENCES

(1) Salamon, M. B.; Jaime, M. The Physics of Manganites: Structure and Transport. Rev. Mod. Phys. 2001, 73, 583−628. (2) Coey, J. M. D.; Viret, M.; von Molnár, S. V. Mixed-Valence Manganites. Adv. Phys. 1999, 48, 167−293. (3) Xiong, Z. H.; Wu, D.; Vardeny, Z. V.; Shi, J. Giant Magnetoresistance in Organic Spin-Valves. Nature 2004, 427, 821− 824. (4) Krempaský, J.; Strocov, V. N.; Patthey, L.; Willmott, P. R.; Herger, R.; Falub, M.; Blaha, P.; Hoesch, M.; Petrov, V.; Richter, M. H


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The Journal of Physical Chemistry C (23) Li, Z. R.; Wang, X. C.; Dai, H. T.; Mi, W. B.; Bai, H. L. Spin Dependent Transport and Magnetic Properties in Fe4N/Tris(8Hydroxyquinoline) Aluminum/Co Organic Spin Valves Fabricated by Facing-Target Sputtering. Thin Solid Films 2015, 588, 26−33. (24) Jiang, J. S.; Pearson, J. E.; Bader, S. D. Absence of Spin Transport in the Organic Semiconductor Alq3. Phys. Rev. B: Condens. Matter Mater. Phys. 2008, 77, 035303. (25) Schoonus, J. J. H. M.; Lumens, P. G. E.; Wagemans, W.; Kohlhepp, J. T.; Bobbert, P. A.; Swagten, H. J. M.; Koopmans, B. Magnetoresistance in Hybrid Organic Spin Valves at the Onset of Multiple-Step Tunneling. Phys. Rev. Lett. 2009, 103, 146601. (26) Santos, T. S.; Lee, J. S.; Migdal, P.; Lekshmi, I. C.; Satpati, B.; Moodera, J. S. Room-Temperature Tunnel Magnetoresistance and Spin-Polarized Tunneling through an Organic Semiconductor Barrier. Phys. Rev. Lett. 2007, 98, 016601. (27) Sun, X.; Wang, B.; Yamauchi, Y. Electronic Structure and Spin Polarization of Metal (Mn, Fe, Cu) Phthalocyanines on an Fe(100) Surface by First-Principles Calculations. J. Phys. Chem. C 2012, 116, 18752−18758. (28) Kohn, W.; Sham, L. J. Self-Consistent Equations Including Exchange an Correlation Effects. Phys. Rev. 1965, 140, A1133−A1138. (29) Kresse, G.; Furthmüller, J. Efficient Iterative Schemes for Ab Initio Total-Energy Calculations Using a Plane-Wave Basis Set. Phys. Rev. B: Condens. Matter Mater. Phys. 1996, 54, 11169−11186. (30) Perdew, J. P.; Burke, K.; Ernzerhof, M. Generalized Gradient Approximation Made Simple. Phys. Rev. Lett. 1996, 77, 3865−3868. (31) Blöchl, P. E. Projector Augmented-Wave Method. Phys. Rev. B: Condens. Matter Mater. Phys. 1994, 50, 17953−17979. (32) Wang, C.; Stojić, N.; Binggeli, N. Optimal Interface Doping at La2/3Sr1/3MnO3/SrTiO3(001) Heterojunctions for Spintronic Applications. Appl. Phys. Lett. 2013, 102, 152414. (33) Bučko, T.; Hafner, J.; Lebégue, S.; Á ngyán, J. G. Improved Description of the Structure of Molecular an Layered Crystals: Ab Initio DFT Calculations with van der Waals Corrections. J. Phys. Chem. A 2010, 114, 11814−11824. (34) Wortmann, D.; Heinze, S.; Kurz, P.; Bihlmayer, G.; Blügel, S. Resolving Complex Atomic-Scale Spin Structures by Spin-Polarized Scanning Tunneling Microscopy. Phys. Rev. Lett. 2001, 86, 4132− 4135. (35) Tsui, F.; Smoak, M. C.; Nath, T. K.; Eom, C. B. StrainDependent Magnetic Phase Diagram of Epitaxial La0.67Sr0.33MnO3 Thin Films. Appl. Phys. Lett. 2000, 76, 2421−2423. (36) Picozzi, S.; Ma, C.; Yang, Z.; Bertacco, R.; Cantoni, M.; Cattoni, A.; Petti, D.; Brivio, S.; Ciccacci, F. Oxygen Vacancies and Induced Changes in the Electronic and Magnetic Structures of La0.66Sr0.33MnO3: a Combined Ab Initio and Photoemission Study. Phys. Rev. B: Condens. Matter Mater. Phys. 2007, 75, 094418. (37) Zheng, B.; Binggeli, N. Effects of Chemical Order and Atomic Relaxation on the Electronic and Magnetic Properties of La2/3Sr1/3MnO3. J. Phys.: Condens. Matter 2009, 21, 115602. (38) Henkelman, G.; Arnaldsson, A.; Jónsson, H. A Fast and Robust Algorithm for Bader Decomposition. Comput. Mater. Sci. 2006, 36, 354−360. (39) Sanville, E.; Kenny, S. D.; Smith, R.; Henkelman, G. Improved Grid-Based Algorithm for Bader Charge Allocation. J. Comput. Chem. 2007, 28, 899−908. (40) Tang, W.; Sanville, E.; Henkelman, G. A Grid-Based Bader Analysis Algorithm without Lattice Bias. J. Phys.: Condens. Matter 2009, 21, 084204. (41) Zhang, Q.; Mi, W. B.; Wang, X. C. Antiferromagnetic Order at the First Fe4N Atomic Layer in Benzene Adsorbed Fe4N Structures. J. Phys. Chem. C 2015, 119, 23619−23626. (42) Ikeda, S.; Hayakawa, J.; Ashizawa, Y.; Lee, Y. M.; Miura, K.; Hasegawa, H.; Tsunoda, M.; Matsukura, F.; Ohno, H. Tunnel Magnetoresistance of 604% at 300 K by Suppression of Ta Diffusion in CoFeB/MgO/CoFeB Pseudo-Spin-Valves Annealed at High Temperature. Appl. Phys. Lett. 2008, 93, 082508.

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