Orbital Redistribution Enhanced Perpendicular Magnetic Anisotropy of

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Orbital Redistribution Enhanced Perpendicular Magnetic Anisotropy of CoFe3N Nitrides by Adsorbing Organic Molecules Zirun Li, Wenbo Mi, and Haili Bai ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.8b05198 • Publication Date (Web): 27 Apr 2018 Downloaded from http://pubs.acs.org on April 27, 2018

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ACS Applied Materials & Interfaces

Orbital Redistribution Enhanced Perpendicular Magnetic Anisotropy of CoFe3N Nitrides by Adsorbing Organic Molecules

Zirun Li, Wenbo Mi, * and Haili Bai *

Tianjin Key Laboratory of Low Dimensional Materials Physics and Preparation Technology, School of Science, Tianjin University, Tianjin 300354, China

*

Authors to whom all correspondence should be addressed. E-mail: [email protected] (W.B. Mi) and [email protected] (H.L. Bai)

ACS Paragon Plus 1 Environment

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

ABSTRACT

Organic/ferromagnetic spinterface plays a significant role in organic spintronics and the manipulation of spinterface will help to optimize the performance of molecular devices. Here, we systematically investigate how the magnetic anisotropy can be tailed by adsorbing different organic molecules on CoFe3N surface. It is found that the adsorption of C6H6, C6F6, and SC4H4 molecules on the FeCo hollow site enhances the perpendicular magnetic anisotropy (PMA) of CoFe3N. The redistribution of Fe/Co d orbitals near Fermi level has an important effect on the modulation of PMA. Asymmetric SC4H4 adsorbed system has a larger PMA than symmetric C6H6 and its halide due to the hybridization between S pz and Fe d z 2 orbitals instead of C atom. Our results indicate that appropriate organic molecule adsorption can improve the magnetic properties of ferromagnets which benefits to organic spintronic devices.

KEYWORDS: organic molecules, perpendicular magnetic anisotropy, CoFe3N, hybrid states, orbital redistribution


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INTRODUCTION

Organic spintronics aims to combine the potential applications in spintronics with the advantages of molecular electronics due to long spin lifetime and mechanical flexibility of organic materials.1-7 The multi-functionality of organic spintronics provides the possibility that allows the reduction of device sizes to a few molecules even one molecule.4-6 Organic spin valve is a typical device of organic spintronics and its magnetoresistance effect has been extensively studied.8-10 A crucial factor that determines the performance of organic spin valve is the hybrid organic/ferromagnetic interface, which affects the magnetic properties and spin transport process.11-15 Therefore, tuning the interfacial properties by the appropriate combinations of ferromagnetic surfaces and organic molecules is necessary to optimize the performance of organic spintronic devices.14-20 The hybrid organic/ferromagnetic state has a strong impact on the magnetic anisotropy of ferromagnets.16,18-21 Atodiresei et al. demonstrated that C6F6 can stabilize the out-of-plane magnetization of Fe surface.18 Pang et al. pointed that a C60 overlayer can enhance the perpendicular magnetic anisotropy of Ni film.20 Magnetic anisotropy, one of the most important parameters of future data storage system, defines the thermal stability of spin and the energy required for magnetization switching.22 Perpendicular magnetic anisotropy (PMA) can realize



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high thermal stability and low energy consumption, which is very important in spintronics.23 Therefore, the study of PMA is of great interest. In our previous works, the hybrid states of molecules/Fe4N24,25 and C6H6/Co2MnSi interfaces26 have been studied. Fe4N and Co2MnSi have strong interactions with organic molecule, but C6H6 adsorption didn’t increase the magnetic anisotropy of Co2MnSi.24-26 Fe4N is a promising ferromagnetic material due to its high magnetization, Curie temperature, and spin polarization, which exhibits an inverse tunneling magnetoresistance,27-29 but cubic symmetry makes it have a small magnetocrystalline anisotropy. It is predicted that the substitution of Co in the Fe4N lattice induces a large PMA of tetragonal CoFe3N.30 Therefore, in this work, we will report the modulation of organic molecules on the magnetic anisotropy of CoFe3N. Atodiresei et al. pointed out that there is a direct connection between electronegativity of organic molecule and magnetic properties at organic/ferromagnetic interface.18 Wang et al. found that reduced symmetry in the thiophene molecule leads to nonuniform spatial distribution of spin polarization.17 Inspired by these reports, we consider that the electronegativity and symmetry of organic molecules might influence the magnetic anisotropy of molecule/CoFe3N

spinterface.

Therefore,

the

impact

of

molecular

electronegativity and symmetry on magnetic anisotropy is investigated. C6H6 and its halides are used to study the impact of electronegativity on PMA of CoFe3N. SC4H4 is basic unit of polythiophene that is always used in organic spintronics,31-33


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and it has less symmetric than C6H6. So, we choose SC4H4 to study the effect of asymmetrical molecular structure on magnetic anisotropy. The results show that C6H6, C6F6, and SC4H4 molecules absorbed on FeCo-hollow site can enhance PMA of CoFe3N. SC4H4 adsorption has a stronger effect on PMA than C6H6 and C6F6, which originates from the orbital redistribution near Fermi level.

CALCULATION DETAILS AND MODELS

The calculations are carried out by the projector augmented wave method implemented

in

Vienna

Ab

Initio

Simulation

Package

code.34-36

The

generalized-gradient-approximation proposed by Perdew, Burke, and Ernzerhof for the exchange-correlation energy is used.37 Van der Waals force is considered by DFT-D2 method.38 CoFe3N(001) surface is modeled by a five layer-slab with a 2×2 in-plane periodicity, where a 15-Å vacuum layer is added along z axis. The atoms in the bottom layer are fixed at its bulk positions, whereas the others are relaxed until the force is weaker than 0.01 eV/Å. The magnetic anisotropy energy (MAE) is determined by the magnetic force theorem39 with the consideration of spin-orbit coupling and a 9×9×1 k-point grid in Brillouin zone is used. The MAE is calculated by performing two-step procedures.40 First, charge density is acquired by a self-consistent calculation for the collinear case. Second, reading the self-consistent charge density, two non-self-consistent calculations are performed



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for different orientations of the magnetization direction by considering spin-orbit coupling. Finally, MAE was obtained by MAE=E[100]–E[001], where E[100] and E[001] are the energies with the magnetization in the [100] and [001] directions. The MAE

on

the [100 ]

MAE iλ = ( E iλ

λth

atom

orbital

of

the

ith

atom

is

calculated

by

[ 001]

− E iλ ) / a 2 , where a represents the in-plane lattice constant.41 The

MAE of the ith atom can be obtained by MAE i = ∑ λ MAEiλ .

RESULTS AND DISCUSSION

Figure 1a shows the lattice structure of tetragonal CoFe3N. There are two adsorption terminations on CoFe3N(001) surface, namely, FeN and FeCo. For each termination, two adsorption sites are considered where C6H6 locates at the center and hollow sites of CoFe3N(001) surface. So for C6H6 molecule adsorption, four models FeN-center, FeN-hollow, FeCo-center, and FeCo-hollow are included. Among them, the FeN-center and FeCo-hollow models are studied since they are energetically favorable and they have stronger molecule-surface interaction. FeCo-hollow is the most stable model, so FeCo-hollow site is used to study the C6F6, C6Cl6 and SC4H4 molecules adsorption. For C6Cl6 adsorption, the C-Cl bond length increases greatly, even one C-Cl bond breaks, indicating that the C6Cl6 molecule is destroyed after adsorption on the FeCo surface. Compared with C6H6 and C6F6 molecules, the strength of C-Cl bond is weaker than C-H and C-F bonds.


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Figure 1. (a)-(b) The structure of bulk CoFe3N and molecules. (c)-(f) Top and side views of four adsorption models.

Due to the strong electronegativity of Cl atom, when C6Cl6 molecule is adsorbed on the FeCo surface, the charge density is displaced from C toward Cl. The charge transfer between Cl and Co atoms makes the C-Cl bond broken. So the C6Cl6 molecule adsorbed on the FeCo-hollow site is instability. Next, we will mainly discuss C6H6, C6F6, and SC4H4 molecules. Figure 1c-f shows the top and side views of molecules/CoFe3N interfaces. After molecules adsorption, molecule planes are no longer flat and the H/F atom shifts upward. The distances between molecules and FeN/FeCo surface is less than 2.00 Å, indicating four adsorption models are chemisorbed. Particularly, C6F6 molecule deviates away from FeCo-hollow site, which may be attributed to the change of charge density from C toward F because of a stronger electronegativity of F atom.18 In three FeCo-hollow models, a large position deviation of Fe/Co



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atoms appears in the interfacial layer, even inner layer. The large displacement of Fe/Co atoms (geometrical effect) brings a change of interlayer magnetic exchange coupling and then influences the magnetic anisotropy of CoFe3N.42 The adsorption energies (Eads ) of Figure 1c-f show that four adsorption models are energetically favorable. Molecule adsorption can change magnetic exchange interaction between the magnetic atoms,15,16 so we mark Fe/Co atoms that influenced by the molecules adsorption, where the magnetic moments are listed in Table 1. In C6H6-FeN-center and C6H6-FeCo-hollow models, Fet and Feb/Cob atoms have a decreased magnetic moment of 0.2 µB, which is consistent with C6H6/Fe/W(110) interface.18 For C6F6-FeCo-hollow model, Fet1 atom locates below the C-C bond, where its magnetic moment is reduced by 0.7 µB. The magnetic moment of Cob also decreases greatly. SC4H4 molecule locates FeCo-hollow site, where the change of

Table 1. The magnetic moments of Fe/Co atoms of four adsorption models and corresponding surfaces. Moment (µB)

FeN surface

C6H6-FeN-center

FeCo surface

C6 H 6

C 6 F6

SC4H4

Fet

2.302

2.049

3.040

2.773

2.963

2.855

Feb /Cob

2.424

2.268

1.343

1.019

0.789

0.953

Fet1

2.423

2.114

3.040

2.769

2.216

2.905


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magnetic moment is similar to C6H6 adsorption. The decrease of magnetic moment originates from the hybridization between pz states of molecules and d z 2 states of ferromagnetic atoms. Figure 2a shows the MAE of four adsorption models and clean surfaces. Positive value stands for PMA. The PMA of C6H6-FeN-center model slightly decreases with respect to FeN surface. However, in three FeCo-hollow models, molecule adsorptions make the PMA of CoFe3N increase greatly. The PMA of C6H6-FeCo-hollow model increases by nearly four times than FeCo surface. Compared with C6H6 adsorption, C6F6 adsorption slightly decreases PMA while PMA of SC4H4 adsorbed system is further increased, which has the largest PMA.

Figure 2. (a) The total MAE of four adsorption models and corresponding surfaces. (b)-(c) Orbital-resolved MAE and DOS of C6H6-FeN-center model and FeN surface.



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The variation of PMA is closely related to redistribution of Fe/Co d orbitals near Fermi level. Magnetic anisotropy depends sensitively on the details of the electronic states near Fermi level. Since the unoccupied states near Fermi level is mainly from minority-spin, we will focus on the contribution to MAE from the coupling between minority-spin unoccupied and majority- or minority-spin occupied states. Based on the second-order perturbation theory, MAE is expressed as43,44

MAE ∝ ξ 2 ∑

ψ o | Lˆz | ψ u

2

− ψ o | Lˆ x | ψ u

2

(1)

Eu − Eo

o ,u

where ψ o and ψ u indicate the occupied and unoccupied states, Eo and Eu are the eigen energies of the occupied and unoccupied states, respectively. ξ is the spin-orbit coupling constant. The MAE depends on the couplings between the occupied and unoccupied states through the orbital angular momentum operators

Lˆx and Lˆz . The small energy separation (Eu-Eo) between the occupied and unoccupied states is responsible for the variation of MAE.45 The nonzero coupling matrix elements of equation (1) include

z 2 Lˆx yz = 3 , of

xy Lˆx xz =1 , and

occupied/unoccupied

states,

xz Lˆz yz =1 ,

x 2 − y 2 Lˆz xy =2 ,

x 2 − y 2 Lˆx yz =1 .43 By considering the spin Yang

et

al.

further

pointed

out

that

unoccupied/occupied states from the same and opposite spins have opposite MAE contribution.46 It means that the variation of spin direction of unoccupied/occupied


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states can inverse the sign of MAE. Owing to the hybridization between C pz and Fe d z 2 orbitals, the local electronic structure of Fe changes significantly. Therefore, combined with spin-resolved local density-of-states (DOS), the orbital-resolved MAE is analyzed. In Figure 2b-c, compared with clean FeN surface, unoccupied minority-spin d z 2 states of Fet/Feb atoms in C6H6-FeN-center model disappear. Unoccupied minority-spin d z 2 state forms the nonzero matrix element

yz Lˆx z 2 , which contributes to the in-plane anisotropy (IMA).47,48 So,

IMA of d z 2 states decreases even reverses the MAE from in-plane to out-of-plane. In C6H6-FeN-center model, the energy separation between e1 (dxy+ d x2 − y 2 ) orbitals increases, hindering electron hopping between occupied and unoccupied e1 states. Since the coupling

x 2 − y 2 Lˆz xy

between minority-spin e1 states favors

PMA,46-48 PMA of dxy and d x2 − y 2 orbitals decreases. After C6H6 adsorption on FeN surface, majority-spin occupied e2 (dyz+dxz) states at Fermi level disappears, and e2 states appear in the form of nonzero matrix elements

yz Lˆz xz , and

yz Lˆx x 2 − y 2 ,

yz Lˆx z 2 . The superimposed contributions of these matrix

elements change MAE of dyz and dxz states. Next we mainly focus on the molecule adsorption on FeCo surface. Figure 3a gives the interfacial MAE of adsorption systems and clean FeCo surface. The interfacial MAE has a similar tendency to total MAE, indicating that hybrid organic/ferromagnetic interface plays an important role in increased PMA. Furthermore, the increased PMA is not just at the interface, even extends to inner



Figure 3. (a) Interfacial MAE of C6H6, C6F6, and SC4H4 adsorption models and FeCo surface. (b) Top view of clean FeCo surface. (c)-(f) Orbital-resolved MAE of Fe/Co atoms in C6H6, C6F6, and SC4H4 adsorption models and FeCo surface.

layer. In facts, in inner layer of adsorption systems, the large displacement of Fe/Co atoms occurs, but no Fe/Co displacement in clean FeCo surface exits [see Figure 3b]. The change of interlayer exchange interaction due to the geometrical effect is responsible for the increase of PMA in inner layer. In Figure 3c-f, orbital-resolved MAE of Fe/Co atoms in different adsorption systems are compared with clean FeCo surface. The MAEs of Fe/Co atoms on FeCo surface include the contribution of PMA and IMA, but C6H6 and SC4H4 adsorbed systems only have PMA contribution. In C6F6 adsorbed system, few


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orbitals show a particularly large PMA. Combined with DOS in Figure 4, the MAE contribution of different d orbitals is discussed. In Figure 3c and 4a, DOS of Fet/Fet1 atoms in FeCo surface around Fermi energy mainly comes from the minority-spin e1 state, whereas d z 2 state keeps away from Fermi level. In C6H6 adsorbed system, unoccupied d z 2 state of Fe t/Fet1 atoms disappears, but occupied d z 2 state appears. Due to the large energy separation (∆2=1.71 eV) of d z 2 and dyz

z 2 Lˆx yz

states, the contribution of

is weak. The shift of Fet e1 orbitals towards

to Fermi level in C6H6 adsorbed system decreases the energy separation (∆1) from 1.33 to 1.23 eV, thus enhancing PMA contribution of atom in clean FeCo surface,

z 2 Lˆx yz

x 2 − y 2 Lˆz xy . For Cob

between occupied d z 2 states and

Figure 4. (a)-(d) Orbital-resolved DOS of C6H6, C6F6, and SC4H4 adsorption models and FeCo surface.



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unoccupied e2 states is the main contribution and favors IMA, while disappeared

d z 2 and e2 states in C6H6 case lead to a reversal of magnetic anisotropy [see Figure 3d]. C6H6 adsorbed on FeN surface decreases PMA while increases PMA on FeCo surface. Due to the existence of hybrid states, the orbital-resolved DOS of Fe/Co atoms on different adsorption surfaces show a distinct variation and thus induce different PMA behavior. Maybe the magnetic interaction of Fe and Co atom in interfacial FeCo layer is responsible for the opposite behaviors. In C6F6 adsorbed system, the most significant change is that d z 2 state of Fet1 atom has a large PMA [see Figure 4c]. The hybridization of C6F6 adsorbed system is just at Fermi level, casing the appearance of majority-spin d z 2 states. In above discussions,

z 2 Lˆx yz

from minority-spin d z 2 occupied state favors IMA.

Yang et al. point out that the opposite spins of occupied and unoccupied states can change the MAE sign.46 So

z 2 Lˆx yz

from majority-spin d z 2 occupied state

supports PMA. Together with a small energy separation (∆2=0.52 eV) of d z 2 and dyz states, Fet1 d z 2 orbital exhibit a large PMA [see Figure 3e]. Below Fermi level,

the majority-spin occupied Fet1 e1 state arises, making

x 2 − y 2 Lˆz xy

turn to

favor IMA, so the MAE of d x2 − y 2 orbital decreases. The other notable change is the increased d x2 − y 2 orbital contribution of Cob atom [see Figure 3e]. Compared with C6H6-FeCo-hollow, the disappearance of unoccupied e1 state leads to the IMA contribution of

yz Lˆx x 2 − y 2

decrease, so the MAE of d x2 − y 2 orbital increases.

C6H6 and C6F6 molecules with different electronegativity are adsorbed on


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FeCo surface to study the impact of electronegativity on magnetic anisotropy. In the case of C6H6, the hybrid organic/ferromagnetic state is far below the Fermi level. While the hybridization of C6F6 adsorbed system is just at Fermi level. The difference of electronegativity gets rise to the change of charge distribution, and brings

the

shift

of

hybrid

organic/ferromagnetic

states.

The

hybrid

organic/ferromagnetic states lead to the orbital redistribution of Fe/Co atoms, finally casing the variation of PMA. Different from C6H6 and C6F6 molecules, C atom does not hybrid with Fe atom in SC4H4 adsorbed system, only S pz orbital has a strong hybridization with Fet d z 2 orbital. Fet d z 2 orbital also has a large PMA due to majority-spin d z 2 occupied states (-0.58 eV), similar to C6F6 adsorption system. The asymmetry of SC4H4 structure caused by S atom leads to the asymmetric magnetic interaction with Fe atom, thus inducing the asymmetry of magnetic anisotropy of Fet and Fet1 atoms. But more importantly, the coupling to S pz orbital does not alter Fe e1/e2 orbitals. Conversely, the hybridization with C pz orbital can influence the occupation of other orbitals. So PMA contribution of Fet/Fet1 e1 orbitals preserves. To clarify the discrepancies of MAE in three molecule adsorption models, Figure 5a gives the main d orbital-MAE contribution. The orbital-resolved Fet MAE (the Fet1 atom for C6F6 adsorption) in Figure 5b is used to explain the origin of enhanced PMA. The red and blue arrows represent spin-up and spin-down electrons. The green dotted arrows indicate the coupling between occupied and



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Figure 5. (a) Schematic of dominate d-orbital MAE contribution in three adsorption models and FeCo surface. (b) Orbital-resolved MAE of Fet atom in three adsorption models and FeCo surface. The inset gives total MAE of Fet d-orbitals. (c)-(d) SSP and line profiles at 0.33 Å above molecules in ([-0.4, 0.0]

eV) energy interval in C6H6 and SC4H4 adsorption models. (e) Schematic model of CoFe3N/polythiophene/CoFe3N spin valves.

unoccupied states and the values show energy separation. With the decrease of energy separation ∆1 in C6H6 and SC4H4 cases, PMA of d x2 − y 2 orbital increases [see Figure 5b]. However, the majority-spin d x2 − y 2 state in C6F6 case leads to the IMA contribution of

x 2 − y 2 Lˆz xy , so the MAE of d x2 − y 2 orbital decreases,

even reveals IMA. In SC4H4 and C6F6 cases,

z 2 Lˆx yz

from the majority-spin

d z 2 occupied states favors PMA, resulting in a large PMA of d z 2 orbital. The


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total MAE of Fe t d orbitals in the inset of Figure 5b show that molecules adsorption increases the Fet PMA, the trend of Fe t PMA agrees well with total and interfacial MAE of adsorbed systems. In SC4H4 case, owing to the hybridization with S pz orbital, the PMA contribution of

z 2 Lˆx yz

x 2 − y 2 Lˆz xy

increases and

appears to favor PMA. So in these molecule adsorbed systems, SC4H4

adsorption has the largest PMA. Figure 5c-d show the spatial spin polarization (SSP) at 0.33 Å above molecule surface of C6H6 and SC4H4 adsorption systems. The SSP at C/S sites that directly interacts with Fet atom is inversed. It means that the C/S pz orbitals strongly hybridize with the Fet d z 2 orbitals. Line profiles at the top also show change of spin polarization. Furthermore, the SSP of SC4H4 case exhibits asymmetry compared with C6H6. In C6H6 case, two C-Fe bonds are symmetric while one S-Fe bond in SC4H4 case brings the asymmetry of magnetic interaction of Fe atoms. So the asymmetry of SC4H4 structure induces a nonuniform SSP spatial distribution. By considering the potential applications of SC4H4/CoFe3N interface in organic

spintronics,

Figure

5e

shows

the

schematic

model

of

CoFe3N/polythiophene/CoFe3N organic spin valve. SC4H4 is adsorbed on FeCo and FeN surfaces to form different spinterfaces. SC4H4 adsorbed on FeCo surface increases the PMA while decreases PMA on FeN surface, so top and bottom electrodes have different coercive fields, allowing the creation of parallel and



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antiparallel states in organic spin valves by magnetic field. Similar to CoFeB/MgO/CoFeB perpendicular magnetic tunnel junction with good memory retention

and

low

write

current,22,23

the

perpendicular

CoFe3N/polythiophene/CoFe3N organic spin valve might be an important attempt for developing multifunctional organic spintronic devices.

CONCLUSION

In summary, our results demonstrate that employing an appropriate organic molecule can effectively enhance PMA of CoFe3N, which mainly originates from the redistribution of Fe/Co d orbitals near Fermi level due to the formation of hybrid organic/ferromagnetic states. Especially, asymmetric SC4H4 adsorbed on the FeCo surface has a stronger effect on PMA than C6H6 and it halide. The S pz-Fe d z 2 hybridization leads to the appearance of majority-spin Fe d z 2 states

near Fermi level, enhancing PMA of SC4H4 case. These results suggest that the detailed structure of organic molecule should also be considered when engineering the organic spintronics.

AUTHOR INFORMATION

Corresponding Author


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*

(W.B. Mi) E-mail: [email protected]

*

(H.L. Bai) E-mail: [email protected]

Author Contributions Z.L. and W.M. designed the outline of the manuscript and wrote the main manuscript text; H.B. contributed detailed discussions and revisions. All authors reviewed the manuscript.

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS

This work is supported by National Natural Science Foundation of China (51671142, U1632152), Key Project of Natural Science Foundation of Tianjin City (16JCZDJC37300).



REFERENCES

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(6) Camarero, J.; Coronado, E. Molecular vs. Inorganic Spintronics: the Role of Molecular Materials and Single Molecules. J. Mater. Chem. 2009, 19, 1678–1684. (7) Cinchetti, M.; Dediu, V. A.; Hueso, L. E. Activating the Molecular Spinterface. Nat. Mater. 2017, 16, 507–515.

(8) Ding, S.; Tian, Y.; Li, Y.; Mi, W.; Dong, H.; Zhang, X.; Hu, W.; Zhu, D. Inverse Magnetoresistance in Polymer Spin Valves. ACS Appl. Mater. Interfaces 2017, 9, 15644–15651.

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Table of Contents Graphic (TOC)


Orbital Redistribution Enhanced Perpendicular Magnetic Anisotropy of

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