Fe3O4 Organic Magnetic Tunnel Junctions

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Large Magnetoresistance in FeO/4,4’-Bipyridine/ FeO Organic Magnetic Tunnel Junctions 3

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Meifang Sun, Xiaocha Wang, and Wenbo Mi J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.7b11583 • Publication Date (Web): 23 Jan 2018 Downloaded from http://pubs.acs.org on January 24, 2018

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The Journal of Physical Chemistry

Large Magnetoresistance in Fe3O4/4,4’-Bipyridine/Fe3O4 Organic Magnetic Tunnel Junctions

Meifang Sun†, Xiaocha Wang‡, Wenbo Mi*,†

†

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

‡

School of Electrical and Electronic Engineering, Tianjin University of Technology, Tianjin 300384, China

*

Author to whom all correspondence should be addressed. E-mail: [email protected]

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The Journal of Physical Chemistry 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 magnetic tunnel junctions (OMTJs) are promising systems thanks to their chemically tunable electronic property, long spin lifetime and easy functionalizations. Here, the spin-dependent electronic transport properties in Fe3O4/4,4’-bipyridine/Fe3O4 OMTJs are investigated by first-principles quantum transport calculations. Since the transport properties of junctions are sensitive to the device details, two types of terminations of Fe3O4 electrodes are considered. The device with tetrahedral Fe termination shows an anomalous negative tunnel magnetoresistance (TMR), i.e., which has a higher and lower junction resistance in the parallel and anti-parallel magnetization configurations, respectively. When the contact termination is octahedral Fe, a large positive TMR of 180% appears. The difference in TMR sign of two OMTJs originates from the electrons transmission mediated by frontier molecular levels coupled differently to Fe d states. Furthermore, TMR can be effectively controlled by applied electrical bias by changing states of octahedral Fe involved in transport, which can reach 22000% at 0.1 V. Moreover, a perfect spin-filter effect is demonstrated irrespective of the contact geometry. The results contribute to a fundamental understanding of spin-dependent transport properties in OMTJs.


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INTRODUCTION

The magnetic tunnel junction (MTJ), as one of the spintronic devices, has aroused great interest due to its important applications such as magnetic sensors, read heads of hard disk drive and magnetic random access memory.1 The basic MTJ consists of two ferromagnetic (FM) electrodes separated by a non-magnetic layer. Owing to the match and mismatch of density of states (DOS) for the spin-up and spin-down electrons of two FM electrodes, the resistance of MTJ depends on the relative orientation of magnetization in two FM electrodes. Thus, the MTJ can work as a spin valve and a high tunneling magnetoresistance (TMR) value is desired. In traditional MTJs, FM electrodes are usually Fe, Co, Ni and other alloys like CoFeB, while the tunnel barriers are typically Al2O3 or MgO.2-4 More recently, the spin-dependent transport properties in organic magnetic tunnel junctions (OMTJs), where the non-magnetic layer is an organic material, have received much attention since the junctions can be chemically manipulated for better functionality.5-7 Particularly, the organic molecules have a long spin-relaxation time due to its low weak spin-orbit coupling and hyperfine interactions, which is very promising for spintronic applications.8-10 Meanwhile, the usage of organic molecules opens up a route to downscale these devices into the nanometer scale by using single molecule. The spin transportation in a single organic molecule provides insights into the microscopic understanding of the device operation.11 A large amount of effort has been made to improve the TMR value in OMTJs.12-22 A TMR of up to 80% has been observed in single C60 molecule transistors with ferromagnetic Ni electrodes.21 In a Ni/hydrogen phthalocyanine (Pc)/Ni magnetic junction, a TMR of ~60% was measured by the 3 ACS Paragon Plus Environment


spin-polarized scanning tunneling microscopy.22 It should be noted that the sign and magnitude of TMR in an OMTJ are sensitive to the microscope details between organic molecules and FM electrodes at the interface.7,23,24 Owing to the spin-dependent filtering effects caused by FM/molecule interface hybridization, a positive MR of 300% at 2 K25 appeared in the (La, Sr)MnO3/Alq3/Co structure although the MR was once reported as -40% at 11 K in the same structure.9 The Co/AlOx/NaDyClq/NiFe devices exhibit a negative TMR, whereas swapping the NiFe and Co electrodes results in a positive TMR signal.26 Indeed, the electronic structures of the FM/molecule interfaces play an important role in the spin-dependent transport properties. Accordingly, the term “spinterface” has been coined for that behavior.27 The interfacial spin polarization can vary from a simple enhancement to a complete inversion due to the strong interfacial bonding.25,28 Hence, understanding the properties of spinterface on the spin-polarized charge carrier transport in OMTJs is extremely desired. In this work, the connection between FM/molecule interfacial characteristics and TMR effects are established through the first-principles quantum transport calculations. It is well known that the FM electrodes with a high spin polarization can enhance TMR response. In this context, compared with the generally used FM electrodes Fe, Co and Ni, the metal oxide CrO2, La2/3Sr1/3MnO and Fe3O4 has attracted much interest due to their large spin polarization near 100% at EF. Among these candidates for spintronics, Fe3O4 is especially promising since it has a good room temperature conductivity and high Curie temperature of 850 K.29,30 Here, a prototypical OMTJ device is constructed by sandwiching single planar organic molecule 4,4’-bipyridine (BP) between two Fe3O4 electrodes, and two model geometries with different terminations are considered to investigate the 4 ACS Paragon Plus Environment

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influence of contact geometry on the electronic structures and transmission properties. It is found that even though the main difference between two considered OMTJs is the contact terminations, the anomalous negative TMR behavior appears in one OMTJ, while the other has a positive TMR of 180% at equilibrium. Moreover, the transmission of OMTJs is affected by electrical bias, which leads to an effective electrically controlled TMR. Irrespective of the contact geometry, a highly spin-polarized current is always obtained in OMTJs.

COMPUTATIONAL DETAILS AND MODELS

The present OMTJ consists of a scattering region connected to the left and right semi-infinite Fe3O4(111) electrodes which extend to z = ±∞ . Particularly, bulk Fe3O4 has an inverse spinel structure, where O ions constitutes a close-packed face-centered cubic structure with Fe2+ and Fe3+ cations located in the interstitial sites. One site is occupied by tetrahedrally coordinated FeA (Fe3+ ions), and the other is occupied by octahedrally coordinated FeB (Fe3+ and Fe2+ ions), as shown in Figure 1b. The spin coupling between FeA and FeB is antiferromagnetic. Owing to the lattice symmetry, Fe3O4 lattice consists of periodic units of six atomic planes of either only O2- anions or Fe3+

and

Fe2+

cations

along

the

(111)

direction

with

the

stacking

sequence

FeA1-O1-FeB1-O2-FeA2-FeB2,31 as shown in Figure 1c. However, the recent results show that the FeA1 and FeB2 terminations are the most stable ones.32,33 Therefore, the organic molecule in this work is considered to be contacted by FeA1 or FeB2 on both sides to investigate the effect of microscopic contact configuration. For simplicity, these device models are named as A-Configuration and 5 ACS Paragon Plus Environment


Figure 1. (a) Spin-resolved density of states (DOS). EF is indicated by vertical lines and set to zero. (b) Cubic inverse spinel structure of bulk Fe3O4. (c) Side view of Fe3O4(111) surface structure. (d) BP molecule adsorbed on the Fe3O4(111) surface in a periodic supercell.

B-Configuration, respectively (See Figure 2a,c). The atomic structure optimization is performed Vienna Ab Initio Simulation Package using Perdew-Burke-Ernzerhof generalized gradient approximation.34-36 The structure of bulk Fe3O4 is first relaxed. The lattice constant obtained is 8.483 Å, and the calculated band structures (Figure 1a) are in well agreement with the previous results.37-39 Then two models of BP molecule adsorbed on Fe3O4(111) surface is constructed based on the optimized bulk, and the FeA1 terminated-adsorption model as an example is shown in Figure 1d. The N atom of BP just locates above the FeA (FeB) atom. During the structure relaxations, the bottom 5 layers atoms of Fe3O4 are frozen to their bulk positions while all other atoms is allowed to relax until the forces are smaller than 0.02 eV/Å. Sufficient vacuum is included in the supercell to avoid the coupling between adjacent slabs. 6 ACS Paragon Plus Environment

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Furthermore, the van der Waals interactions are included in the calculations. The relaxed N-FeA and N-FeB bond lengths are 2.08 and 2.02 Å, respectively. Finally, the two-probe geometry is obtained by means of mirroring since the BP molecule and OMTJs models are perfectly symmetric structures, as shown in Figure 2a,c. The contact details of two configurations are shown in the dashed rectangles of Figure 2. The scattering region consists of the BP molecule plus two units (thirty-six

Figure 2. The atomic structure of (a) A-Configuration with FeA1 termination and (c) B-Configuration with FeB2 termination after relaxation. The transport direction is along the z-axis and the two electrodes extend to z = ±∞ as shown by the left/right arrows. (b), (d) Computed spin-resolved transmission spectra Tσ(E) for PC and APC configurations of the A- and B-Configurations at zero bias, respectively. Two dashed rectangles in right panels correspond to the enlarged view of the dashed rectangles in (a) and (c), respectively. 7 ACS Paragon Plus Environment


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units (thirty-six layers) of Fe3O4 on either side. A 20-nm vacuum layer is chosen around the electrode in the transverse x and y directions to suppress the interaction between device and its mirror image. So the Fe3O4 electrodes in this study are quasi-one dimensional Fe3O4 wires with a 2×2 cross section, composed of periodic units along the (111) direction. The quantum transport calculation is based on real-space density functional theory (DFT) carried out within the nonequilibrium Green’s function (NEGF) framework using Nanodcal transport package.40 The energy E and spin channel σ ( σ =↑, ↓ representing the spin up and spin down) resolved current is calculated by Landauer formula

µ

Iσ (Vb ) =

e R Tσ ( E, Vb )( f L − f R )dE h µ∫

(1)

L

where uL(R) are the chemical potentials of the left (right) electrodes, Tσ the transmission coefficient with spin σ, e the electron charge, fL(R) the Fermi-Dirac functions of the left (right) electrodes, and h the Planck’s constant. The applied bias voltages VL and VR are applied to the left and right electrodes, which is similar to an external potential field that shift the corresponding potential of the electrodes from its original potential values. The potential difference Vb = VL − VR is dropped at the scattering region. Then Nanodcal solves the Hartree potential according to the boundary conditions of the Hartree potential of the scattering region at the electrode/central interface is equal to that of the electrode. The relationship between Vb and chemical potentials satisfies the relation of

µ L − µ R = eVb . 8 ACS Paragon Plus Environment

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The spin filter efficiency and TMR ratio at a voltage are defined as

η% =

| I↑ (V ) − I ↓ (V ) | I ↑ (V ) + I ↓ (V )

TMR% =

%

I PC (V ) − I APC (V ) % I APC (V )

(2)

(3)

Herewith, IPC and IAPC are the total current for the OMTJs in parallel (PC) and anti-parallel magnetization configurations (APC), respectively. The PC and APC configurations of two OMTJs are visualized by the spatial distribution of spin density, as shown in Figure 3. In the equilibrium state, the current value is obtained by using T(EF), where EF is Fermi level. The Brillouin zone for the electrodes is sampled by a 1×1×100 k-point grid. The valence electrons are treated by linear combination of atomic orbital basis with double-zeta plus polarization orbital basis functions for all the atoms. The charge is calculated through density matrix and real space charge density in Nanodcal and can be further decomposed into contributions from the different atoms. The charge transfer is obtained through calculating the difference between the charge of the two-electrode transport system (OMTJ) and the charge of the isolated molecule or the two-electrode geometry but remove the molecule. In the self-consistent DFT or NEGF-DFT calculation, the convergence criteria are set so that every element of the Hamiltonian matrix and density matrix are restricted to 5×10-5 eV.



RESULTS AND DISSCUSSION

Figure 2b,d show the energy-resolved transmission coefficient Tσ(E) at zero bias of A- and B-Configurations, respectively. In both cases, EF locates in the gap of the Tσ(E), where T(EF) takes extremely small values (see inset), indicating a tunneling regime of OMTJs. In PC, the most prominent difference between A- and B-Configurations in the transmission near EF is the peak in the spin-down channel, as shown in Figure 2b,d. In A-Configuration, a peak appears just above EF, whereas the broader peak is located below EF in B- Configuration. Both peaks can be traced back to the interfacial states derived from the lowest unoccupied molecular orbital (LUMO) of organic BP molecule which is coupled to Fe d states at the binding site. The differences in the contact geometry of two OMTJs result in different bonding characteristics between Fe3O4 electrode and BP molecule, which have a great effect on the spin-dependent transport properties. More details will be given in the following text. Furthermore, the transmission at EF with a large weight is spin-up in A-Configuration, whereas it is the spin-down dominance in B-Configuration (inset of Figure 2). It means that the spin filter efficiency η of two OMTJs contributes from different spin channels, but they are very substantial, η of ~100%. That’s to say, the present OMTJs show an excellent spin injection effect at equilibrium. In APC, the spin-up and spin-down channels display the similar transmission due to the symmetry of the device models, so the η is zero. Moreover, the calculated TMR at equilibrium is -44% and 180% for A- and B- Configurations, respectively. This negative TMR in A-Configuration is striking because the junction is perfectly symmetrical. Based on Julliere model,41 TMR can be expressed as 10 ACS Paragon Plus Environment

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TMR% =

2 PL PR ×100% (1 − PL PR )

(4)

where PL(R) is the spin polarization of left (right) electrodes. In the symmetric MTJ, the weighted spin polarization at the left and right interfaces is theoretically the same. Thus, the positive TMR should be observed in the MTJs with the same electrodes. Recently, the abnormal negative TMR also appears in a symmetric Co/CoPc/Co OMTJ.42 This work shows that the dissimilar FM/molecule geometry induced by experimental fabrication condition causes the different coupling strength at Co/CoPc and CoPc/Co spinterfaces, which results in the inversion of the TMR sign. However, the left and right electrodes in present OMTJs are obtained by means of mirrors, which exactly have the same left and right spinterfaces. Up to now, other results on negative TMR in symmetric OMTJs were related to the inversion of hybrid interfacial states, or the presence of applied bias or strain.19,21,23,43-45 Whereas the anomalous negative TMR in present OMTJs is at equilibrium condition, and it also cannot be ascribed to the inversion of interfacial states since the interfacial FeA and FeB keep the original sign of spin polarization. Nevertheless, the hybrid states between organic BP molecule and Fe3O4 cannot be ignored. In order to analyze the hybridization, the density of states (DOS) of two OMTJs for PC and APC are shown in Figure 3, which are projected onto the FeA, FeB and N atoms at the left electrode/molecule interface (labeled FeA1, FeB1 and N1) and at the right molecule/electrode interface (labeled FeA2, FeB2 and N2). The alignment of magnetic moment between FeA and FeB is 11 ACS Paragon Plus Environment


Figure 3. (a), (e) Spin-up and (b), (f) spin-down DOS of the OMTJs projected onto the interfacial atoms of N, FeA and FeB for PC in A- and B-Configurations at equilibrium. Spin-up DOS of the OMTJs projected onto (c), (g) N1, FeA1, FeB1 and (d), (h) N2, FeA2, FeB2 atoms for APC in A- and B-Configurations at equilibrium. Subscripts 1 and 2 represent left and right molecule/electrode interface atoms. To facilitate comparison, the DOS of Fe atom is divided by 10. The plots above PDOS correspond to the spatial distribution of spin density of OMTJs for both PC and APC in Aand B-Configurations.

antiparallel. Owing to the interaction with Fe3O4 electrodes, the organic molecular states have a spin-dependent energy shifting and broadening. In A-Configuration, since the electronic states of FeA atoms with a large weight are spin-up near EF, a peak at EF produced by the LUMO of BP molecule hybridized with FeA d states is spin-up for PC (see Figure 3a). Note that the LUMO here is defined according to the energy distribution of the DOS of N atom at the molecule/electrode interface. However, the DOS of N atom includes the characteristics of all the frontier orbitals of BP 12 ACS Paragon Plus Environment

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molecule, which makes it easy to explain the influence of interfacial hybridization on the molecular electronic structures and thus the change of the transport properties of OMTJs. Furthermore, the organic BP molecule/Fe3O4 interactions accompany with the charge transfer. The result shows that although the whole BP molecule loses charge of 0.23e in A-Configuration, an N atom gains 0.04e due to the strong electronegativity of N atom. This agrees with the shift of the spin-up LUMO state of N atom from above to slightly below EF, causing the LUMO state to be partially occupied, as shown in Figure 3a. Furthermore, perceptible hybridizations are found near EF in Figure 3a, in which the states of BP molecule and Fe3O4 electrode share similar DOS distributions. However, there is almost no hybridization between the LUMO of molecule and FeA d state in the spin-down channel, and the LUMO remains unoccupied (see Figure 3b). Differently, the state of the FeB atom directly bonded to the N atom at the molecule/electrode interface is the spin-down dominant for PC in B-Configuration. Therefore, the hybridization of molecule with electrode mainly occurs in the spin-down channel. The calculated charge transfer indicates that the BP molecule gains 0.36e from electrodes, where one N atom gets 0.1e. Compared to the A-Configuration, the N atom gets more electrons in B-Configuration. This results that not only the LUMO state, but also the LUMO+1 state shifts toward the shallow level, as shown in Figure 3f. As a consequence, the LUMO state is fully occupied and the LUMO+1 state is partially occupied at EF. Similarly, no available FeB states are hybridized with the molecule near EF in the spin-up channel, and the molecular states remain unoccupied, as displayed in Figure 3e. These main hybridized peaks for PC in A- and B-Configurations form the transferring channels at equilibrium condition. Consequently, the conductive electrons from one electrode across organic BP 13 ACS Paragon Plus Environment


molecule/Fe3O4 interface to the other electrode in two OMTJs have an opposite spin channel, which supports the transmission results where the transmission of two OMTJs show opposite spin polarization, as mentioned above (inset of Figure 2). The change in the magnetization of Fe3O4 electrodes does not affect the charge transfer, but cause the DOS of BP molecule to be redistribution near EF, as shown in the right panels of Figure 3. Here, only the spin-up PDOS of the left and right interface atoms is given because the PDOS of FeA1,2, FeB1,2 and N1,2 atoms for spin-up are the same as the PDOS of FeA2,1, FeB2,1 and N2,1 for spin-down, respectively. In A-Configuration, the molecular energy level splits in one spin direction near EF when the magnetic configuration is change from PC to APC, as shown in Figure 3d, where one of the states shift towards EF. So in this case the electronic states of the left and right electrodes are well matched (see Figure 3c,d). The same argument is valid for the spin-down states. The transmission spectrum depends on the overlap of atomic states of the left and right electrodes. Therefore, both spin-up and spin-down electrons can flow through the OMTJs, which indicates a high conductance in APC configuration. Differently, although similar molecular energy splitting occurs in B-Configuration as the magnetic configuration of electrode changes, the states at EF are not significant, as shown in Figure 3g,h. So a high resistance in APC is expected as compared to the PC case in B-Configuration. In order to further illustrate the effect of different electronic interactions at molecule/electrode interfacese on the transport properties of OMTJs, localized density of states (LDOS) is analyzed by different colors along transport direction (z direction) at zero bias, which visualizes how electrons traverses the OMTJs. In A-Configuration (Figure 4a-d), the energy level of organic BP molecule is 14 ACS Paragon Plus Environment

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Figure 4. LDOS by different colors along the transport direction (z direction) of A-Configuration at equilibrium. (a) Spin up, (b) spin down states in PC and (c) spin up, (d) spin down states in APC configurations. Black dashed lines indicate the EF.

indicated by the bright regions in the center area, which obviously shows that the conductance in OMTJ is mostly dominated by LUMO of BP molecule. Moreover, except for the spin-down in PC where the calculated EF goes through the band gap of organic BP molecule, EF aligns well with the LUMO of molecule, indicating that the electrons can easily transmit through the LUMO of organic BP molecule from one electrode to the other. Thus the spin-up and spin-down electrons can be easily traversed through OMTJ in APC, which is consistent with the PDOS results in Figure 3. In PC, however, only spin-up electrons can flow through the OMTJ, and the transmission of spin-down electrons is completed blocked. Consequently, the total conductance in APC is larger than that in PC, which results in the negative TMR sign in A-Configuration. Figure 5 shows the LDOS of B-Configuration. In PC, the LUMO+1 level of organic BP 15 ACS Paragon Plus Environment


Figure 5. LDOS of the B-Configuration at equilibrium. (a) Spin up, (b) spin down in PC and (c) spin up, (d) spin down states in APC Configurations. Black dashed lines indicate the EF.

molecule for spin-down channel are very close to EF although the density of states is not large (see Figure 5b). Therefore, in this case the spin-down electrons is partially transmitted, which is well consistent with the transmission results of Figure 2d. Different from the A-Configuration, a high resistance appears when the magnetic configuration changes from PC to APC in B-Configuration, as shown in Figure 5c,d. The EF locates between LUMO and LUMO+1, thus the electrons to transmit through the electrode-molecule interface at EF will be hindered. Consequently, the APC one is expected to be more conductivity than the PC one and a large positive TMR appears. The above results suggest that the TMR sign is strongly dependent on the FM/molecule coupling and the resulting hybrid spin-dependent states. Additionally, since the states are sensitive to the degree of coupling of FM electrode with BP molecule at interfaces, the magnitude of the positive TMR (180%) is clearly different than the negative one (-44%). Furthermore, the 16 ACS Paragon Plus Environment

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Figure 6. (a), (c) Spin-resolved current IPC(APC), TMR and (b), (d) spin filter efficiency η versus bias voltage in A-Configuration (top panels) and B-Configuration (bottom panels), respectively. Here, IPC and IAPC is the total current obtained by

∑ σ Iσ

for the PC and APC spin configurations,

respectively. The black spheres in (a) are used to highlight the negative TMR.

bias-dependent TMR is shown in Figure 6a,c for A- and B-Configurations, respectively. The TMR does not monotonically decrease when the bias of the OMTJs increases as usually appeared in experiments, reaching a maximum value of 1900% at bias of 0.3 V in A-Configuration and of 22000% at 0.1 V in B-Configuration. A similar variation of the TMR as a function of bias has been observed in C60 bilayer barrier sandwiched between two Fe electrodes,46 however, where the calculations shown that the maximum of TMR can only reach 21%. The calculated total current-voltage curves for A- and B-Configurations in both PC and APC are displayed in Figure 6a,c, respectively. In both OMTJs, the currents increase non-monotonically with the bias. Particularly, IPC is larger than IAPC above 0.1 V in A-Configuration, which results in a transition from negative to large positive TMR. 17 ACS Paragon Plus Environment


In B-Configuration, an appreciable larger IPC value than IAPC leads to a maximum of TMR at 0.1 V. What’s more, a common feature of two OMTJs is that both IPC and IAPC notably reduce when the bias increases from 0.1 to 0.15 V, which implies a negative differential resistance (NDR) effect. Additionally, as the bias increases from 0.1 to 0.25 V in A-Configuration, the decay of TMR versus bias is transparently from the decreased growth rate of IPC and quick increase of IAPC, as shown in Figure 6a. A similar case can also be found in B-Configuration as the TMR changes with bias. What’s more, Figure 6b,d display the spin filter efficiency η at different bias for both PC and APC in two OMTJs. The obvious features can be found: the η almost always reaches 100% for PC and the minimum is also up to 77% for APC in two OMTJs, showing a perfect spin filter effect. In

Figure 7. Spin-up (red) and spin-down (blue) transmission spectra for PC at the bias of (a), (d) 0.1, (b), (e) 0.15 and (c), (f) 0.3 V in A- (left panels) and B-Configurations (right panels). The vertical lines indicate the bias window.


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order to explain the current-voltage characteristics, the transmission properties of OMTJs within the bias window are important. Since both OMTJs have very similar bias-dependent transport behaviors for PC and APC, we only present the spin-resolved transmission coefficient of PC at 0.1, 0.15 and 0.3 V for two OMTJs, as shown in Figure 7. The dotted lines represent the bias window. Note that the transmission peaks within the bias window are mainly spin-down states to dominate currents, so a large η appears for both OMTJs. The appearance of such a large η can be linked to the negative spin polarization of Fe3O4 which is the spin-down electrons of FeB to contribute to the conductance. Similar explanations for individual Fe3O4 have been reported,47 in which the Fe3O4 acts as an efficient spin filter that only the spin-down states of FeB can transport through the Fe3O4. Further explanations will be given through calculating the density of scattering states (DOSS) in the following text. At 0.1 V, the transmission is characterized by a main broad peak above EF, which corresponds to the conduction orbital LUMO of BP molecule. However, this peak height decreases dramatically when the bias increases from 0.1 to 0.15 V, which leads to an obvious reduction in IPC(APC) and resultant a NDR behavior in OMTJs. Particularly, as the bias reaches 0.3 V in A-Configuration, the integral area of T↓ reaches the maximum values. As a result, the largest IPC appears, leading to a large TMR value. However, the spin-down transmission peak is smaller at a bias of 0.3 V in B-Configuration, resulting in a decrease in the current, as shown in Figure 6c. Nevertheless, the molecule/electrode coupling is not affected by bias since the molecular energy levels are almost pinned on the electrochemical potentials of the Fe3O4 electrodes (not shown here). For further understand the transport mechanism, the DOSS is calculated. Scattering states are eigen-states of the Hamiltonian of the open device structure. The DOSS gives the number of 19 ACS Paragon Plus Environment


Figure8. DOSS of atoms in (a), (b) central region (C-DOSS), (c), (d) left electrode (L-DOSS) and (e), (f) right electrode (R-DOSS) for PC in B-Configuration at bias of 0.1 V (left panels) and 0.15 V(right panels).

scattering states per interval of energy, which is proportional to the conductance at that energy. Figure 8 shows the DOSS of atoms in central region (C-DOSS), left electrode (L-DOSS) and right electrode (R-DOSS) for PC in B-Configuration at bias of 0.1 V (left panels) and 0.15 V(right panel). The similar discussion can be obtained in A-Configuration. Only the spin-down DOSS is presented since the spin-up states are zero, consistent with the transmission results. According to the DOSS at applied bias, it can be seen that the participating transport states are mainly FeB-dominant. At 0.1 V, the FeB states that contribute to the conduction effectively transport from one electrode through LUMO of BP molecule to the other electrode, which leads to the appearance of the transmission peak, as discussed above. However, increasing the bias to 0.15 V, there is almost no DOSS of FeB in the right electrodes within bias windows to participate in transport. Consequently, the transmission


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peak is reduced at 0.15 V, appearing a NDR effect. This suggests that the FeB states involved in transport affect the conduction of OMTJs at applied bias. Quantitatively, the calculated TMR value of present OMTJs reaches a maximum of 22000% at a bias of 0.1 V. Experimentally, a similar two-electrode structure of Fe3O4/graphene/Fe3O4 junction has been fabricated with MR values of -1.6%.48 It should be noted that the existence of uncontrollable formation of growth defects and chemical-off stoichiometry in experiments may affect the device performance. For example, numerous studies have shown that the presence of antiphase domain boundary defects in Fe3O4 leads to their transport and magnetic properties strongly deviating from the single-crystal bulk.49-51 However, a simplified model is adopted in this paper where the metal electrode Fe3O4 has an ideal single-crystal structure. From an applicable point of view, more comprehensive studies such as the effects of structural defects and surface disorder are necessary in the future work. Furthermore, recent calculations on [(Ge5)Fe]∞-BDT-[(Ge5)Fe]∞ molecular junctions showed that the highest MR ratio can reach 21100%.52 However, a highly spin-polarized current is lacked when the magnetization configuration changes. The OMTJs present here show that in addition to the extremely large TMR value, a perfect spin filter effect can be obtained independent of the contact geometry and magnetization configuration. The Fe3O4/BP/Fe3O4 junction should be a promising candidate for future organic molecular spintronic devices and the results provide the theoretical guidance for experiments toward its practical realization.

CONCLUSIONS 21 ACS Paragon Plus Environment


In conclusion, the spin-dependent electronic transport in BP molecule contacted with Fe3O4 electrodes with two types of terminations is investigated by first-principles quantum transport calculations. The contact details are found to be crucial to determine the sign of TMR at equilibrium. The anomalous negative TMR of -44% appears in A-Configuration, but of a large positive of 180% in B-Configuration. The difference in TMR induced by the contact configurations is derived from the electron transport mediated by the frontier molecular levels coupled with the interfacial Fe atoms. Furthermore, the bias-dependent TMR is calculated, with the maximum TMR reaching 22000% in B-Configuration. It is found that the FeB states that effectively involve in the transport mediate the conductance of OMTJs through the LUMO of BP molecule. Additionally, a highly spin-polarized current is obtained in A- and B-Configurations, that is, only the spin-down carriers can transport through the junctions. The results highlight the importance of contact details between molecule and electrode for the spin-polarized quantum transport properties in OMTJs, which is necessary for the design of future organic molecular spintronic devices.


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AUTHOR INFORMATION

Corresponding Author *E-mail: [email protected]

Notes The authors declare no competing ﬁnancial interest.

ACKNOWLEDGEMENTS

This work is supported by National Natural Science Foundation of China (51671142 and U1632152), Key Project of Natural Science Foundation of Tianjin (16JCZDJC37300), Program for New Century Excellent Talents in University (NCET-13-0409).



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Fe3O4 Organic Magnetic Tunnel Junctions

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