Magnetic Polymer Chains of Transition-Metal Atoms and Zwitterionic

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C: Physical Processes in Nanomaterials and Nanostructures

Magnetic Polymer Chains of Transition-Metal Atoms and Zwitterionic Quinone Hassan Denawi, Mathieu Abel, and Roland Hayn J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.8b12433 • Publication Date (Web): 23 Jan 2019 Downloaded from http://pubs.acs.org on February 6, 2019

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The Journal of Physical Chemistry

Magnetic Polymer Chains of Transition-Metal Atoms and Zwitterionic Quinone Hassan Denawi*, Mathieu Abel and Roland Hayn Aix Marseille Université, CNRS, IM2NP UMR 7334, 13397, Marseille, France.

ABSTRACT Ab initio calculations based on density functional theory (DFT) including an explicit treatment of the strong electron correlation in the d-shell of the transition metal ions have been conducted using the spin polarized generalized gradient approximation with Hubbard term U (SGGA+U) to investigate systematically the electronic and magnetic properties of a new material class representing one-dimensional transition-metal zwitterionic quinone (TM-ZQ) polymers having many potential applications, especially in spintronics. The complete class of 3d transition metals (TM) are investigated from Sc to Zn. Zn-ZQ is nonmagnetic since it has a 3d10 configuration. All the other TM-ZQ polymers are antiferromagnetic semiconductors with the exception of Mn-ZQ that is metallic for the ferromagnetic (FM) and the antiferromagnetic (AFM) spin arrangement, and Sc-ZQ and Ti-ZQ which are FM semiconductors. All these polymer chains have the potential to be produced by on-surface synthesis on metallic surfaces as was recently shown for Fe-ZQ (Koudia, M; et al. Nano Res. 2017, 10, 933–940) .

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INTRODUCTION Organic molecules with a quinonoid structure constitute a large and important class of compounds with remarkable chemical and physical properties. These zwitterions appear to be reagents of choice: in organic chemistry,1 in coordination chemistry, as new ligands in the stepwise synthesis of polynuclear complexes,1,2 and in biochemistry as precursors to a bioinhibitors-related OH-substituted aminoquinone.3 The electrically neutral molecules carry positive and negative charges on opposite parts of the central ring, the positive charge being delocalized between the nitrogen moieties over four bonds involving 6 π-electrons, while the negative charge is likewise spread between the oxygen atoms. The zwitterions form molecular arrangements whose structures are thought to be determined by their strong dipole and by the Hbonding intermolecular interactions involving the O and N−H groups of adjacent molecules.4,5 Recently, it could be experimentally demonstrated that zwitterionic quinone (ZQ) and Fe may build long one-dimensional (1D) polymer chains on the Ag(111)6 and the Au(110) surfaces7 by on-surface polymerization being not possible in the gas phase or in wet conditions. The Fe-ZQ polymers are bound by covalent bonds between the Fe2+ ion and the 4 neighboring ligands, two O and two N. The polymers build also a well-ordered arrangement on the metallic substrates of micrometer size. It is important to note that the ab-initio calculations for Fe-ZQ on the 2 different substrates indicate a spin state transition between S=1 (per Fe atom) on Ag(111) and S=2 on Au(110) since the polymer adapts its lattice constant to the underlying substrate. On the other hand, there is no remarkable charge transfer between Fe-ZQ polymer and the substrate. The electronic and magnetic structures for a free-standing polymer or one absorbed on the metal surface are nearly identical, once the lattice constants coincide.


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Low-dimensional metal-organic covalent networks are very actively studied at present due to their high potential of interesting applications in many fields reaching from catalysis to spintronics.8,9,10,11 For instance, one-dimensional conducting polymers could be an essential ingredient of nanoelectronics.12,13 Other examples are spin chains, which are one of the most fascinating low-dimensional systems of solid-state physics, both for fundamental and applied reasons. They are expected to behave completely different from higher dimensional magnets whose excitations are characterized by spin waves around a long-range ordered ground state. In contrast, the spin-1/2 antiferromagnetic Heisenberg chain shows a continuum of spinon excitations. Very spectacular is also the difference between spin chains with integer or noninteger spin, having a gap in the spin excitation spectrum or not. That is the content of the famous Haldane conjecture whose deep reason can be traced back to a topological invariant.14,15 The case of a ferromagnetic nearest neighbor coupling, being eventually connected with metallic conductivity could open the interesting perspective of one-dimensionally directed electronic spin-polarized transport. Another highly discussed question is the use of local magnetic moments in molecular memory devices. For that purpose, a high magnetic anisotropy energy is demanded to prevent thermal fluctuations of the local moments. In the spirit to discover new 1D materials, we investigate here the complete class of 1D TM-ZQ polymer chains where TM stands for a 3d transition metal between Sc and Zn. We investigate free-standing chains since the example of Fe-ZQ shows that any possible metallic substrate does not change the electronic structure. Also, the obtained lattice constants of freestanding chains will help to choose the most convenient metal surface for the on-surface synthesis. And, finally, the synthesized polymers can also be lift-off of the surface and act as free-standing wire.16


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Using first-principles calculations within density functional theory (DFT) we study TMZQ chains with periodic boundary conditions. We choose the Vienna Ab initio Simulation Package (VASP) being based on a plane-wave basis and consider the strong correlation of the electrons in the 3d shell by a Hubbard U term added to the spin polarized generalized gradient approximation (SGGA+U method). To investigate the magnetic anisotropy energy (MAE) we apply the SGGA+U method in combination with spin-orbit coupling (SOC). The results uncover TM-ZQ as a fascinating material class with a surprisingly rich behavior. One can find: (i) 1D metallic behavior (Mn-ZQ) allowing for charge transport being of highest importance for potential applications in nanoelectronics, (ii) ferromagnetic chains (Sc-ZQ and Ti-ZQ) with a very small energy gap to construct 1D spin valves at room temperature, (iii) spin cross-over transitions (Fe-ZQ, Mn-ZQ, and Ni-ZQ), (iv) nearly ideal realizations of antiferromagnetic spin ½ (Cu-ZQ) or spin 1 (Ni-ZQ) chains to test the Haldane conjecture within the same material class, and (v) a spin chain with a high MAE (V-ZQ) which is important for possible applications in magnetic memory devices. That is outlined in detail below based on our calculations of the total energy curves for different lattice constants and spin states, the densities of states, the total and local magnetic moments, the spin exchange couplings and MAEs. It is shown that for TM = V, Cr, Mn, Fe, Co, Ni and Cu, the magnetic nearest neighbor coupling is AFM. METHOD The first principle calculations had been performed using the pseudo potential code VASP (Vienna Ab initio Simulation Package) to study the metal-organic 1D chains. We used the spin resolved Generalized Gradient Approximation (SGGA) of Perdew and Wang (PW91)17,18 for the exchange and correlation potential and the Projector Augmented Wave (PAW) pseudo potentials.19,20 Due to the well-known problems of standard Spin polarized Density Functional


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Theory (DFT) in describing strongly correlated systems, we use the Spin polarized Gradient Approximation with Hubbard term U (SGGA+U). The SGGA+U corrections were introduced by Liechtenstein et al.21 where U and J enter as independent corrections in the calculations. The value of J = 0.9 eV is fixed by the properties of the TM atom and only slightly reduced with respect to the value for a free atom. On the other hand, the correlation energy U for a 3d orbital is considerably screened in a solid and not easy to determine. We choose here U = 5eV as it was justified for Fe-ZQ.7 The electronic wave functions were expanded in plane waves with a kinetic energy cutoff of 480 eV, the polymers were relaxed until the residual forces were below 10 ―6 eV/Å and the convergence criteria for the energy deviations was 10 ―7 eV. The Brillouin zone was determined by a set of 8×1×1 k-points in the unit cell using the Monkhorst-Pack points.22 The Gaussian smearing method was used in these calculations and a width of 0.01 eV was adopted. The geometry optimization was performed by relaxing all atomic coordinates for a fixed lattice constant (Figure 1).

Figure 1: Geometrical structure of a free-standing polymer chain of Mn atoms and zwitterionic Quinone (C: grey, O: red, N: blue, H: white, and Mn: purple).


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RESULTS AND DISCUSSION We calculated the total energies of the 1D transition-metal (TM) zwitterionic quinone (ZQ) polymer as a function of lattice constant. Between V and Co we find two magnetic solutions, a lower-lying high spin (HS) solution at larger lattice constant and a higher-lying intermediate spin (IS) solution at smaller lattice constant (see Table 1). The corresponding energy curves are presented in the Supplementary Material (Figure S2). The energy difference between HS and IS solution is small for Fe-ZQ (Δ𝐸(𝐼𝑆 ― 𝐻𝑆) = 73meV per Fe ion) as it was already reported before7 and the two energy minima are well separated corresponding to a spin-state transition. There is a similar situation for Mn-ZQ with slightly higher Δ𝐸(𝐼𝑆 ― 𝐻𝑆) and in Ni-ZQ between S=1 (stable at 15.50 Å) and S=0 (or NM solution) at 15.07 Å. In the cases of V-ZQ, Cr-ZQ, and Co-ZQ the local energy minimum of the IS solution is higher than the HS solution at the same lattice constant. Therefore, we expect no spin-state transition there. A possible spin-state transition would be visible by quite important changes of the metal-ligand distances as shown in Table 1. All magnetic solutions correspond to a FM arrangement in Table 1 where SO coupling was neglected. The energy gain due to the intra-atomic exchange interaction is defined as Δ𝐸 (𝑁𝑀 ― 𝐹𝑀) and it can take important values when we have a significant magnetic moment as high as 3.263 eV and 3.170 eV per metallic atom in Cr-ZQ and Mn-ZQ, respectively. On the other hand, at the beginning of the 3d series, the energy gain due to magnetism is very small for Sc-ZQ (only 0.072 eV). That is connected with a very small part of the total magnetic moment of 𝑀 = 1𝜇𝐵 (or S=1/2) sitting at the metal site, namely 𝑀𝑚 = 0.121 𝜇𝐵 (𝑀𝑑 = 0.095 𝜇𝐵 corresponds to the magnetic moment in the 3d orbital, see Table 2). The values of 𝑀𝑚 and 𝑀𝑑 indicate that the magnetic moments become more and more localized on the 3d orbital if we go


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from Sc to Cu. The magnetization density is very largely spread in Sc-ZQ but very localized in Cu-ZQ. For a better understanding of the chemical bonding and the magnetization distribution we show in Figure 2a the charge density and in Figures 2b and 2c the spin densities for Mn-ZQ (Figure 2b) and Sc-ZQ (Figure 2c). After relaxation, all polymers remain completely planar and we choose a cut through the plane of the polymer to present charge and spin densities. The charge density varies between 0 and 4 Å-3 and the spin density between 0 and 0.1 Å-3. The charge density is defined as the sum of the electron densities for spin up and spin down (ρspin-up + ρspindown)

and the magnetization density as the difference between the electron densities for spin up

and spin down (ρspin-up - ρspin-down). The charge density illustrates well the strong bonding in the benzene ring and to its ligands. The charge density between ZQ molecule and metal is a little bit lower indicating weaker bonding. Nevertheless, it remains a covalent bond, also supported by the high bonding energy above 3 eV (per Fe) found in Ref. 7 and confirmed here for all the 3d series of TM-ZQ. Concerning the spin density, its highest value is found at the metal site as expected. But there is an important difference. The magnetization distribution in Sc-ZQ is nearly equally distributed between Sc, the two ligands O and N, and four C atoms in the benzene ring. On the other hand, the magnetization in Mn-ZQ is concentrated on the Mn site with some contribution on the O atoms. These tendencies are in good agreement with our calculated local magnetic moments listed in Table 2.


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Table 1. Total magnetic moments (M per TM atom, in µB), lattice constant (a, in Ǻ), distances between the central TM atoms and the ligands N (dTM-N, in Å) and O (dTM-O, in Å), energy difference between IS and HS states (E(IS-HS), per TM, in eV) as well as between NM and HS solution (E=E(NM-HS), per TM atom in eV) for 1D TM-ZQ polymer in the SGGA+U method. The IS solution for Cr-ZQ is special since it corresponds to alternating magnetic moments along the chain (Md = 1.816 µB and 3.466 µB).

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

dTM-N dTM-O IS M (µB) a (Å) dTM-N dTM-O HS M (µB) a (Å) ΔE (IS- HS)

2.16 2.08 1 16.33 -

2.09 2.01 2 16.10 -

2.02 1.99 1 15.80 2.11 2.10 3 16.16 0.397

2.00;2.02 1.99;2.00 2 15.79 2.06 2.03 4 15.90 0.842

1.94 1.90 3 15.48 2.10 2.14 5 16.20 0.276

1.93 1.93 2 15.45 2.02 2.08 4 15.90 0.073

1.91 1.93 1 15.40 1.98 2.06 3 15.75 0.271

1.92 2.01 2 15.50 -

1.91 1.97 1 15.41 -

1.93 2.08 0 15.65 -

dTM-N dTM-O a (Å) ΔE (NM-HS)

2.17 2.08 16.36 0.072

2.03 1.96 15.90 1.0165

2.01 1.92 15.77 2.100

2.00 1.96 15.743 3.263

1.90 1.92 15.40 3.170

1.90 1.89 15.32 2.071

1.82 1.90 15.16 1.293

1.84 1.85 15.07 0.106

1.89 2.00 15.44 0.516

1.93 2.08 15.65 -

NM

a


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b

c


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d

Figure 2: Charge density (a) and spin density (b) of Mn-ZQ and spin density of Sc-ZQ (c). Shown are cuts through the plane of the polymers with a color code as an inset. Bader charge distribution (d) for all atoms in the Mn atoms zwitterionic quinoidal polymer.

Information about the chemical bonding in TM-ZQ polymers can also be obtained from an analysis of the Bader charge (Figure 2d). The example of Mn-ZQ is chosen; information on the other compounds can be found in the Supplementary Material (Figure S3). Each Mn atom in the structure loses 1.41 electrons confirming the +2 valence. It is also revealed that 0.39 electrons are transferred from H to N or O. Concerning the C-N and C-O bonds, they are clearly visible by the 0.54 or 0.74 electrons, respectively, which were transferred from C to N or O atoms. Correspondingly, the atoms of N and O gain 1.16 and 1.29 electrons, respectively. The ferromagnetic arrangement is not necessarily the most stable magnetic order. Depending on the initial conditions of the self-consistent calculations we consider the following two magnetic configurations: ferromagnetic (FM) and antiferromagnetic (AFM), where the SO coupling was not included into the spin-polarized calculation. The energy difference between the


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FM and AFM states 𝐸𝑒𝑥 = 𝐸𝐹𝑀 ― 𝐸𝐴𝐹𝑀 is listed in Table 2. That energy difference allows estimating the nearest neighbor exchange coupling. Positive (negative) exchange energy indicates that the ground state of the system is AFM (FM). The total energy calculations showed that most TM-ZQ polymers are AFM with the exceptions of Sc-ZQ and Ti-ZQ, which are FM. It is quite natural that Zn-ZQ is nonmagnetic since it has a completely filled 3d shell. Table 2. Exchange energies (Eex =EFM-EAFM per TM atom, in meV), exchange coupling constants within chain direction (J per TM atom, in meV), total magnetic moments (M per TM atom, in µB), local magnetic moments of the d orbital at the TM atoms (Md per TM atom, in µB), local magnetic moments at the TM atoms (Mm per TM atom, in µB), energy band gaps for spin up (Ea) and spin down (Eb, in eV), total energy gaps involving eventual spin flip processes or being the minimum of Ea and Eb (Eg , in eV) and magnetic anisotropy energy (MAE, in meV) for 1D TM-quinone polymer.

(HS)

Sc

Ti

V

Cr

Mn

Fe

Co

Ni

Cu

Zn

Eex J M Mm Md

-73.17 1 0.121 0.095

-74.47 2 1.199 1.111

6.49 0.72 3 2.777 2.684

14.21 0.89 4 3.746 3.649

4.27 0.27 4 3.720 3.651

1.05 0.12 3 2.709 2.682

12.37 3.09 2 1.678 1.670

5.38 5.38 1 0.638 0.638

0 0 0

Ea

1.06

1.22

0.96

0.75

1.23

1.25

1.26

1.14

1.29

Eb Eg MAE (Ex-Ez) MAE (Ex-Ey)

1.18 0.25

1.22 0.22

1.44 0.96

1.36 0.75

3.49 0.14 5 4.613 4.536 No gap 1.4 0

1.27 1.20

1.25 1.25

1.02 1.02

1.12 1.07

1.29 1.29

0

0.47

-21.72

1.36

-0.67

-4.31

-4.26

5.85

0

-

0

0.10

-0.36

0.15

0.13

3.72

-4.38

-0.22

0

-

To determine the nearest neighbor exchange couplings J we map the energy differences 𝐸𝑒𝑥 = 𝐸𝐹𝑀 ― 𝐸𝐴𝐹𝑀 onto the Heisenberg Hamiltonian: 𝐻 = ∑ < 𝑖𝑗 > 𝐽𝑖𝑗 𝑆𝑖 𝑆𝑗

,

(1)

where the sum over < 𝑖𝑗 > counts each bond only once and where 𝑆𝑖 are the spin operators. Neglecting quantum corrections, the energy difference between FM and AFM states per TM ion Δ𝐸 = 𝐸𝐹𝑀 ― 𝐸𝐴𝐹𝑀 = 4𝐽𝑆2 can be used to determine the nearest neighbor exchange coupling within chain direction 𝐽. The results are shown in Table 2 for all antiferromagnetic TM-ZQ. The


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values of the exchange coupling constants reveal a weaker coupling between Co (Mn) spins 𝐽 = 0.12 (0.14) 𝑚𝑒𝑉, as compared to the coupling between Ni (Cu) spins 𝐽 = 3.09 (5.38) 𝑚𝑒𝑉 respectively. The mechanism for all AFM spin chains can clearly be established to be Anderson’s super exchange mechanism. On the other hand, the Heisenberg Hamiltonian cannot describe the ferromagnetism in Sc-ZQ and Ti-ZQ. First of all, as seen in Figure 2c, the magnetization density is not well localized in these cases. As a consequence, the exchange energies Δ𝐸 = 𝐸𝐹𝑀 ― 𝐸𝐴𝐹𝑀 become very large and would lead to unrealistic (negative) exchange couplings. To calculate the magnetic anisotropy energy (MAE), one has to evaluate the difference between the total energies of a magnetic material for different orientations of the magnetization. For that purpose we use the SGGA method including spin–orbit coupling (SOC) and taking into account the Hubbard term (U). The SO coupling is only used to calculate the MAE; all other data in the Tables are calculated without it. The structural data and the gap energies are not at all influenced by the SO coupling and the local magnetic moments change by less than 10 ―2𝜇𝐵. The anisotropy energies (calculated for a FM spin arrangement) are summarized in Table 2 and can be interpreted as follows: we find an easy axis z perpendicular to the plane of the polymers for Ti-ZQ, Cr-ZQ, and Ni-ZQ. The magnetic anisotropy disappears for Cu-ZQ and Sc-ZQ as it should be the case for spin ½ systems. The difference between Ex and Ey was found to be much smaller than Ex-Ez for all compounds, with the two exceptions of Fe-ZQ and Co-ZQ for which both differences are comparable. The magnetic moment prefers to be in y- or x-direction for FeZQ or Co-ZQ, respectively. Very remarkable is the high easy-plane (x-y) anisotropy of -21.72 meV for V-ZQ.


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The analysis of the total and partial densities of states (DOS) is very instructive for a detailed understanding of the remarkable electronic and magnetic properties of TM-ZQ polymers. Let us start with Mn-ZQ, which has no gap at the Fermi level. That is surprising since the S=5/2 spin moment points to a 3d shell which is completely filled with spin up electrons but empty of spin down electrons, which in other Mn compounds leads to insulating behavior. But Mn-ZQ is an exception being visible in Figure 3. There one can see that Mn-ZQ is a half-metal in its FM state with the highest dxy orbital half-filled. The reason for the incomplete filling of the spin up dshell is a ligand orbital of N- and O-character lying also at the Fermi level and being also halffilled. That is visible in the partial DOS (PDOS) and in the band-structure (Figure 3d). Interestingly, the two bands have a crossing point for spin-up electrons not far from the Fermi level. That indicates different spatial symmetries of the two bands such that hybridization is not allowed. The near degeneracy of these two bands is at the origin of the metallic behavior of MnZQ. One has to mention that both bands are also at the Fermi level for the AFM solution (not shown) which, as a consequence, is also metallic. All other TM-ZQ polymers have a gap at the Fermi level. Its value can be determined from the DOS curves (Supplementary Material Figure S5) and is collected in Table 2. The gap is given for the FM solution and is usually different for spin up and spin down. Of course, that is not the case for Zn-ZQ, which is nonmagnetic. Zn-ZQ can serve as a reference material since it has no 3d states in the neighborhood of the Fermi level and the largest gap in the series. The FM states of Sc-ZQ and Ti-ZQ are very remarkable since they have very small total gaps Eg of the order of 0.2 eV, much smaller than the gaps for spin up and spin down. It means that the lowest excitations should involve spin flip processes. Such small gaps should allow electric transport at


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room temperature. So, these two polymer chains are close to ferromagnetic metals with a Stoner mechanism of ferromagnetism. To understand the magnitude of the magnetic moments listed in Table 2, the projected densities of states (PDOS) of the d electrons of the TM atoms in the ground state are presented in Figure 3b and in Figure 4. In the case of Sc-ZQ, the 3d orbitals are nearly empty, indicating again the delocalized character of the magnetization density that cannot be described by the Heisenberg model. Ti-ZQ is similar with only one occupied d-orbital. That gives only half of the total magnetization, the other half is distributed on other than Ti 3d orbitals. Starting with V-ZQ, the magnetization is dominated by the 3d orbitals. That means that the value of the total magnetic moment can be estimated from the filling of the d-shell. For instance, for Cr-ZQ, the value of 𝑀 = 4𝜇𝐵 (𝑆 = 2) corresponds to 4 spin-up electrons in the 3d shell (only 3 dxy is empty) and no spin-down electron. Very instructive is the case of V-ZQ. It reveals two orbitals, dxz and dyz, which are nearly degenerate. Also, they are only partially filled which allows an orbital current in the x-y plane. Correspondingly we find remarkable orbital moments 𝑀𝐿(𝑥) = ―0.10 𝜇𝐵, 𝑀𝐿(𝑦) = ―0.08 𝜇𝐵, and 𝑀𝐿(𝑧) = ―0.29 𝜇𝐵 and a high MAE. The orbital moments of the other TM-ZQ are presented in the Supplementary Material (Table S1). Let us finally discuss some limitations of our approach. It is true that the chosen U value has some ambiguity and variations of U by about ± 2 eV will certainly change the energy difference 𝛥𝐸(𝐼𝑆 ― 𝐻𝑆) in Table 1, as well as some other numbers. For illustration we present in the supplementary material the E(a) curves for different U (U = 3, 4, 5 eV) for Fe-ZQ and we see that the IS state becomes more stable with decreasing U. On the other hand, the main conclusions are not changed by small U variations. For instance, the predicted spin state transition for Fe-ZQ, Mn-ZQ and Ni-ZQ by substrate induced strain do not depend on 𝛥𝐸(𝐼𝑆 ― 𝐻𝑆) since the lattice


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constants are expected to be determined by the substrates. Also, the quantitative values of J and MAE in table 2 might be subject to small changes by U variation but we have checked that the orders of magnitude and the signs remain the same.

a


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b

c


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d

Figure 3: (a) Total DOS (b, c) projected DOS of the atoms (Mn, C, O, N, H) and the d orbitals at the Mn atoms and (d) band structure (spin up [blue] and spin down [red]) in the 1D Mn atoms zwitterionic quinoidal polymer with the PAW-SGGA+U method with high-spin (S = 5/2).

a


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b

c


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e


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g


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h

i

Figure 4: Projected DOS of the d orbitals on the TM atoms in the 1D zwitterionic quinoidal polymer with TM atoms with the PAW-SGGA+U method where the TM atom is (a) Sc, (b) Ti, (c) V, (d) Cr, (e) Fe, (f) Co, (g) Ni, (h) Cu, and (i) Zn.


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CONCLUSIONS The family of TM-ZQ opens a surprising variety of 1D conducting and magnetic systems. Of highest importance for potential applications in nanoelectronics is our finding of metallic behavior in Mn-ZQ independently of the magnetic structure, either ferromagnetic or antiferromagnetic. The preferred magnetic order is found to be AFM, but the exchange coupling J=0.14 meV is the second smallest one in the whole family (after J=0.11 meV for Co-ZQ). So, the magnetic moments can be ordered in a magnetic field which would allow spin polarized onedimensionally directed electric transport in that material. We have shown that the metallicity of Mn-ZQ (which is an exception in the series) is due to the near degeneracy of two bands of different character and symmetry at the Fermi level. Ferromagnetic order was found for the two compounds in the beginning of the 3d-series, i.e. for Sc-ZQ and Ti-ZQ. But they are special in the sense that the local 3d moments are completely absent (in Sc-ZQ with a configuration 3d0) or weakened (in V-ZQ with a configuration 3d1). In both compounds, the ferromagnetism arises mainly due to delocalized spin-polarized p-orbitals but it is very weak for Sc-ZQ. Both compounds have a small gap of about 0.2 eV. Therefore, these ferromagnetic materials allow for spin-polarized transport and could be used as spin-valves. Due to the small energy difference between the nonmagnetic and the ferromagnetic state in Sc-ZQ it seems that the ferromagnetism is not stable at room temperature for that compound. Therefore, Ti-ZQ is certainly a better candidate material to construct a 1D spin-valve since it has stable local moments, a stable tendency towards a ferromagnetic order and a small energy gap of only 0.22 eV. Other interesting phenomena are spin-state transitions which can be used to store information. Similar spin state transitions as in Fe-ZQ7 are also predicted for Mn-ZQ and Ni-ZQ. They can be induced by strain due to different substrates. The example of Fe-ZQ shoes that the adsorption of


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the polymer to the metallic substrate is so strong that the polymer adapts its lattice constant to that one of the substrates. Very interesting are the two antiferromagnetic (electrically insulating) spin chain compounds Cu-ZQ and Ni-ZQ, the first one with spin ½ and the other with spin 1 which could allow to test the famous Haldane conjecture within one and the same material family. The nearest neighbor Heisenberg exchange couplings J are sufficiently high (5.4 and 3.1 meV for the Cu- and Ni-compound, respectively) to allow crucial experiments at reasonable temperatures. As we have shown, the TM-ZQ polymer chains are strongly bond by covalent bonds with a binding energy of the order of 3 eV.7 Therefore, it should not only be possible to synthesize them on a noble metal surface6,7 but also to separate the spin chains spatially and to perform experiments on really separated chains. A very remarkable material is also V-ZQ having spins 3/2 with antiferromagnetic nearest neighbor exchange and a high magnetic anisotropy of about 22 meV with the magnetic moment lying in the plane of the polymers. That is an exceptional high value for 3d electrons and was shown to be caused by the degenerated 3dxz and 3dyz orbitals which are not completely filled and allow a large orbital moment. In summary, we performed accurate calculations and presented in a systematic way the electronic and magnetic properties of 1D transition metal zwitterionic quinone (TM-ZQ) polymer chains. We found a rich variety of different systems, ranging from ferromagnetic semiconductors with small gaps over metallic chains up to antiferromagnetic semiconductors. We have shown the large potential of this new material class for spintronics applications.


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ASSOCIATED CONTENT Supporting Information Figure S1, Total energy vs lattice constant for the free-standing Fe-ZQ polymer calculated with the SGGA+U method for U = (3,4,5 eV). Figure S2, Total energy as a function of the lattice constant; Figure S3, Bader charge distribution for all atoms; Figure S4, Cut plane for a spin density; Figure S5, Total DOS for the TM-ZQ; Table S1. Local Magnetic Moments (Md, per TM, in μB), Orbital moment (ML, per TM, in μB), and Total Magnetic Moments (M, per TM, in μB), for TM-ZQ polymers with spin-orbit coupling (SOC).

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] (H.D.).

ACKNOWLEDGEMENTS This work was supported by the Computer resources of the Centre Informatique National de l’Enseignement Supérieur (CINES), Project No. A0020906873 and the High Performance Computing (HPC) resources of Aix-Marseille University financed by the project Equip@Meso (ANR-10-EQPX-29-01). We thank O. Siri and M. Koudia for helpful discussions. References (1)

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TOC Graphic Mn-ZQ

Sc-ZQ


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Magnetic Polymer Chains of Transition-Metal Atoms and Zwitterionic

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