Exchange Coupling of Spin-Crossover Molecules to Ferromagnetic Co

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Exchange Coupling of Spin-crossover Molecules to Ferromagnetic Co Islands Saber Gueddida, Manuel Gruber, Toshio Miyamachi, Eric Beaurepaire, Wulf Wulfhekel, and Mebarek Alouani J. Phys. Chem. Lett., Just Accepted Manuscript • DOI: 10.1021/acs.jpclett.6b00172 • Publication Date (Web): 19 Feb 2016 Downloaded from http://pubs.acs.org on February 20, 2016

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The Journal of Physical Chemistry Letters

Exchange Coupling of Spin-Crossover Molecules to Ferromagnetic Co Islands Saber Gueddida,† Manuel Gruber,∗,†,‡,§ Toshio Miyamachi,‡,∥ Eric Beaurepaire,† Wulf Wulfhekel,‡,¶ and Mebarek Alouani† Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS), Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France, Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe, Germany, and Institute of Nanotechnology, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany

∗

To whom correspondence should be addressed Institut de Physique et de Chimie des Matériaux de Strasbourg (IPCMS), Université de Strasbourg, CNRS UMR 7504, 23 rue du Loess, BP 43, F-67034 Strasbourg Cedex 2, France ‡ Physikalisches Institut, Karlsruhe Institute of Technology, Wolfgang-Gaede-Strasse 1, 76131 Karlsruhe, Germany ¶ Institute of Nanotechnology, Karlsruhe Institute of Technology, 76021 Karlsruhe, Germany § Present address: IEAP, Christian-Albrechts-Universität zu Kiel, 24098 Kiel, Germany ∥ Present address: The Institute for Solid State Physics (ISSP), The University of Tokyo, Kashiwa 2778581, Japan †

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Abstract The properties of Fe(1,10-phenanthroline)2 (NCS)2 (Fe-phen) molecules deposited on Co/Cu(111) are studied using scanning tunneling microscopy (STM) operated in ultrahigh vacuum at low temperature (4K) and ab initio calculations. Both the experimental and theoretical results are used to identify the high-spin (HS) state of Fe-phen. Additionally, the calculations reveal a strong spin-polarization of the density of states (DOS) and is validated experimentally using the spin sensitivity of spin-polarized STM. Finally, it is shown that the magnetic moment of the Fe-ion within HS Fe-phen is strongly magnetically coupled to the underlying magnetic Co through the NCS groups. These findings enable promising spintronic perspectives.

Graphical TOC Entry

Fe-phen

Co

Keywords

Cu

spin crossover, spin polarization, magnetism, density functional theory, scanning tunneling miscroscopy, scanning tunneling spectroscopy


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Organic spintronics is a fertile research field aiming at using organic materials to host spin transport while benefiting of new advantages such as material flexibility, light weight and potential low production cost. 1 Furthermore, various experiments have recently shown that organic molecules on a ferromagnetic substrate can strongly enhance the spin polarization of the molecule-metal interface or “spinterface”. 2–5 Alternatively, spin-crossover (SCO) complexes have gathered considerable attention for their potentialities as intrinsic switch to build nanoscale electronic components. 6–10 Indeed, SCO complexes can undergo a spin transition upon application of various stimuli such as temperature, light, pressure and magnetic field. 11 In this work, we experimentally and theoretically explore the synergy between the two research fields by studying the (spin) properties of single SCO molecules deposited on a ferromagnetic substrate. The molecular system of interest is Fe-phen (see Fig. 1(a)). This prototypical SCO complex 12 undergoes a thermal SCO from the low-spin (LS) state S=0 to the HS state S=2 when heated above 175 K. 13 The transition is accompanied by changes in the Fe-N bond lengths and angles 14 that lead to changes in molecular conformation. 6,15 We deposited Fe-phen on Co bilayer islands grown over a Cu(111) surface (see methods). The Co islands form triangular-like shapes with opposite orientations corresponding to fcc and hcp stacking structures with a ratio of 3:2. 16,17 In Fig. 1(b), we show a scanning tunneling microscopy (STM) topography recorded at 4K with a home built instrument 18 where the Co bilayer islands are clearly visible. We additionally observe protrusions over the Co islands corresponding to molecular adsorbates upon Fe-phen deposition. In Fig. 1(c) we show the three different types of adsorbates that were observed. Molecules of type I and II are of similar sizes and both present an elongated shape. While type I molecules have their maximum apparent height in the center, type II molecules exhibit two protrusions on their sides (see height profiles along the long axis of the molecules in Fig. 1(d)). We note that the overall shape of type I and II molecules present similarities to that of Fe-phen deposited on Cu(100), Cu(111) and CuN/Cu(100). 6,15 We therefore suppose that type I and II molecules are Fe-phen molecules, possibly in different spin states. In addition, we believe that, similarly



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to Fe-phen on Cu(100), Cu(111) and CuN substrates, the NCS groups of Fe-phen bind to the Co/Cu(111) substrate while the phenanthroline (phen) groups face upwards and are seen with the STM as two lobes. The third kind of molecular adsorbate is much smaller than type I and II adsorbates and is compatible with a single phen group. 19 We note that we observed single phen groups in a non-negligible proportion. We foresee two possible scenarios accounting for the presence of the isolated phen groups: i) phen groups were co-evaporated with Fe-phen and/or ii) some Fe-phen molecules reacted with the Co/Cu(111) substrate and were decomposed into phen and the remaining part of Fe-phen. The precise origin of the single phen groups is out of the scope of this paper, and in the following, we will focus on type I and type II molecules. In Fig. 1(e), we present the dI/dV (V ) spectra recorded on the center of type I and type II molecules and on the Co surface. The Co reference spectrum resembles that of previous studies. 17,20,21 The spectrum over the center of type I molecule does not reveal clear spectroscopic features while that on the center of type II molecule reveals peaks at the following energies: -1.18 V, -0.57 V, -0.26 V, +0.25 V. Since we do not yet know the energy position of the molecular orbitals of HS and LS Fe-phen, we are unable to identify type I and type II molecules to HS and LS Fe-phen. To circumvent this limitation, we performed ab initio density functional theory (DFT) calculations of HS and LS Fe-phen on Co/Cu(111). In Fig. 2 we show the calculated density of states (DOS) within the GGA+U+vdW method for LS and HS Fe-phen (see methods and SI). Within the measured energy interval, the LS Fe-phen DOS has a single wide peak centered at 0.28 eV. These states mainly arise from the phen groups with small contributions from the Fe ion (see Fig. 2(a)). The calculated DOS for HS Fe-phen presented in Fig. 2(b) also exhibits a wide peak for the unoccupied states (centered at 0.28 eV). However, the peak is much wider than that of LS Fe-phen. We observe an additional peak, for HS Fe-phen, located at -0.90 eV. As expected, the difference between HS and LS Fe-phen mainly arises from the Fe DOS. We now compare the calculated DOS to the experimental dI/dV spectra presented in Fig. 1(e). The dI/dV for type II


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(a)

(b)

Fe N C S H (e)

(c) I

phen II

dI/ dV ( arb. unit s)

6

Height (pm)

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4

I

II

2 Co

120 (d) 80 40 0.0

I II

0 -1 .0

1.0 Length (nm)

0 .0

1 .0

Sample volt age ( V)

Figure 1: (a) Three dimensional model of Fe-phen molecule. (b) STM topography of Cu(111)/Co/Fe-phen. The black background corresponds to the Cu(111) surface while orangish triangular-like structures are Co bilayer islands. The yellow spots on the Co islands are molecular adsorbates. (c) STM topography of type I, type II and phen molecules on a Co island. (d) Height profiles along type I and II molecules. (e) dI/dV spectra recorded on the center of type I and II molecules and on the Co island. The image sizes are (b) 92×60 nm2 (V =1.0 V, I=100 pA) and (c) 3.2×5.4 nm2 (V =1.0 V, I=100 pA).



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molecule exhibits a very wide resonance for positive sample voltage (unoccupied states) as well as a resonance located at -1.18 V. The position and the width of these resonances are in qualitative agreement with the calculated DOS for HS Fe-phen. Furthermore, the dI/dV for type II molecule has two additional peaks located at -0.57 V and -0.26 V. These peaks possibly come from states localized at the molecule/metal interface induced by the Co Shockley surface states. 22 Our calculated model has infinite lateral dimensions while the Co islands are of finite size. Therefore, the calculated Shockley-induced states can differ from the experimental one. Considering the qualitative agreement between the dI/dV spectrum of type II molecule and calculated DOS for HS Fe-phen, type II molecule is identified as HS Fe-phen. Note that further evidences for this identification are given below. Concerning type I molecule one may tentatively ascribe it to LS Fe-phen by deduction. However, there is less agreement with the calculations. Below the Fermi energy, the calculated DOS for LS Fe-phen has no strong peak but a small feature around -0.7 eV in agreement with experimental dI/dV at negative voltage. The peak at +0.2 eV could correspond to the shoulder observed in the dI/dV . We also recall the presence of isolated phen (see Fig. 1(c)). Type I molecules could therefore also be a residue of Fe-phen dissociation leading to the isolated phen. Considering the doubts associated to type I molecule, in the following, we only discuss type II molecules, i.e. HS Fe-phen molecules. Confirming the qualitative agreement between the experimental dI/dV and calculated DOS of HS Fe-phen, we stress the strong spin-polarization of the calculated HS Fe-phen DOS. Indeed, the spin-up DOS (solid blue line in Fig. 2(b)) differs from the spin-down DOS (dashed red line in Fig. 2(b)). The spin polarization reaches 84% at the Fermi level, i.e. higher than that of the Co alone in agreement with previous spinterface studies. 4,5 Actually, a spin-polarized DOS is expected for FeII complexes in the HS state since the FeII ion carries a magnetic moment. 11,23 To be of practical spintronic usage, the Fe-ion magnetic moment must be coupled to that of the underlying Co at finite temperature. We estimate the magnetic coupling by performing calculations of HS Fe-phen on Co/Cu(111) with the


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(a) Density of states (states/eV/Cell)

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6

Fe-phen

LS

3 0 1

Fe

0 2

phen

0 1

NCS

0 -1.5

Spin up Spin down -1.0

-0.5 0.0 E-EF (eV)

0.5

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(b) Density of states (states/eV/Cell)

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Fe-phen

HS

3 0 2

Fe

0 phen

2 0 1 0 -1.5

NCS

Spin up Spin down -1.0

-0.5 0.0 E-EF (eV)

0.5

1.0

Figure 2: Calculated spin-dependent DOS of (a) LS Fe-phen and (b) HS Fe-phen adsorbed on Co/Cu(111) using the GGA+U+vdW method. For each sub-figure, the top panel indicates the spin-dependent DOS integrated over the entire molecule. The three next panels report the integrated spin-dependent DOS over the Fe ion, the phen and NCS groups. The spin-up (spin-down) DOS are represented in solid blue (dashed red).



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Fe-ion magnetic moment either parallel or anti-parallel to that of Co. We find an energy difference between the two magnetic configurations of ∆E = E AF M − E F M = 22.6 meV, which implies that i) the magnetic moment of the Fe-ion is preferentially ferromagnetically coupled to that of the underlying Co and ii) the coupling is relatively strong and can be observed at the measurement temperature of ≈ 4 K. Since the magnetic referential of HS Fephen is linked to that of the underlying Co, we can use the spin sensitivity of spin-polarized STS to experimentally confirm the spin-polarization of HS Fe-phen. To do so, we used a Co coated W tip, which possesses a dominant out-of-plane magnetization that is parallel or anti-parallel to the magnetization of the Co islands. The exact magnetic orientation of the tip is unknown, but as in previous spin-polarized STS studies on Co/Cu(111), 21,24–26 Co islands presenting the strongest (lowest) dI/dV signal -0.38 eV are referred to as parallel (anti-parallel). We then acquired dI/dV spectra over HS Fe-phen adsorbed on Co islands. In Fig. 3(a), we show two HS Fe-phen dI/dV spectra for HS Fe-phen lying on parallel and antiparallel spin polarized Co islands, respectively represented in dashed red and solid blue. The amplitudes and the widths of the peaks of the HS Fe-phen dI/dV spectra clearly depend on the magnetic orientation of the underlying Co islands. As pointed out in other spin-polarized STS studies, 25,27,28 the difference in the dI/dV spectra arises from the asymmetry between the spin up and spin down DOS, weighted by the spin-polarization of the tip. Interestingly, the experimental dI/dV presented in Fig. 3(a) are very similar to the calculated DOS of HS Fe-phen presented in Fig. 2(b). For the unoccupied states, we see that the spin-up DOS peak is higher but narrower than that of the spin-down DOS. In addition, the spin-down DOS has a peak around -0.90 eV that is absent for the spin-up DOS. If one assumes that the tunneling current within the STM junction for a parallel (anti-parallel) tip/sample spin polarizations is dominated by spin-up electrons, we have a qualitative agreement between the ab initio calculations and the experiments. We now stress several points: i) as for the HS Fe-phen dI/dV spectrum acquired with a W tip (see Fig. 1(e)), the calculations fail to reproduce the peak at around -0.5 eV; ii) for the experimental HS Fe-phen dI/dV , the


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“parallel” (“anti-parallel”) Co islands had a hcp (hcp) while the calculations were carried out using a fcc stacking. Concerning i), we believe that the peak(s) located around -0.5 eV originate from surface states as described above for the comparison of Fig. 1(e) and Fig. 2(b). ii) The different stacking structures is known to induce an energy shift of the Co occupied states by about 0.03 eV. 20,24 It is also known that the same states can shift by more than 0.1 eV with the size of the islands. 20 Therefore, for the acquired dI/dV spectra, we took care to select islands of similar sizes but ignored the stacking structure of the islands. This refrains us to appreciate shifts lower than 0.03 V. Nevertheless, the experimental data support the calculations concerning two points: i) HS Fe-phen has a spin-polarized DOS close to the Fermi level and ii) the magnetic moment of HS Fe-phen is coupled to that of the underlying Co. For completeness, we also theoretically estimated the conductance of the system composed of a HS Fe-phen molecule on Co/Cu(111) and an ideal Co tip (see Fig. 3(b)). The conductance is estimated by the Jullière model using the calculated DOS of HS Fe-phen and the independently calculated DOS of an ideal Co STM tip (see SI). For the unoccupied states, we observe that the conductance of an anti-parallel spin polarization has a narrower but higher peak than that of a parallel spin polarization. This remains in qualitative agreement with the experimental data and any deviation can be easily understood since i) we calculated an idealized STM tip while the exact shape of the experimental tip apex is unknown, ii) the DOS are calculated for zero voltage and iii) the tunneling probability is not the same for all electrons but depends on their in-plane k vector. Various studies already demonstrated the magnetic coupling between a molecular magnetic moment and a ferromagnetic substrate. 30–34 However, in the present case, the Fe-ion that carries the magnetic moment, is not in direct contact with the substrate but at a distance of 5.1 Å. This distance prohibits direct exchange interaction. In order to have more insights about the origin of the magnetic coupling, we show calculated magnetization density of HS Fe-phen on Co/Cu(111) in Fig. 4. The Fe-ion carries a magnetic moment of 3.5 µB in



(a)

Normalized dI/ dV ( arb. unit s)

Parallel Ant i-parallel

-1 .5

-1 .0

-0 .5 0 .0 0 .5 Sample volt age ( V)

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Est imat ed conduct ance ( arb. unit s)

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Parallel Ant i-parallel

-1 .5

-1 .0

-0 .5 0 .0 E-EF ( eV)

0 .5

1 .0

Figure 3: (a) Experimental normalized dI/dV acquired over Fe-phen lying on “parallel” (dashed red) and “anti-parallel” (solid blue) Co islands. We performed the normalization using the procedure described in Ref. 29. (b) Conductance estimated by the Jullière model of HS Fe-phen and an ideal Co tip for parallel (dashed red) and anti-parallel (solid blue) spin polarizations.


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comparison to 3.6 µB for the gaz-phase molecule. In addition, we observe that the nitrogen atoms of the NCS groups as well as the sulfur atoms have an induced magnetization. To a limited extend, the N atoms of the phen groups are also magnetized. The (induced) magnetization on the Fe, N and S within NCS is of the same sign than that of the underlying Co. The ferromagnetic coupling between the Fe-ion and the Co surface may be explained as follow: the S atoms are magnetized by and coupled to the underlying Co. S and N within NCS are magnetically coupled by double-exchange interaction through the C atoms. Finally the N of the NCS groups couples to Fe by direct-exchange interaction. The remarkable point for this system is the apparent large magnetic-coupling energy of 22.6 meV despite the large distance between the Fe and the Co. We also calculated the magnetization density of LS Fe-phen (see SI). Interestingly, the Fe ion has an induced magnetic moment.

Figure 4: Calculated magnetization density of HS Fe-phen using the GGA+U+vdW method. The positive magnetization density is red, and the negative is blue. The values of magnetic moments of iron, nitrogen, carbon and sulfur are also indicated. In conclusion, we have experimentally and theoretically shown that the magnetic moment of a spin-crossover complex (here Fe-phen) can be strongly magnetically coupled to the underlying ferromagnetic substrate (here Co). We have additionally shown that the spinpolarization of the DOS is relatively important close to the Fermi level revealing promising 11 ACS Paragon Plus Environment


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spintronic perspectives.

Methods Sample preparation Clean and flat Cu(111) surfaces were prepared by several cycles of Ar sputtering (1.5 keV) and subsequent annealing to 500◦ C. The Co was deposited from a e-beam source to the Cu(111) sample held at room temperature. Fe-phen molecules were subsequently sublimed at 180◦ C while maintaining the substrate at a temperature of 50◦ C. Tips STM tips were prepared by chemical etching of a tungsten wire and subsequent flashing in ultrahigh vacuum. The tip used for non spin-polarized measurements were repeatedly indented into the Cu(111) substrate in order to sharpen the tip. For spin-polarized contrast, the flashed W tip was covered by about 12-13 ML of Co. Differential conductance acquisition Differential conductance dI/dV spectra were obtained with a lock-in technique using a modulation voltage ranging from 15 mV to 30 mV and a frequency of 5.03 kHz. When ramping the voltage, the feedback loop was opened. STM topographies All the STM topographies were acquired using the constant-current mode. Ab initio calculations The first-principle calculations were carried out using the Vienna Ab initio Simulation Package (VASP) 35 using the plane-wave pseudo-potential method. 36 For the exchange-correlation functional, we adopted the generalized gradient approximation (GGA) with the Perdew, 12 ACS Paragon Plus Environment

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Burke and Ernzerhof parametrization. 37 In order to account for the strong correlated electronelectron interactions within the central iron atom of the investigated molecule, we used the GGA+U method 38 as implemented in the VASP code, 39 in which an additional on-site Hubbard-U term is included on the iron atom. We also included van der Waals (vdW) interactions in the calculations 40,41 (GGA+U+vdW), which is known to give more reliable structural, electronic and magnetic properties of metallo-organic molecules on a metallic substrate. 42 In this work, we used an effective U of 3 eV and an exchange parameter J=0.9 eV for the 3d electrons of the Fe atom within the molecule. These values have been used in a previous study 23 and correctly describe the properties of the free and adsorbed Fe-phen molecule on a metallic substrate. The kinetic energy cutoff for the expansion of the wave functions is 450 eV at the Gamma point of the Brillouin zone.

Acknowledgement We thank G. Rogez for synthesizing the molecule. We acknowledge funding from the FrenchGerman University and the Baden-Württemberg Stiftung in the framework of the Kompetenznetz für Funktionale Nanostrukturen (KFN), from the Institut Carnot MICA’s “Spinterface” grant, from the Agence Nationale de la Recherche ANR-09-JCJC-0137 and ANR-11-LABX0058 NIE and from the International Center for Frontier Research in Chemistry. This work was performed using HPC resources from GENCI-CINES Grant 2014-gem1100 and Strasbourg HPC Mesocenter.

Supporting Information Available Additional figures and information. This material is available free of charge via the Internet at http://pubs.acs.org/.

Note: The authors declare no competing financial interest. 13 ACS Paragon Plus Environment


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Exchange Coupling of Spin-Crossover Molecules to Ferromagnetic Co

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