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Bulk-Like Magnetic Signature of Individual Fe4H Molecular Magnets on Graphene Fabian Paschke,* Philipp Erler, Vivien Enenkel, Luca Gragnaniello, and Mikhail Fonin*
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Department of Physics, University of Konstanz, 78457 Konstanz, Germany ABSTRACT: Single-molecule magnets (SMMs) incorporate key properties that make them promising candidates for the emerging field of spintronics. The challenge to realize ordered SMM arrangements on surfaces and at the same time to preserve the magnetic properties upon interaction with the environment is a crucial point on the way to applications. Here we employ inelastic electron tunneling spectroscopy (IETS) to address the magnetic properties in single Fe4 complexes that are adsorbed in a highly ordered arrangement on graphene/Ir(111). We are able to substantially reduce the influence of both the tunneling tip and the adsorption environment on the Fe4 complex during the measurements by using appropriate tunneling parameters in combination with the flat-lying Fe4H derivative and a weakly interacting surface. This allows us to perform noninvasive IETS studies on these bulky molecules. From the measurements we identify intermultiplet spin transitions and determine the intramolecular magnetic exchange interaction constant on a large number of molecules. Although a considerable scattering of the exchange constant values is observed, the distribution maximum is located at a value that coincides with that of the bulk. Our findings confirm a retained molecular magnetism of the Fe4H complex at the local scale and evaluate the influence of the environment on the magnetic exchange interaction. KEYWORDS: single-molecule magnet, graphene, scanning tunneling microscopy, inelastic electron tunneling spectroscopy, electrospray deposition
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and revealed how the distribution of adsorption-induced structural modifications plays an important role for the molecular magnetism.14 In parallel to this development, an important advance toward the characterization of molecular magnetism on the single-molecule scale has recently been made by addressing Fe4 deposited on Cu2N by means of inelastic electron tunneling spectroscopy (IETS).15 Analysis of spin-flip excitations provided information about the magnetic anisotropy and exchange coupling, confirming the overall preservation of SMM behavior upon surface deposition. However, the authors observed a significant modification of the exchange coupling constant due to structural distortions upon compression of a Fe4 molecule in the STM contact, caused by the low tip−sample distance combined with the bulky shape of the molecule. This experiment illustrates the particular challenge to implement a local probe as a highly sensitive and interference-free tool for the analysis of polynuclear SMM complexes on surfaces. In this work we refine the access to magnetic properties of the Fe4 SMMs on a single-molecular level. We use a derivative
ingle-molecule magnets (SMMs) are promising building blocks for future device applications based on spintronics and molecular electronics.1−3 Crucial topics to address are the ordered organization of SMMs on surfaces and how the surface deposition affects the electronic and magnetic properties of the molecules.4−7 One of the most studied SMMs in this regard is the tetranuclear iron(III) cluster [Fe4(L)2(dpm)6] (with H3L being of the general form R-C(CH 2 OH) 3 and Hdpm = dipivaloylmethane) that combines chemical stability with magnetic properties that are prototypical for a molecular nanomagnet.8,9 The spins of four iron atoms are antiferromagnetically coupled via oxygenmediated superexchange interaction, which results in a collective spin state that can be described by the giant spin approximation (GSA), having S = 5 in the ground state.10 The surrounding ligand shell favors an easy magnetic axis that is pointing perpendicular with respect to the plane of the four iron atoms. Various studies employing ensemble-averaging techniques such as X-ray absorption spectroscopy or magnetic susceptibility measurements have shown that Fe4 compounds are inert enough to retain their particular bulk magnetic properties upon deposition on surfaces.8,9,11−13 More recently, synchrotron Mössbauer spectroscopy studies provided detailed insights into the intramolecular exchange interactions in Fe4 © 2019 American Chemical Society
Received: October 25, 2018 Accepted: December 31, 2018 Published: January 3, 2019 780
DOI: 10.1021/acsnano.8b08184 ACS Nano 2019, 13, 780−785
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imaging reveals the approximately 3-fold symmetry of single molecules that originates from the pronounced dpm ligands, as can be seen in Figure 1b−d. Thus, all molecules are lying flat on the surface, with their easy magnetic axis pointing perpendicular to the substrate plane.17 Recording conductance (dI/dU) maps in the voltage range of −2.6 V to +3.0 V give access to the electronic properties of Fe4H, with the highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) being shown in Figure 1d. Whereas the HOMO is localized over the molecular center, the LUMO is spread over the dpm ligand shell, which indicates a moderate coupling to the graphene substrate.17 To probe the low-energy excitations, we perform dI/dU measurements in the range of around ±20 mV. Under scanning conditions of +3 V and 20 pA the apparent height of the Fe4H complex is around 750 pm. Due to its low conductivity around the Fermi level,17 the molecule can be easily disrupted upon ramping down the bias voltage, thus substantially decreasing the tip−molecule distance.22 Figure 2
with the protruding tripodal ligand being the shortest possible one (R = H, see Figure 1c), which allows realizing a flat
Figure 1. (a, b) STM topographic images of Fe4H molecules on graphene/Ir(111) at different magnifications (U = +3 V, I = 20 pA, T = 1.9 K). (c) Structure of the Fe4H compound viewed perpendicular to the plane of the four Fe atoms. Color code: Fe = orange, O = red, C = gray, H = white. (d) STM topographic image of a single Fe4H molecule and two corresponding conductance maps recorded at −2.20 V (HOMO) and +2.38 V (LUMO). The intensity circle indicates the molecule’s position to highlight the localization of HOMO/LUMO orbitals (U = +3 V, I = 5 pA, T = 1.9 K).
adsorption geometry for this particular Fe4 compound (named Fe4H) on h-BN/Rh(111)16 and graphene/Ir(111)17 using electrospray deposition (ESD) as an efficient preparation method.18−21 The structural integrity and electronic properties of individual Fe4H molecules are comprehensively addressed by means of scanning tunneling microscopy (STM) and spectroscopy (STS). Relying on these measurements, we are able to ensure a well-defined and reproducible molecular configuration within the tunneling contact, being indispensable for reliable IETS measurements. The spin states of Fe4H are locally probed by IETS, revealing intermultiplet spin excitations that are identified upon comparison with the GSA model. For the Fe4H molecules on the graphene/Ir(111) surface we are able to minimize the effect on the molecular core upon its confinement in the tip−sample junction provided a slow approach of the tip and appropriate set parameters. From the IETS measurements, we determine the exchange interaction constant between the four Fe atoms that shows a pronounced variation, with the maximum of the distribution being in a very good agreement with the predictions of the GSA model with bulk parameters. Our study thus represents a successful implementation of the experimental protocol for IETS measurements on large SMM complexes, giving particularly important insights into the properties of surfacesupported individual nanomagnets. Figure 1a,b show STM topographic images of large molecular islands on the graphene/Ir(111) surface. Within the islands a highly ordered arrangement of molecules is observed with intermolecular distances being close to those in Fe4H single crystals. This implies an effective spatial confinement in the lateral direction, thus minimizing molecules’ mobility at low tip−sample distances. High-resolution STM
Figure 2. STM topographic images recorded before (a) and after (b) approaching and retracting the tip at the indicated position. Scale bar: 1 nm (U = +3 V, I = 20 pA, T = 5.9 K). (c) Tip displacement upon sweeping the bias voltage with 0.5 V/s from +3 V to +20 mV with Iset = 20 pA, recorded on the bare graphene/ Ir(111) (black curve) and over the Fe4H molecule (blue and red curves). The tip position at +3 V is defined as zero.
demonstrates a slow voltage sweep at 0.5 V/s from 3 V (scanning voltage) to 20 mV (spectroscopy set voltage) recorded over one and the same Fe4H complex that is located inside a submonolayer and shows a reversible decrease in zdirection of around 1.3 nm over the molecule and 500 pm over the bare graphene/Ir(111) surface. Due to the flat and defined adsorption geometry of the used Fe4 compound, the tip approach over the center behaves similarly on all measured molecules. By exploring a range of set currents of 5−50 pA we deduce an upper border of 20−30 pA before entering the region of dominant molecular defragmentation. In contrast to the exponential behavior on graphene, the tunneling conductance of Fe4H exhibits jumps around 2 V upon the transition into the low-conductive (∼nS) intragap regime. Small jumps that occur within the gap are denoted as minor reordering of the molecule in the tip−sample junction and were previously reported for another Fe4 derivative.15 Those jumps are commonly observed upon sweeping the voltage at a constant set current of 20 pA, but not during IETS data acquisition. STM topographic imaging before and after the measurement shows no particular changes on the Fe4H molecule, which allows a varying number of repetitions of tip 781
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Figure 3. (a) Representation of the lowest lying spin multiplets of Fe4H in a 1 T external magnetic field applied along the easy magnetic axis. The energies were calculated from /exch + /ZFS for bulk anisotropy and exchange parameters. Arrows indicate allowed transitions from the |S,M⟩ = |5,5⟩ ground state into excited spin states. The intramultiplet transition exceeds the resolution of the measurements presented here. (b) Schematic dI/dU excitation spectra of the first intermultiplet transition. The upper curves show the step broadening at effective temperatures from 0 K (black) to 5 K (red) at B = 1 T. The lower curves display the Zeeman shift when changing the magnetic field from 1 T (black) to 6 T (red) at Teff = 5.5 K. (c) dI/dU spectra recorded on a single Fe4H molecule at 1 and 6 T out-of-plane magnetic field (points), together with a background spectrum recorded on clean graphene/Ir(111) (lower curve) (Uset = 20 mV, Iset = 20 pA, T = 1.9 K). An exponential moving average serves as a guide to the eye (black). A model fit (red curves) with Tfit = 5.5 K yields the excitation energies, which are marked by blue arrows.
approach before disrupting the molecule. Thus, this measurement gives a margin for tunneling spectroscopy set parameters, which ensures that the molecules remain undisturbed during the IETS data acquisition. For all IETS measurements the parameters from this range were used and the imaging of Fe4H molecules was performed before and after the spectroscopic measurement to exclude any deterioration of the complexes. Due to the considerable low conductivity of the molecule (tunneling current is such that the molecule remains intact), we use a modulation voltage of 1 mV for recording dI/dU data sets, which gives a reasonable signal-to-noise ratio. The chosen modulation voltage setting leads to an effective spectral broadening of 2.6 meV or 5.5 K, respectively (see Figure 3 and methods). Representative dI/dU spectra obtained at a single Fe4H molecule at 1 and 6 T out-of-plane magnetic field are shown in Figure 3c. While reference measurements performed on the clean graphene/Ir(111) substrate show no pronounced features in the applied voltage range, the dI/dU spectra obtained on Fe4H exhibit symmetric steps at around ±5 mV, which are a fingerprint of inelastic excitations. A slight asymmetry that is present in some of the dI/dU spectra between positive and negative bias voltages we assign to the different couplings of the molecules to the substrate and the tip, respectively.23 Although we observe a small change in the line shape upon the application of the external magnetic field, the signal-to-noise ratio does not allow unambiguously determining a shift of the excitation energy. We therefore record dI/dU spectra on a large number of molecules (N = 191) to obtain reliable statistics (see Figure 4), giving the possibility to quantitatively evaluate the impact of the magnetic field. At the same time this procedure captures the variation of step energies that are present within the submonolayer. The excitation energy values are determined from a leastsquares fit of the spectra based on the standard equation describing the contribution of inelastic processes to the tunnel current.24 In the small energy window of the measurement, we assume the elastic contribution to the current to be linear, therefore contributing as a simple offset constant in dI/dU spectra. The corresponding conductance curves are shown as solid red lines in Figures 3 and 4. Following this procedure, we
Figure 4. IETS measurements of Fe4H SMMs on graphene/ Ir(111). (a) Typical dI/dU spectra recorded on different Fe4H molecules at 1 and 6 T out-of-plane magnetic field (points) (Uset = 20 mV, Iset = 30 pA, T = 1.9 K). An exponential moving average serves as a guide to the eye (black curve). (b) Histograms showing the statistical distribution of the excitation energy of the step feature. Average values and standard deviations are highlighted by Gaussian curves; the Zeeman shift is indicated by arrows.
obtain excitation energies of a large number of Fe4H molecules, which are plotted as a histogram in Figure 4b. The average value and standard deviation are found to be 4.3 ± 1.4 meV and 5.0 ± 1.5 meV in a 1 and 6 T out-of-plane magnetic field, respectively. This energy shift reveals the magnetic nature of the observed inelastic features and identifies their origin as spin-flip transitions and at the same time rules out vibronic excitations as a main contribution. To interpret the IETS results of the Fe4H complex, we recall the magnetic properties of bulk Fe4 SMMs that are known to be well described by a combination of Heisenberg exchange and zero-field splitting: / = /exch + /ZFS.10 The first term, /exch = J1Sc∑i Si + J2 ∑i ≠ j Si ·Sj , reflects the intramolecular spin−spin interactions that are mediated by the oxygen ions and takes into account the nearest and next-nearest neighbor exchange constants J1 and J2. Here, Sc is the spin of the central Fe atom and Si are the spins of the three peripheral Fe atoms. The coupling of the spins to the molecular crystal field splits up each spin multiplet, described by the anisotropy term /ZFS = DSz2 + gμB S·H , where Sz is the out-of-plane component of the total spin S, D is the anisotropy constant, g is the Landé factor, and H is the external magnetic field. In a first782
DOI: 10.1021/acsnano.8b08184 ACS Nano 2019, 13, 780−785
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ACS Nano order approach, D is typically considered to be constant in adjacent spin multiplets.15 Based on the bulk values of D, g, J1, and J216 as well as on the selection rule ΔM = 0, ±1 for spinflip transitions induced by tunneling electrons, the three lowest lying states that can be addressed by spin-flip excitations are (I) the adjacent M = 4 state of the S = 5 ground state multiplet |5,4⟩, (II) the M = 4 state of an excited S = 4 multiplet |4,4⟩, and (III) the M ∈ {4,5,6} states of an excited S = 6 multiplet |6,4...6⟩ (see Figure 3a). The corresponding excitation energies calculated for bulk parameters at 1 T magnetic field are ΔE|5,5⟩→|5,4⟩ = 0.59 meV, ΔE|5,5⟩→|4,4⟩ = 4.96 meV, and ΔE|5,5⟩→|6,4···6⟩ = 11.9−13.2 meV. All measurements are performed at 1.9 K; thus we can assume that the |S,M⟩ = |5,5⟩ ground spin state of Fe4H is initially occupied. On the basis of the step energy we identify the excitation observed in our measurement as a transition from the ground spin state into the first excited S = 4 multiplet. We note that the lowest energy transition into the |5,4⟩ state cannot be resolved in our measurement, since the energy resolution of the experiment of 2.6 meV greatly exceeds the excitation energy. In some measurements, steps at energies around ±12 meV were observed, which might correspond to the transition to the high-energy spin states |6,4···6⟩. However, the obtained statistics does not allow recognizing a clear trend in the behavior upon application of a magnetic field, and thus these features will not be further discussed. Therefore, we focus on the steps at energies of ±5 meV in our discussion. The average energy shift of the |5,5⟩ → |4,4⟩ excitation and its standard error of 0.76 ± 0.21 meV between 1 and 6 T are in agreement with the Zeeman shift expected for a Landé factor of g ≈ 2. The excitation energy ΔE|5,5⟩→|4,4⟩ is a function of the quantity J1′ = J1 − 3J225 and therefore provides direct information on the exchange coupling in Fe4H on graphene/ Ir(111). The term J1′ can be interpreted as the nearest neighbor exchange constant of a simplified model that neglects the next-nearest neighbor term in /exch . By using the value of D as extracted from XMCD measurements on comparable samples,17 we obtain an average value and standard deviation of J1′ = 1.5 ± 0.4 meV. This is very close to the value of 1.7 meV measured on Fe4H bulk material16 and is in very good agreement with values reported for other Fe4 derivatives.26 Since J1 ≫ J2, the same applies to J1. A small reduction of J1′ compared to the bulk value was previously explained by the structural relaxation of Fe4 upon adsorption.15 To explain the rather large standard deviation obtained in our experiment, several possible effects need to be considered. Variations of the molecule−substrate coupling due to different adsorption sites on the graphene moiré might lead to a renormalization of the spin state eigenenergies, as it also alters the HOMO/LUMO energies.17,27 However, the bulky and inert nature of the large Fe4 complex does not favor a strong influence on the electron spins like it is the case for planar, single-core SMMs28,29 due to its location at the shielded Fe atoms. More likely are structural distortions due to different adsorption geometries on the substrate as well as those due to the presence of the neighboring molecules in the close-packed submonolayer. Structural deformations have been sensitively addressed for another Fe4 derivative on Au(111),14 where a distribution of coordinations, and thus different exchange interactions have been measured on the different Fe sites within the molecule. This deformations might be related to differences in the adsorption geometry and coincide with our observation of a variety of molecular appearances in STM; see
Figure 1b. While the molecules are arranged into a perfect hexagonal lattice, the individual rotation of each molecule varies around 8 ± 4° with respect to the unit cell, giving rise to slightly different appearances and possibly leading to sitedependent strain on the molecular cages. Additionally, the STM tip itself can distort the molecular structure if placed in close proximity to the complex. This leads to changes in the intramolecular atomic distances including the oxygen bridges that mediate the exchange interaction between the Fe atoms. Such an effect has been recently suggested to explain unexpectedly high excitation energies found in IETS measurements on another Fe4 derivative on Cu2N.15 Although a fraction of our molecules shows comparably large excitation energies, the absence of a systematical increase of J1′ in our measurements supports the assumption that the flat geometry of our Fe4 derivative is less prone to structural deformation caused by the STM tip. To underline this conclusion, we observe no systematic effect upon the variation of the set current (20 and 30 pA), corresponding to two different tip− molecule distances. In contrast to the previous study, we therefore suggest the environmental effects on the observed variations to be dominant compared to the confinement in the STM junction. Furthermore, the defined imaging of the adsorption geometry combined with a less bulky tripodal ligand allows the tip to approach closer to the molecular orbitals located at the Fe atoms, increasing the efficiency of inelastic electron tunneling without a considerable distortion of the molecule. These results highlight the importance of specifically tailored molecular structures that are suitable for low-impact measurements of surface-supported SMM devices, in order to study electronic and magnetic properties by scanning probe methods such as IETS. In this context, the approach presented here using bulky polynuclear compounds complements other recent STM-based experiments investigating planar SMMs30 and single-core nanomagnets.31
CONCLUSIONS We successfully performed IETS on a Fe4 SMM complex, whose adsorption geometry is unambiguously identified by STM topographic imaging. For this purpose we used an Fe4H derivative deposited on graphene/Ir(111), which allows for a flat adsorption geometry. In all IETS measurements the important parameters (tunneling voltage, tunneling current, and tip approach speed) were set to minimize the influence of the STM tip on the molecular structure. We determined the exchange interaction constant between the Fe atoms, J1′ = 1.5 ± 0.4 meV, and thus revealed a mostly unaffected molecular magnetism with characteristics close to that of the bulk. This result is in line with the observed bulk-like anisotropy constant D that was previously measured by XMCD on Fe4H submonolayers17 and extends the observation of preserved magnetic properties to a molecular scale. By quantifying the range of exchange interactions that are present within a closepacked submonolayer of Fe4H we provide a measure of the environmental effects that influence the molecular magnetism upon deposition on a surface. Our work therefore highlights a set of important issues to deal with in order to reliably study molecular nanomagnets with IETS. Furthermore, these results refine the insight into the weak interaction of Fe4H with the graphene/Ir(111) substrate, making it a very attractive system for further investigations related to molecular magnetism. 783
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METHODS AND EXPERIMENTAL Scanning tunneling microscopy and spectroscopy experiments were performed in a two-chamber UHV system (base pressure 5 × 10−11 mbar), equipped with an Omicron Cryogenic-STM. The STM was operated at a temperature of 1.9−6 K. All STM measurements were carried out in the constant-current mode using ground and polished PtIr tips (Nanoscore GmbH). The sign of the bias voltage (U) corresponds to the potential applied to the sample. For conductance maps, I(U) spectra were collected on a 30 × 30 grid over an area of 1.6 × 1.6 nm2. The obtained spectra were numerically differentiated and normalized to the I/U signal. Spatial maps at specific bias voltages were derived from the spectroscopic data and smoothed by using a Gaussian filter. Differential conductance curves for IETS were recorded by modulating the gap voltage ( f mod = 693.7 Hz, Umod = 1 mV (rms)) and using standard lock-in detection. For the fitting procedure, an effective temperature was calculated using (5.4 kBTeff)2 = (5.4 kBT)2 + (2.4 eUmod)2,32 based on the modulation voltage Umod (rms) and the experimental temperature T. Samples were prepared in situ for all experiments presented. The Ir(111) single crystal (Surface Preparation Laboratory B. V.) was cleaned by repeated cycles of Ar+ sputtering at 2 kV, heating in an O2 atmosphere of 5 × 10−7 mbar at 900−1150 °C, and flash annealing in UHV up to 1150 °C. Graphene was prepared by exposing the clean Ir(111) surface to an ethylene partial pressure of 1 × 10−7 mbar for 20 min while keeping the sample at T = 1100 °C. A good quality of the graphene layers was verified by means of STM imaging. Electrospray deposition of Fe4H was performed as described elsewhere.16,17
AUTHOR INFORMATION Corresponding Authors
*E-mail:
[email protected]. *E-mail:
[email protected]. ORCID
Fabian Paschke: 0000-0002-9710-170X Luca Gragnaniello: 0000-0003-1150-3941 Mikhail Fonin: 0000-0003-3015-0045 Notes
The authors declare no competing financial interest.
ACKNOWLEDGMENTS This work was supported by the Deutsche Forschungsgemeinschaft (DFG) through SFB 767. REFERENCES (1) Bogani, L.; Wernsdorfer, W. Molecular Spintronics Using SingleMolecule Magnets. Nat. Mater. 2008, 7, 179−186. (2) Raman, K. V.; Kamerbeek, A. M.; Mukherjee, A.; Atodiresei, N.; Sen, T. K.; Lazić, P.; Caciuc, V.; Michel, R.; Stalke, D.; Mandal, S. K.; Blügel, S.; Münzenberg, M.; Moodera, J. S. Interface-Engineered Templates for Molecular Spin Memory Devices. Nature 2013, 493, 509−513. (3) Gruber, M.; Ibrahim, F.; Boukari, S.; Isshiki, H.; Joly, L.; Peter, M.; Studniarek, M.; Da Costa, V.; Jabbar, H.; Davesne, V.; Halisdemir, U.; Chen, J.; Arabski, J.; Otero, E.; Choueikani, F.; Chen, K.; Ohresser, P.; Wulfhekel, W.; Scheurer, F.; Weber, W.; et al. Exchange Bias and Room-Temperature Magnetic Order in Molecular Layers. Nat. Mater. 2015, 14, 981−984. (4) Bogani, L.; Cavigli, L.; Gurioli, M.; Novak, R. L.; Mannini, M.; Caneschi, A.; Pineider, F.; Sessoli, R.; Clemente-Leon, M.; Coronado, E.; Cornia, A.; Gatteschi, D. Materials Based on Single-Molecule Magnets Monitor Strong Environmental Effects. Adv. Mater. 2007, 19, 3906−3911. (5) Salman, Z.; Chow, K. H.; Miller, R. I.; Morello, A.; Parolin, T. J.; Hossain, M. D.; Keeler, T. A.; Levy, C. D. P.; MacFarlane, W. A.; Morris, G. D.; Saadaoui, H.; Wang, D.; Sessoli, R.; Condorelli, G. G.; 784
DOI: 10.1021/acsnano.8b08184 ACS Nano 2019, 13, 780−785
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DOI: 10.1021/acsnano.8b08184 ACS Nano 2019, 13, 780−785
