Supplementary Information. for. Magnetic memory of a single-molecule quantum magnet wired to a gold surface

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1 Supplementary Information for Magnetic memory of a single-molecule quantum magnet wired to a gold surface Matteo Mannini, 1 Francesco Pineider, 1 Philippe Sainctavit, 2 Chiara Danieli, 3 Edwige Otero, 4 Corrado Sciancalepore, 3 Anna Maria Talarico, 3 Marie-Anne Arrio, 2 Andrea Cornia, 3 Dante Gatteschi, 1 Roberta Sessoli 1 1 Department of Chemistry and INSTM research unit, University of Florence, 50019, Sesto Fiorentino (FI), Italy. 2 Institut de Minéralogie et de Physique des Milieux Condensés, CNRS UMR7590, Université Pierre et Marie Curie, Paris Cedex 5, France. 3 Department of Chemistry and INSTM research unit, University of Modena e Reggio Emilia, 41100, Modena, Italy. 4 Synchrotron Soleil, Saint Aubin BP48, Gif sur Yvette Cedex, France Supplementary Information for Magnetic memory of a single-molecule etc.. by M. Mannini et al. 1

2 Synthesis of Fe 4 and monolayer preparation For these experiments we employed the tetrairon(iii) complex [Fe 4 (L) 2 (dpm) 6 ] with H 3 L = 11- (acetylthio)-2,2-bis(hydroxymethyl)undecan-1-ol and Hdpm = dipivaloylmethane, synthesized in accordance to the reported preparation methods S1,S2. The complex, obtained as large crystals, was dissolved in pure dichloromethane (99.8% purity, purchased from Fluka) to give a M solution. 1 H NMR analysis confirmed that the complex is completely stable in solution over a 24-h period, which exceeds the timeframe of submonolayer preparation (see Fig. S1). Fig. S1. 1 H-NMR spectrum (200 MHz, K) recorded on a fresh 1 mm solution of Fe 4 in CD 2 Cl 2 and after 24 h of aging at room temperature under inert atmosphere. The inset shows an expanded view of the NMR signal arising from the paramagnetically shifted t-bu protons of dipivaloylmethane, whose position and shape are diagnostic of structural intactness S1,S3. Au(111)-mica substrates (Agilent inc.) were treated with a hydrogen flame and immersed in anhydrous EtOH in order to allow proper surface cleaning and reconstruction prior to deposition. The monolayer of Fe 4 on gold was prepared by soaking a freshly-annealed gold substrate in the solution for 20h and by subsequent repeated washings of the sample with fresh dichloromethane. Thick films were prepared as previously described S4. All operations related to the XMCD sample preparation were carried out under inert atmosphere, using a portable glove box filled with nitrogen gas and directly connected to the preparation chamber of the end-station. Samples studied by STM were prepared following the same protocol described above in the controlled inert atmosphere of a fixed dry box apparatus. Supplementary Information for Magnetic memory of a single-molecule etc.. by M. Mannini et al. 2

3 STM analysis STM imaging was carried out on a NT-MDT Solver P47pro (NT-MDT, Zelenograd, Moscow, Russia; equipped with a custom built low-current head operated in air. Tips were prepared by mechanical sharpening of Pt/Ir 90:10 wire. Wide-area scans are reported in Fig. S2 to show the presence of Au(111) reconstructed terraces below the monolayer of Fe4. Fig. S2. Consecutive scans of the same area of the Fe4 monolayer deposit on gold surface evidencing the typical Au(111) reconstruction over the mica substrate. XAS and XMCD setup and method The TBT-XMCD setup used in these experiments, one of the few end-stations capable to reach mk temperaturess5, has been described in detail elsewheres6,s7. Briefly, it consists of a 4He immersion cryostat to reach temperatures around 2 K under helium pumping, and of a 3He-4He dilution refrigerator especially designed for UHV uses, to reach temperatures of the order of 500±5mK. Thin Al foils (0.7 µm, Lebow Company, Goleta, CA, USA) with very low iron impurity content (< 0.01 % ) have been employed as thermal shield for the windows, in order to avoid a spurious absorption of the incoming X-ray at the iron L-edge. The magnetic field has been applied with superconducting Helmholtz coils along the direction of the X-rays with a maximum amplitude of 7±0.001 Tesla; this end-station has been connected to the employed beamlines. A first set of experiments was done in the temperature range K at the UE46-PGM beamline S8 of BESSY synchrothron, in Berlin (Germany). A second set of experiments was carried out down to sub-kelvin temperatures at the X11MA-SIM beamlines9,s10 of the Swiss Light Source (SLS), Villigen (Switzerland). Supplementary Information for Magnetic memory of a single-molecule etc.. by M. Mannini et al. 3

4 At BESSY, the X-ray source was a single Apple II undulator. At SLS, it consisted of a set of two Apple II undulators providing in both cases intense flux of circularly polarized photons and allowing the fast switching of polarization required to record the field and time dependent signals. XAS spectra were recorded in Total Electron Yield mode S11,S12. In order to reduce possible experimental artifacts and to improve the signal-to-noise ratio a series of eight spectra has been recorded by changing the combination of circular polarization and field direction, σ - and σ + being then evaluated by averaging over four spectra each. The XMCD has been derived as (σ - - σ + ). Fig. S3 shows the average ½(σ - + σ + ) XAS spectra of a Fe 4 monolayer and a Fe 4 thick film recorded in similar conditions (BESSY UE46-PGM beamline, T = 4.5 K). A base-line correction and normalization was applied to allow a better comparison. Fig. S3. Comparison of the average XAS spectra (calculated as (σ - + σ + )/2) of the thick film and of the monolayer deposit recorded at T = 4.5 K and with an induction of 5T. The XMCD/XAS ratio in Fe 4 can be regarded as a reliable fingerprint of the spin alignment within the tetrairon(iii) core. In fact, each Fe center contributes to the overall signal XMCD(E) with a specific pattern which reflects its coordination environment and whose amplitude is proportional to local magnetic polarization, according to Eq. 1 S13 : XMCD(E) = Σ i δ i S i (E) (1) In the above formula, i runs over the four Fe ions, the S i (E) s are the XMCD contributions of fully field-polarized Fe ions and the δ i s describe the extent of field polarization, which in a classical picture can range from parallel (δ i = +1, ) to antiparallel (δ i = -1, ) to the applied field. Due to finite size effect, the actual coupling coefficients have to be considered for antiferromagnetic coupling of an inner Fe ion with three outer Fe ions, affording δ outer = and δ inner = at full Supplementary Information for Magnetic memory of a single-molecule etc.. by M. Mannini et al. 4

5 polarization S14. Due to their very similar coordination environment, the central and peripheral ions can be safely assumed to have similar S i (E) s. As a consequence, the nature of the magnetic interaction is expected to simply modulate the amplitude of the overall XMCD signal. Within this simplified picture, it can be argued that complete fragmentation to isolated Fe ions or ferromagnetic coupling in the tetrairon(iii) core would lead to an approximately two-fold increase of the XMCD signal ( ) as compared with antiferromagnetic coupling ( ). References S1. Barra, A.L. et al. New single-molecule magnets by site-specific substitution: incorporation of "alligator clips" into Fe 4 complexes. Eur. J. Inorg. Chem (2007). S2. Cornia, A. et al. Energy-barrier enhancement by ligand substitution in tetrairon(iii) singlemolecule magnets. Angew. Chem. Int. Ed. 43, (2004). S3. Accorsi, S. et al. Tuning anisotropy barriers in a family of tetrairon(iii) single-molecule magnets with an S=5 ground state. J. Am. Chem. Soc. 128, (2006). S4. Mannini, M. et al. X-ray magnetic circular dichroism picks out single-molecule magnets suitable for nano-devices. Adv. Mater. in press ((DOI: /adma ). S5. Funk T. et al. Soft x-ray magnetic circular dichroism at 2 K: A tool in biological inorganic chemistry. Rev. Sci. Instrum. 75, , (2004). S6. Letard, I. et al. Remnant magnetization of Fe 8 high-spin molecules: x-ray magnetic circular dichroism at 300 mk. J. Appl. Phys. 101, (2007). S 7. Sainctavit, Ph. & Kappler, J.-P. X-ray magnetic circular dichroism at low temperature in Magnetism and Synchrotron Radiation Proceedings (Springer, Berlin, D, 2000), pp S8. Englisch, U. et al. The elliptical undulator UE46 and its monochromator beam-line for structural research on nanomagnets at BESSY-II. Nucl. Instr. Meth. A , (2001). S9. Quitmann, C. et al. A beamline for time resolved photoelectron microscopy on magnetic materials at the Swiss light source Surf. Sci. 480, 173 (2001). S10. See S11. Nakajima, R., Stöhr, J., & Idzerda, Y.U. Electron-yield saturation effects in L-edge x-ray magnetic circular dichroism spectra of Fe, Co, and Ni. Phys. Rev. B 59, (1999). S12. Ufuktepe, Y., Akgül, G, & Lüning, J. X-ray photoabsorption and total electron yield of Fe thin films at the L2,3 edge. J. Alloys Compd. 401, (2005). S13. Mannini, M. et al. XAS and XMCD investigation on Mn12 monolayers on gold. Chem. Eur. J. 14, (2008). S14. Arrio M.-A. et al. Soft x-ray magnetic circular dichroism in paramagnetic systems: elementspecific magnetization of two heptanuclear Cr(III)-M 6 (II) high-spin molecules. J. Am. Chem. Soc. 121, (1999). Supplementary Information for Magnetic memory of a single-molecule etc.. by M. Mannini et al. 5