single molecule magnet-based microarrays

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1 SUPPORTING INFORMATION Low temperature magnetic force microscopy on single molecule magnet-based microarrays Michele Serri, 1,2 Matteo Mannini, 1,2 * Lorenzo Poggini, 1,2 Emilio Vélez-Fort, 3 Brunetto Cortigiani, 1,2 Philippe Sainctavit, 4 Donella Rovai, 1,2 Andrea Caneschi, 1,2 Roberta Sessoli 1,2 1 Laboratory for Molecular Magnetism (LA.M.M.), Department of Chemistry Ugo Schiff, Università degli Studi di Firenze via della Lastruccia 3-13, Sesto Fiorentino, Italy. 2 INSTM Research Unit of Firenze, via della Lastruccia 3, Sesto Fiorentino, Italy. 3 European Synchrotron Radiation Facility, 71 Av. Martyrs, F Grenoble 9, France 4 Institut de Mineralogie, de Physique des Materiaux et de Cosmochimie, UMR 7590, CNRS, UPMC, IRD, MNHN, F Paris, France & Synchrotron SOLEIL, L Orme des Merisiers, Saint-Aubin BP 48, Gif-sur-Yvette, France matteo.mannini@unifi.it Experimental methods Synthesis and homogeneous sample preparation. Microcrystalline powders of the neutral Tb(III) bis-phthalocyaninato complexes were prepared following procedure of Weiss et al. S1 Silicon (100) wafers were sonicated in acetone and then in isopropanol, dried with nitrogen and 1

2 then used as substrates. Polycrystalline gold films ( 100 nm) with preferential (111) orientation were thermally evaporated on glass substrates at a pressure of about 10-6 mbar and at a rate of ca. 2Å/min monitored by a quartz microbalance (QCM). The continuous PTCDA films ( 20 nm) were deposited on Si by Organic Molecular Beam Deposition with a Kurt J. Lesker SPECTROS system at pressure <10-6 mbar and a rate of 0.2Å/s. Shadow masking-based deposition. A home built evaporator was used for the deposition of TbPc 2 and of the patterned PTCDA films. Patterned PTCDA films (12 nm) were deposited at an average rate of 0.6 Å/min using a 2000 mesh copper TEM grid as a shadow mask, heating the powders to 360 C in a quartz crucible at a pressure <5x10-7 mbar. Depositions of the TbPc 2 layers were done on substrates after exposure to air, using powders that were degassed for several days before sample preparation. The dots pattern was created using a R 1.2/1.3 TEM grid (Quantifoil Micro Tools GmbH) as a shadow mask. The base pressure during the sublimation was <10-6 mbar and the deposition rate was 0.2 Å/min, as measured by QCM before exposing the substrates. SPM characterizations. The topography of the samples was measured in air with AFM imaging (NT-MDT P47-PRO). Low temperature magnetic force microscopy (LT-MFM) characterization was performed using the attoafm/mfm Ixs system (Attocube systems AG) based on the physical properties measurement system (PPMS-9, Quantum Design). High coercivity (>0.5 T/μ 0 ) MFM probes (ASYMFMHC Asylum Research) with a 30 nm coating of CoPt/FePt were used (radius 32 ± 7 nm, spring constant 2 ( ) N/m, resonant frequency f 0 =73.4 khz). The cantilever resonance Q factor was larger than 2000 during the LT measurements. 2

3 ToF-SIMS analysis. ToF-SIMS analysis was carried out with a TRIFT III time-of-flight secondary ion mass spectrometer (Physical Electronics, Chanhassen, MN, USA) equipped with an Au liquid-metal primary ion source. Positive ion spectra were acquired with a pulsed, bunched 15 kev primary ion beam at 600 pa by rastering the ion beam over a 100 μm x 100 μm sample area. Positive ion images were acquired with a pulsed, unbunched 25 kev primary ion beam at 600 pa by rastering the ion beam over a 200 μm x 200 μm sample area. The primary ion dose was kept below ions/cm 2 to maintain static SIMS conditions. Positive data were calibrated to CH + 3 ( m/z), C 2 H + 3 ( m/z) and AuSCH + 2 ( m/z). The reported images were convoluted using the WinCadence software (Physical Electronics). Synchrotron experiments. XMCD spectra were extracted from X-ray absorption spectra (XAS) measured at the Tb M 4,5 edges using circularly polarized light at normal and 45 incidence to the sample surface and applying a magnetic field of 3 T along the beam. The experiments were performed using the XAS dedicated branch of the ESRF-ID32 beamline and working in the total electron yield (TEY) mode to guarantee surface sensitivity. A strongly reduced flux density (5x10 8 ph s -1 mm -2 ) obtained by defocusing the X-ray beam was employed to prevent sample damage S2 and photon induced relaxation. S3 The resulting measured drain current of the sample was of the order of 3pA at the M 5 edge of Tb on gold substrates. The magnetic field sweep rate used during the hysteresis measurement was 2.6T/min. MFM image simulations. The simulation of the MFM images in Figure 3c was performed by numerical calculation of the gradient of the magnetic force between a point dipole, representing the tip, and an ideal model of the experimental TbPc 2 dot, i.e. a magnetic structure with a truncated cone shape 25 nm high, having 2.04 µm diameter at the base and 1.40 µm at the top. We notice that the expected demagnetizing field of a magnetic object is less uniform around 3

4 sharp edges, while it is perfectly homogeneous in a spherical object. The truncated cone shape of the TbPc 2 dots thus contribute positively to the enhancement of the stray field contrast in the MFM image. The topography of the structure is described by the Z(x,y) function; the x and y dimensions were sampled with a pixel size Δx= Δy=20 nm. For ease of calculation, we considered the structure to be homogeneously magnetized, with the magnetic moment pointing along the direction normal to the substrate, as it is expected in the case of a TbPc 2 dot on PTCDA at saturation. The MFM probe is tilted by 15 along the y direction and oscillates by 80 nm along the same direction. At each point of the image, the tip oscillation trajectory is sampled by 21 points, with the lowest point of the trajectory being at a lift height of 85 nm from the sample surface Z(x,y). The stray field projection along the tip axis was calculated at each point of the image and of the tip oscillation trajectory by integrating the dipolar field emanated by all the microscopic volumes of dimension Δx, Δy, δz into which the dot is divided; the integral along the z dimension was calculated analytically, while along x and y it was performed numerically. At each point (x,y) of the image, the stray field values calculated along the tip oscillation trajectory were fitted with a second order polynomial. The coefficient of the second order term of the polynomial is proportional to the gradient of the magnetic force experienced by the magnetic point dipole that represents the tip, hence to the MFM f signal. Since real MFM tips are not point dipoles but have a finite size with a typical a radius >30 nm, the experimental images have a much lower resolution than in this simulation. An accurate calculation of the tip sample interaction would require a detailed knowledge of the tip magnetic structure and is outside the scope of this publication. For an easier comparison between simulation and experiment, the simulated image has been blurred with a Gaussian filter, with radial σ of 120 nm, and random 4

5 noise was added to the simulation (rms of the noise 1/65 times the maximum contrast observed in the simulated image after Gaussian blurring). Data-mask generation algorithm. An area in the MFM image of the TbPc 2 dot on PTCDA at 3 T (Figure 4a) was selected in order to exclude the signal from the neighboring dots. The selection consists of a circle covering the dot and a semicircle, 1.5 times larger in diameter, that includes the crescent shaped region of repulsive magnetic interaction outside of the dot. The distribution of the MFM frequency shift (f) values within this selection, shown in Figure 4c, was fitted with three Gaussian functions (G A, G B, G C ) which are assumed to represent the regions (R A, R B, R C ) of strongest attraction, moderate attraction and repulsive interaction between tip and sample. The regions are defined by data-masks created with the following algorithm, which does not make any a priori assumption on their shape and position. Each pixel is randomly assigned to one of the three masks with a probability given by the relative contribution of the three Gaussian components of the fit at the f value measured at that pixel. In other words, the probability of a pixel (i,j), with frequency shift f i,j, to belong to R k is equal to the ratio G k (f i,j )/ G n (f i,j ). The masks created in such a way may show fragmentation into small islands or pixels that are poorly connected to the rest of the region. To correct this, all the pixels that are part of such defects are removed from the masks and reassigned again with the probabilistic method described above. This process is reiterated until the resulting masks do not presents fragmentation, or the maximum number (500) of iterations is performed. 5

6 Figure S1. a) Time-of-Flight Secondary Ion Mass Spectrometry (ToF-SIMS) mass spectrum (histograms) of the molecular peaks of TbPc 2 measured on a 50 nm high TbPc 2 dot array deposited on a continuous PTCDA film (20 nm) on silicon. The theoretical isotopic distribution is shown on top. b) ToF-SIMS mass spectrum of the molecular peaks of PTCDA measured on the same sample, with its theoretical isotopic distribution. c) Schematics (not in scale) of the Quantifoil TEM grid used as shadow mask for the patterned TbPc 2 film. d) Spatial distribution of the TbPc 2 mass signal on the patterned 50 nm TbPc 2 /PTCDA structure. e) spatial distribution of the PTCDA mass signal on the structure. f) overlay of the TbPc 2 (red) and PTCDA (green) signals. g) AFM topography image of the TbPc 2 dot microstructures found inside the square superstructures observed by ToF-SIMS. 6

7 Figure S2. a) Topography of the TbPc 2 dot microarray (24 nm thick) on patterned PTCDA layer (12 nm thick) on silicon, measured at 10 K, 0 Tesla. b) MFM image at a lift height of 85 nm from the surface, measured together with a) at 10 K and 0 Tesla. The color scale was set as in Figure 2b. 7

8 Figure S3. a) Topography image of a single TbPc 2 dot on silicon and profiles along the horizontal and vertical diameters, following the lines highlighted in the image. b) MFM image of the dot measured at 85 nm lift height and profiles along the lines highlighted in the image. The measurements were simultaneously performed at 10 K and 4 T magnetic field. 8

9 Figure S4. a) Topography image (10 K, 0 T) of a single TbPc 2 dot on PTCDA. b) MFM image (10 K, 0 T) of the TbPc 2 dot on PTCDA. c) Topography image (10 K, 0 T) of a single TbPc 2 dot on silicon. d) MFM image (10 K, 0 T) of the TbPc 2 dot on silicon. All measurements were performed after a demagnetizing sequence obtained by swapping the field across zero field and gradually reducing it down to zero; MFM signal was measured at 85 nm lift height from the topography. 9

10 Figure S5. a) Topography and b) MFM image (lift height 85 nm) of the TbPc 2 dot microarray (24 nm thick) on patterned PTCDA layer (12 nm thick) on silicon, measured in a horizontal scan direction at 10 K, 4 Tesla. c) Topography and d) MFM image measured in a vertical scan direction under the same conditions. 10

11 Figure S6. Maps of the different channels collected in parallel during the dual pass MFM measurement of a TbPc 2 dot on a PTCDA island at 10 K, 4 Tesla, in the forward (a-e) and in the backward (f-j) scans along the vertical direction (forward is from top to bottom): a, f) Topography (1 st pass), b, g) MFM image (2 nd pass), c, h) phase (2 nd pass), d, i) amplitude (2 nd pass), e, j) amplitude (2 nd pass) after plane subtraction. The phase signal is almost flat, except noise, due to the activated PLL feedback. The dash-dot lines in the MFM images indicate the directions along which the profiles in figure S7 were measured. 11

12 Figure S7. MFM signal along the a) vertical and b) horizontal directions passing through the center of a TbPc 2 dot on PTCDA (see figure S6) in the forward (black) and the backward (red) scan images (figures S6 b, g). Additional references (S1) De Cian, A.; Moussavi, M.; Fischer, J.; Weiss, R. Inorg. Chem. 1985, 24, (S2) Mannini, M.; Pineider, F.; Sainctavit, P.; Danieli, C.; Otero, E.; Sciancalepore, C.; Talarico, A. M.; Arrio, M.-A.; Cornia, A.; Gatteschi, D.; Sessoli, R. Nat. Mater. 2009, 8, (S3) Dreiser, J.; Westerström, R.; Piamonteze, C.; Nolting, F.; Rusponi, S.; Brune, H.; Yang, S.; Popov, A.; Dunsch, L.; Greber, T. Appl. Phys. Lett. 2014, 105,