X-ray absorption and magnetic circular dichroism investigation of bis(phthalocyaninato)terbium single-molecule magnets deposited on graphite

R. Biagi, J. Fernandez-Rodriguez, M. Gonidec, A. Mirone, V. Corradini, F. Moro, V. De Renzi, U. del Pennino, J. C. Cezar, D. B. Amabilino, and J. Veciana

Phys. Rev. B 82, 224406 – Published 6 December 2010

Abstract

Bisphthalocyaninato terbium complexes show a long magnetization relaxation time at relatively high temperatures—which makes them very interesting as magnets at single-molecule level. Their technological exploitation, however, requires the addressing of the individual molecules, therefore the deposition of single-molecule magnets on surfaces is a topic of great interest as the interaction with the substrate can play a crucial role in the definition of the molecule properties. In this work we investigate the electronic and magnetic properties of anionic and neutral forms of a bis(phthalocyaninato)terbium derivative deposited on graphite by means of x-ray absorption spectroscopy and x-ray magnetic circular dichroism, performed at low temperature and high magnetic field at the $M_{4, 5}$ edge of Tb. We were able to reproduce the experimental spectra by means of multiplet calculations and to validate the applicability of sum rules to the present case. Sum rules were then used for determining the orbital and spin moments of thick (several monolayers) and of thin films (submonolayer range). Calculations of spectra as a function of the molecule orientation with respect to the impinging x-ray beam, allowed us to ascertain the adsorption geometry of molecules. For both compounds, molecules stay essentially flat when adsorbing as thin film on graphite. This result is also confirmed by scanning probe microscopy, which also finds a very interesting ordered arrangement for the molecules of the neutral form. In the thick film of the neutral compound the molecules keep the same orientational order, arranging almost flat as well. On the contrary, in the thick film of the anionic compound their orientation appears to be random. The origin of this different behavior can be related to the hindrance of the counterion moiety and/or to the different solvent used for each compound. Finally, the comparison of the magnetization values and their dependence on the external magnetic field and temperature suggest that the magnetic properties of molecules are preserved when adsorbed onto the graphite surface.

Received 18 December 2009

DOI:https://doi.org/10.1103/PhysRevB.82.224406

Authors & Affiliations

R. Biagi^1,2,*, J. Fernandez-Rodriguez³, M. Gonidec⁴, A. Mirone³, V. Corradini², F. Moro^1,2,†, V. De Renzi^1,2, U. del Pennino^1,2, J. C. Cezar³, D. B. Amabilino⁴, and J. Veciana⁴

¹Dipartimento di Fisica, Università di Modena e Reggio Emilia, Modena, Italy
²Istituto Nanoscienze-S3, CNR, Modena, Italy
³European Synchrotron Radiation Facility, BP 220, F-38043 Grenoble Cédex, France
⁴Campus de la UAB, ICMAB-CSIC, Bellaterra, Spain

^*Corresponding author; roberto.biagi@unimore.it
^†Present address: University of Nottingham, School of Chemistry, United Kingdom.

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Vol. 82, Iss. 22 — 1 December 2010

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Images

Figure 1
(Color online) Thin-films topography as taken by using scanning tunneling (STM, top panel) and atomic force (AFM, bottom panel) microscopies, performed on the monolayer samples of the neutral and of the anionic compounds, respectively. The inset on the left bottom within each topography image displays the molecular structure of each derivative whereas the one on the right bottom shows the height along the line path marked on the topography image. In the STM image of the neutral monolayer $({}^{ML}1)$ , contour lines are reported with a height step of 0.7 nm, in order to help recognizing the different layers. The inset on the left top shows the ordered arrangement of molecules.Reuse & Permissions
Figure 2
(Color online) XAS spectra taken at the $M_{4, 5}$ edges of Tb, for the neutral and anionic forms, thick and thin films, with helicities parallel $(I^{↑ ↑})$ and antiparallel $(I^{↑ ↓})$ to the applied magnetic field, together with their difference $(I^{↑ ↓} - I^{↑ ↑})$ normalized to the maximum intensity of the unpolarized spectrum $(I^{↑ ↓} + I^{↑ ↑}) / 2$ , i.e., the XMCD spectrum. Temperature and external magnetic field were 7 K and 5 T, respectively. Next to each spectra set are reported the values of the corresponding angular moments (in $ℏ$ units) as derived from sum rules (see text).Reuse & Permissions
Figure 3
(Color online) Comparison of experimental (dots) XAS and XMCD spectra taken on thick films of neutral (left) and anionic (right) ${Pc}_{2} Tb$ compounds with the results of multiplet calculations (solid lines). For both experiment and theory, temperature and external magnetic field were 7 K and 5 T, respectively.Reuse & Permissions
Figure 4
(Color online) Top: experimental unpolarized spectra for thin and thick films of both anionic and neutral compounds. Bottom: calculated unpolarized spectra as a function of the angle between the molecule axis and the x-ray beam direction. The angular step between adjacent spectra is $6 °$ . The spectrum resulting from the isotropic average of the molecule orientation is also reported.Reuse & Permissions
Figure 5
(Color online) Comparison between the experimental and theoretical circularly polarized XAS and XMCD spectra. The left panel displays the experimental spectra of ${{}^{ML}1}^{-}$ , ${}^{TF}1$ , and ${}^{ML}1$ in comparison with a single set of spectra which is representative of tilt angles ranging between $0 °$ and $24 °$ . On the right panel the experimental XAS and XMCD spectra of ${{}^{TF}1}^{-}$ are compared with the theoretical ones resulting from the isotropic average.Reuse & Permissions
Figure 6
(Color online) Magnetization as a function of the external field (top panel, taken at $T = 7 K$ ) and of the temperature (bottom panel, taken at $H = 5 T$ ). Circles and squares refer to neutral and anionic compound, solid and open symbols refers to thick and thin films, respectively. Dashed and dotted lines display the dichroic signal corresponding to the reported magnetization measurements of the neutral thick and thin films, respectively. These curves make reference to the right axis.Reuse & Permissions

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