Reversible Change of the Spin State in a Manganese Phthalocyanine by Coordination of CO Molecule

Abstract

We show that the magnetic state of individual manganese phthalocyanine (MnPc) molecules on a Bi(110) surface is modified when the Mn2þ center coordinates to CO molecules adsorbed on top. Using scanning tunneling spectroscopy we identified this change in magnetic properties from the broadening of a Kondo-related zero-bias anomaly when the CO-MnPc complex is formed. The original magnetic state can be recovered by selective desorption of individual CO molecules. First principles calculations show that the CO molecule reduces the spin of the adsorbed MnPc from S ¼ 1 to S ¼ 1=2 and strongly modifies the respective screening channels, driving a transition from an underscreened Kondo state to a state of mixed valence. DOI: 10.1103/PhysRevLett.109.147202 PACS numbers: 75.20.Hr, 68.37.Ef, 82.37.Gk, 68.43.Bc Control over the magnetic moment of a molecule and its interaction with a substrate is a key issue in the emerging field of molecular spintronics [1]. In metal-phthalocyanines and metal-porphyrins the metal center is usually coordinatively unsaturated and presents a local reactive site, which opens a unique possibility of controlling the magnetic moment in situ by external chemical stimuli [2][3][4][5][6][7][8]. The axial coordination of small molecules like CO, NO or O 2 to those complexes substantially alters their electronic properties, which has led to a successful implementation of phthalocyanines and porphyrins in gas sensors [9,10]. Studies specifically addressing the magnetic state are, however, scarce. Only recently it has been shown that the attachment of a NO molecule to a cobalt-tetraphenylporphyrin (CoTPP) quenches its spin due to the oxidation process [6]. The general picture is however more complicated, as the chemical bond to the reactant molecule causes the redistribution of charge in the d orbitals of the metal center and modifies the ligand field of the metal ion. This has critical consequences for the magnetic ground state of the complex [7,8]. On metal surfaces, the formation of a new ligand bond may additionally alter the hybridization of molecular and substrate states, thus affecting the electronic and magnetic coupling of the metal ion to the substrate [3][4][5]. Understanding the response of these effects to the change in chemical coordination is crucial to gain full control over the functionality of the magnetic system.Here, we show that the magnetic moment of a manganese phthalocyanine (MnPc) molecule [see Fig. 1(a)] on a Bi(110) surface is reduced when coordinated to a CO molecule. Using a combination of a low temperature scanning tunneling microscopy (STM) and density functional theory (DFT) we find, first, that the spin of the MnPc molecule is reduced from S ¼ 3=2 to S ¼ 1 upon adsorption and, second, that CO further reduces the spin of the MnPc from S ¼ 1 to S ¼ 1=2. The change in the magnetic ground state upon CO attachment is detected by the broadening of a characteristic zero-bias anomaly (ZBA). We interpret this broadening as a transition from a Kondo regime, for the bare MnPc on...

Surface chemistry of porphyrins and phthalocyanines

“…A change of the spin state from S=1 to S=1/2 upon CO coordination was observed for MnPc on Bi(1 1 0) [773]. TERS (cf.…”

Probing the Electronic Properties of Individual MnPc Molecules Coupled to Topological States

“…Similar to MnPc molecules adsorbed on Bi(110), its broadening is though to be a result of the hybridization with the substrate. 12 Interestingly, we found clear signature of how electronic correlations on the central 3d metal ion influence the overall electronic structure of the system. This is done by studying the MnPc energy levels once the central Mn atom is adsorbed on top of a defect, which may be view as a perturbation potential influencing the e-e-interaction.…”

“…Our choice was motived by the fact that transition metal phthalocyanines are representative of a broad class of planar metal-organic compounds where the presence of partially empty d orbitals in the central metal atom gives them interesting magnetic properties. [11][12][13] Bi 2 Te 3 , on the other hand, is a well studied TI characterized by a single Dirac cone centered around the Γ point which makes it a In fact, a slight distortion of the molecule's four-fold symmetry can already be recognized in the overview scan of 1(a). This is confirmed by the atomically resolved images reported in 1(b-d) which also show the inequivalent appearance of the axis, with the one aligned along a dense-packed rows of Te atoms appearing slightly brighter than the other.…”

Influence of film thickness and air exposure on the transport gap of manganese phthalocyanine

“…8 Further theoretical studies on a single MnPc molecule sandwiched between two single walled carbon nanotubes reveal its nearly perfect spin filtering properties. 6 It was already shown that the high spin ground state of MnPc S = 3/2 can be changed to S = 1 by adsorption on Bi(110) surface, 9 or S = 1/2 by reaction with CO, 9 or O 2 . 10 A higher spin state of S = 5/2 can, on the other hand, be obtained by doping with alkali metals.…”