Exchange coupling of magnetic molecules to magnetic and non-magnetic substrates

Abstract

This thesis summarizes an experimental investigation of magnetic behaviour of square-planar paramagnetic molecules (e.g. metallo-phthalocyanines and metallo-porphyrins) adsorbed on magnetic and non-magnetic substrates. Magnetic and electronic properties of the molecules are studied by means of X-ray Photoelectron Spectroscopy, Scanning Tunnelling Microscopy, Low-Energy Electron Diffraction and synchrotron based X-ray Absorption Spectroscopy and X-ray Magnetic Circular Dichroism. The experimental results are complemented by Density Functional Theory calculations conducted by Kartick Tarafder, Mohammed Ali Ehesan and Peter Oppeneer from Uppsala University, Sweden. Main results of the thesis include studies on: Exchange coupling of chromium-tetraphenylporphyrin chloride (CrTPPCl) molecules to bare ferromagnetic Co(001) thin films. Adsorption of CrTPPCl species on the bare Co substrate leads to an induced magnetic moment on Cr ion. The magnetic moment is shown to couple antiferromagnetically with the substrate magnetization. Furthermore, CrTPPCl molecules adsorbed on the bare cobalt thin film change their oxidation state from 3+ to 2+ upon release of chlorine ligand from the Cr ion. Microscopic origin of the observed AFM coupling is described by a 90° indirect cation-anion-cation exchange coupling between less than half-filled 3d shell of the Cr(II) ion, 2p orbitals of molecular nitrogen and out-of-plane 3d orbitals of cobalt atoms. Exchange interaction of manganese- and iron-tetraphenylporphyrin chloride (Mn- & FeTPPCl) molecules to nitrogen and chlorine terminated Co(001) thin films. Insulating spacer inserted between the paramagnetic molecules and the cobalt substrates is formed by an atomically thin layer of cobalt nitride or cobalt chloride, respectively. The layers are shown to alter molecule-substrate exchange coupling resulting in reduced electronic interaction of the ad-molecules with both substrates as evidenced by formation of self-assembled molecular 2D arrays. Magnetic moments resolved on Mn and Fe ions of the molecules are observed to couple antiferromagnetically with N/Co substrates. Microscopic origin of the interaction is explained by an indirect 180° cation-anion-cation exchange coupling between 3d orbitals of Mn(Fe) ions, 2pz orbital of substrates’ nitrogen atoms and out-of-plane orbitals of cobalt atoms. The pre-adsorbed nitrogens possess sizeable magnetic moment on the 2pz orbital which is induced due to the exchange interaction with the cobalt atoms. The magnetic moment of pre-adsorbed nitrogens is shown to couple ferromagnetically with that of cobalt. Adsorption of MnTPPCl molecules at chlorine terminated Co film has revealed an antiferromagnetic coupling between a relatively small magnetic moment induced on Mn ion and the substrate magnetization. Surprisingly, the XMCD signal measured at Fe L3,2 edges of FeTPPCl species adsorbed on Cl/Co substrate, resolves a subtle molecule-substrate coupling. Our magnetic measurements reveal ferromagnetic coupling between magnetic moment of Fe and that of the cobalt thin film. Analysis of XAS/XMCD spectra acquired at L3,2 and K edges of chlorine suggest that the exchange interaction between adsorbed molecules and the underlying substrate is mediated by 3d-orbitals of the chlorine atoms pre-adsorbed at the surface, i.e. 180 degree indirect exchange coupling. Exchange coupling of manganese-tetraphenylporphyrin chloride (MnTPPCl) molecules to Co(001) substrates through a chromium spacer. Chromium spacer in the form of a wedge is shown to alter magnetic exchange coupling between MnTPPCl molecules and ferromagnetic cobalt substrates as a function of increasing thicknesses of chromium spacer. Magneto-chemical origin of the exchange coupling at the organic-inorganic interface is investigated by X-ray Photoelectron Emission Microscopy, i.e. spectro-microscopy correlation approach. At higher thicknesses of the chromium layer, the magnetic coupling between the molecules and the substrate is reduced due to the fact that induced spin waves decay exponentially with increasing distance. Long-range ferrimagnetic order in supramolecular 2D Kondo lattice consisting of paramagnetic manganese-phthalocyanine (MnPc) and hexadeca-flourinated iron-phthalocyanine (FeFPc) molecules self-assembled into checkerboard array on Au(111) substrate. Magnetic moments of the molecules show magnetic remanence in absence of magnetic substrate or external magnetic field. Exchange coupling between the molecules, leading to a long-range ferrimagnetic order, is attributed to RKKY coupling mediated by the conduction electrons of the underlying gold substrate. Remanent molecular magnetic moments align in out-of-plane direction and reveal a notable contribution from orbital magnetic moment. Interestingly, the magnetic measurements performed for two different geometries (i.e. normal and grazing) suggest easy-plane anisotropy. Furthermore, the interaction of molecular magnetic moments with the substrate electronic states leads to a profound many body Kondo effect. Presented results demonstrate the ability to control magnetic properties of paramagnetic molecules by means of exchange coupling to magnetic or via non-magnetic surfaces.