Molecular Nanomagnets

Phthalocyanine molecules with a 3d transition metal in the centre, like MnPc, FePc, CoPc, NiPc and CuPc, have attracted a huge interest in the last decades due to the large number of possible applications. Experimental and theoretical gas phase studies are an important reference to understand the properties of the molecules, as well as how they can be modified and manipulated upon deposition on substrates or in supramolecular conformations. However, in several 3d metal phthalocyanines, the electronic structure of the single molecule is still under debate even after several spectroscopic studies and computational works have been performed. This is mostly due to the highly correlated 3d electrons of the metal atoms, which pose a challenge for the theory. In addition, the experiments to determine the electronic structure are often carried out in different conditions (on thick films or in gas phase for example), and this can lead to different results. The following chapter provides an overview of the theoretical and experimental results and debates related to the electronic structure of gas phase MnPc, FePc, CoPc, NiPc and CuPc.

Different spectroscopic methods can be used to characterize the electronic structure of a system of interest, i.e. molecular, solid or adsorbate samples. The different techniques give complementary information about the geometric and electronic structure of the system. By Photoelectron Spectroscopy (PES), Auger and resonant photoemission (RPES) the occupied electronic levels can be studied, whereas X-ray Absorption Spectroscopy (XAS) gives information about the unoccupied valence states of the system in presence, however, of a core hole. Magnetic information can be obtained from X-ray Magnetic Circular Dichroism (XMCD).

In this chapter, the theoretical methods required for the description of structure, electronic structure and magnetism of magnetic molecules in the gas phase and in the adsorbed configurations will be discussed. The main workhorse of the theoretical methods is the density functional theory that provides a materials-specific description of electronic structure, which is quite sufficient for many of the materials. However, in the present context of magnetic molecules, one needs to go beyond standard approximations in density functional theory. In this regard, some of the crucial characteristics in the electronic structure and magnetism will be discussed such as electron correlation, van der Waals interaction, band gaps, magnetic anisotropy and spin-dipole moments.

Phthalocyanine molecules with a 3d transition metal in the center, like MnPc, FePc, CoPc, NiPc and CuPc, have attracted a huge interest in the last decades due to the large number of possible applications. Experimental and theoretical gas phase studies are an important reference to understand the properties of the molecules, as well as how they can be modified and manipulated upon deposition on substrates or in supramolecular conformations. However, in several 3d metal phthalocyanines the electronic structure of the single molecule is still under debate even after several spectroscopical studies and computational works have been performed. This is mostly due to the highly correlated 3d electrons of the metal atoms, which pose a challenge for the theory. In addition, the experiments to determine the electronic structure are often carried out in different conditions (on thick films or in gas phase for example), and this can lead to different results. The following chapter provides an overview of the theoretical and experimental results and debates related to the electronic structure of gas phase MnPc, FePc, CoPc, NiPc and CuPc.

Theoretical treatment of functional metalorganics is non-trivial for the metal centers with narrow bands (3d, 4d of transition metals or 4f bands of rare-earth metals), featuring a sizeable Coulomb interaction. An interplay between crystal field, spin-orbit coupling and Coulomb interaction expresses the properties of the molecule. Correlated metal centers, immersed in the electron bath of organic ring makes it ideal to treat with Anderson’s impurity model. In this chapter, we will focus on the description of electron correlation in functional metalorganics with the aid of density functional theory, combined with a many body approach. For most of the illustrative purposes, we will consider iron porphyrin (FeP) molecule. The chapter will reveal the importance of the treatment of explicit electron correlation in order to accurately identify the spin transition, magnetic anisotropy, Kondo effect etc., which are key ingredients for molecular spintronics and electronics.

Organometallic molecules have attracted interest because their properties can be varied by changing ligands, metal center, end groups etc. which makes them candidates for various applications. Special attention has been paid to hybrid systems of molecules and substrates as possible building blocks for future electronic and magnetic devices. In view of such devices phthalocyanine molecules are advantageous because they can adsorb flat on metallic or semiconducting substrates. Aiming to understand the magnetic properties of the molecules and their interplay with substrates and ligands the focus will be on the paramagnetic Pc molecules i.e. Mn, Fe, Co and CuPc and their interaction with (metallic) substrates and nonmagnetic TMPCs such as NiPc and ZnPc are only briefly mentioned.

The adsorption of small molecules on the metal center of 3d metal phthalocyanines alters the bonding scheme of the molecule and can induce different spin states and magnetic moments, as well as trigger more exotic effects like Kondo resonance. Soft X-ray spectroscopy studies and computational studies have highlighted this kind of effects in magnetic molecules like FePc, CoPc and MnPc by adsorption of for example CO, NO, O\(_2\) molecules.

In this chapter, a short introduction to possible applications utilizing organic molecules interacting with ferromagnetic substrates will be given. By explaining the spin filtering concept, studies with Cu-phthalocyanine as well as zinc methyl phenalenyl molecules will be reviewed. The hybridization of the electronic states of the molecules at the interface to those of substrates is a crucial point to realize functional hybrid metalorganic interfaces. A large magnetic anisotropy is found for the hybridized phenalenyl layer, which is important for the interface magnetoresistance effect used in this spin filter concept study.