A systematic study of single molecule metallocenes with 4d and 3d transition metal atoms

Abstract

The realization of spin based devices is one of the most aspiring goals of spintronics research. Single molecule magnets are an important class of nanoscale magnetic systems with potential to realize different spintronic devices where each molecule can be used as a fundamental building block for devices. In this work, we have systematically investigated metallocenes, a class of single molecule magnets, with 4d and 3d transition metal elements for their electronic and magnetic anisotropic properties, using first-principles density functional theory. Among the seven 4d elements studied in this work, the largest anisotropy of about 20 Kelvin is obtained for Mo and Rh with uniaxial anisotropy. We found that the anisotropy does not increase with an increasing number of $d$ electrons; rather, it depends strongly on the orbital ordering of the $d$ states of the transition metal. Our calculations also show that the anisotropy of Mo-metallocene increases for cationic charge states to 60 Kelvin but with an easy-plane anisotropy. For 3d elements, the anisotropy of the molecules is calculated to be less than 10 Kelvin. We also have studied the role of ligands on the structural stability of these molecules and have provided a clear guideline to construct an appropriate model of molecules for theoretical studies.

Figures (6)

• Figure 1: Metallocene with transition metal elements denoted as red ball at the center. The green and blue balls correspond to carbon and hydrogen, respectively.
• Figure 2: Double-well potential energy surface of ZrCp$_2$ for different displacements Q along one imaginary vibrational mode and the corresponding optimized ground-state structure.
• Figure 3: Schematics of energy levels of $^*$MoCp$_2$, Mo$^+$Cp$_2$, VCp$_2$ and Cr$^+$Cp$_2$ metallocenes around the Fermi level and the corresponding spatial distribution of molecular orbitals (the same isosurface cutoff is used for all plots). In the inset the HOMO of $^*$MoCp$_2$, and the LUMO of VCp$_2$ are replotted with smaller cutoff to show the contribution of the Cp$_2$$\pi$ orbitals.
• Figure 4: Energy level diagrams of 4d metallocenes showing the splitting and occupation of the metal $d$ orbitals. The left and right columns correspond to the spin-up and spin-down channels, respectively. Arrows indicate the occupation of each orbital.
• Figure 5: Energy level diagrams of 3d metallocenes showing the splitting and occupation of the metal $d$ orbitals. The left and right columns correspond to the spin-up and spin-down channels, respectively. Arrows indicate the occupation of each orbital.