Disputation: Dynamics of Magnetic Molecules under Electrical Control

  • Datum:
  • Plats: Polhemsalen, Ångströmlaboratoriet, Lägerhyddsvägen 1, Uppsala
  • Doktorand: Hammar, Henning
  • Om avhandlingen
  • Arrangör: Materialteori
  • Kontaktperson: Hammar, Henning
  • Disputation

This thesis theoretically studies the dynamics of molecular magnets under electrical control.

Molecular magnets are nanoscale magnets that can, e.g., consist of single-molecules or single-atoms. In these magnets, the electronically mediated exchange and transport can be controlled by external fields. In this thesis, we study the effect of electrical control and voltage pulses on the transport properties, spin dynamics and the exchange of the molecular magnets.

Nonequilibrium Green's functions is the method used to describe the underlying electronic structure of the magnetic molecule. The studied systems consists of single-molecule magnets in a tunnel junction between metallic leads. Here, the transport characteristics are derived for charge, spin and heat currents in the system for time-dependent voltage pulses. Furthermore, a generalized spin equation of motion is derived for the molecular spin moment, using nonequilibrium field theory. The equation of motion incorporates nonequilibrium conditions and is of nonadiabatic character. The effective model for the spin moment can be decomposed into effective magnetic field, isotropic Heisenberg interaction, and anisotropic Ising and Dzyaloshinskii-Moriya interactions. These effective fields depends on the electronic structure of the molecule and can be controlled by, e.g., gate and bias voltages.

The thesis encompasses studies on the effect of a sudden on-set of a voltage pulse for a single-molecule magnet and its effect on the spin dynamics and transport properties of the molecule. Different approximations schemes for the spin equation of motion and their regimes of validity are investigated. Moreover, spin-dependent signatures in the heat transport characteristics of the single-molecule magnet are connected to the dynamics of the molecular spin moment. A phase induced switching mechanism of the molecular moment is shown for voltage pulses of varying temporal length. In the stationary limit, it is shown that one can electrically control the interaction and transport of two molecular magnets in a series. Furthermore, investigations on the electronically mediated anisotropy in a vibrating single-molecule magnet show that the anisotropy can be tuned by a temperature difference or a voltage bias.