Transport phenomena in nonitinerant magnets

Abstract

Traditionally, the role of information carrier in spin- and electronic devices is taken by respectively the spin or the charge of the conduction electrons in the system. In recent years, however, there has been an increasing awareness that spin excitations in insulating magnets (either magnons or spinons) may offer an interesting alternative to this paradigm. One of the advantageous properties of these excitations is that they are not subject to Joule heating. Hence, the energy associated with the transport of a single unit of information carried by a magnon- or spinon current could be much lower in such insulating magnets. Additionally, the bosonic nature of the magnon quasi-particles may be advantageous. Three crucial requirements for the successful implementation of spintronics in insulating magnets are the ability to create, detect, and control a magnon- or spinon current. The topic of creation and read-out of such currents in insulating magnets has been discussed elsewhere, in this thesis we will mainly focus on the third requirement, that of the ability to control magnon- and spinon currents. In the first part of this thesis is we aim to draw a parallel between spintronics in nonitinerant systems and traditional electronics. We do this by considering the question to which extent it is possible to create the analog of the different elements that are used in electronics for magnetic excitations in insulating magnets. To this end, we consider (in Ch. 2 and 3 respectively) rectification effects and finite-frequency transport in one-dimensional (1D) antiferromagnetic spin chains. We mainly focus our attention on the effects of impurities, which are modeled by local changes in the exchange interaction of the underlying Hamiltonian. Using methods from quantum field theory, which include renormalization group analysis and functional field integration, we determine the effect of such impurities on the transport properties of the spin chains. Our findings allow us to propose systems which behave as a diode and a capacitance for the magnetic excitations. In Ch. 4 we introduce a setup which behaves as a transistor for either magnons or spinons: a triangular molecular magnet, which is weakly exchange-coupled to nonitinerant spin reservoirs. We use the possibility to control the state of triangular molecular magnets by either electric or magnetic fields to affect tunneling of magnons or spinons between the two spin reservoirs. The second part of this thesis is devoted to the study of thermal transport in two-dimensional (2D) nonitinerant ferromagnets with a noncollinear ground state magnetization. More specifically, our interest is in thermal Hall effects. Such effects can be used to control a magnetization current, and arise because the magnons (which carry the thermal current) experience a fictitious magnetic field due to the equilibrium magnetization texture. We consider the different magnetic textures that occur in ferromagnets with spin-orbit interaction, and discuss which of them give rise to a finite thermal Hall conductivity.