Combined two-dimensional electron gas and artificial spin ice structures

• Main Author: Riley, Susan Tania
• Other Authors: Cunningham, John E. ; Marrows, Christopher H.
• Published: University of Leeds 2016
• Subjects: 621.3815
• Online Access: http://ethos.bl.uk/OrderDetails.do?uin=uk.bl.ethos.694110

This thesis presents research into the electron transport properties of hybrid semiconductor / ferromagnetic structures. Periodic arrays of ferromagnetic stripes and artificial spin ice (ASI - arrays of geometrically frustrated nanomagnets) are patterned atop GaAs-AlGaAs wafers containing a two-dimensional electron gas (2DEG), and resistance measurements are performed under cryogenic temperatures and applied magnetic fields. The effects of piezoelectric strain on the transport properties of 2DEGs are investigated by comparing the resistances of magnetic and non-magnetic stripes patterned atop a 2DEG. Piezoelectric strain manifests itself as electric commensurability oscillations in the longitudinal resistance of a 2DEG. These oscillations are independent of temperature and are caused by stress acting upon the edges of the stripes. Transport measurements of combined 2DEG / ASI structures reveal the first observations of commensurability oscillations (COs) caused by ASI in the longitudinal resistance of a 2DEG. These oscillations are periodic on length-scales commensurate with the length of the individual nanomagnets that form the ASI. The COs are temperature dependent, but independent of the angle of applied magnetic field for our particular samples. Models based upon a Fourier analysis of Maxwell's equations help explain our results. This thesis also addresses the thermally-activated magnetization dynamics behaviour of ASI. We show exactly how the proportion of each vertex type changes as an ASI is heated, and moves from an ordered state to a ground state. We compare the results from two different alloys of PdFe and three different lattice spacings. The way in which arrays of ASI change to a ground state is dependent upon the material composition of the ASI, with little dependence upon the period of the ASI. A material with a large magnetization requires a higher temperature to cause any magnetic spins to flip, after which the ASI abruptly changes from an ordered state to a ground state.