Novel Instrumentation for High Frequency Electrical Conductivity and Magnetisation Dynamics Characterisation

Abstract

The drive toward smaller devices and faster operational speeds places a challenge on techniques for probing electrical conductivity and magnetisation dynamics. This thesis attempts to address this challenge by developing novel instrumentation capable of accessing higher frequencies on-chip and in bulk devices and further expands conventional thin-film characterisation techniques to higher frequencies for analysis of non-continuous media.

To begin, contactless broadband microwave techniques, conventionally used to characterise thin-film devices, are adapted for non-destructive conductivity characterisation of randomly assembled arrays of metallic nanowires. Using a microstrip transmission line carrier, inductive broadband (CW (Continuous Wave) VNA (Vector Network Analysis) < 20 GHz and pulsed TDR (Time-Domain Reflectometry) < 50 GHz) measurements of conductance and distributed capacitance are performed for spray deposited arrays of poly(vinylpyrrolidone) (PVP) surface coated Ag nanowires.

As a follow on, rectangular cavity waveguides are designed, simulated, constructed and used to evaluate inkjet printed silver films as potential coatings for EMI shielding applications in three distinct frequency bands (X-Ku: 7-14 GHz, K: 15-20 GHz and Ka: 21-42 GHz). Comprehensive microwave reflectometry analysis is performed for films of varying thickness, in order to establish the threshold at which the films transition from a microwave transparent state to a reflective state. It is shown that films with a thickness beyond 120 nm are consistently effective EMI shields but the rate at which they transition to shielding behaviour depends on the frequency of the stimulus.

Finally, two new techniques for the detection of broadband (100 MHz - 20 GHz) ferro/ferrimagnetic resonance in single-crystalline and polycrystalline materials, which rely on SQUID-based gradiometry detection of small changes in the magnetisation, are developed. In the first method small changes in the along-the-applied-field projection of the coupled magnetic moment (dmz) are detected as the material is driven into resonance. Absolute measurement of the longitudinal component of the magnetisation and the resonance induced lowering of this moment makes estimation of the precession cone angle accessible, which is typically difficult to extract using conventional cavity or stripline based detection methods. The second method invokes the change in mz with the resonance-induced thermal heating (dmz/dT). Magnetisation dynamics in bulk Y3Fe5O12 is observed over a broad range of experimental temperatures (4 K - 400 K) and fields (10 - 500 mT). The inhomogeneous microwave excitation allows for the observation of higher magnetostatic modes and the convenient tracking of very broad resonances. The two SQUID-detection techniques when combined with conventional broadband VNA-FMR, low-frequency magnetic susceptibility and DC magnetometry, all easily realised using the same module, greatly expand the amount of static and dynamic information accessible.