Novel Synthetic Magnetic Materials Optimised for RF and Microwave Applications

Abstract

Understanding and controlling the dynamic magnetisation behaviour of magnetic systems is fundamental to current and future technological applications. This thesis examines the underlying physics and explores the potential of magnetic thin-film systems for high frequency applications. Particular focus was directed towards exploiting interfacial phenomena, to enhance the frequency bandwidth response, and lithographic patterning to control the resonant frequency.

The design, development and functional capabilities of a purpose built integrated system to probe the dynamic response of magnetic thin-film systems as a function of applied magnetic field, excitation frequency (up to 15~GHz) and sample orientation is detailed. Sample motion, with respect to the measurement transmission line, was achieved with m vertical motion precision and an angular step precision of 1.8.

Physical mechanisms and parameters involved in damping were investigated on multilayers combining ferromagnetic (Co, CoFeB) and heavy metal layers (Pt, Ru) to quantify interfacial spin-transport. Enhanced interfacial transparency, leading to higher damping, is observed for more closely matched FM/HM crystal structures. Including a thickness-dependent spin-diffusion length gives a bulk value of ~nm for Pt. The propagation of spin current into Pt was shown to be suppressed beyond a nominal SiO insulating barrier of 2~nm corresponding to the formation of a continuous SiO layers, from x-ray reflectivity analysis.

The role of the induced moment in Pt with respect to interfacial damping was explored using synchrotron radiation. The nature of the induced moment was first explored by investigating the alignment of the Pt moment across the transition-metal-rare-earth ferrimagnetic magnetisation compensation transition. It was shown that the moment aligns with the transition-metal regardless of the dominant sub-lattice. The induced moment was correlated with the magnetic damping in CoFe/Pt and NiFe/Pt systems with Au and Cu spacer layers. The relation between an induced moment and enhanced damping highlights the role of hybridisation.

Photolithography was used to pattern materials to enhance the effective magnetic field. An almost linear relation was observed between the aspect ratio and the induced anisotropy field, and hence the resonant frequency. This relation was used to produce tessellated patterns with varying aspect ratio, that demonstrated an isotropic, broadband and field-free dynamic magnetisation response.