Developing novel materials for spin transfer torque devices

Abstract

Ferromagnetic thin films consisting of binary alloys with L10 crystallographic ordering are currently being used as the medium in heat assisted magnetic recording (HAMR) and have significant potential in spintronic devices and more generally as permanent magnets in electric machines. This research focuses on the formation of the L10 phase in FePt and MnAl thin films, which is motivated by their potential for use in energy assisted data storage devices. FePt and MnAl binary alloy magnetic properties in their L10 phase have highly desirable magnetic properties; FePt has a high magnetic moment and very high magnetic anisotropy, MnAl has a moment comparable to permalloy, high magnetic anisotropy and low spin orbit coupling in addition to being composed of relatively abundant elements. The research presented in this thesis details the development of fabrication procedures to achieve L10 ordered thin films of FePt and MnAl using magnetron sputtering. Whilst the fabrication of L10 ordered thin films of FePt with either random or perpendicular magnetic anisotropy is well known, thin films with uniaxial in-plane magnetic anisotropy is unreported using sputter deposition. The fabrication of uniaxial in-plane FePt thin films is investigated as a foundation for their use in exchange spring structures. The growth of the L10 phase is controlled by seed layers and choice substrates, along with deposition and annealing temperatures. The phase formation within the films is characterised by X-ray diffraction, with a novel approach of using XRD with in-situ heating for feedback into the fabrication procedure. The optimised sputter fabrication procedure is shown to be comparable with literature values for MBE grown samples. Fabrication of sputtered L10 ordered MnAl thin films was investigated using both an alloy target and Mn/Al co-deposition. Forming the desired phase is highly dependent on composition, where both methods show an unexpected substantial loss of Mn during fabrication, which it is proposed to be due to adhesion issues on the substrate surface. Further exploration of deposition and annealing conditions found key links between phase formation and film surface roughness, where reducing deposition pressure plays a key role in formation of low roughness films. However, the magnetic properties of the films fabricated fell short of the theoretical values for MnAl. This work moves the state-of-knowledge forward in the fabrication of L10 ordered MnAl thin films and details the challenges faced producing ferromagnetic L10 ordered MnAl film with key findings on compositional uncertainties.

Date of Award	1 Aug 2022
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Thomas Thomson (Supervisor)