Exploring the electrical and physical properties of 2D atomically layered materials with advanced scanning probe microscopy

Abstract

The atomic force microscope (AFM) was first created in 1986 and was initially used to measure topography and magnetisation, but has continually evolved over the 30 years since its introduction. A common application is in precise measurements of electrical properties including the local resistivity of semiconducting materials, however metallic samples cannot presently be characterised due to the complexities of applying nanoscale resistivity measurements to such surfaces. This missing technique needs to be addressed, and in this work a solution is developed and evaluated which uses a voltage divider to measure the contact resistance between a solid-metal probe and a metallic conducting surface. The oxide layer of the FeRh sample under investigation has to be removed prior to characterisation, but following this the expected pattern is resolved and it is concluded that any non-oxidising metallic sample with resistivities varying by at least one order of magnitude should be resolvable with this methodology. Antiferromagnetic (AF) spintronic devices are an active area of study at present and FeRh is one of the most promising materials for these devices. Equiatomic FeRh is antiferromagnetic at room temperature and becomes ferromagnetic at around 100°C, with the transition temperature being ~10°C higher during heating than cooling. The room temperature magnetic ordering may be manipulated by irradiating the surface with noble gas ions, and the possibility of using lithography to produce magnetic patterns is being explored. This work provides nanoscale magnetic characterisation of an FeRh thin film which has been patterned into 100 nm stripes. The patterning is shown to be successful and is not disrupted by exposure to magnetic fields of up to 2T nor by temperatures above the FeRh transition temperature. It is seen that the specific FM domain structure of the irradiated regions remains stable throughout the heating and cooling cycle. An artificial magnetoelectric multiferroic may be formed by depositing FeRh onto the ferrolectric substrate PMN-PT, where the coupling between ferroic layers is mediated through strain. The local piezoelectric coefficient and coercive field are related to the local stoichiometry so the distributions of these characteristics are therefore needed to be modelled and characterised on the nanoscale. It is shown that rapid charging and discharging causes damage to the surface and that electrostatic effects are dominant over relatively thick materials with high electric permittivity, so a series of OFF-field scans are determined to be the optimal solution with the localised spectroscopy achieved through concatenation of the measured piezoelectric response values. This characterisation agrees with the models which suggest the inhomogeneity in piezoelectric coefficient to be negligible, and so PMN-PT is an ideal candidate for the ferroelectric substrate used in artificial magnetoelectric multiferroics.

Date of Award	1 Aug 2023
Original language	English
Awarding Institution	The University of Manchester
Supervisor	Thomas Thomson (Supervisor)