Frustration: more ways to emergent behaviour.

Functional magnetic properties are constantly exploited in technological applications. The phenomena of magnetism was originally used in a compass and magnetic properties are still being developed to help store digital data on hard drives. Indeed the hard drive industry in 2013 is expected to generate $33 billion of sales, importantly as more information is stored digitally the bit density needs to carry on increasing. Novel methods of magnetic data storage need to be developed, requiring fundamental research which can be fed directly into industry.

One such avenue of research is magnetic frustration, where pairwise interactions within a system cannot be simultaneously satisfied. If these frustrated interactions occur on the intersections of a triangular crystal lattice (1 nanometre in size) the net moment can be used as a bit with an incredible bit density. Indeed artificial frustrated systems (3 orders of magnitude bigger than the crystal lattice) are currently being developed to test this idea of storing data. However the fundamental physical processes on the nanometre scale still need to be understood in order to manipulate this technology. Fundamental investigations of frustrated systems at low temperature will redefine how we understand magnetic materials, and how we store and manipulate information in the future.

Fundamental properties of magnetic systems are still delivering twists and surprises. Specifically, magnetism has recently generated excitement because of the concept of emergent behaviour. This is essentially a description of unexpected properties that fundamentally challenge how to understand and manipulate magnetism on an every day basis. In the frustrated material know as spin ice (so called because the lowest energy ground state has 16 different spin configurations, the same number of proton configurations as water ice) emergent magnetic monopoles have been discovered. These monopoles act like electric charges in that they can be driven apart by a magnetic field, in much the same way electricity is controlled by electric fields. However to understand these properties the sample of spin ice needs to be measured at very low temperatures, much less than 1 K as the excitation which creates monopoles has a thermally activated behaviour with an energy gap around 4 K. Therefore to be in the dilute limit to enable charges to be manipulated the magnetisation has to be measured with a low temperature (< 500 mK) dilution fridge. One question to be to answered is how 'magnetricity' can be manipulated by controlling experimental parameters, such as cooling rate.

One way to get a handle on the properties is to measure the magnetization. Specifically how long does the sample take to respond to an applied field which can be used as a measurement of spin ice dynamics. Because the creation of monopoles depends on how quickly spin ice is cooled through the magnetic freezing temperature it is important to understand how the dynamics respond afterwards. This enables the manipulation of the monopole density and the subsequent dynamics. Moreover the dynamics present need to be investigated over a wide dynamical regime and an instrument will be built to measure the changes in the susceptibility at frequencies up to 1MHz.

The instrument that will be developed, which will be unique in the UK and one of very few worldwide, will also allow the study of quantum properties in magnetic materials. The low temperature is required as thermal fluctuations will swamp the quantum excitations. Emergent behaviour is expected in quantum spin liquid magnets which contain tightly packed magnetic ions with frustrated interactions. Quantum behaviour can also be measured in lone magnetic ions with magnetometers. This allows the manipulation of different energy levels and understanding of the fundamental properties.

The research described in this proposal fulfills two criteria of the EPSRC grand challenges themes. Specifically it meets the 'Quantum Physics for New Quantum Technologies' and the 'Emergence and Far From Equilibrium Phenomena' themes, and is a novel direction for work which has led to a deeper understanding of quantum behaviour and frustration in magnetic systems. It also has a direct relevance to data storage, a $33 billion a year industry. To potentially increase data storage density frustrated interactions are being manipulated on micron sized artificial islands which have antiferromagnetic interactions with each other. However to fully exploit this idea the fundamental properties of interactions in frustrated single crystals on the nano metre length scale need to be fully understood. Moreover it is expected that emergent or unexpected behaviour will be seen on frustrated lattices which can be manipulated and allow the design of future artificial systems.

The impact of the project is three-fold.

The manipulation of emergent electromagnetism is a core area of fundamental physics, this project will learn how to manipulate states in both the classical and quantum regimes. The exploratory experiments already performed have strongly guided the direction to pursue to observe new physics, with some theories even suggesting the creation of artificial light is possible with such systems, opening a vast area of potential exploitation. Although this work is based on bulk single crystals, the techniques to control monopole density and manipulate the resultant magnetic charge have a direct analogy in artificial spin ice structures. These two dimensional structures already show signatures of controllable data storage and device fabrication and any fundamental understanding produced by this project will most likely be highly relevant to future artificial devices.

To open a new time window, up to 1 MHz for the study of frustrated systems with susceptibility as a bulk probe. If any devices are to be fabricated it is essential to understand any persistent dynamics which may affect device performance.

The manipulation of quantum states is essential in the development of quantum computing, a likely future tool in complex problems with examples including cyber security, cryptography, information management and complex molecule design. This project will investigate the quantum state of dense magnetic spins and also investigate quantum state population in dilute single ion systems such as LiY1-xHoxF4.

This diverse portfolio of fascinating physics will involve the development of a novel magnetometer that will allow new physical processes to be measured at temperatures below 500mK. The ground work has already been performed in this field with the applicant already having developed techniques to demonstrate the existence of magnetic monopoles in condensed matter systems and the manipulation of the magnetic charge. The applicant has a proven track record in technique development and the system will also allow the investigation of quantum correlations in both dense and dilute magnetic systems. This will help to ensure that the United Kingdom stays at the front emergent technologies, such as quantum computing in both the short and the long term.

The results will be disseminated using a variety of means. It is expected that there will be a continued stream of publications in high impact journals such as Nature and Physical Review Letters and interactions with the community in general will be maintained via participation in colloquium and international conferences. To ensure the public remains engaged novel work will be distributed via press releases and through the Science Made Simple project based at Cardiff University and outreach work at local schools ensuring an academic link between those at the start of their career and cutting edge discoveries.