Exchange spring magnetic thin films: The future recording media

Data storage is right at the centre of the digital age and a core developer and user of nanoscale technology. The ability to store and retrieve vast amounts of information on demand, and at miniscule cost, has revolutionised the way society functions. The device at the heart of this revolution is the hard disk drive (HDD) where over the last 50 years data densities have increased by a factor of 100,000,000 so that 1TB of storage capacity is now available in a single 3.5 inch device. The key magnetic components that have enabled this explosion in capacity are the recording head and the storage medium. The research work which forms the basis of recording head transducers, the giant magnetoresistive (GMR) effect was recognised with the 2007 Nobel prize in physics.In the work proposed here, the potential of new, highly engineered magnetic media based on thin film exchange springs will be explored. These materials allow the relationship between medium thermal stability and switching field to be tailored, so that thin films with sufficient anisotropy to avoid thermally activated reversal can still be reversed by the fields available from a write head. In order to take full advantage of these materials there is a pressing need to address the exciting fundamental questions in thin film exchange spring magnets. Specifically, what is the optimum exchange spring structure for thin films at technologically relevant thicknesses (~10 nm) that achieves maximum thermal stability whilst retaining addressability; in dense packed granular materials how does intergranular or, for patterned structures, inter-island exchange coupling modify the reversal behaviour and the thermal stability; what are the details of the spring structure during reversal; how resistant are exchange spring thin film to reversal from stray fields; what other application areas can thin film exchange springs provide enhanced functionality. The goal of our research is to provide quantitative answers to the important questions surrounding thin film exchange spring magnets. We plan to achieve this by building on the innovative vector magnetometry measurement protocols developed by the PI to determine the magnetic properties of specially designed samples, where we will systematically control the thin film exchange spring by choice of materials, coupling layers and lithographic processing. The proposed measurement programme makes full use of the vector magnetometer's ability to track the position and moment of the magnetisation vector whilst applying a field at an arbitrary angle and maintaining the sample at a set temperature. This capability allows the reversal process to be accurately characterised so that, for example, the relationship between nucleation and domain wall processes can be quantified.A critical part our programme is the work to model the behaviour of thin film exchange spring magnets and so obtain the maximum scientific output from our unique data. A simulation framework based on a kinetic Monte Carlo scheme to compute energy barriers and remanent hysteresis loops will be developed for thin film exchange spring media. This expansion of the existing simulation code will make it possible to study the effect of exchange and magnetostatic interactions on the magnetization reversal behavior. Using this newly developed capability, remanent hysteresis loops of exchange spring media will be computed for different field angles and different intergrain exchange interactions and compared directly with the experimental results.