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 200,000,000 so that 1TB of storage capacity is now available in a single 3.5 inch device at a cost of less than $100. 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 our research we explore the potential of new, highly engineered magnetic media based on thin film exchange springs. 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 switched 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 do this by building on our expertise in the innovative vector magnetometry measurement protocols to determine the magnetic properties of specially designed samples. We are able to systematically control the thin film exchange spring by choice of materials, by deposition conditions, through the use of coupling layers and lithographic processing. Our work 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.
In order to develop a full understanding of our data we collaborate with Sheffield Centre for Advanced Magnetic Materials and Devices (SCAMMD) on a project to simulate magnetic exchange spring materials. This collaboration allows the maximum scientific output from our unique data.
The need to make progress on understanding the physical origins of the SFD and how it can be tailored for technological applications is critical to any nanomagnetic device that relies for its function on switching of magnetic elements. Bit patterned media which is a dense array of nanoscale magnetic elements, provides an excellent platform to study SFD’s as the field required to switch each individual island can be isolated and studied. It is already clear that in most of the systems studied to date the intrinsic variation in material properties and/or fabrication and processing is responsible for the overwhelming majority of the observed SFD.