Heat induced phase change exchange coupled composite media (HIP-ECC)

Today the data storage market is dominated by magnetic hard disk drives (HDDs) due to their cost effectiveness and utility compared with competitor technologies (eg. solid state flash drives). The basic layout of a HDD has remained the same since they were invented more than 50 years ago, but the technology of the components has changed beyond imagination and this has led to a 200 million-fold increase in data storage capacity since the first hard disk drives. Today's information society would not be possible without this extraordinary accomplishment which has occurred as a result of scientific advancement and engineering prowess working hand in hand. As an example, the discovery of the giant magnetoresistance (GMR) effect used in HDD data readers for which the Nobel Prize in Physics was awarded in 2007. This project aims to explore new ideas for magnetic disk media to allow a continuation of the phenomenal growth in data storage capacities that is required for societal progress in the future.

The success of HDDs has been built on the scientific and technological progress that has allowed each of the components to be scaled to ever decreasing size. The materials used in conventional magnetic recording media are nanoscale (~8nm) granular magnets where a single bit is stored on multiple grains using an electromagnet designed to fly a few nanometres above the surface of the disk. These grains cannot be scaled down in size indefinitely and as the volume of the grain is limited by the super-paramagnetic effect, where individual magnetic grains may reverse due to thermal excitations, results in data loss and device failure. Recent research has focussed on circumventing this problem.

In this joint project between the University of Manchester and the University of Sheffield we propose a new design for a tuneable exchange coupled composite (ECC) medium for heat assisted magnetic recording (HAMR); a heat induced phase change ECC medium (HIP-ECC). An exchange coupled composite medium typically consists of several nanometre thick layers of magnetically hard and soft materials in intimate contact. Magnetic switching of the hard layer is assisted by coupling with the soft layer, resulting in a lower overall switching field and a higher thermal stability based on the properties of the hard layer. The proposed tuneable ECC medium has an intermediate layer between the soft and hard layer that will allow control of the exchange energy/coupling between both layers using a change in temperature. This thermal switch will allows us to dramatically reduce the heat requirements for recording, thereby avoiding many of the difficulties of more conventional approaches to HAMR. The key advantage of this design is that an extremely thermally stable material can be used to store the data with no loss in writeability.

HAMR is the leading technological candidate for achieving higher data storage densities in magnetic recording. This technology has the advantage that it can be used with both existing and future data recording technologies i.e. conventional magnetic media and bit patterned media (the magnetic material is patterned into individual nanometre-scale islands, each recording a single bit of data). HAMR makes use of the reduction in the magnetic field required to switch a ferromagnet at elevated temperatures. This phenomenon allows the use of the highest magneto-crystalline anisotropy materials such as highly ordered FePt and CoPt alloys to maintain long term stability. Magneto-crystalline anisotropy is an internal property of the material that determines its magnetic thermal stability.

Through this project we aim to deliver scientific progress that will result in clear applications in magnetic data storage, enabling the next generation of HDD products to be produced. Using this technology data storage density can theoretically be increased to 20Tbit/in2, 40 times larger than current commercial disk drives.

Who will benefit from this research:
The proposed research has both academic and industrial beneficiaries. There is a direct value to both the data storage industry and key equipment suppliers which is very well demonstrated by the letters of support from Seagate, HGST (a WD company) and Xyratex.
There is also a direct benefit to the UK and global academic community of the novel approach we propose to energy assisted magnetic reversal. Currently, there is significant interest in methods that allow additional energy (other than applied magnetic field) to be introduced to control the reversal process in nanoscale magnetic materials. The experimental realization and theoretical understanding we develop will add a new element to the stock of global knowledge on highly engineered nanoscale magnetic materials.
In addition to the direct applicability to the hard disk industry, there is real potential for benefit in related areas such as magnetic random access memory (MRAM). Here the same physical challenges of the need to reverse a nanoscale magnetic element and maintain long term thermal stability are encountered. Application of heat to assist reversal in MRAM has already started to be explored by start-up company Crocus Technology and our approach has the potential to make a contribution through the reduced heat load needed for the energy assist.

How will they benefit from this research:
There are two channels where benefits can be identified (i) the creation of new materials, knowledge and understanding and (ii) the provision of highly trained individuals able to work at the leading edge of science and technology. Our proposal will provide benefit in both these channels. Research aimed at underpinning the development of thermally assisted recording by exploring alternative ideas allows the data storage community to understand the merits of alternative approaches and incorporate them into their own research and development programmes. More generally there is a scientific benefit in creating new understanding as to how heat can affect the exchange coupling between two ferromagnetic layers. Exchange coupling between two ferromagnetic layers is of high scientific and technological interest as it not only forms the basis of magnetic exchange spring systems but also exchange bias materials and hard/soft phase permanent magnet materials. Past work has shown that progress aimed at one particular application area is of valuable across the entire subject area. Our work thus has the potential for significant indirect benefit in a number of related areas.
Highly skilled researchers and technologists are an essential requirement for progress in both industry and academia. This proposal directly provides an opportunity for two early career researchers to enhance their experience and expertise thereby enhancing the human resources seed corn for our technological future. Indirectly, there is benefit in maintaining a globally competitive research profile in Manchester and Sheffield so that future generations of students and postdocs are able to undertake research projects that are scientifically state-of-the-art and motivated by the technological needs of society.