Heat Assisted Magnetic Recording Using Exchange Bias

Heat Assisted Magnetic Recording (HAMR) is considered to be the next technology to improve current magnetic recording densities. The principle of HAMR is that a laser delivers heat pulses via near field optics to the surface of a conventional disc with grains oriented in the perpendicular direction. Although the concept of heating the recording layer close to its Curie temperature during the writing process to reduce the energy barrier to reversal is over 50 years old, it is only recently that such an approach has become viable thanks to the introduction of near field optical transducers. By heating to elevated temperatures, ~750K, the anisotropy of the material is thereby reduced enabling a conventional write-head to switch the grains. Once the grains have cooled, the anisotropy rises allowing higher data density. For the last 15 years the material of choice for the recording layer has been the ferromagnetic alloy FePt mainly because of its high magnetocrystalline anisotropy with a strong temperature dependence. It also has a magnetic moment comparable to CoCrPt-based alloys currently used in perpendicular magnetic recording.

In this project, a novel approach to replace FePt is proposed based on our previous work in collaboration with Seagate Media Research (Fremont) [1,2]. It makes use of a phenomenon known as exchange bias. This is where a ferromagnetic (F) layer is grown in contact with an antiferromagnetic (AF) one. Usually exchange bias is used in the read-head sensor that reads the information stored in the media. The idea of using exchange bias in the recording layer is totally new. Due to the interaction at the F/AF interface, the hysteresis loop of the F layer is shifted along the field axis. There are several advantages to this approach compared to FePt:
• No phase transformation is required.
• Significantly lower writing temperature (~500K).
• Lower power consumption.
• Easy implementation.

In previous work we have provided proof of principle for this technology, enabling a 2015 patent in collaboration with Seagate Media Research in Fremont. Although the original work shows promising results complete grain segregation in the CoCrPt alloy was not achieved which results in partially exchange coupled CoCrPt grains. Complete grain segregation is needed to minimise bit transition widths and, hence, further work is required to optimise this structure.

The student will acquire experimental skills on thin film deposition and structural/magnetic characterisation via transmission electron microscopy, X-ray diffraction and a wide range of magnetometry techniques. The project will be done in collaboration with Seagate Technology in Northern Ireland and the student will be encouraged to spend time in Seagate Technology as part of their personal development.