Heat Assisted Magnetic Recording using Exchange Bias

In this work a study of a new paradigm for the recording layer in a heat assisted
magnetic recording (HAMR) media based on the use of exchange bias is presented. Exchange
bias occurs when an antiferromagnetic (AF) layer such as IrMn is grown in contact with a
ferromagnetic (FM) layer, which in our case happens to be also the recording layer resulting
in a hysteresis loop shifted along the field axis. The FM layer when magnetised, serves to align
the AF layer in the direction required to store the information and then provides a readout
signal indicating in which direction the AF layer is oriented. Hence in a complex way the
“recording layer” is actually part of the read/write head. By achieving spin alignment of the
IrMn such that the spins are aligned perpendicular to the plane of the film, the required
perpendicular exchange bias can be induced for information storage in the AF layer. A series
of 14 set ofsamples have been prepared and evaluated to determine the optimised structure.
A segregated CoCrPt-SiO2 sample was used as the recording layer from a pressed
powder target in a HiTUS deposition system. Several seed layers were tested such as Ru, Pt
and Cu for their ability to favour perpendicular anisotropy in double or combined form of 8
nm and 12 nm thicknesses respectively. They were deposited using 3 mTorr and 30 mTorr
process pressure. XRD and XRR techniques were used in order to evaluate the structural
properties of the multilayers. Alternating gradient force magnetometry and vibrating sample
magnetometry at room and low temperature measurements were also used to characterise
their magnetic response.
The key feature of this media is that the recorded information is impossible to be
erased by a demagnetising field. In order to achieve this requirement, the hysteresis loop has
to be completely shifted to a negative field. In the current study a small shift up to Hex=325
Oe was achieved at 100K without the need of a Co interlayer. A complementary attempt was
made to induce the desired exchange bias by depositing an ultrathin (0.8 nm) Co interlayer
above a Pt and a Ru seed layer resulting in an exchange field of Hex=40 Oe. In addition, mixed
seed layers were deposited using Ru/Pt and Ru/Cu with the first case showing the most
promising results of squareness around 70% and coercivities of 1800 Oe. Because of the fact
that loop shifts were observed at 100K, further work is required to optimise those structures.