Heat Assisted Magnetic Recording Media Based on Exchange Bias

A study of a new paradigm for 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 (F) layer and the system if field cooled resulting in a hysteresis loop shifted along the field axis. In this concept the temperature dependent anisotropy is provided by IrMn. Therefore the information is stored in the AF layer and the recording layer is actually part of the read/write structure when field cooled. The F layer when magnetised, serves to align the F layer in the direction required to store the information and then provides a read out 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. The key to achieve a structure of this kind is the control of the orientation of the Mn ions of the IrMn such that they are aligned perpendicular to the plane of the film. In this way a perpendicular exchange bias required for information storage in the AF layer has been achieved. Over 300 samples have been prepared and evaluated to determine the optimised structure.
A segregated sample CoCrPt-SiO2 was sputtered using a pressed powder target in a HiTUS deposition system. Dual Ru seed layers of 8nm and 12nm were deposited using 3mTorr and 30mTorr process pressure, respectively. The median grain size of D_m = (6.2 ± 0.2)nm was achieved using these sputtering conditions. High resolution cross section TEM imaging has been used to show that the CoCrPt grains remains segregated by SiO2 after the deposition of IrMn.
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. The highest shift or exchange bias of H_ex = (240 ± 5)Oe was measured at room temperature. This was achieved by depositing an ultrathin (0.8nm) Co interlayer above a Pt seed layer.