Superior bit error rate and jitter due to improved switching field distribution in exchange spring magnetic recording media.

Magnetic recording at high density requires small magnetic grains or islands and narrow distribution of the material properties. In addition, intrinsic noise due to the thermal fluctuations during writing has to be minimized. This is a particular challenge in heat-assisted recording, where the write process occurs at elevated temperature1,2. In heat-assisted recording for smaller grain sizes, both (i) the fundamental jitter due to thermal fluctuations during writing as well as (ii) the Tc distributions are expected to increase. In order to break the dilemma of designing a media that has a good writability and good thermal stability for non-heat-assisted recording, exchange spring media were proposed3. The original goal of exchange spring media was (i) to allow for a media that is stable, even for smaller grain sizes, and (ii) to obtain an improved switching field distribution (SFD) due to the Kondorsky dependence of the switching field4 as a function of field angle3,5. Experimentally exchange spring media was first demonstrated by Wang et al.6,7 showing a significantly reduced coercivity by maintaining a high thermal stability. Furthermore in this work it was demonstrated the predicted lower angle dispersion of the coercivity. Soon after the first experimental realization the same group demonstrated exchange spring media on the disk substrate with soft underlayer8. Since about 2007, exchange spring media have been used in current products of hard disk drives. Interestingly, the grain size in these media has not decreased. Nevertheless, exchange spring media lead to an improved signal-to-noise ratio. This can obviously not attributed to the main original goal of a small grain size. We believe the reason can be found in an improved switching field distribution in exchange spring media, which was predicted by micromagnetic simulations9. Experimentally, the improved switching field distribution was reported by various groups10,11. The switching field distribution is essential in order to improve the areal density in both bit-patterned magnetic recording and magnetic recording on granular films. A further improvement beyond two layer exchange spring media was the usage of a media design with varying anisotropy as function of the layer depth. As pointed out in ref. 12 the coercive field can be much further reduced if a graded anisotropy is used. Graded media where FePt as hard magnetic layer is used was reported experimentally by various groups13,14,15,16,17.