1/35 Future Magnetic Storage Media Jim Miles Electronic and Information Storage Systems Research Group

2/35 Future Magnetic Storage Media 1.Media requirements for very high density 2.Model description 3.Predicted effects of grain size distribution 4.Patterned media: possible routes 5.Conclusions

3/35 Granular or Patterned Media?

4/35

5/35 Granular Media Limitations W D The transition from one bit to another follows the grains… (or maybe clusters of grains). Jitter Small grains are needed for low noise.

6/35 Writing to Media Anisotropy K u Magnetisation M S 0MS0MS Field H > H K = 2K U A sufficiently large field is needed to overcome the anisotropy of the material, which keeps magnetisation aligned along one axis

7/35 Thermal Stability of Media Energy barrier E B = K U V Thermal energy ~ K B T Spontaneous switching when E B < 70K B T Require E B ~70 K B T

8/35 To Increase the Density: Decrease the bit length: Jitter must decrease Decrease the track width W: Jitter must not increase. Jitter,  grain diameter D must fall Volume V =  D 2 t/4  Volume falls  K U must rise to keep E B = K U V high enough  bigger write field H > 2K U /  0 M S is needed. Density can only rise by increasing write field.

9/35 Perpendicular Recording Increases write field, but only by ~ x2…

10/35 Other Problems of Granular Media Media are granular. Grains are not equal-sized. Typically  D ~ 0.2,  V ~ 0.4 Hypothesis - Irregularity in media structure produces noise: –Big grains give big transition deviations; –Different grain volumes switch more or less easily; –Different grains see different local interaction fields.

11/35 Perpendicular Media Modelling Real Storage Medium Model Storage Medium (not to identical scale)

12/35 Landau-Lifshitz dynamic and M-C thermal solvers. Arbitrary sequences of uniform vector applied fields Recording simulation with FEM or analytical head fields. Soft underlayer by perfect imaging Microstructural clustering and texturing. Fully arbitrary grain positions and shapes. Full account of grain shape in interaction fields Allows vertical sub-division and tilted columns (MET like) Manchester MicroMagnetic Multilayer Media Model (M6)

13/35 Magnetostatic Interaction - Pairs of Grains Magnetostatic interaction tensors D are computed numerically ‘Field’ grain experiences a field that varies through the volume. Surface charge from each polygon face of the source generates field. Typically 48 faces per polygon. Top and bottom faces computed similarly by division into strips. Interaction Field: H j = D ij M i Integrate over the surface charge of i and the volume of j. Underlayer included by incorporating images into D ij MiMi HjHj

14/35 Exchange Interaction - Pairs of Grains Exchange interaction factors are computed numerically Integral term computed numerically from polygon geometry x x d ij i (source) j (field) Grain j experiences an exchange field due to grain i

15/35 Varying Grain Size Voronoi seed positions randomised Minimum grain boundary width 0.7nm fixed Number of grains/m 2 and packing fraction fixed Mean grain volume remains constant  H ex remains constant σv/ = 0% σv/ = 15% σv/ = 39%

16/35 Grain Size Distributions σv/ = 0% σv/ = 4.7% σv/ = 10.2% σv/ = 15.5% σv/ = 22.6% σv/ = 29.4% σv/ = 38.7%

17/35 Exchange Field Distributions Average exchange field does not change as the microstructure changes. H E = 0.5 H D A = 1.85x for all structures σv/ = 0% σv/ = 4.7% σv/ = 10.2% σv/ = 15.5% σv/ = 22.6% σv/ = 29.4% σv/ = 38.7%

18/35 Exchange Interaction Between Pairs of Grains Width of line  H ex Uniform grains, perfect hexagonal lattice. Exchange field is identical between all pairs. Thermally decayed from DC saturated σv/ = 0 H E /H D = 0.5

19/35 Exchange Interaction Between Pairs of Grains Width of line  Hex Large volume distribution: σv/ = 39% Irregular structure, Large variation in H E / = 0.5

20/35 Magnetostatic (Demag) Field Distributions σv/ = 0% σv/ = 4.7% σv/ = 10.2% σv/ = 15.5% σv/ = 22.6% σv/ = 29.4% σv/ = 38.7%

21/35 Energy Barrier Distributions σv/ = 0% σv/ = 4.7% σv/ = 10.2% σv/ = 15.5% σv/ = 22.6% σv/ = 29.4% σv/ = 38.7%

22/35 Recorded Transitions, b=20nm, Tp = 80nm, 411 Gb/in 2 σv/ = 39% σv/ = 0%

23/35 Effect of Irregularity on Data Signal σv/ = 0% σv/ = 4.7% σv/ = 10.2% σv/ = 15.5% σv/ = 22.6% σv/ = 29.4% σv/ = 38.7%

24/35 Effect of Irregularity on Noise σv/ = 0% σv/ = 4.7% σv/ = 10.2% σv/ = 15.5% σv/ = 22.6% σv/ = 29.4% σv/ = 38.7%

25/35 Grain Microstructure Conclusions Grain size distributions give rise to decreased signal and increased noise (BAD) Media with small grain size distributions are needed Patterned media are needed Additional advantage: switching volume is the bit size, not the grain size  lower switching field is possible.

26/35 Tom Thomson

27/35 Tom Thomson

28/35 Direct Write e-beam 1.Form master by direct write e-beam on resist layer 2.Evaporate gold coating 3.Lift-off gold from unexposed areas 4.Etch to remove magnetic layer except where protected by gold 50 nm diameter islands B. Belle et. al. University of Manchester

29/35 Patterned Media Potential Provides a route to regular arrays of thermally stable low noise 1Tb/in 2 requires 12.5nm lithography Not feasible using semiconductor manufacturing technology for some years to come…

30/35 Self-Organised Magnetic Assembly (SOMA Media) 1.FePt nanoparticles manufactured in aqueous suspension. 2.Very narrow size distribution. 3.Deposited onto substrate. 4.Self-Assemble into ordered structure.

31/35 FePt Particle Growth

32/35 FePt problems FePt manufactured in solution has low Ku. Very high Ku can be developed by annealing: Much ongoing research in low temperature formation of high coercivity FePt…

33/35 Other Potential Technologies Electro-chemical deposition in self- ordered templates: University of Southampton. Electroplating into self-ordered pores in Alumite: R. Pollard et. al, Queens University Belfast. Vacuum deposition through self-assembled nanosphere templates: Paul Nutter, Ernie Hill, University of Manchester.

34/35 Self-Assembly – Long Range Order 40nm diameter CoCrPt nanoparticles. Mask made from a diblock co-polymer (polystyrene/PMMA), self-assembled in nanoimprinted grooves. (Naito et al, Toshiba, IEEE Trans. Magn 38 (5) (2002) Self-assembled pattern using a diblock co-polymer (in nanoimprinted grooves. (C. Ross et al, MIT, 2002) Self-assembly produces only local order. Over long ranges order breaks down at dislocations.

35/35 Conclusions Conventional media can only be extended so far. Patterned media overcome thermal stability issues. Higher stability granular materials could be used with heat assisted recording (HAMR) …but patterned media might still be needed to avoid excessive transition noise. Patterned media are likely to be necessary in ~5 years