Acknowledgements. Presentation Title Date 2

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1 Extensions of Perpendicular Recording Olle Heinonen and Kaizhong Gao Recording Head Operations Seagate Technology

2 Acknowledgements We gratefully acknowledge contributions and learning from Mark Kief, Robert Lamberton, Mark Gubbins, Alex Wong, Johannes van Ek Werner Scholz, Shezhaad Kaka, Sharat Batra, Mike Seigler Seagate RHO Advanced Transducer Development team Seagate Pittsburgh Research HAMR team many others and any errors are of course or our own making Date 2

3 Recording Head Writer Roadmap Perpendicular Recording Longitudinal Recording (Mark Kief, Mark Kryder) Date 3

4 Limits on conventional perpendicular recording So far, advances in perpendicular recording have been made by simple scaling: by reducing all dimensions - head geometry, grain size, head-tomedia separation (HMS), head-to-keeper separation (HKS) signal-to-noise remains constant (assuming the writer can supply enough field) Scaling has allowed us to trade off favorably in the writer trilemma diagram: balancing adequate head field, thermal stability, and SNR. Write-ability Large field or low anisotropy K k U B V T > 60 Thermal Stability SNR (grain size) Smaller grains Large grains or high anisotropy Date 4

5 Limits on conventional perpendicular recording There are now signs that scaling breaks down around 500 Gbit/in 2 : HMS scaling is expensive and difficult to maintain with active fly clearances approaching 1 nm. There simply are not that many more atoms to shave off from head and media overcoats, and media lube. Evidence of write-ability issues reduction in the poletip surface (unaccompanied by concomitant HMS scaling) means smaller field magnitude delivered by the writer. Evidence that scaling of transition curvature and adjacent track interference changes unfavorably Read-back thermal magnetic noise is becoming a problem and scales as 1/(Free layer volume) and ratio of signal to thermal magnetic noise is independent of reader magnetoresistive ratio and reader stiffness (sensitivity) Maintaining reader shield-to-shield spacing ~2x (bitlength) is difficult less space for antiferromagnet and biasing magnet leads to reader instabilities. Date 5

6 Getting out of the write-ability tri-lemma: Add energy to assist reversal in thermally stable grains with increasing anisotropy (HAMR) Write-ability Make volume of each bit large one grain per bit: Bit-patterned media (BPM) Thermal Stability SNR (grain size) Or: use non-stoner-wohlfarth switching to reduce necessary switching field while maintaining thermal stability and SNR. (MAMR, ECC media/domain-wall assisted recording, ) Date 6

7 Perpendicular recording extensions: Energy-assisted recording: Microwave-Assisted Magnetic Recording (MAMR) Heat-Assisted Magnetic Recording (HAMR) Spin torque driven layer Jimmy Zhu Non-Stoner-Wohlfarth recording Exchange-Coupled Composite media (ECC) or Domain-Wall Assisted Magnetic Recording (DWAMR) Mike Seigler What is the underlying physics? What are system requirements? What can be solved by engineering? What requires invention? What is a likely path for magnetic recording? M1 M2 Date 7

8 Microwave Assisted Magnetic Recording Basic physics: less field amplitude is required to reverse a noninteracting Stoner-Wohlfarth particle if it is first made to precess by applying an AC field in the plane perpendicular to the anisotropy axis H K -H H bias bias =H loc e.a. H K M x-ac field AC field: H x = ±0.1 T f x = 11.2 GHz DC bias: H z = T H z <<H K x z y H K = 1.1 T f K = 30.8 GHz cf.: G. Bertotti, C. Serpico, I. D. Mayergoyz, Nonlinear Magnetization Dynamics under Circularly Polarized Field, Phys. Rev. Lett. 86 (2001) Werner Scholz X. Zhu, J.-G. Zhu, Localized Microwave Field Generation for Perpendicular Recording at Deep Subcoercivity, Intermag 2006; J.-G. Zhu, Z. Zhu, and Y. Tang, Microwave assisted magnetic recording, TMRC Conf Date 8

9 MAMR proof of concept C. Thirion, W. Wernsdorfer, and D. Mailly, Nature Materials, 2, 524 (2003): Switching field of single 20 nm Co particle is reduced below the SW switching field in the presence of an AC field H K =1.1 T α= SW boundary 10 f (GHz) 5 non-switched switched 2 ns delay between ac and bias field (ac after bias) Werner Scholz 0 H ac =0.11 T H bias (T) 1 ns field rise time of ac and bias field Date 9

10 MAMR pros and cons basic physics Pros: Reduction in switching field demonstrated for single particles; theory for single particle well understood Assist-energy is coupled directly into the magnetization mode. Cons: The switching field and switching dynamics depend on the damping (LLG α). Basically, the spin has to be able to perform well-defined precessional motion. Small damping -> lower switching field but longer switching time; larger damping -> larger switching field but faster switching. What is the damping in real media? Can it be controlled? What are the field amplitude and frequency requirements for the AC field? Resonance (FMR) frequency goes up as ~H 0.5 K and quickly get into the tens of GHz. As a rule of thumb, the field amplitude needs to be ~0.1 H K, which is of the order of 10 3 Oe. Not well known: Damping and dissipative processes in real media. Work by N. Mo et al. (MMM-2007 FC-12) on continuous CoCrPt sheet films indicate small intrinsic FMR damping (LLG α) but large contribution to damping by two-magnon processes. - What is the damping in higher-anisotropy granular (eg Co/Pt multilayers)? What is the intrinsic damping and damping due to magnon emissions? - How does the AC field interact with granular dispersive media? Date 10

11 MAMR integration and recording physics considerations Need a low-power high-frequency oscillator to deliver AC field to media. Need sufficient field amplitude (a few koe?) and high frequency. - SMT oscillator integrated with transducer (J. Zhu et al). Only need to deliver DC power small losses, small power consumption Small physical size (~50 nm or less) made out of basic reader stack material and process Proof of concept of SMT oscillator with perpendicular polarizing layer [D. Houssameddine et al., Nature Materials 6, 447 (2007)] How do we tune oscillator frequency in the presence of time-dependent stray field from writer? Will the field/power delivery be sufficient? Can we make these integrated devices at a sufficient yield? - Other oscillator (electronic) Not easy to find electronic oscillators with frequency ranges up to tens of GHz. Power consumption in oscillator may be a problem Need high-frequency waveguide to deliver AC field to the recording layer in vicinity to transducer Must avoid coupling losses to transducer structure and media (heating and protrusion!). Integration of this device may be difficult process complexity increases Cost integrated device cannot increase cost of HGA by much (tens of cents?) Reliability? Recording physics - Switching time may be long limits achievable data rates - Induced precession in grains near the writing may increase jitter what is SNR trade-off between enhanced write-ability and increased jitter and transition broadening? - AC illumination may increase erasure risks on-track and off-track Date 11

12 HAMR Basic Physics Reduce the media coercivity at writing by heating it up to ~T C. Requires integration of laser and optical system (waveguide and aperture/antenna), and coupling to media. PSIM Sidewall I Cladding Core Cladding I Magnetic Pole Coil Mike Seigler Date 12

13 HAMR Basic Physics Issues Superparamagnetic traps: the media needs to be heated up to ~T C in order to reduce the coercivity so that lower field is required. But at T C the media grains are paramagnetic and do not respond to a field. As the grains cool down from T C, the magnetization fluctuates rapidly. If the cooling rate is too fast, the magnetization will be quenched in an arbitrary state and the thermoremanent magnetization will be low. M z /M S Cooling rate [K/ns] Cartoon of field-cooled thermoremanent magnetization (after Lyberatos and Chantrell) Date 13

14 HAMR Basic Science Issues Near-field optics: For high areal density (~1 Tbit/in 2 and beyond), the track width will be defined by the optical spot size. At 400 ktpi, the track width is ~60 nm, so the optical spot size must be much smaller than the diffraction limit of available laser wavelengths (eg 830 nm or 1,300 nm). A near-field transducer couples surface plasmons to an optical field and radiates an evanescent field with a spot size defined by physical dimensions, not by the wavelength. The design of high-efficiency near-field transducers is still an active research field and is not a mature technology field. The field from the near-field transducer decays exponentially and is very sensitive to HMS. The near-field transducer and optical system has to be carefully designed so that the near-field transducer emits an evanescent mode that couples well to the recording layer. Media, lube, and overcoat Need high-anisotropy media (L 10 CoPt) with small grains (~ 3 4 nm for 1 Tbit/in 2 ) and controlled inter-granular separation. Need lube and overcoat that can withstand the necessary heating and allow for efficient coupling of the near-field transducer to the recording layer Date 14

15 HAMR Engineering and system issues Thermal recording: The recording (track width and transitions) will be determined by the thermal spot size and thermal gradients. This means that the thermal diffusion has to be controlled very precisely need to engineer recording media with very specific thermal properties. Trade-off between high thermal cooling rate to avoid thermal erasure and low thermal cooling rate to avoid superparamagnetic traps. Too low cooling rate ensures high thermoremanent magnetization but will take long and will limit data rates. Efficiency of near-field transducer and optical system? A lot of heat dissipated in the transducer means thermal protrusions and heat dissipations have to be controlled. Carefully have to trade of placing the near-field transducer close to the writer pole (to ensure writing at hot media) while avoiding too much heat dissipation into the transducer. HMS has to be controlled in the presence of thermal protrusions from the transducer and thermal expansion/buckling of the media What is the effect of HMS variations on the thermal spot size, the thermal diffusion, and track width and transition widths? How will HMS modulations affect jitter and erase bands? Date 15

16 HAMR Engineering and system issues Power dissipation: Laser: Can power dissipation (especially from the optical system) be acceptably low for all market segments (especially CE and Mobile)? Can the laser be placed on the transducer while maintaining high coupling efficiency into optical waveguides? Too much heat dissipated into the transducer may make the laser unstable. Does the laser have to be placed somewhere else, eg on the E-block or near the voice coil? If so, how will the light be coupled to the transducer? What are the mechanical tolerances and coupling loss? How will such a coupling affect the mechanical and flyability properties of the slider? Near-field transducer: Spot size is determined by geometrical features. Can we have processing with adequate tolerances of features ~20 30 nm at the ABS? Critical dimension in a Ridge waveguide (aperture near-field transducer); the CD ~ spot size Date 16

17 BPM basic physics One single continuous dot per bit Physics is simple: one dot of volume V per bit, much larger than volumes of grains in granular media. Larger volume -> anisotropy energy can be decreased while maintaining thermal stability so switching field is reduced Date 17

18 BPM basic science issues Anisotropy distribution must be controlled very tightly in the patterned dots (σ K /K < 5%) to avoid unwanted writing or failing to write. It is not clear what the anisotropy distribution is in small patterned dots, nor is it clear what controls and affects it (eg damage from patterning process, stacking faults during growth) Media Switching Field Distribution Write field limit Direct written in error Date 18

19 BPM engineering issues with patterning A master resist pattern on a silicon wafer is generated by a rotating stage electron-beam lithography system. Lithography systems able to handle a whole disk with ~10 nm features are currently not available. A template is etched from the original master resist pattern and replicated several thousand times into imprint stamps. stamp Hard mask Magnetic Ru stack Nano-Imprint Lithography (NIL) is used to transfer the stamp pattern onto the magnetic disk; more than one thousand disks can be imprinted per stamp. NIL is presently under development and does not exist as a mature technology. Magnetic Islands are formed by either etching the stamped resist pattern into the magnetic film or by electroforming dots into etched nanoholes. Planarization followed by appropriate overcoat and lubricant deposition completes the fabrication process Date 19

20 BPM engineering issues Tolerances on placement error and timing errors are very challenging. For example, at 1 Tbit/in 2 and 50% duty cycle, the dot size is ~12.5 nm. A 5% placement error means cumulative errors from lithography, pattern transfer, nanoimprint, and patterning must be ~0.6 nm. That is of the order of two to three atomic planes. A bit aspect ratio of one is favorable for lithography (dot placement and shape) but leads to very small transducer dimensions, high track pitch, and low data rate. Example: 1 Tbit/in 2 with 50% duty cycle gives dots 12.5 nm in diameter. The track pitch is 1000 ktpi but only a linear density of 1000 kbpi. A disk velocity of 25 nm/ns gives a data rate of 1 Gbit/s. Servo will be very difficult (discontinuous jump from granular/dtm), transducers have to have physical dimensions ~20 nm or less and still be able to write and read. A 15% timing error ~ 3 20 nm/ns disc velocity give as timing control of 150 ps. Those are very tight tolerances for electronics, and have to be stable against thermal drifts. Write synchronization is extremely challenging: how can one ensure staying on dot? Use a feedback system? What will the feedback reader look like? How can one correlate feedback signal to writing under thermal drift and different skew conditions? Date 20

21 Recording Roadmap Speculations There is always inertia to extend current technologies as far as possible Simplest extension in terms of cost, reliability and complexity is ECC or DWAMR, which only involves media development but no other added subsystem components. Current estimates (Victora and co-workers, Suess, Dobin) estimate capabilities ~1 Tbi/in 2 ; more realistic estimates for real disc drives probably end up around 600 Gbit/in 2. MAMR may be promising provided a low-power device (SMT oscillator) can be used to supply the AC field. Many basic questions regarding media-ac field interactions and recording physics remain and are just starting to be worked. HAMR and BPM are both very intensive in development cost and capital cost (especially BPM), therefore they can probably not be developed in parallel. One scenario is HAMR as entry point either at ~600 Gbit/in 2 or later (if MAMR can deliver) on granular media, followed by HAMR+BPM. Discrete-track recording may change this as it may provide a more natural entry point for BPM (BPM can draw on track patterning and servo development) Date 21

22 Recording Roadmap Speculation MAMR? DTR + HAMR HAMR+BPM Conv. recording ECC + DTR BPM Gbit/in 2 Date 22