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Magnetic Data Storage and Nanotechnology

Magnetic Data Storage and Nanotechnology. Hard Disk. Sensors Media. Switching layer 5 nm. Magnetic grain 10 nm. Faster than exponential. Technology Changes in Magnetic Data Storage. GMR Reading Head. 5 nm NiFe Optimum. Giant Magnetoresistance (GMR) and Spin Filters.

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Magnetic Data Storage and Nanotechnology

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  1. Magnetic Data Storage and Nanotechnology Hard Disk SensorsMedia Switching layer 5 nm Magnetic grain 10 nm

  2. Faster than exponential

  3. Technology Changes in Magnetic Data Storage

  4. GMR Reading Head 5 nm NiFe Optimum

  5. Giant Magnetoresistance (GMR) and Spin Filters Parallel Spin Filters  Resistance Low B>0 M Opposing Spin Filters  Resistance High B=0 M NiFe Cu Co • Two filtering mechanisms: • Bulk: Spin-dependent scattering (Scattering length ℓ>ℓ ) • Interface: Spin-dependent reflection(Spin-dependent potential step)

  6. GMR and TMR Reading Heads • General principle: • Two ferromagnetic layers separated by a non-magnetic layer. • For B=0 they have opposite magnetization (1800). • An external B-field forces them parallel (00). • The switch from opposite spin filters to parallel spin filters • reduces the resistance. • Obtain maximum sensitivity right at the switching point (900). • Implementation: • GMR = metallic spacer layer(Giant Magneto-Resistance) • TMR = insulating spacer layer(Tunnel Magneto-Resistance)

  7. (GMR) Current in plane (CIP) (TMR) Current perpendicular to the plane (CPP) TMR has taken over GMR in hard disk reading heads: Larger effect with the current perpendicular to the layers, no shorting by the metal layer. GMR vs. TMR

  8. Magnetic Storage Media Magnetic Force Microscope (MFM) Image Need 102 magnetic particles per bit to average out size and shape variations. Can’t make them smaller (next slide). The ultimate limit: one particle per bit ! 10 nm CoPt particles (magnetically isolated by a Cr coating)

  9. Flip Rate =fexp[-E/kT] = Attempt frequency  Larmor 109 s-1 40kT for several years retention frequency (proportional to the volume) (Lect. 24) Probability per attempt Superparamagnetic Limit of the Particle Size: Thermal Switching Energy Barrier E Preferred axis = “easy axis” (c-axis in hcp Co, shape anisotropy)

  10. B-field B-field A large external B-field costs energy. Close the field lines internally. (A more quantitative description requires the demagnetizing field Hd .) Magnetic Shape Anisotropy (Dipole Interaction) • Thin film: M parallel to the film • Rod-shaped particle: M parallel to the rod

  11. Unfavorable when shrinking bit size. More volume for the same area. Favored by the shape anisotropy.

  12. The next Step: Patterned Magnetic Storage Media: One Particle per Bit

  13. “Perfect” Magnetic Particles Can use fewer particles per bit FePt Nanocrystal with Organic Shell 3D stacking 2D stacking

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