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 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)

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)

GMR vs. TMR 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. (TMR) (GMR) Current in plane (CIP) Current perpendicular to the plane (CPP) Both GMR and TMR had significant technological impact on magnetic sensors. GMR was recognized by the 2007 Nobel Prize in Physics and TMR by the 2009 Buckley Prize, the highest honor awarded by the American Physical Society.

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)

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

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

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

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

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