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Magnetic Data Storage
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5 nm Optimum
Hard Disk Reading Head
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Filtering mechanisms:
• Bulk: Spin-dependent Scattering
• Interface: Spin-dependent Reflection
Parallel Spin Filters → Resistance Low
Opposing Spin Filters → Resistance High
Giant Magnetoresistance (GMR): Two Spin Filters
2007 Nobel Prize in Physics to Fert and Grünberg
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TMR has taken over GMR in hard disk reading heads. Get larger effect with the current perpendicular to the layers, no shorting.
GMR vs. TMR (Tunnel Magnetoresistance):Replace Metal Spacer by Insulating Spacer
(TMR)
(GMR)
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50 nm 10 nm CoPt particles (≈ superparamagnetic limit)
Magnetic Storage Media
600nm
Need about a hundred particles per bit (particles not uniform).
Magnetic ForceMicroscope (MFM)Image
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Barrier ΔE
Energy
Superparamagnetic Size Limit for Magnetic Particles
Flip Rate ≈ νAttempt • exp[-ΔE/kT]
≈109s-1 ≥40kT ~ Volume
A superparamagnetic particle has all its spins aligned internally, but thermal energy keeps flipping the magnetic orientation of the whole particle. The magnetization of an ensemble of such particles is zero, as for a paramagnetic arrangement of very large spins.
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Shape anisotropy:
The magnetization prefers to be parallel to the axis of a needle-shaped particle or in the plane of a thin film.
Crystalline anisotropy:
The magnetization prefers to align with a specific crystallographic direction (e.g. the hexagonal axis in cobalt)
Surface anisotropy:
The magnetization at a surface/interface is often perpendicular to the interface (opposite to the shape anisotropy)
Hard magnet (large anisotropy): Permanent magnet (NdFeB), storage medium (Co).
Soft magnet (small anisotropy): Transformer core (pure Fe), sensor (permalloy).
Magnetic Anisotropy:The Energy to Rotate the Magnetization
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Blocking Temperature
When cooling a superparamagnetic particle, the flip rate drops rather suddenly. The blocking temperature defines the point where the magnetization of a superparamagnetic particle becomes “frozen”.
Such behavior resembles the transition from paramagnetism to ferromagnetism at the Curie temperature, but there is a conceptual difference: The Curie temperature defines a sharp phase transition, while the blocking temperature depends slightly on the time scale of the experiment (a bit fuzzy). The magnetization of a particle will flip even below the blocking temperature if one waits a very long time.
An example from magnetic data storage: For a reasonable lifetime of stored data one needs an energy barrier ΔE ≈ 40 kBT . A typical attempt frequency of 109 s-1 gives a flip rate of 109 e-40 s-1 or about one flip in 7 years. Reducing the diameter of a magnetic particle by a factor of 2, their volume decreases by a factor of 8 and likewise ΔE in the exponent. The resulting flip time is only 150 nanoseconds !
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3 atomic layers of Ru for antiferromagnetic coupling (AFC)
Antiferromagnetically Coupled (AFC) Storage Media
Make bits smaller while keeping the volume:Need to go deeper
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Want a Storage Medium like this:
Deep, Regular, Flat Top
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http://www.hitachigst.com/hdd/research/recording_head/pr/
As the bit size shrinks, the shape anisotropy works against shorter in-plane bits and favors perpen-dicular magnetization.
Adjacent perpendicular bits with opposite magnetization repel each other, like bar magnets. A soft underlayerconnects the field lines, like an iron bar across a horse-shoe magnet.
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http://www.seagate.com/docs/pdf/whitepaper/TP-549_PerpRecording_Feb-06.pdf
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Patterned Media: The next Step
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Europhysics News 39, 31 (2008)
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Reading the Spin of a Single Atom by
Scanning Tunneling Spectroscopy (STS)
Polarized atom, unpolarized STM tip: See transitions between different mS
as energy loss (inelastic tunneling).
Use polarized atom, polarized STM tip for readout (not shown): Asymmetry reveals spin orientation (TMR, same as in hard disk reading heads, Slide 4). IBM Almaden Group, Science 317, 1199 (2007)
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Something really far out: A Magnetic Virus
S.D. Bader, Rev. Mod. Phys. 78, 1 (2006); Liu et al., JMMM 302, 47 (206)