Transcript Lecture 12 ( ppt ) - UW-Madison Department of Physics

Magnetic Data Storage
Hard Disk Reading Head
5 nm
Optimum
Giant Magnetoresistance (GMR):
Two Spin Filters
Parallel Spin Filters  Resistance Low
Opposing Spin Filters  Resistance High
Filtering mechanisms:
• Bulk:
Spin-dependent Scattering
• Interface: Spin-dependent Reflection
2007 Nobel Prize in Physics to Fert and Grünberg
GMR vs. TMR (Tunnel Magnetoresistance):
Replace Metal Spacer by Insulating Spacer
(GMR)
(TMR)
TMR has taken over GMR in hard disk reading heads.
Get larger effect with the current perpendicular to the layers, no shorting.
Magnetic Storage Media
Magnetic Force
Microscope (MFM)
Image
600
nm
Need about a hundred particles
per bit (particles not uniform).
50 nm
10 nm CoPt particles ( superparamagnetic limit)
Superparamagnetic Size Limit for Magnetic Particles
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.
Energy
Barrier E
Flip Rate  Attempt
109s-1
•
exp[-E/kT]
40kT ~ Volume
Magnetic Anisotropy:
The Energy to Rotate the Magnetization
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).
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 E ≈ 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
E in the exponent. The resulting flip time is only 150 nanoseconds !
Antiferromagnetically Coupled (AFC) Storage Media
3 atomic layers of Ru for antiferromagnetic coupling (AFC)
Make bits smaller while keeping the volume:
Need to go deeper
Want a Storage Medium
like this:
Deep, Regular, Flat Top
As the bit size shrinks, the
shape anisotropy works
against shorter in-plane
bits and favors perpendicular magnetization.
Adjacent perpendicular bits
with opposite magnetization
repel each other, like bar
magnets. A soft underlayer
connects the field lines, like
an iron bar across a horseshoe magnet.
http://www.hitachigst.com/hdd/research/recording_head/pr/
http://www.seagate.com/docs/pdf/whitepaper/TP-549_PerpRecording_Feb-06.pdf
Patterned Media: The next Step
Europhysics News 39, 31 (2008)
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)
Something really far out: A Magnetic Virus
S.D. Bader, Rev. Mod. Phys. 78, 1 (2006); Liu et al., JMMM 302, 47 (206)