the physics of perpendicular recording
TRANSCRIPT
The Physics of Perpendicular RecordingH.J. Richter
Seagate Technology, Fremont
Overview
Introduction
Perpendicular Recording and the “tri-lemma”
Microstructure of perpendicular media
The role of intergranular exchange
Recording geometry and angle effects
A way to postpone the tri-lemma: composite (ECC) media
Summary
Slide 2Perp. RecordingIdema Dec 2006 H.J. Richter
Areal Density ProgressResearch frontier:≥500 Gbits/in2
Commercial products:≤130 Gbits/in2, 80-130 GB/3.5” Platter
Demonstrations: up to 421 Gbit/in2
Technology optionsSingle Particle Superparamagnetic Limit
Slide 3Perp. RecordingIdema Dec 2006 H.J. Richter
1990 1995 2000 2005 2010 2015 2020 20250.1
1
10
100
1000
10000 10 Tbit/in2
INSIC
||⊥
1 Tbit/in2
100 Gbit/in2
Products
Max
. Are
al D
ensi
ty (G
bit/i
n)
Availability Year
Demos
HAMR & BPM
BPM or HAMR
Perpendicular(tilted, composite)
Longitudinal
2
421
BPM = bit patterned mediaHAMR = heat assisted magnetic recording
What gives perpendicular recording the lead over longitudinal recording?
Slide 4Perp. RecordingIdema Dec 2006 H.J. Richter
High SNR Requires Small GrainsDue to its size, each grain is a “single domain particle” and cannot be cut magnetically.Hence, the boundary between the bits can at best follow the grain structure.
Slide 5Perp. RecordingIdema Dec 2006 H.J. Richter
Superparamagnetic Limit
Increasing SNR requires smaller grains Sm
aller grain
A Dilemma: a situation requiring a choice between equally undesirable alternatives (Webster)
s
KV too small –media are ther-mally unstable
Head fields are too weak to write media
Higher K (HA)
Slide 6Perp. RecordingIdema Dec 2006 H.J. Richter
"Tri-Lemma" In Magnetic Recording
The “Superparamagnetic limit”
Is it postponed by perpendicular recording?
SNR
write-ability thermal decay
conflicting requirements
H.J. Richter
Slide 7Perp. RecordingIdema Dec 2006 H.J. Richter
Perpendicular Recording GeometryHeads and media are different!
Longitudinal Recording Perpendicular Recording
Slide 8Perp. RecordingIdema Dec 2006 H.J. Richter
Demagnetization
In longitudinal recording increasing the layer thickness increases demagnetization and causes poorer SNRVery mild SNR penalty in perpendicular recording – can make small grains and thick media and store volume “in the depth”
Slide 9Perp. RecordingIdema Dec 2006 H.J. Richter
Perpendicular Media: Microstructure
Slide 10Perp. RecordingIdema Dec 2006 H.J. Richter
Cross-section
Plan view
CoPt-TiO2 magnetic layer
Ru – interlayerSeedlayerSUL
Medium Magnetics: Hysteresis Loop
Hysteresis loop along film normal is sheared due to demagnetizing effects
Increasing exchange makes the hysteresis loop squarer
Exchange coupling leads to square hysteresis loops
Square hysteresis loops are beneficial, but collective reversal
Slide 11Perp. RecordingIdema Dec 2006 H.J. Richter
Optimized Exchange
Slide 12Perp. RecordingIdema Dec 2006 H.J. Richter
Wide transitions & small clusters
Narrower transitions & big clusters
optimum
More on Exchange Interaction
magnetostatics destabilize Miexchange field aligned with Mi
Slide 13Perp. RecordingIdema Dec 2006 H.J. Richter
grain i
Less field required to switch the media
Exchange helps to stabilize magnetization in the bit
Thermal Decay
For amplitude decay, longitudinal recording and perpendicular recording are mirror images.
Slide 14Perp. RecordingIdema Dec 2006 H.J. Richter
Perpendicular Oxide Media
• Longitudinal media use CoPtXCr with Cr as segregant for decoupling; perpendicular media use CoPt + oxide to decouple grains– Less tendency of the grains to grow wider for thick films
– Decoupling by oxide, not Cr Segregation – narrower SFD
• Very high orientation ratio - a FWHM of 3° corresponds to an orientation ratio of more than 40!
• High magnetization, high anisotropy
• Dual layer media to control intergranular exchange (CGC)
Slide 15Perp. RecordingIdema Dec 2006 H.J. Richter
High Orientation: Angles Matter!
It is important to realize that the angle dependence matters a lot for perpendicular recording. A tilt of the head field helps to switch the media => hence a mild sensitivity to interlayer thickness
Slide 16Perp. RecordingIdema Dec 2006 H.J. Richter
Shielded HeadsTrailing shield cuts the tail of the head field => high field gradient, shaper transitions
Fields of shielded heads are lower, but angle effects mitigate write-ability.
Slide 17Perp. RecordingIdema Dec 2006 H.J. Richter
A Way to Postpone the Tri-Lemma
magnetically soft (low anisotropy)δ2
H
magnetically hard (high anisotropy)δ1
Exchange Coupled Composite (ECC) media
The soft layer is a “switching assist” for the hard layer
R.H. Victora and X. Shen, IEEE Trans. Magn. vol. 41, pp 537-542 (2005)
Slide 18Perp. RecordingIdema Dec 2006 H.J. Richter
How Do Composite Media Work?
hard
soft
Reversal process:
1. Nucleation
2. Domain wall formation and compression of wall
3. Switch: wall moves through hard layer
Applied field compresses wall
Slide 19Perp. RecordingIdema Dec 2006 H.J. Richter
ECC Media: Reducing Switching Field at Will?
( )
hardhard
softsoft
hardSW
AMAM
HH
≡
+=
γ
γ2
1Theoretically” yes!
Graded anisotropies even more efficient! (Suess APL, 2006)
Infinite layers: Kronmüller, Goll(2002)
switc
hing
fiel
dthickness of soft layer
1/4
Slide 20Perp. RecordingIdema Dec 2006 H.J. Richter
Composite Media Gains• Gain must consider the
constraint: δconv = δcomp
(and similar Mrδ)
• Absolute limit*: Gmax = 4Using graded anisotropy app. K~x2 and non-existing materials
• G = 2…2.5 (estimate for a hard material with an anisotropy field of ~ 100kOe)
δ
Note: gains are in grain size limit.
*P. Visscher, U. of Alabama
Slide 21Perp. RecordingIdema Dec 2006 H.J. Richter
Summary: Perpendicular Recording1. Perpendicular media have better microstructure
High orientation (orientation factor >40)Smaller grains with narrower distributions=> sharper transitions
2. Can use thicker magnetic layer without SNR penalty => can reduce grain size and maintain KV for thermal stability
3. Some intergranular exchange is desirable for SNR4. Intergranular exchange helps to stabilize magnetization5. Can use composite media (ECC)6. Higher output
Slide 22Perp. RecordingIdema Dec 2006 H.J. Richter