Title: Magnetic Nanostructures for Spin Valve Applications
1Magnetic Nanostructures for Spin Valve
Applications
- Gary J. Mankey
- MINT Center and Department of Physics and
Astronomy - The University of Alabama
S. Al-Ghamdi, H. Alouach, P. Mani, Z. Zhao, I.
Zoto V.V. Krishnamurthy
This project was funded by grants from NSF and
DOE.
2Center for Materials for Information Technology
(MINT) at The University of Alabama
- A multidisciplinary research program focusing on
new materials for advanced data storage. - 22 faculty, 10 postdocs, and 40 graduate students
from 7 academic programs in science and
engineering. - Support federal grants (including an NSF
Materials Research Science Engineering Center
grant), industry (IBM, Seagate, Quantum, Sony,
Fujitsu, Hitachi Maxell, INSIC), and university
support.
3Abstract
- Spin valves are used as magnetic sensors for
readout of information in hard disk drives. To
meet the demands of increased storage capacity
and lower cost, the bit density in hard disk
drives has increased at a rate even faster than
that of Moores law. The incredible shrinking
bit in these devices has reached truly nanoscopic
dimensions with the recent 100 gigabit per square
inch demo by Seagate having a bit size of
approximately 37 nm x 170 nm. These advances are
partly due to the development of spin valve
technology, which enables the measurement of tiny
changes in magnetization by the giant
magnetoresistive effect. This presentation will
cover the basic operating principles of spin
valve sensors and the basic physics applied
toward materials selection. Challenges which
must be met to stay on the bit density growth
curve will also be discussed.
4Research 20 more years??
- Moores Law for semiconductors has density of
transistors doubling every 18 months. - Recently, for magnetic recording storage density
doubles every 12 months. - Currently 100 Gigabit per square inch-- 1 bit is
37 nm x 170 nm - 10 years 10 Terabit per square inch
- 20 years 1 Petabit per square inch-- 1 bit will
be 1 nm x 1 nm!!
Ref IBM Website
5Head Technology
- The introduction of MR sensor technology created
the discontinuity of slope in the areal density
evolution curve. - The change in slope demonstrates the
revolutionary nature of the new technology.
Ref Hitachi Global Storage Website
6Head, Media and Servo
- The read/wire head, media and servomechanism with
associated signal processing electronics are
integrated into the disk drive system.
Ref Hitachi Global Storage Website
7The Magnetic Recording Process
http//www.storage.ibm.com/hardsoft/diskdrdl/techn
olo/gmr/gmr.htm
8MR and GMR Head Structures
http//www.storage.ibm.com/hardsoft/diskdrdl/techn
olo/gmr/gmr.htm
9The Current In Plane Head Structure
- The spin valve sensor is sandwiched between two
magnetic shields and electrically isolated with
oxide layers. - The hard bias rotates the free layer to 90º--this
gives a more linear response to changes in the
magnetization of the free layer.
Ref Hitachi Global Storage Website
10The Read Write Head
- A thin film head structure consists of 20
material layers with patterns for each layer
defined by photolithography and either additive
processing (electroplating, liftoff masking) or
subtractive processing (ion milling, wet etching,
reactive ion etching, chemical mechanical
processing). - This figure shows a generic head structure and an
actual cross section of a Hitachi thin film head
product.
Ref Hitachi Global Storage Website
11Head Processing Challenges
- Critical Thin Film Head Features Two critical
features in the thin film head, the width of the
read sensor (MRw) and the width of the write pole
tip (P2w), determine areal density performance.
This figure (4) shows SEM views of these
features. The lithography techniques for the MR
sensor are comparable to gate requirements in
integrated circuits. The lithography processing
for the write pole tip can be compared with the
interconnect processing strategy in the
integrated circuit.
Ref Hitachi Global Storage Website
1240,000 Heads on an 8" Wafer
- Slicing, bonding and lapping are all proprietary
technologies. - The sensor is bonded to an air bearing slider and
leads are connected for writing and reading of
magnetic bits.
Ref Hitachi Global Storage Website
13Staying on the Growth Curve
- Areal Density Environment -- Process
ImplicationsFor the last 5 years the areal
density of recording head structures has
sustained annual increases as high as 100. From
a process perspective this requires that the
width of the critical read sensor and write pole
tip decrease by 20 to 30 each year. By
comparison, integrated circuit dimension decrease
at an annual rate of 10 to 12. This figure (5)
shows a historical plot of integrated circuit
dimensions and thin film head dimensions over the
last 20 years. Projections shows that the
dimensional requirements for the thin film head
will merge with those of the integrated circuit
in the middle of this decade. The Advanced
Recording Head Processing Department is
developing processing techniques which will
sustain the areal density growth through this
decade.
Ref Hitachi Global Storage Website
14The First GMR Device
- An antiferromagnetically coupled multilayer
exhibited a 100 DR/R for a field of 0.3 Tesla at
room temperature. - Ten years of extensive research resulted in
increased field sensitvity for thinner layers
incorporated into nanoscale sensors.
Ref M.N. Babich et al. Phys. Rev. Lett. 61, 2472
(1988).
15F/AF Exchange Bias
- When a ferromagnet (F) is deposited on an
antiferromagnet (AF) in an applied field, the
hysteresis loop of the F film is altered in two
ways - There is a bias (or shift) of the hysteresis loop
by an amount called Hp or the pinning field. - There is an enhancement of the coercive field,
Hc, particularly along the direction of the
applied field. - The origin of this effect is FM defects in the
antiferromagnet. - The anisotropy of the antiferromagnet controls
the magnitude of the effect.
16The Basic Spin Valve
17The Magnetoresistance Curve
Anti-parallel gives high resistance
f
f
p
p
DR/R
Parallel gives low resistance
f
f
p
p
Happ
Happ
18Band Engineering
- The magnitude of the GMR effect is determined by
the relative alignment of spin-polarized energy
bands in the ferromagnet and the conduction
electrons in the copper spacer layer.
- Inserting a thin Co layer on both sides of the Cu
layer aligns the energy bands better and improves
the magnitude when using permalloy which has a
better field sensitivity.
Ref Hitachi Global Storage Website
19Equivalent Circuit of a MR Sensor
Equivalent Circuit
Protective
Active
Free
Conductor
Pinned
Ancillary
Antiferromagnet
Seed
Substrate
I
- The active part changes with applied field while
the ancillary part does not.
V
20Magnetoresistance of a Spin Valve Stack
- A more complex structure is necessary to achieve
high MR ratios.
Ref H. S. Cho, et al., IEEE Trans. Magn., 34,
1414 (1998).
21The UA MR Sensor
- Separate current inputs and voltage outputs allow
measurements of magnetotransport independent of
contact resistance. - Effect of Sense current on MR ratio was studied.
22Minor Loop
- In the sensor, the applied field is smaller than
the field necessary to switch the pinned layer,
and the magnetoresistance reflect the change in
magnetization direction of the free layer only.
23A New Global Perspective
IBM Almaden was recently acquired by HitachiSome
players remained at IBM to develop new data
storage paradigms such as MRAM and probe storage.
Ref Hitachi Global Storage Website
24Microdrive for Portable Applications
- Pros Large capacities fast
- Cons Expensive CompactFlash capacities are
quickly catching up - Supporting manufacturers Various, usually for
pro and semipro cameras
Ref Hitachi Global Storage Website
25Toshiba Makes Smallest HDD (for the time being)
- An employee of Japan's comprehensive electrical
machinery manufacturer Toshiba Corporation shows
off its 0.85-inch hard disk drive (HDD) at the
headquarters in Tokyo, 16 March 2004. - Toshiba announced that the Guinness World Records
has certified it as the smallest HDD in the
world. - .more to come (or less).
26Casio QV2000Plus
- With an IBM Microdrive, users can snap up to 400
high-quality images or 19 minutes of 7
frame-per-second video without having to switch
memory cards or download. One 340 MB IBM
MicroDrive could replace over 11 rolls of film.
This storage capacity accommodates anyone who
needs to take a large number of pictures or who
plans on being away from a computer for an
extended period of time, such as while on
vacation.
27Fast Storage Application Gaming
- Stand alone and on-line gaming require huge
amounts of memory for rich graphics. - Seagate's Cheetah is a favorite among providers.
28The Cheetah
- One site that has taken advantage of the Seagate
Cheetah X15 disc drives is C/Net affiliate
Gamers Depot (www.gamersdepot.com), a wildly
popular and successful site for gaming
enthusiasts. - Gamers Depot brings high-tech hardware and
software reviews to the Web in an easy-to-read
format that appeals to techies and hard-core
gamers alike. - Naturally, fast response times are absolutely
necessary to retain such a sophisticated
audience. - After all, the video-game generation isnt used
to waiting for anything, and a Web site that
doesnt serve up information with lightning speed
will discourage repeat visitors.
Seagate Cheetah
29TiVo
199.99
30Hitachi Travelstar 1.8-inch hard drive makes
beautiful music for new Dell digital music player
- Customer Dell
- Challenge Maximum capacity and minimum power
usage in a small, portable form factor - Solution Hitachi Travelstar 1.8-inch hard disk
drive - The market for digital music players is
burgeoning From a base of 6.8 million units sold
in 2002, analysts forecast sales to grow to 36
million in 2007. - 179 (1 GB) - 259 (30 GB)
31Whoever Controls the Materials Controls the
Science and Technology
Design
Science DrivenNanofabrication
Synthesis
Characterization
Ref E.W. Plummer
32Ferromagnetic Band Structures
- This experimental technique, performed in
collaboration with F.J. Himpsel at UWM/SRC,
directly probes the spin-dependent Fermi surface.
- The measurement here was used to identify the
nature of the spin polarization in the conduction
electrons in Ni. - We apply this technique to the study of the
spin-split energy bands involved in
magnetotransport.
Ref K.N. Altmann, et al., Phys. Rev. B 61,
15661 (2000).
33CO Chemisorption Effect on Magnetic Properties
- The magnetization of 7 angstrom ultrathin films
is reduced by the adsorption of CO. - The slope of the magnetization curve changes upon
heating above the desorption temperature for CO. - CO chemisorption also modifies the magnetic
anisotropy. - The processing challenge is to produce clean
films.
93 Surface Science Letters
34H2 Chemisorption Effects on Magnetic Properties
- The in-plane magnetization of Co films decreases
with H-adsorption. - The perpendicular magnetization of Fe films
increases with H-adsorption.
Mankey et al., 93 JVST
R. Vollmer and J. Kirshner, 00 PRB.
35Reproducible Results
48 Films x 1 day/film x 2 20 weeks
Huang, et al., 94 PRB
Renjun Zhang and R.F. Willis, 01 PRL
- Process control and advanced characterization
techniques are essential for the development of
phenomenological models describing key parameters
for device development.
36The Pixie Disk
- The recording layer is now two magnetic layers
exchange coupled to each other. - Perpendicular media is even more complex, with a
flux-closing soft underlayer under the storage
layers.
Ref Hitachi Global Storage Website
37Advantages of Pixie Dust
- Stability is increased without increasing the
magnetization per bit. - The coercivity is also smaller than the
conventional analog, so write fields can be
reduced.
Ref Hitachi Global Storage Website
38Pixie Dust
- Recent experiments have probed the origin of
biquadratic coupling in Co/Ru/Co and these
experiments demonstrate that we can control the
magnitude of this effect.
Co
Ru
Co
Ta
Ref Z. Zhao,et al., J. Appl. Phys. 95, 7157
(2004)
39Uniaxial Synthetic Antiferromagnetic Films
- Comparison of experimental remanence with a
calculation which only considers bilinear
coupling. The difference is due to biquadratic
coupling. - Dependence of easy axis critical fields Hcr1 and
Hcr2 on top layer Co thickness. The dotted line
is a fit to a minimize energy model.
Ref Z. Zhao et al., Phys. Rev. B 71, 104417
(2005).
40Interlayer Coupling
Bilinear
Biquadratic
Applied Field
H0
Easy Axis
Hard Axis
41Tuning the Coupling
- Strong antiferromagnetic exchange coupling was
observed in USA structures. - Distinguishable in plane easy and hard axis
loops were also observed. - We can tune the biquadratic coupling effect.
Ref Z. Zhao et al. , J. Appl. Phys. 95, 7157
(2004),
42Perpendicular Field Effect
Simulation
- Hard axis loops show Mr/Ms increases with a
increasing perpendicular field. - Energy minimization model confirms that the
increase of J2 is the origin of the observed
trends. - It shows the magnitude of biquadratic coupling
can be controlled with external field applied
during deposition.
43Current In Plane vs Current Perpendicular to the
Plane
- For sub-200 nm track width, making contacts to
the current-in-plane sensor and isolation of the
leads is beyond the limits of conventional
lithography techniques. - The solution is to apply the sense current
perpendicular to the plane of the sensor-now the
magnetic shields become the leads. - When an insulator is used as the spacer, the
sensor is called a tunnel-valve.
Ref Hitachi Global Storage Website
44Spin Tunneling
The tunneling current is proportional to A B
cos Q, where Q is the angle between the
magnetizations of the magnetic layers. A and B
depend on spacer thickness, materials, and
interface quality.
Ref Hitachi Global Storage Website
45The Basic Tunnel Junction
The resistance of the structure is significantly
larger than a metal CPP device. This is good for
increasing the sense voltage, but puts
limitations on minimum device size for future
applications. The insulating layer also
introduces additional noise sources.
Ref Hitachi Global Storage Website
46Micrograph of Tunnel Valve Sensor
- The tunnel valve sensor is a much simpler design.
- Fine tuning of the design to achieve superior
performance is underway.
Ref Hitachi Global Storage Website
47Patent 6,560,077, Fujiwara and Mankey
- The insulating layer is replaced by a composite
structure with conducting pillars separated by
and average distance approximately equal to the
exchange length of the ferromagnetic free layer. - The design has the advantages of an increased
resistance without introducing additional noise
sources.
48New Challenge Get Perpendicular
- For perpendicular recording, the media becomes
part of the write head. - Now, additional criteria are added to media
design - Soft underlayer
- Perpendicular anisotropy
- Even smaller grain sizes
49Relevant Research
- The classic rectifierwe dont perform
proprietary research. - ICs need X
- Better antiferromagnetic materials and artificial
antiferromagnetic coupled layers (sensors). - 3-6 nm decoupled magnetic grains (media).
- High moment materials (writer).
- Pinning without the AF material (sensor).
- 30-60 nm magnetic nanorods (tape).
- High anisotropy self assembled magnetic particle
arrays (media). - Steering committee.
Hard bias
Anisotropy too high
Demag field
Spin transfer torque (noise)
Coupling too high
Switching speed
Conductivity Too low
Contact resistance
MRAM and/or CPP GMR Sensor
50MINT Student Placements
Ph.D. Degrees
T. Scharf, MtE, Ph.D., 00 NRL S. Stinnett, PH,
Ph.D., 00 Millsaps College M. Han, CH, Ph.D.,
00 KRICT D. Orgassa, PH, Ph.D., 01 Cal Tech T.
Selby, CH, Ph.D., 01 Univ. Utah C. Liu, PH,
Ph.D., 01 Read-Rite C. Tidwell, CH, Ph.D.,
01 Montevallo Univ. M. Sun, MTLS, Ph.D.
01 Read-Rite B. Chae, ChE, Ph.D., 01 Univ.
Washington M. Kim, CH, Ph.D., 01 Altair J. Huh,
CH, Ph.D., 01 Penn State M Chen, CH, Ph.D.,
02 Carnegie Mellon Univ. A. Bandhar, ChE, Ph.D.,
02 Cornell B. Xu, PH, Ph.D., 02 Seagate K.
Zhang, PH, Ph.D., 02 Headway (TDK) H. S. Jung,
MTLS., Ph.D., 03 Read-Rite G. Wei, MtE, Ph.D.,
03 Ohio State X. Liu, MtE, Ph.D., 03 Headway
(TDK) M. Piao, ChE, Ph.D., 03 Southern
Mississippi I. Zana, MtE., Ph.D., 03 Georgia
Tech F. Liu, MTLS Headway (TDK) S. Wang, PH,
Ph.D., 03 Seagate Y. Ding, PH, 03 U. MN. J.
Rantshler, PH, 03 NIST X. Feng, PH, 03 Seagate
- J. Cates, PH, Ph.D., 94 StorageTek
- M. Tan, MtE, Ph.D., 94 Quantum
- E. Haftek, MtE, Ph.D., 94 Seagate
- S. Hossain, EE, Ph.D., 96 Read-Rite
- A. Chacko, CH, Ph.D., 96 CTS Corp
- L. He, PH, Ph.D., 96 Maxtor
- J. Arroyo, MtE, Ph.D., 96 Texas Instruments
- H. Deng, MtE., Ph.D., 97 IBM
- J . Cain, CH, Ph.D., 97 Read-Rite
- J. Jarratt, MtE, Ph.D., 97 IBM
- V. Inturi, MtE, Ph.D., 97 Seagate
- L. Varga, PH, Ph.D., 97 Fujitsu
- Y. Yu, PH, Ph.D., 95 Russel Tech
- F. Li, CH, Ph.D., 98 Headway
- K. Minor, MtE, Ph.D., 98 Seagate
- C. Hou, PH, Ph.D., 99 Seagate
- Y. Hu, CH, Ph.D., 99 Applied Materials
- A. Catalina, MtE, Ph.D., 00 NASA/MSFC
- S. Maat, PH, PhD., 00 IBM
Student Internships are Encouraged
5120 More Years of Research
- Stay on the growth curve.
- Remember the cost per bit curve.
- Consumer driven demand.
- New paradigmsMRAM, HAMR?