Magnetic Nanostructures for Spin Valve Applications

1
Magnetic 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.
2
Center 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.

3
Abstract

• 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.

4
Research 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
5
Head 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
6
Head, 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
7
The Magnetic Recording Process
http//www.storage.ibm.com/hardsoft/diskdrdl/techn
olo/gmr/gmr.htm
8
MR and GMR Head Structures
http//www.storage.ibm.com/hardsoft/diskdrdl/techn
olo/gmr/gmr.htm
9
The 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
10
The 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
11
Head 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
12
40,000 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
13
Staying 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
14
The 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).
15
F/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.

16
The Basic Spin Valve
17
The Magnetoresistance Curve
Anti-parallel gives high resistance
f
f
p
p
DR/R
Parallel gives low resistance
f
f
p
p
Happ
Happ
18
Band 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
19
Equivalent 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
20
Magnetoresistance 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).
21
The 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.

22
Minor 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.

23
A 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
24
Microdrive 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
25
Toshiba 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).

26
Casio 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.

27
Fast 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.

28
The 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
29
TiVo

199.99
30
Hitachi 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)

31
Whoever Controls the Materials Controls the
Science and Technology
Design
Science DrivenNanofabrication
Synthesis
Characterization
Ref E.W. Plummer
32
Ferromagnetic 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).
33
CO 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
34
H2 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.
35
Reproducible 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.

36
The 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
37
Advantages 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
38
Pixie 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)
39
Uniaxial 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).
40
Interlayer Coupling
Bilinear
Biquadratic
Applied Field
H0
Easy Axis
Hard Axis
41
Tuning 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),
42
Perpendicular 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.

43
Current 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
44
Spin 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
45
The 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
46
Micrograph 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
47
Patent 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.

48
New 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

49
Relevant 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
50
MINT 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
51
20 More Years of Research

• Stay on the growth curve.
• Remember the cost per bit curve.
• Consumer driven demand.
• New paradigmsMRAM, HAMR?