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SEMINAR REPORT On Giant Magnetoresistance Effect
Post: #1

Giant Magnetoresistance Effect
Submitted in partial fulfillment for the award of the degree of
Master of Technology
Computer and Information Science
Department of Computer Science
Cochin University of Science and Technology
Cochin-22, Kerala
2008Page 2

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
This is to certify that the report entitled
Giant Magnetoresistance Effect
is a
bona fide record of the seminars presented by Mr. Tomsy Paul in partial fulfillment of the
requirements for the award of M.Tech. Degree in Computer and Information Science of
Cochin University of Science & Technology, during the academic year 2007-2008.
G. Santhosh Kumar
Prof. Dr. K. Poulose Jacob
Seminar Guide Head of the Department
24-06-2008.Page 3

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
First of all, I thank GOD ALMIGHTY, for showering his grace upon me to
successfully carry out this work, everything in time.
I take this opportunity to thank Prof. Dr. K. Poulose Jacob, Head of the Dept. of
Computer Science, CUAST for providing me with the necessary facilities to do this work.
I am deeply indebted to Shri. G. Santhosh Kumar, Lecturer, Dept. of Computer
Science, CUSAT for the excellent guidance and timely suggestions.
Finally, I express my deepest gratitude to all my family members and to my friends
for their encouragement, which helped me to keep my spirit alive and to complete this work

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
The Giant Magnetoresistance Effect was
discovered in 1988. It took around 10 years of laborious research for its first fruits to be
ripened. But once they were on, they started to cause unprecedented increase in the
storage capacity of hard disks. Another area of application is the primary memory. The
seminars presents the details of this discovery and the impact it had on the industry. To
point out the relevance of this discovery, it suffices to mention that the discoverers were
awarded the Nobel Prize for Physics in 2007. Page 5

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
Page No.
18Page 6

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
All modern hard disks are equipped with two
different heads “ one for writing and the other for reading. The principle of the writing
head is quite simple, i.e., generation of a magnetic field as electricity passes through the
head. The head focuses this magnetic field generated to the area on the disk surface
where the bit is to be written.
Conceptually the technique for reading is the
reverse of that of writing, i.e., using electromagnetic induction and it was the technique
used in earlier hard disks. But as the storage density increased, it became very difficult to
read a bit from the disk surface as there was the interference of magnetic fields from the
neighboring bits. In this seminars I present a discovery that revolutionized the hard disk
industry by solving this problem. It was the discovery of Giant Magnetoresistance Effect
- The phenomenon of significant decrease in electrical resistance in the presence of a
magnetic field.
A small electrical current is kept flowing
through the reading head. When a bit passes under the head, due to the presence of the
magnetic field associated with the bit, the electrical resistance of head changes which
alters the current flowing through it. This change in the current flow is so significant that
it can be easily detected.
The report consists of brief descriptions
about the history and discovery of GMR and two of its applications “ The hard disk
reading head and The Magnetoresistance RAM.Page 7

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
Magnetoresistance is the property of a
material to change the value of its electrical resistance when an external magnetic field is
applied to it. The effect was first discovered by William Thomson (more commonly
known as Lord Kelvin) in 1856, but he was unable to lower the electrical resistance of
anything by more than 5%. This effect was later termed Anisotropic Magnetoresistance
(AMR) to distinguish it from GMR.
Fig. 1 - William Thomson (Lord Kelvin)Page 8

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
GMR was independently discovered in 1988
in Fe/Cr/Fe trilayers by a research team led by Peter Grünberg of the Jülich Research
Centre (DE), who owns the patent, and in Fe/Cr multilayers by the group of Albert Fert
of the University of Paris-Sud (FR), who first saw the large effect in multilayers (up to
50% change in resistance) that led to its naming, and first correctly explained the
underlying physics.
The discovery of GMR is considered as the birth of Spintronics.
Grünberg and Fert have received a number of prestigious prizes and awards for their
discovery and contributions to the field of Spintronics, including the Nobel Prize in
Physics in 2007.
Fig 2 - Prof.Dr.Peter Grunberg (1988)
(Change of 6%)
Fig 3 - Prof.Dr.Albert Fert (1989)
(Change upto 50%) Page 9

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
The figure is self explanatory. The x axis
gives the magnetic field intensity. And y axis gives the resistance relative to the
resistance in the absence of the magnetic field. The effect is maximum in the case of Iron
with 0.9nm Chromium.
Fig 4 - R/R0 Vs Magnetic fieldPage 10

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
Two or more ferromagnetic layers are separated by a very thin
(about 1 nm) non-ferromagnetic spacer (e.g. Fe/Cr/Fe). At certain thicknesses the RKKY
coupling between adjacent ferromagnetic layers becomes antiferromagnetic, making it
energetically preferable for the magnetizations of adjacent layers to align in anti-parallel.
The electrical resistance of the device is normally higher in the anti-parallel case and the
difference can reach more than 10% at room temperature. The interlayer spacing in these
devices typically corresponds to the second antiferromagnetic peak in the AFM-FM
oscillation in the RKKY coupling.
The GMR effect was first observed in the multilayer configuration,
with much early research into GMR focusing on multilayer stacks of 10 or more layers.
Granular GMR is an effect that occurs in solid precipitates of a
magnetic material in a non-magnetic matrix. In practice, granular GMR is only observed
in matrices of copper containing cobalt granules. The reason for this is that copper and
cobalt are immiscible, and so it is possible to create the solid precipitate by rapidly
cooling a molten mixture of copper and cobalt. Granule sizes vary depending on the
cooling rate and amount of subsequent annealing. Granular GMR materials have not
been able to produce the high GMR ratios found in the multilayer counterparts.
Two ferromagnetic layers are separated by a thin (about 3 nm)
non-ferromagnetic spacer, but without RKKY coupling. If the coercive fields of the two
ferromagnetic electrodes are different it is possible to switch them independently.
Therefore, parallel and anti-parallel alignment can be achieved, and normally the
resistance is again higher in the anti-parallel case. This device is sometimes also called a
spin valve.Page 11

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
Spin valve GMR is the configuration that is industrially
most useful, and is used in hard drives.
The discovery of GMR by Grunberg an Fert took the scientific
community by surprise; physicists did not widely believe that such an effect was
physically possible. These experiments were performed at low temperatures and in the
presence of very high magnetic fields and used laboriously grown materials that cannot
be mass-produced, but the magnitude of this discovery sent scientists around the world
on a mission to see how they might be able to harness the power of the Giant Magneto
resistance effect.
Stuart Parkin and two groups of colleagues at IBM's Almaden
Research Center, San Jose, Calif, quickly recognized its potential, both as an important
new scientific discovery in magnetic materials and one that might be used in sensors
even more sensitive than MR heads.
Fig 5 - Spin-valve GMRPage 12

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
Parkin first wanted to reproduce the Europeans' results. But he did
not want to wait to use the expensive machine that could make multilayers in the same
slow-and-perfect way that Gruenberg and Fert had. So Parkin and his colleague, Kevin
P. Roche, tried a faster and less-precise process common in disk-drive manufacturing:
sputtering. To their astonishment and delight, it worked! Parkinâ„¢s team saw GMR in the
first multilayers they made. This demonstration meant that they could make enough
variations of the multilayers to help discover how GMR worked, and it gave Almaden's
Bruce Gurney and co-workers hope that a room-temperature, low-field version could
work as a super-sensitive sensor for disk drives.
The key structure in GMR materials is a spacer layer of a non-
magnetic metal between two magnetic metals. Magnetic materials tend to align
themselves in the same direction. So if the spacer layer is thin enough, changing the
orientation of one of the magnetic layers can cause the next one to align itself in the same
direction. Increase the spacer layer thickness and you'd expect the strength of such
Fig 6 “ Dr. Stuart Parkin
Fig 7 “ The GMR Reading HeadPage 13

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
"coupling" of the magnetic layers to decrease. But as Parkin's team made and tested
some 30,000 different multilayer combinations of different elements and layer
dimensions, they demonstrated the generality of GMR for all transition metal elements
and invented the structures that still hold the world records for GMR at low temperature,
room temperature and useful fields. In addition, they discovered oscillations in the
coupling strength: the magnetic alignment of the magnetic layers periodically swung
back and forth from being aligned in the same magnetic direction (parallel alignment) to
being aligned in opposite magnetic directions (anti-parallel alignment). The overall
resistance is relatively low when the layers were in parallel alignment and relatively high
when in anti-parallel alignment. For his pioneering work in GMR, Parkin won the
European Physical Society's prestigious 1997 Hewlett-Packard Europhysics Prize along
with Gruenberg and Fert.
Searching for a useful disk-drive sensor
design that would operate at low magnetic fields, Bruce Gurney and colleagues began
focusing on the simplest possible arrangement: two magnetic layers separated by a
spacer layer chosen to ensure that the coupling between magnetic layers was weak,
unlike previously made structures. They also "pinned" in one direction the magnetic
orientation of one layer by adding a fourth layer: a strong anti ferromagnet. When a weak
magnetic field, such as that from a bit on a hard disk, passes beneath such a structure, the
magnetic orientation of the unpinned magnetic layer rotates relative to that of the pinned
layer, generating a significant change in electrical resistance due to the GMR effect. This
structure was named the spin valve.
Gurney and colleagues worked for several
years to perfect the sensor design that is used in the new disk drives. The materials and
their tiny dimensions had to be fine-tuned so they 1) could be manufactured reliably and
economically, 2) yielded the uniform resistance changes required to detect bits on a disk
accurately, and 3) were stable -- neither corroding nor degrading -- for the lifetime of the
drive. "That's why it's so important to understand the science," Parkin says. "IBM's
intensive studies of GMR enabled us to enhance considerably the performance of some
low-field sensors."Page 14

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
The chief source of GMR is "spin-
dependent" scattering of electrons. Electrical resistance is due to scattering of electrons
within a material. By analogy, consider how fast it takes you to drive from one town to
another. Without obstacles on a freeway, you can proceed quickly. But if you encounter
heavy traffic, accidents, road construction and other obstacles, you'll travel much slower.
Depending on its magnetic direction, a
single-domain magnetic material will scatter electrons with "up" or "down" spin
differently. When the magnetic layers in GMR structures are aligned anti-parallel, the
resistance is high because "up" electrons that are not scattered in one layer can be
scattered in the other. When the layers are aligned in parallel, all of the "up" electrons
will not scatter much, regardless of which layer they pass through, yielding a lower
To see animations of how MR and GMR
heads work and of how electrons of different spins scatter within a GMR structure,
please follow the link
Fig 8 “ The Physics of GMRPage 15

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
These are two important successors of GMR.
They stand for Tunnel MR and Current Perpendicular to the Plane MR respectively.
In physics, the tunnel magnetoresistance
effect (TMR), occurs when two ferromagnets are separated by a thin (about 1 nm)
insulator. Then the resistance of the tunneling current changes with the relative
orientation of the two magnetic layers. The resistance is normally higher in the anti-
parallel case. It was discovered in 1975 by Michel Julliere, using iron as the ferromagnet
and germanium as the insulator. Room temperature TMR was discovered in 1995 first by
Terunobu Miyazaki and independently by Moodera et al. following renewed interest in
this field fueled by the discovery of the giant magnetoresistance effect. In 2005, the first
drives to use TMR heads were introduced by Seagate allowing 400 GB drives with 3
disk platters (60Gb/square inch).
CPP-GMR is the HITACHI version of PMR
(Perpendicular Magneto Recording). Hitachi believes CPP-GMR heads will enable hard
disk drive (HDD) recording density of 500 gigabits per square inch to one terabit per
square inch, a quadrupling of today's highest areal densities. Today the company has
achieved 345 Gbits/square inch in their Desktop hard disk Deskstar 7K1000 (1TB
capacity) and by 2011 they expect to release 4TB disks.Page 16

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
Magnetoresistive Random Access Memory
(MRAM) is a non-volatile computer memory (NVRAM) technology, which has been
under development since the 1990s. Continued increases in density of existing memory
technologies -- notably Flash RAM and DRAM -- kept MRAM in a niche role in the
market, but its proponents believe that the advantages are so overwhelming that MRAM
will eventually become dominant.
Unlike conventional RAM chip technologies,
in MRAM data is not stored as electric charge or current flows, but by magnetic storage
elements. The elements are formed from two ferromagnetic plates, each of which can
hold a magnetic field, separated by a thin insulating layer. One of the two plates is a
permanent magnet set to a particular polarity; the other's field will change to match that
of an external field. A memory device is built from a grid of such "cells".
Reading is accomplished by measuring the
electrical resistance of the cell. A particular cell is (typically) selected by powering an
associated transistor which switches current from a supply line through the cell to
ground. Due to the magnetic tunnel effect, the electrical resistance of the cell changes
due to the orientation of the fields in the two plates. By measuring the resulting current,
the resistance inside any particular cell can be determined, and from this the polarity of
the writable plate. Typically if the two plates have the same polarity this is considered to
mean "0", while if the two plates are of opposite polarity the resistance will be higher
and this means "1".
On comparison with existing memory
technologies, MRAM is faster than SRAM, have a higher storage density than DRAM,
the power requirement is less than that of DRAM and it is faster than FLASH.
MRAM is the Memory of the future. If the
researches turn up, it will replace both Volatile and Non Volatile Primary memories.Page 17

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
The introduction of the GMR head in 1996
by IBM lead to a period of rapid areal density increases of about 100% per year. GMR
based primary memory shall be replacing both the volatile and non volatile primary
memories. Since the discovery of GMR effect has influenced both the primary and
secondary storage of the computer, it may be considered the discovery meant for
computer memory. Page 18

Giant Magnetoresistance Effect
Department of Computer Science CUSAT
7. A dictionary of Chemistry, Oxford University Press,4th Ed., page 348

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